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Cheap LiFePO4 BMS?


jetzi

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25 minutes ago, IanD said:

I looked at this last year and talked to Beta at some length about it. It turned out that the solution with lowest cost and least worry about belt loading was to use 2 of the standard Beta alternators (external controller optional and expensive), not one of the really big non-standard jobbies like Balmar (external controller required) -- I was looking at 2 24V alternators in series to generate 48V, but I expect the same will apply to 2 12V ones in parallel.

 

Also note that a lot of the high-power alternators/Travelpower need higher revs than normal to achieve their headline rating -- you can even see this with the Iskra 80A/100A alternators, unless you rev the engine the 100A one doesn't give any more current than the 80A one.

 

For any of these cases, providing cool air to the alternators (fan plus ducting?) is a very good idea to try and prevent overheating, especially if you're drawing a lot of current at low rpm. Even so they're going to get pretty hot, they like being spun faster so the internal cooling fan is more effective.

 

These figures are for a Beta 43, you can also see that there's a pretty heavy torque loading at low rpm which will pull the engine revs down, so an external controller which can dial back current at idle would help -- the Wakespeed is the best at this but cost about £700 in the UK last time I looked... 😞

 

alternators.png

 

Thanks Ian, as it happens I've sort of stumbled down the route of two alternators to avoid the expense and complication of a single bug alternator, but I'll be pretty much wringing their necks when charging, and I'll try to remember to put on some revs, and not leave it at tickover. 

 

I have wondered about the bilge blower/cooling fan idea, but the ones I've looked at draw 5 or 6 amps. From earlier tests, I think if I reduce the alternator output by 5 amps from say 50ish to 45 ish, then it probably wont overheat very much in the first place. 

 

I read a huge thread here that explained the use of these fans, and even the optimum positioning etc, and it seems they can knock about 10 degrees or so off of the running temp. 

I think its one for me to give some serious thought to. 

I f a 5 amps fan meant I could push another 15 amps from the alternators, that would be a good shout. 

I do remember though, last January, I had my engine board off and was experimenting with directing a strong fan at the alternator whilst it was putting out 60 amps and 70 amps (I was adjusting the output current by the very belt and braces method of using varying lengths of 16mm and even 10mm cable), but if memory serves, at those higher output levels the fan didnt do enough, and I had to dial back the output.

 

 

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I spoke to Beta about an alternator upgrade on my 38, they were quite insistent that 75A + 40A is the highest it can go. My engine is an older model that isn't the same as the current Beta 38s. It has two V belts (not poly-vee).

 

My engine electrics are in a state of disorganised equilibrium. My ignition "on" doesn't work, neither do temperature and oil pressure lights. I put a switch on my domestic alternator and when I turn on my engine now I can choose whether to run the alternator or not (the battery light for my domestic alternator does work when I turn this on). The second battery light - which I assume is for my starter alternator - is not currently connected, and my starter alternator doesn't do anything at the moment (and has never since I bought the boat - I installed a VSR to charge my starter battery from my domestic alternator immediately after buying the boat).

 

Where I went "wrong" is installing so much solar that I have no need for alternators for 8 months of the year. That's kind of meant that the electrics have dropped way down on my very long boat maintenance to do list. Which is a shame because I think it's probably the most enjoyable job!

 

I did play around with trying to get the starter alternator to work, with the domestic totally disconnected, and I was unsuccessful. I haven't tried very hard yet. I have a spare of each alternator, so I can switch out the starter alternator if necessary. My first goal will be to get the starter alternator working in isolation with just my LA battery. I can then see the voltage it's regulated at, and at this point I can decide whether to just straight parallel them or if I need to do something smarter.

 

My domestic alternator sits at about 40A of current only. I have got it higher by putting thicker cables on it but then I run into overheat. So again I left it as "tomorrow's job".

 

However, 40A is not a lot to charge 640Ah of LiFePOs in the dead of winter. So I do need to sort this out.

Edited by jetzi
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4 hours ago, jetzi said:

 

My domestic alternator sits at about 40A of current only. I have got it higher by putting thicker cables on it but then I run into overheat. So again I left it as "tomorrow's job".

