Cheap LiFePO4 BMS?

[nicknorman]

A few glitches with the new alternator controller software, I sorted most of them but it’s still not quite right in terms of the softness of voltage and field current limit rates of change, and the new feature that limits field current at lower rpm when the charge rate switch is set to Fast.. I’ll take that home and work on it.
I thought I’d give it a run whilst still on shore power, to see what the Li would take. This is the Masterview display which can be configured to show parameters from whatever Mastervolt gadgets one has - in my case, the Combi and the Mastershunt. The top 2 rows are data from the Mastershunt, so the Battery A is the current in/out of the batteries. The bottom row is the Combi data so the Battery A is the current in/out of the Combi:

 

 

So 223A into a battery bank that is already at 93%. Not bad, I thought. It would probably have gone higher if I’d increased the revs (that was about 1300) or increased the voltage on the Combi,  but I had Scotty’s voice in my head telling me the engines couldna take much more of it and something was gonna blow! Of course this is with supplement from the Combi on shore power, but it could equally be the Combi fed by the travelpower.

[MoominPapa]

Looking good.

 

My latest battery activity has been to give in to Victron envy and put my batteries on the internet. I can access the BMS to get data from logs etc over the net, and there's even a web page with a summary status so I can access it from my phone. I draw the line at writing Android apps.

 

MP.

 

[nicknorman]

On 02/04/2021 at 00:21, MoominPapa said:

Looking good.

 

My latest battery activity has been to give in to Victron envy and put my batteries on the internet. I can access the BMS to get data from logs etc over the net, and there's even a web page with a summary status so I can access it from my phone. I draw the line at writing Android apps.

 

MP.

 

Sound interesting, any screenshots? Do you have a web server on board, or do you ftp the data to a server somewhere else?

[MoominPapa]

55 minutes ago, nicknorman said:

Sound interesting, any screenshots? Do you have a web server on board, or do you ftp the data to a server somewhere else?

 

The screenshot shows that I'm a grizzled old systems programmer, having difficulty finding my inner hipster web-designer.

 

The chain is quite long.

 

The serial port on the Arduino is connected to the console serial on the boat's internet router. via an opto-isolated half duplex current loop interface.

The boat's router runs a little daemon which listens on a TCP port and copies data to/from the serial port. There's no protocol translation, so once the TCP connection is set up, the code to talk to this is identical to talking directly over the USB virtual serial line.

The tools I had to talk to the BMS over the serial line got extended to talk over TCP. This is mainly useful because I don't need to plug my laptop into the BMS USB port to get data whilst on the boat now.

 

It turns out that Three mobile internet still gives a real, routable, IP address, and the USB stick modem I use allows port forwarding. I use dynamic DNS to set a record in my DNS zone to the current IP address of the modem, and port-forward to the router behind it, so the BMS is now directly accessible from the global internet with the same TCP interface as before.

 

I have a virtual private server in the cloud running Linux which has a web server on it and I just made a trivial CGI program which runs the data-access tool to get the data, and reformats the data as brain-dead html.

 

So, the chain is webserver -> CGI -> BMStool -> internet -> boat router -> relay daemon -> serial line -> BMS

 

MP.

 

 

 

[jetzi]

On 01/04/2021 at 00:18, nicknorman said:

I mean taking the cells up the knee a bit (which amplifies any SoC differences) and lopping charge off the higher cells using a resistor or whatever. Yes I guess you more or less did this by connecting all in parallel and getting the voltage up to 3.65. As you have found, there is very little charge between 3.4 and 3.6v.

 

Anyway i suppose that is one way of doing it, but not the normal or best way. Not the best way because whilst all the cells were at 3.65v, you don’t know what the current through each cell was. If one cell is at a much higher current than another, even though they are at the same voltage, they aren’t at the same SoC.

 

OK I am being a bit pedantic because in your case the charger was only 5A and so there isn’t going to be a big difference in current or SoC, fractions of a %. I’m just making this point because it would work less well at a higher charge current.

 

So i would say your cells are as well top balanced as can reasonably be expected, no need to leave them connected in parallel.