 

However, 40A is not a lot to charge 640Ah of LiFePOs in the dead of winter. So I do need to sort this out.

 

 

Perhaps you could fit the spare domestic alternator in place of the 40 amp unit? 

If you can make it fit and find a belt to fit, that could bring your charge up to 80 amps. 

 

Mine is a canaline 38 (a rebadged Kioti), and my two were rated at 100 amps and 50 amps. 

 

My solution (at least for this year) was to have the smaller alternator replaced with a 90 amp Prestolite unit (it was fairly easy to adapt to fit), and then to set up two B2B systems, one running from each alternator.  Having two separate systems meant I didnt need to find a way of matching the voltages to make them both work together, but the B2B units arent cheap.. 

 

I think I use about 120Ah per day, so in winter I had been looking at 3 or more hours of engine running per day, but hopefully now it'll be more like 90 mins, or less if I get any decent solar.

 

 

Edited by Tony1
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4 hours ago, jetzi said:

I spoke to Beta about an alternator upgrade on my 38, they were quite insistent that 75A + 40A is the highest it can go. My engine is an older model that isn't the same as the current Beta 38s. It has two V belts (not poly-vee).

 

My engine electrics are in a state of disorganised equilibrium. My ignition "on" doesn't work, neither do temperature and oil pressure lights. I put a switch on my domestic alternator and when I turn on my engine now I can choose whether to run the alternator or not (the battery light for my domestic alternator does work when I turn this on). The second battery light - which I assume is for my starter alternator - is not currently connected, and my starter alternator doesn't do anything at the moment (and has never since I bought the boat - I installed a VSR to charge my starter battery from my domestic alternator immediately after buying the boat).

 

Where I went "wrong" is installing so much solar that I have no need for alternators for 8 months of the year. That's kind of meant that the electrics have dropped way down on my very long boat maintenance to do list. Which is a shame because I think it's probably the most enjoyable job!

 

I did play around with trying to get the starter alternator to work, with the domestic totally disconnected, and I was unsuccessful. I haven't tried very hard yet. I have a spare of each alternator, so I can switch out the starter alternator if necessary. My first goal will be to get the starter alternator working in isolation with just my LA battery. I can then see the voltage it's regulated at, and at this point I can decide whether to just straight parallel them or if I need to do something smarter.

 

My domestic alternator sits at about 40A of current only. I have got it higher by putting thicker cables on it but then I run into overheat. So again I left it as "tomorrow's job".

 

However, 40A is not a lot to charge 640Ah of LiFePOs in the dead of winter. So I do need to sort this out.

With that alternator configuration (two V belts only) your only option is to squeeze as much current as you can out of your existing alternators (maybe via B2B?) while trying to keep them cool (fans and ducting?).

 

Other than that you'd have to look at installing a poly-V drive pulley, but this may not be possible with your Beta 38 depending on how the crankshaft front-end is built -- basically, whether Beta have left the splined power takeoff on or cut it off...

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10 hours ago, IanD said:

With that alternator configuration (two V belts only) your only option is to squeeze as much current as you can out of your existing alternators (maybe via B2B?) while trying to keep them cool (fans and ducting?).

 

Other than that you'd have to look at installing a poly-V drive pulley, but this may not be possible with your Beta 38 depending on how the crankshaft front-end is built -- basically, whether Beta have left the splined power takeoff on or cut it off...

 

I also spoke to an alternator expert on the phone recommended by someone on the forum here (can't remember his name, I think it was Ed). He was very full of enthusiasm and gave me half an hour of free advice - clearly something he knows plenty about.

 

I feel very sure that it's not possible (or not advisable) to change the pulleys or try to upgrade the alternators on the Beta 38. Everything is sized to the current configuration, including the skin tanks, so upgrading them might result in engine overheat. I really don't know what I'm talking about here, so I have to rely on the experts - and if both Beta and an independent alternator expert with no skin in the game (actually, he could have done the work - so he stood to benefit if anything) say that I shouldn't do it, then I'm going to listen to them.

 

I got through the last winter OK and it wasn't so bad, I was just running my engine more than I'd have liked. I feel like incremental steps will help a lot - 1) adding the 40A starter alternator into the mix, and 2) getting more out of my 75A alternator with a thicker cable.

 

I have gotten over 60A out of my 75A alternator simply by connecting a 70mm^2 negative cable directly from my LiFePOs to the alternator body. So somewhere along the circuit there is a negative connection that is acting as a resistor, probably getting hot, and reducing the alternator output to a very cool 40A, which in the absence of a blower has suited me just fine.

 

I think I might just go and have a tinker now.

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First off, trying to get the original starter alternator to work in isolation.

 

Couldn't get the old starter alternator to do anything in situ, so I installed my replacement. I removed the "energiser wire" from the domestic alternator and put it on the new alternator, so that the domestic alternator wouldn't confound the results and so that I have a wire that I know works.

 

Turning on the "ignition" turns on the battery light (energiser), and when I start the engine the light goes out which suggests that the alternator has started to generate current. However, voltage measured across the alternator body and the +ve stud measures just a tad less than the battery voltage, which suggests that it's not doing anything, and my clamp ammeter doesn't measure anything significant out of the +ve either.

 

I wonder if it might due be a slight difference between the old and new alternator terminals. Not including the main +ve stud, the old starter had 2 terminals, each with a connector that had 2 wires (2 pairs - total of 4 wires). I expect the one is for the energiser, and the other a tachyometer (that I don't have). I don't know why there would be pairs. The new alternator has 3 terminals and the main positive stud.

 

So far I have only connected the energiser wire and the positive. That's all the domestic requires to function but I am going to re-read @Tony Brooks's alternator page and perhaps I'll learn there is something essential about the other terminals. Taking a lunch break now, but if anyone has any ideas that would be great :)

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We need to know make and model of the alternator, in case it's a six diode machine that needs another positive feed. Alternatively, how are the terminals labelled or post a photo of the terminal end.

 

You may even have a proper insulated return alternator with two large stud terminals  (B+ & B-) and if so you need to measure between the two terminals.

If your engine and domestic bank negatives are not linked, then using a domestic warning lamp on an engine alternator may have odd results.

 

A typical 9 diode alternators will have a large B+ terminal, a small D+ (W/L) terminal and possibly a small phase tap terminal to feed a rev counter (this one can be left disconnected). It may also have a large B- terminal.

 

A typical six diode machine usually has an additional Ign terminal that takes a live feed from the relevant battery when you turn on the ignition.

 

Rare battery sensed alternators also have a small S terminal that is connected to the battery or master switch positive, but you are only likely to find them on split charge systems that use a passive split charge diode.

 

 

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Thanks Tony, this is the alternator (https://www.ebay.co.uk/itm/233826618617

 

I first of all tried to connect to the centre terminal ("ignition") but the lamp didn't come on, so then I connected it to the top terminal ("lamp"), which produced the behaviour I mentioned (light comes on when ignition on, light goes off on engine start, but alternator doesn't generate current).

 

Like I said the one it's replacing only had 2 connectors so I'm just hacking it together for now - literally the wire goes from +ve battery terminal, to a switch, to the battery light, to the alternator.

 

The wiring of my engine is an unholy mess :(

image.png.1a5b80915dfdf382123947db6001f3a9.png

 

image.png.23d4bc7c9f4b6d1d2307e781f8b95d5a.png



 

 

26 minutes ago, Tony Brooks said:

If your engine and domestic bank negatives are not linked,

 

The negatives are linked, the positives are linked with a VSR that kicks in once the starter is charged. The "domestic" alternator (my only alternator) currently charges the starter battery first. I want to set this up so that both alternators charge the starter first, then both alternators charge the Lithiums.

 

In fact, I could probably just add the starter battery to the bank and just start the engine using the lithiums, get rid of the VSR and simplify the whole thing that way.

Edited by jetzi
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Looks like a 6 diode machine. You need to connect the warning light to L, and separately a feed direct from the ignition switch to IG. The presence or absence of 12v on this latter is what turns the alternator on and off.

 

P is for feeding a tacho, which you don’t have.

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Great, thanks. I will try that. I managed to get the small alternator running in isolation by connecting the warning lamp feed to the middle "ignition" terminal, and no lamp connected to the lamp terminal. In this case the lamp doesn't light (strange) but the alternator does begin generating.