Thanks, I think I understand what the downside of paralleling them in this way is.

 

In the end I left them overnight, then charged them again to 3.65V (they had subsided to ~3.5V overnight, left them for a few more hours and then charged again to 3.65V. So they were at 100% for about 36 hours which is longer than I'd like but I'm feeling pretty confident that they were well top balanced.

 

Although the lab PSU said it was rated at 5A, the charge current was actually in the 3.5A to 4A range.

 

I'm now taking them down to 3V each before I charge them up again, to check capacity and to try to reset any memory effect. I think this will be about 20%. They have been wonderfully in sync the whole way down so far.

 

 

After reinstalling, I'm a bit concerned about my GWL cell performance monitor. When I turned it on, my Tyco relays failed to trigger, so I don't know that I can trust it to shut off my battery. I tested each of the relays by applying current manually and they are all working, so the problem seems to be in the CPL board. I'm going to contact GWL and see if they know what the problem is. As a precaution I think I'm going to buy one of @Dr Bob's motorised BEP switches and drive it from my BMV712 on low voltage.

[nicknorman]

10 hours ago, jetzi said:

Thanks, I think I understand what the downside of paralleling them in this way is.

 

In the end I left them overnight, then charged them again to 3.65V (they had subsided to ~3.5V overnight, left them for a few more hours and then charged again to 3.65V. So they were at 100% for about 36 hours which is longer than I'd like but I'm feeling pretty confident that they were well top balanced.

 

Although the lab PSU said it was rated at 5A, the charge current was actually in the 3.5A to 4A range.

 

I'm now taking them down to 3V each before I charge them up again, to check capacity and to try to reset any memory effect. I think this will be about 20%. They have been wonderfully in sync the whole way down so far.

 

 

After reinstalling, I'm a bit concerned about my GWL cell performance monitor. When I turned it on, my Tyco relays failed to trigger, so I don't know that I can trust it to shut off my battery. I tested each of the relays by applying current manually and they are all working, so the problem seems to be in the CPL board. I'm going to contact GWL and see if they know what the problem is. As a precaution I think I'm going to buy one of @Dr Bob's motorised BEP switches and drive it from my BMV712 on low voltage.

Just had a look at the GWL spec. It is unsuitable for directly driving the Tyco relay. The Tyco takes about 3A at say 12v, ie 36 watts, just momentarily of course but it needs that belt of power to work. The GWK spec says the maximum short time (150mS) load is 10 watts for some of the outputs, and 0.5watts for others (not sure which one you have used). So it was never going to work!

 

It could be that you have damaged the output stage, or just that you need to insert some kind of relay between the GWL and the Tyco. I’d be reluctant to insert a mechanical relay because of the risk of contacts welding closed, although I suppose if it’s a quality relay and it is only very infrequently going to be used, it would be better than nothing.

Edited April 6, 2021 by nicknorman

[MoominPapa]

24 minutes ago, nicknorman said:

I’d be reluctant to insert a mechanical relay because of the risk of contacts welding closed, although I suppose if it’s a quality relay and it is only very infrequently going to be used, it would be better than nothing.

 

A switching transistor would be better. I used MOSFETs because of the low leakage current, but in reality bipolar would be fine too.

 

MP.

 

[nicknorman]

13 minutes ago, MoominPapa said:

 

A switching transistor would be better. I used MOSFETs because of the low leakage current, but in reality bipolar would be fine too.

 

MP.

 

Definitely! But that might be beyond the understanding of the OP (no offence intended!) which is why I talked about a relay.

[jetzi]

No offence taken! It's definitely is beyond my understanding, though to be fair it all was at the start (and evidently still is, given my oversight with the matching of the Tycoes to the GWL CPM). So I could consider trying to understand how MOSFETs work.

However I think the actual answer is that I need to replace either the Tyco relays with a relay compatible with the GWL CPM, or I need to replace the GWL CPM with something that works with the Tycos. Probably the latter, since I may have damaged the CPM by having the relays attempt to draw too much current. My setup is complicated enough without inserting yet another group of relays (I guess I would need 6, one for each coil of each of the three relays).