 

In isolation, the starter 40A alternator shows 14.39V over the +ve stud and the alternator body while running. I read that it's generating 14A with my ammeter on the cable from the +ve stud. This dropped to about 12A once the alternator reached 60 degrees (as measured with an IR thermometer aimed at the windings).

 

In isolation, the domestic 75A alternator shows 14.49V over the +ve stud and the alternator body while running, and generating 20A (only) at the moment.

 

I tried to parallel them to see what happens, by connecting ignition on -> warning lamp -> IG on the starter alternator & D+ on the domestic alternator. In this case, both alternators read 14.45V across +ve and body, the domestic continues to put out 20ish amps, but the starter produces nothing.

 

So I deduce from this that the starter alternator is regulated at a lower voltage than the domestic, and they can't simply be paralleled.

(Of course the larger problem is that my domestic is performing poorly, I know how to fix that - I have my "winter negative cable" that I'll connect either once the ambient temperature is a bit lower or when I can set up fans and ducting)

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Not strange, it is the expected behaviour for a 6 diode machine. You should connect things as per my previous post.

 

The voltage differences between the alternators is not great, but it means that the 75A alternator will take more of the “load” than the smaller one. At the moment you have so much voltage drop in the wiring that the voltage at the alternator output isn’t getting pulled down below the regulated voltage, hence the 20A. If you parallel the alternators and then insert your “winter wiring” then the smaller alternator will start to produce output, but with that 0.1v regulated voltage difference the 75A one will be operating at or close to its rating, the smaller one rather less so.

 

If you wanted to be clever, the way to get the alternators to “share nicely” would be to eliminate the excessive resistance in the charging wiring and then intentionally insert some resistance in the form of an appropriate length of not-too-fat wiring in the feed from the larger alternator, whilst keeping the smaller one directly connected. With the correct selection of wiring resistance, the larger alternator’s wiring would create a voltage drop of 0.1v or so at some sensible current eg 40A (to stop the alternator overheating) and meanwhile the lower voltage encountered by the smaller alternator would encourage it to produce some output. 

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25 minutes ago, nicknorman said:

Not strange, it is the expected behaviour for a 6 diode machine. You should connect things as per my previous post.

The reason I said it was strange was this logic - if there is current flowing through the ignition->lamp->IG terminal wire, which it must be in order to energise the alternator, at it must be energising given that it's producing current, then the lamp should light?

 

25 minutes ago, nicknorman said:

If you wanted to be clever, the way to get the alternators to “share nicely” would be to eliminate the excessive resistance in the charging wiring and then intentionally insert some resistance in the form of an appropriate length of not-too-fat wiring in the feed from the larger alternator, whilst keeping the smaller one directly connected. With the correct selection of wiring resistance,

My "winter wiring" actually has this length already to keep the current to around 50A in order not to overheat the alternator.

 

Interestingly the resistance on my domestic alternator is on the negative. The current seems to not like flowing through the engine block to the negative cable.

 

Thanks for the encouragement, I will try this and see if it makes an improvement.

 

image.png.8e0100bab9b697e06047a633a2153e77.png

 

Edited by jetzi
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10 minutes ago, jetzi said:

The reason I said it was strange was this logic - if there is current flowing through the ignition->lamp->IG terminal wire, which it must be in order to energise the alternator, at it must be energising given that it's producing current, then the lamp should light?

 

 


The IG terminal senses voltage, it doesn’t take any significant current. Thus when the light is connected, the resistance of the lamp doesn’t case any voltage drop (no current) and so doesn’t light up (no current).

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1 minute ago, nicknorman said:

The IG terminal senses voltage, it doesn’t take any significant current.

If the IG terminal is a voltage sense, then perhaps I can abuse that to get the alternator to put out current even when the domestic is running by connecting it to a lower voltage source?

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Just now, jetzi said:

If the IG terminal is a voltage sense, then perhaps I can abuse that to get the alternator to put out current even when the domestic is running by connecting it to a lower voltage source?

Possibly but it will depend on the detail of the alternator internals (above my pay grade to know that specific alternator). It might be that the IG terminal is merely on/off according to whether the voltage is above or below a threshold. Or maybe as you suggest it remotely senses voltage.

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After a little experimentation, I connected my winter wire directly between the negative bus of my LiFePO4 bank, and the housing of the starter alternator.

This has resulted in ~40A from the domestic and ~20A from the starter, which seems sufficiently low output to keep both alternators below 50 degrees centigrade.

 

I tried a thicker cable here and got a similar result, but it looked like more of the current was coming from the domestic alternator in this case, so it doesn't seem like an improvement if I do that.


I think I'll install a big on/off on my winter wire, so that I can have an extremely rudimentary version of Nick's "fast/slow" charge toggle, and then combined with my separate switch on for my alternators, I can have no charging, slow charging, fast charging. Rather ugly but it will do for the time being!
 

I may spend a bit more time hunting down other sources of resistance to try to bring that 60A number up a bit, especially if I can do it in combination with more advanced alternator cooling, but 60A is sort of sufficient for my needs when outside of solar season anyway.

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23 minutes ago, jetzi said:

After a little experimentation, I connected my winter wire directly between the negative bus of my LiFePO4 bank, and the housing of the starter alternator.

This has resulted in ~40A from the domestic and ~20A from the starter, which seems sufficiently low output to keep both alternators below 50 degrees centigrade.

 

I tried a thicker cable here and got a similar result, but it looked like more of the current was coming from the domestic alternator in this case, so it doesn't seem like an improvement if I do that.


I think I'll install a big on/off on my winter wire, so that I can have an extremely rudimentary version of Nick's "fast/slow" charge toggle, and then combined with my separate switch on for my alternators, I can have no charging, slow charging, fast charging. Rather ugly but it will do for the time being!
 

I may spend a bit more time hunting down other sources of resistance to try to bring that 60A number up a bit, especially if I can do it in combination with more advanced alternator cooling, but 60A is sort of sufficient for my needs when outside of solar season anyway.

With the alternators charging you can check the voltage difference (voltage drop) between the alternator positive and battery positive, and then between the alternator casing/negative and battery negative. This will tell you whether the problem is in the negative line or positive line. Then you can work along the circuit looking for where the voltage drop is. Battery isolator switches are always a good place to start!

 

As to the no charge - 1/2 charge - full charge thing, bear in mind that big alternator is a 9 diode machine and therefore can’t be switched on and off like the 6 diode smaller alternator can. Yes you can disconnect the warning light wire and this will probably stop the alternator from starting, but it won’t stop it once the engine is running. And residual magnetism in the rotor might cause it to start working anyway, at high rpm.

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I will try hunt down the volt drop. But it is really kind of a feature rather than a bug, because especially in the absence of a cooling system i dont want to overload the alts.

 

Yeah, I'm aware that disconnecting the warning light wire only stops the alternator from starting, not from continuing to charge once the engine is running. I have been using it this way for some time now since during solar season I almost never need to use the alternator, I have to restart the engine if I want to stop the alternator from charging. I'm aware that they can spontaneously start due to residual charge, but so far I haven't experienced this. In the case this happens, id be relying on the BMS shutoff.

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  • 2 weeks later...
On 15/09/2021 at 22:53, nicknorman said:

The clue will surely be to measure the input voltage to the B2Bs. If the alternator is keeping up, the input voltage will be up around mid to high 13s. If the alternator can’t keep up then surely the B2B will just shut down due to low voltage at the input. It then takes 5 seconds to restart again. So you would see large slow fluctuations in the charge current on the BMV.

 

Hi Nick- apologies for hassling you once again, but I have an electronic-type query that I've been wrestling with today- so far without success- and I suspect you may understand the issue immediately, if you can possibly find time to help.

 

You may remember I ended up with a right old mish mash of a charging system, using four B2Bs and two MPPTs, all of which could be switched off via a COM wire leading from the BMV712. 

The plan is that when stationary charging I use all four B2Bs and set the engine to about 1400rpm. 

At this speed the alternators dont get too hot, and the total charge is about 90 amps, which is ok for my needs.

During normal cruising the engine rpm is very variable, so I just use two of the B2Bs in order to obtain a lower charge, but also to reduce the stress on the alternators.