[MoominPapa]

4 minutes ago, jetzi said:

No offence taken! It's definitely is beyond my understanding, though to be fair it all was at the start (and evidently still is, given my oversight with the matching of the Tycoes to the GWL CPM). So I could consider trying to understand how MOSFETs work.

However I think the actual answer is that I need to replace either the Tyco relays with a relay compatible with the GWL CPM, or I need to replace the GWL CPM with something that works with the Tycos. Probably the latter, since I may have damaged the CPM by having the relays attempt to draw too much current. My setup is complicated enough without inserting yet another group of relays (I guess I would need 6, one for each coil of each of the three relays).

 

Just a thought. Did you pay attention to the polarity of the Tyco coils? They have built in snubber diodes and if you connect them the wrong way round the diodes will just shunt the coil current and nothing will happen.

 

 

MP.

[jetzi]

58 minutes ago, MoominPapa said:

Just a thought. Did you pay attention to the polarity of the Tyco coils? They have built in snubber diodes and if you connect them the wrong way round the diodes will just shunt the coil current and nothing will happen.

 

MP.

 

Yes I did but I can check this again, thanks. I may have got it wrong. Even so, the specs suggest that the relays draw too much current, so I need to change the equipment.

[jetzi]

On 01/04/2021 at 00:18, nicknorman said:

Anyway i suppose that is one way of doing it, but not the normal or best way. Not the best way because whilst all the cells were at 3.65v, you don’t know what the current through each cell was. If one cell is at a much higher current than another, even though they are at the same voltage, they aren’t at the same SoC.

It's interesting that you say this is not the normal or best way, because this is the way that I have seen most recommended. Notably, by Will Prowse and by GWL power who sell single cell chargers for this purpose (I almost got a 20A one but glad I stuck with the borrowed 5A lab charger in the end). I guess it is probably recommended because it is the easiest way, with the cells in parallel you can be sure that they are at the same voltage. My understanding was that the emptier cells would take more of the charge coming in. The big downside for me is that the cells had to stay near 100% for 36 hours, which I'm sure took some time off their lifespan.

 

I did my initial balancing last year using a immersion heater element to take off charge from the top cells while my solar MPPT was charging the batteries. I had to do it this way as I didn't have shore power. This was a really tricky operation. Swapping the element between the cells while charging was happening was difficult and probably a bit dangerous. In the end I deemed the cells balanced "enough" but I have a much better result now I feel, because I was able to take the cells right up to 100% instead of trying to balance around the 3.45-3.5V mark. As the cells have been discharging their voltages have also remained closer together.

[nicknorman]

1 hour ago, jetzi said:

It's interesting that you say this is not the normal or best way, because this is the way that I have seen most recommended. Notably, by Will Prowse and by GWL power who sell single cell chargers for this purpose (I almost got a 20A one but glad I stuck with the borrowed 5A lab charger in the end). I guess it is probably recommended because it is the easiest way, with the cells in parallel you can be sure that they are at the same voltage. My understanding was that the emptier cells would take more of the charge coming in. The big downside for me is that the cells had to stay near 100% for 36 hours, which I'm sure took some time off their lifespan.

 

I did my initial balancing last year using a immersion heater element to take off charge from the top cells while my solar MPPT was charging the batteries. I had to do it this way as I didn't have shore power. This was a really tricky operation. Swapping the element between the cells while charging was happening was difficult and probably a bit dangerous. In the end I deemed the cells balanced "enough" but I have a much better result now I feel, because I was able to take the cells right up to 100% instead of trying to balance around the 3.45-3.5V mark. As the cells have been discharging their voltages have also remained closer together.

That’s why I said it, because relatively small currents flow between the batteries with small differences in SoC when the cells are all held at the same voltage and so it takes a long time held at high voltage. Better IMO to do it by shunting current away from the higher cells as they start to climb the knee at eg 3.45v, so that no cell gets a voltage above that. You can shunt as much current away as you current shunting device can cope with, so the whole process can be fairly quick.