 

It was all working fine, but I have recently created a snag. After seeing the voltage creep up to 14.3v in recent days (with both solar and engine charging), I tried to set up an additional switch off feature using a BMV700 unit that I had lying around (I later swapped to a BMV712). 

My plan was to use the BMV700 to force the chargers to switch off at dangerously high voltages- an emergency measure. 

I got the third shunt and the BMV700 plugged in last night, but it was too late to play around with it, and this morning I noticed that the MPPTs were no longer responding to the BMV712, (the one that I had set up for a high SoC switch off). 

 

I think the relay on the BMV700 was set open overnight because of a default setting working with my battery state at the time), and this seems to have buggered up my MPPTs. 

Every time I plug in their control cable from the BMV712 (they use special Victron cables), they think the BMV712 is telling them to switch off- even when the other end of the control cable is unplugged from the BMV712 and hanging loose.

I've tried a factory reset, and disconnecting them completely (including from the panels), but they seem to be stcuk with this incorrect response. 

 

Now this is the bit that I think will give you a clue as to what is going wrong:  the B2B units all still switch on and off as expected when the BMV712 relay opens and closes, but here's the odd thing- when the B2Bs are switched on, the MPPTs seem to stop thinking they are constantly being told to switch off, and they switch on and work normally.

But as soon as the B2Bs stop running, the MPPTs very quickly switch off as well. even when the BMV712 relay is closing and telling them to work. 

 

Its as if the B2Bs being switched on is providing an extra current that is helping the MPPTs to respond as they should, but when the B2Bs are off, the MPPTs refuse to switch on at any time, until I take out the control cable (it uses the VE socket).

Once I take out the control cable the MPPTs start charging again. 

 

I think I've changed something inside the MPPTs by plugging in the BMV700, but I have no idea what. 

Can you recognise from this description what the fault might be? 

 

 

Edited by Tony1
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Apologies Nick, I tried to edit the post above to say that its now behaving normally again, but it wont let me edit. 

 

I've no idea why its started working again-  I connected up a new BMV712 this afternoon, and the MPPT auto switch off function (via the BMV) started working again after I did a short engine run to try a few settings out, and test that the new BMV would switch off the chargers at the right values.

So I now have one BMV that will disconnect only the loads if a low SoC or low voltage event occurs. 

I have a second BMV that will switch off all the chargers if a high SoC event occurs (meant to be used as a way of day-to-day management of the SoC, to keep it within reasonable limits during the summer solar surge, without me having to check it periodically). 

And I now have a third BMV, which is also set to switch off the chargers (I spliced into the same control wires used by the second BMV), if the voltage goes above 14.5v.

This one is meant as an emergency disconnect, and to make it more reliable I'm considering getting a Victron battery protect, which will physically disconnect all the chargers in the event of a dangerously high voltage- which should never happen once I start fine tuning the various charger settings. 

 

Having now seen that the current charger switch off can be a bit flaky, I'm not sure about relying on it for the 'emergency' last-ditch high voltage disconnect- the one that we hope never happens, of course.

 

 

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3 hours ago, Tony1 said:

Apologies Nick, I tried to edit the post above to say that its now behaving normally again, but it wont let me edit. 

 

I've no idea why its started working again-  I connected up a new BMV712 this afternoon, and the MPPT auto switch off function (via the BMV) started working again after I did a short engine run to try a few settings out, and test that the new BMV would switch off the chargers at the right values.

So I now have one BMV that will disconnect only the loads if a low SoC or low voltage event occurs. 

I have a second BMV that will switch off all the chargers if a high SoC event occurs (meant to be used as a way of day-to-day management of the SoC, to keep it within reasonable limits during the summer solar surge, without me having to check it periodically). 

And I now have a third BMV, which is also set to switch off the chargers (I spliced into the same control wires used by the second BMV), if the voltage goes above 14.5v.

This one is meant as an emergency disconnect, and to make it more reliable I'm considering getting a Victron battery protect, which will physically disconnect all the chargers in the event of a dangerously high voltage- which should never happen once I start fine tuning the various charger settings. 

 

Having now seen that the current charger switch off can be a bit flaky, I'm not sure about relying on it for the 'emergency' last-ditch high voltage disconnect- the one that we hope never happens, of course.

 

If you want to connect multiple BMV relays to the same devices, it needs careful thinking. I am getting confused between your system, Jetzi's and Richards, so can you remind me what is connected to the 3 relay terminals (NO, COM, NC) of the original BMV?

If there is just something connected to NO and COM (positive to NO and the devices to COM) then it will be easier. One of you has the NC conntected to 0v, if that's you we will have to rethink.

 

Bottom line with the MPPT is that when you plug in the cable, it recognises that it is being remotely controlled. If the wires at the other end aren't connected to anything, they are pulled down to 0v and the devices will be off by defauilt. So with the cable plugged in, you have to have 12v across the control wires for it to switch on. Fortunately this is the same for the B2Bs, which is why you can connect all 4 device control inputs together. So to allow any of the BMVs to turn the devices off, the first BMV needs to be connected as I mentioned above, and the second BMV needs to be in series with the wire from COM on first BMV to the devices, so that wire comes in on NC and out on COM to the devices. That way, when either of the relays operate, the +12v signal to both the MPPTs and the B2Bs will be removed and the pulldown to 0v built in to the MPPT interface, and probably also to the B2B interface, will turn everyting off.

 

If this doesn't work I think you are really going to have to sketch out how you have got the relays etc connected, because at the moment I've had quite a bit of GandT and am rather confused between your setup and the other folk's setups!

Edited by nicknorman
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28 minutes ago, nicknorman said:

If you want to connect multiple BMV relays to the same devices, it needs careful thinking. I am getting confused between your system, Jetzi's and Richards, so can you remind me what is connected to the 3 relay terminals (NO, COM, NC) of the original BMV?

If there is just something connected to NO and COM (positive to NO and the devices to COM) then it will be easier. One of you has the NC conntected to 0v, if that's you we will have to rethink.

 

Bottom line with the MPPT is that when you plug in the cable, it recognises that it is being remotely controlled. If the wires at the other end aren't connected to anything, they are pulled down to 0v and the devices will be off. So with the cable plugged in, you have to have 12v across the control wires for it to switch on. Fortunately this is the same for the B2BS,

 

Thanks Nick- as I understand it, I have a small fused red wire (an unused spare taken from the BMV712 box actually) that goes from battery live to the NO port. 

There is also a black wire from the neg bus bar to the NC port. 

From the COM port I have a cable that is split off into six, so that there is one leading into each of the three Sterling B2Bs, one into the remote switch port on the Victron B2B (they say you need a special £25 cable, but it works with a single strand of 18AWG). The MPPTs each have a dedicated victron cable that joins onto the the COM wire, and plugs into the VE port on the MPPT.

 

I have the high SoC disconnect set such that it actually uses the 'low SoC' parameter on the BMV712- it works like this because the relay is not inverted, so that it is closed below 85% SoC, but it then opens when the SoC gets above the set 85% value for low SoC- so its all in reverse.

 

The high voltage disconnect, which is the job of the new BMV712, currently uses the same 'reverse' principle, so it uses the 'low voltage' relay parameter, and in fact uses the same three control wires as does the high SoC BMV unit. I used tiny junction boxes to split each of the existing three cables into two. 

 

So its a bit of a dogs breakfast of wiring, but on my last test this afternoon (the only one where I actually ran the engine and fired up the B2Bs properly), for reasons I cant understand but would be very interested to know, both BMV712s actually started to work with the MPPTs as I planned.

Both BMV712s were able to switch the MPPTs on and off again (tested as I normally do by varying the set cutoff values for the relay). 

Prior to that final test , I could not get the MPPTs to switch on for as long as their control cables were plugged in. I tried the cables fully plugged in and 'enabled', and I tried disonnecting the MPPTs during the 'switch off' triggers, and then reconnecting them before I sent the switch on trigger by causing the relay to close. No matter what I did, as long as the VE cable was plugged in, the MPPTs would not switch on.

The only time that behaviour changed was when I switched on the B2Bs and ran the engine, at which time the MPPTs become responsive to ON and OFF signals from the BMV. 

 

I think the issue has resolved itself somehow during the engine run test, but I've no idea how, and it has rather lessened my confidence in using these tiny control cable systems as a high voltage protection. 