 

Of course when you are starting out with cells at unknown SoC it is a good idea to connect them all in parallel at some medium SoC and leave them for several days, that gets the “rough edges” off. But most suppliers will deliver cells that are already roughly in balance.

[jetzi]

Since the balance I let the battery run down over 4 days without charging it to try to measure capacity. The BMV showed 470Ah consumed (out of a 640Ah battery) before one of the cells dipped below 3.0V. I probably could have gone lower than that but I'm going to consider that the battery capacity.

[nicknorman]

Well day 2 out and about with my Li batteries after 7 months of not being allowed out of Scotland. Yesterday I charged to nearly 100% - well, one cell hit 3.6v and BMS started beeping, the others were lower so a bit of top balancing needed. BMV 712 and Mastershunt both reset to 100% anyway. We stopped about 4pm, obviously there is the fridge and lighting, plus the sat box and TV (to watch that programme about the flooring company, Lino Duty), gas fan oven for my dins, then electric blanket on number 3 all night (we didn’t bother to light the stove and it was a chilly night) then Mikuni on 7:15 to 9am, the electric kettle for a large pot of tea, Nespresso machine for Jeff, then electric kettle again for Jeff’s porridge-in-a-pot, and then electric kettle on again because he didn’t like that pot of fancy Waitrose “mixed grain” porridge so wanted a different one. Laptop, 2 phones and 2 iPads charged, finally set off at about 9:30 with batteries on 75%. I will have to think of a way to use more power!

Tinkered a bit with the alternator controller, everything fine on slow charge (about 85A) but on fast charge the temperature compensation (reducing max output as the alternator gets hot) wasn’t working after the last code edit. But now fixed. And the output reduction at low revs was a bit too sluggish during a sudden drop in rpm (flicking to idle as moored boats approached etc) so that was tweaked. The ramp up to max field current is nice and slow so you hardly notice it. It seems the Iskra can hold about 130A at about 80C which is not bad IMO.

BMS all wired up to Tyco relay and BMV 712, and I installed a longer battery temperature probe that fits down the gap between 2 cells created by the corrugations in the casing of the CALB cells, thus giving me “core” battery temperature. Alternator is currently using the Mastershunt SoC to decide when to go to float but I think I’ll switch it to using the BMV SoC, or really I should have it using both/either / the most recent data, to give redundancy.

All in all, quite happy and no magic smoke escaped. Yet.
 

Edited April 26, 2021 by nicknorman

[MoominPapa]

7 minutes ago, nicknorman said:

Well day 2 out and about with my Li batteries after 7 months of not being allowed out of Scotland. Yesterday I charged to nearly 100% - well, one cell hit 3.6v and BMS started beeping, the others were lower so a bit of top balancing needed. BMV 712 and Mastershunt both reset to 100% anyway. We stopped about 4pm, obviously there is the fridge and lighting, plus the sat box and TV (to watch that programme about the flooring company, Lino Duty), gas fan oven for my dins, then electric blanket on number 3 all night (we didn’t bother to light the stove and it was a chilly night) then Mikuni on 7:15 to 9am, the electric kettle for a large pot of tea, Nespresso machine for Jeff, then electric kettle again for Jeff’s porridge-in-a-pot, and then electric kettle on again because he didn’t like that pot of fancy Waitrose “mixed grain” porridge so wanted a different one. Laptop, 2 phones and 2 iPads charged, finally set off at about 9:30 with batteries on 75%. I will have to think of a way to use more power!

Tinkered a bit with the alternator controller, everything fine on slow charge (about 85A) but on fast charge the temperature compensation (reducing max output as the alternator gets hot) wasn’t working after the last code edit. But now fixed. And the output reduction at low revs was a bit too sluggish during a sudden drop in rpm (flicking to idle as moored boats approached etc) so that was tweaked. The ramp up to max field current is nice and slow so you hardly notice it. It seems the Iskra can hold about 130A at about 80C which is not bad IMO.

BMS all wired up to Tyco relay and BMV 712, and I installed a longer battery temperature probe that fits down the gap between 2 cells created by the corrugations in the casing of the CALB cells, thus giving me “core” battery temperature. Alternator is currently using the Mastershunt SoC to decide when to go to float but I think I’ll switch it to using the BMV SoC, or really I should have it using both/either / the most recent data, to give redundancy.

All in all, quite happy and no magic smoke escaped. Yet.
 

All sounds good. Be warned that you'll still be "tweaking" the code two years hence, if my experience is anything to go by.

 

MP.

 

 

 

[nicknorman]

38 minutes ago, MoominPapa said:

All sounds good. Be warned that you'll still be "tweaking" the code two years hence, if my experience is anything to go by.

 

MP.

Definitely! And I still have a lot to do, eg setting a float voltage appropriate to the target SoC so the current in/out of the battery is zero at float, ditto for the Combi charger, setting charge and float volts according to the SoC selector switch. And ramping the target voltage between charge and float voltage to avoid abrupt changes. And and and...

[jetzi]

Last weekend I put in a new calorifier that has a immersion element. It is dual voltage, 1kW 240V and 600W 12V.

 

I want to connect this up as a dump load since now it is summer by 1.5kW panels charge my 640Ah LiFePOs almost by the time I wake up in the morning.

 

The immersion element came with a digital thermostat for the 12V elemenr where you can set the on and off temperatures, but it only has a 10A relay on the board so it doesn't seem fit for purpose (600W at 12V is 50A).

 

As for the voltage I need to set it up so that it starts the heater just before the solar goes into float (currently set at 13.8V) and then turns it off again before the heater has drawn down to say 60% SoC. I am not using the relay functionality on my BMV712 so I thought that might be something I could use.

 

Owing to the currents involved I think I'm going to have to learn about using MOSFETs to turn on Tyco 190A bistable relays. I have two or three spare that I bought during my BMS installation.

 

I'll have to find an old copy of "electrical engineering for dummies"!!

[nicknorman]

1 hour ago, jetzi said:

Last weekend I put in a new calorifier that has a immersion element. It is dual voltage, 1kW 240V and 600W 12V.

 

I want to connect this up as a dump load since now it is summer by 1.5kW panels charge my 640Ah LiFePOs almost by the time I wake up in the morning.

 

The immersion element came with a digital thermostat for the 12V elemenr where you can set the on and off temperatures, but it only has a 10A relay on the board so it doesn't seem fit for purpose (600W at 12V is 50A).

 

As for the voltage I need to set it up so that it starts the heater just before the solar goes into float (currently set at 13.8V) and then turns it off again before the heater has drawn down to say 60% SoC. I am not using the relay functionality on my BMV712 so I thought that might be something I could use.

 

Owing to the currents involved I think I'm going to have to learn about using MOSFETs to turn on Tyco 190A bistable relays. I have two or three spare that I bought during my BMS installation.

 

I'll have to find an old copy of "electrical engineering for dummies"!!

I think you want to use the BMV to “authorise” the heater, but you will also need a thermostat function to avoid boiling the water in the calorifier. Therefore you could insert the thermostat relay in the control line from the BMV. So then it is a matter of changing from a voltage level signal (on or off) to the 75ms pulses required to turn the bistable relay on and off. MP designed and built such a circuit, I think he published it somewhere on this thread but I’ll leave you to trawl through and find it! FYI the Tyco needs about 3A switching current (but only for 75mS, obviously).

[Markblox]

On 08/05/2021 at 08:30, jetzi said:

Last weekend I put in a new calorifier that has a immersion element. It is dual voltage, 1kW 240V and 600W 12V.

 

I want to connect this up as a dump load since now it is summer by 1.5kW panels charge my 640Ah LiFePOs almost by the time I wake up in the morning.

 

The immersion element came with a digital thermostat for the 12V elemenr where you can set the on and off temperatures, but it only has a 10A relay on the board so it doesn't seem fit for purpose (600W at 12V is 50A).