 

 

 

 

 

Edited by Tony1
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13 hours ago, nicknorman said:

 

...to allow any of the BMVs to turn the devices off, the first BMV needs to be connected as I mentioned above, and the second BMV needs to be in series with the wire from COM on first BMV to the devices, so that wire comes in on NC and out on COM to the devices. That way, when either of the relays operate, the +12v signal to both the MPPTs and the B2Bs will be removed and the pulldown to 0v built in to the MPPT interface, and probably also to the B2B interface, will turn everyting off.

 

 

Sorry Nick, forgot to add my thanks for this advice, I'll give it a try shortly.

 

At the moment the two BMVs have all three control wires connected in parallel, which is clearly not correct. 

I was messing about with it earlier and I noticed that one MPPT will work at a time, but not both of them at once.  

 

So I have to take the COM wire emerging from the first BMV, and insert that into the NC port on the second BMV (I currently have a black wire from the neg bus in that port).

 

Then from the COM port on the second BMV I lead a cable out to all of the MPPTs and chargers.....

 

Thanks a million, and I'll keep you posted!

 

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15 hours ago, nicknorman said:

If you want to connect multiple BMV relays to the same devices, it needs careful thinking. I am getting confused between your system, Jetzi's and Richards, so can you remind me what is connected to the 3 relay terminals (NO, COM, NC) of the original BMV?

If there is just something connected to NO and COM (positive to NO and the devices to COM) then it will be easier. One of you has the NC conntected to 0v, if that's you we will have to rethink.

 

Bottom line with the MPPT is that when you plug in the cable, it recognises that it is being remotely controlled. If the wires at the other end aren't connected to anything, they are pulled down to 0v and the devices will be off by defauilt. So with the cable plugged in, you have to have 12v across the control wires for it to switch on. Fortunately this is the same for the B2Bs, which is why you can connect all 4 device control inputs together. So to allow any of the BMVs to turn the devices off, the first BMV needs to be connected as I mentioned above, and the second BMV needs to be in series with the wire from COM on first BMV to the devices, so that wire comes in on NC and out on COM to the devices. That way, when either of the relays operate, the +12v signal to both the MPPTs and the B2Bs will be removed and the pulldown to 0v built in to the MPPT interface, and probably also to the B2B interface, will turn everyting off.

 

If this doesn't work I think you are really going to have to sketch out how you have got the relays etc connected, because at the moment I've had quite a bit of GandT and am rather confused between your setup and the other folk's setups!

 

Hi Nick, just an quick update- the rewiring in series as described hasnt resolved the issue, so what I've done is unplug the wires from the second BMV (except for the live wire into NO port), so its just acting as a monitor only. 

My thinking is that before trying to wire up a second BMV, I need to resolve the problem that has recently arisen with the first BMV (the one I set up as a high SoC disconnect). 

 

The MPPTs are not responding to this BMV in the way they did a few days ago, before I started messing about with a second one. 

Before my sabotage, all the MPPTs and B2Bs would switch off and on correctly as per the relay setting changes on the BMV. 

Since my experiments, only one of the MPPTs can successfully be controlled- if I even plug in the cable to the second MPPT, both switch off. Both units behave this way- I reversed their roles and it doesnt matter which unit has the control cable plugged in. If either one of them is plugged in, it works as expected. If the other cable is plugged in, they both switch off. 

 

Another thing I noticed is that if the B2Bs are running, both of the MPPTs will behave correctly, as they used to do (ie both respond to the BMV in unison).

But when the B2Bs switch fully off, the MPPTs go back to the incorrect behaviour. 

 

So before even considering wiring in the second BMV again, I wonder if you can work out why- with a single BMV connected- I can only control one of the MPPTs at a time, even though my wiring setup has been returned to its original state of a few days ago? 

 

The wiring setup is as described above- I tried removing the negative wire from the bus bar that goes into the NC port, but if I do that, the chargers no longer respond to the BMV at all. 

 

I'm now thinking that your view of a few weeks ago was very valid- it might not be wise to rely on the control cables to switch off chargers in the event of a high voltage.

I think what I need, as Richard suggested, is a Victron battery protect unit, which will provide a physical disconnection, and which I can control remotely using one of my BMV712s.  

 

 

 

 

 

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