 

As for the voltage I need to set it up so that it starts the heater just before the solar goes into float (currently set at 13.8V) and then turns it off again before the heater has drawn down to say 60% SoC. I am not using the relay functionality on my BMV712 so I thought that might be something I could use.

 

Owing to the currents involved I think I'm going to have to learn about using MOSFETs to turn on Tyco 190A bistable relays. I have two or three spare that I bought during my BMS installation.

 

I'll have to find an old copy of "electrical engineering for dummies"!!

Just been reading part of this thread having stumbled upon it, very interesting.  Only yesterday I ordered a combined 230V/ 12V immersion element from the same makers of my Calorifier (Surejust) and that has a second safety cutoff for the higher temperatures gained from a second engine coil.  It's also 2.25" BSP.  It's 750W @ 230V and 300W at 12V and I intend to use the relay from my Smart Solar 150/85 controlled by the daylight function at this time of year.  Building a DIY system is beyond me so have installed two 300Ah Victrons with Cerbo etc and use a DC to DC 50A charger to throttle down the 175A Beta 43 alternator. I have the smart BMV but will probably change this to a Smart 712 at a later date.       I have 3 X 460W tilting PV panels and like you want to use the abundance of energy at this time of year to heat the water.

BTW, I have a trad stern and the LI's are in the original battery box and I have two 50W heat pads in there but this winter the temperature never got to below 7C, helped by the Webasto nearby.  Just ordered some specialist aluminium heat reflector to protect from radiant heat as they are so high they are above the battery box and the temperature on top of the engine near the alternator gets to 50C and the batteries get up to just above 30C after a few hours, hopefully this will improve when I install the barrier.   I will probably have another ventilation hole cut and have an extract fan connected to lower the engine bay temperature as I don't want those batteries to get hot and it will help the engine to breath and keep it dryer in the winter.  Here is my system for anyone interested:      https://vrm.victronenergy.com/installation/94592/share/4e3e40ee   

[Markblox]

As I said previously, I have ordered a dual voltage immersion element and want to use the 12V part for a solar dump.  I have a Victron MPPT smart solar and also a Cerbo GX.  The Cerbo has two relays and currently the relay no. 1 which is very programmable, is used to control the heat pads in the winter but would only ever be used for a few days in the winter.  I have used a relay with a diode to avoid a voltage spike damaging the Cerbo relay for the heat pads.  I could use the Cerbo number one relay in the summer months to control a large relay for the 300W, 25A,  12V immersion element and have ordered a Durite 0-772-10 which is rated at 100A.  This is where I need advice as although electrics I am fine with, electronics I struggle with. This Durite relay doesn't have a diode so do I need a different relay entirely or do I need to connect a diode and if so, how, and what one etc etc etc.  This Durite relay is a normally open make or break version I think.  Can anyone help with this as whether I use the Cerbo relay or the MPPT relay I know I have to avoid a voltage spike when it is turned off.

I hope this isn't off topic. 

Regards, Mark.

 

Edited May 12, 2021 by Markblox

[BEngo]

You need to put a diode in reverse polarity across the relay coil.  Nothing huge, perhaps an IN 4146.

 

N

[nicknorman]

Can you still get 1N4146, I suspect they may be obsolete? But anyway as you say, non-critical provided it can cope with the peak current of 400mA or so. A 1N4001 type might be easier to source.

 

As said, to be put in reverse polarity across the relay coil, ie the cathode (normally marked with a bar/line) to the positive of the relay coil, and the anode to the negative

[Markblox]

2 hours ago, nicknorman said:

Can you still get 1N4146, I suspect they may be obsolete? But anyway as you say, non-critical provided it can cope with the peak current of 400mA or so. A 1N4001 type might be easier to source.

 

As said, to be put in reverse polarity across the relay coil, ie the cathode (normally marked with a bar/line) to the positive of the relay coil, and the anode to the negative

Thank you both for that.

[Markblox]

Trying to find a NC relay greater than 30A for this element and Durite don't do one and I can't find one anywhere.  Can anyone point me in the right direction please.  The reason I need a NC is that it will be from relay one of a Cerbo generator control so I need reverse logic.