Cheap LiFePO4 BMS?

[jetzi]

1 hour ago, Dre said:

I have left a lead acid connected to the alternator to which I connected my sterling b2b to charge the Lifepo4 when needed. The sterling can autosense and start charging automatically but I have used the manual function via a little switch so I start it when I want. My model charges at about 50 amps with the option to go half power at the press of a button. It works just fine. 

There are so many ideas in the last 55 pages!! I'm now second guessing why I decided against the B2B! I recall it was just because my 70A alternator is too pathetic. But I'm very happy to hear it is working for you. It does sound that this is mostly a manual process of keeping an eye on your batteries and topping them up with the B2B when the solar isn't cutting it? I'm very curious to hear what your experience of that will be like in the winter.

 

1 hour ago, Dre said:

For top balancing I used a cheap (but pretty nifty) power supply that works with anything up to 32V DC. Like you I got the bank charged with solar and stopped when the first cell got up to about 3.5v (that's when the out of balance increased quite dramatically). Then connected the power supply to the lead acid and top balanced the whole bank. I'm also full time liveaboard so no mains either!

That's great. I think I probably have a similar power supply unit - it's advertised as 0-5A at 0-32V. I did consider going LA-> inverter -> PSU -> LiFePO but I decided that I'd probably asked far too much of my single, 2 year old, 110Ah, much abused cheapo lead acid battery! I guess with the engine running this is fine? But I can't have the engine running for the potentially days it would take so I'd have to charge it most of the way with my MPPT. But yeah - maybe I should do that. 

 

1 hour ago, Dre said:

All my DC goes through a 120A bms that also has low temperature protect. The bms has a little Bluetooth dongle that allows me to monitor all cells and check and change all the parameters I could possibly need. It also has a good coulomb meter. But I've actually disconnected the Bluetooth now that I'm happy the setup is working as I want a fit and forget system and I didn't want to keep looking at the app! 

Do you shut off the load at low temperature? I thought it was only charging that was really an issue at low temperature.

So you actually have two different load undervoltage protection here, one for the inverter and one for the DC. I also have a victron inverter so I could connect remote cut off to it as well. When designing the system I simply considered a load a load, which is actually not that sensible given that I'd have to really try hard to flatten my battery with just DC loads. 

 

1 hour ago, Dre said:

I've just installed a 1kw element in my calorifier and been getting free hot water (draws about 80amps) while we're getting a lot of solar. 

This is something I want to do too! I bought a dual 12V / 240V heating element for my calorifier that I'm yet to fit. How do you "detect" that you need to dump some load here? Do you just manually turn it on when it's a sunny day? I'd ideally like my system to detect when my battery has gone past 80% SoC, then turn on the calorifier element. That could be another protection against overvoltage, not to mention free hot water. 

 

1 hour ago, Dre said:

A good top balance allowed me to set the mppt to 14.2 and still be perfectly balanced.

Jealous! That does mean you're taking your cells to 3.55V each which is higher than I'd like to routinely go, just in case something does knock the cells out of balance and you end up overcharging one of them. But if it works for you then it works for you. 

 

My approach is belt and braces - be able to cope with 14.2V but then only charge to 13.8. I realise I'm probably being overcautious but I'm also enjoying the process of creating a really robust system.

 

1 hour ago, Dre said:

As for alternator charging the Sterling is plenty good. It probably doesn't have the best profile for Lifepo4s but unlike solar I wouldn't be leaving it charging all day so it probably will never reach full charge anyway. Infact I hope to use it the least possible even in winter. 

Are you expecting to get a decent amount of power from solar in winter? This is going to be my first winter with solar power so I'm verrry curious to see if the pessimism of how much I can expect holds true. I've got about 1500W of panels so I'm hoping that what I get will at least be non-negligible. But I am expecting to need to run my engine a bit, and cruise, and possibly do a bit of our washing at the launderette!

 

1 hour ago, Dre said:

I'm about to order it all again for my friend's boat and at less than a grand for the whole of the above system (not including the solar and inverter of course) is crazy good value considering the capacity (300a) and the performance if lithium! 

Yes that is very cheap, a lot less than I paid in total. (though I do have double the capacity). I should actually go back and add it all up. I guess I'll never know if the second hand Thunderskys were a better bet than buying new cheap cells, and I think I was a bit overzealous with some of the extras. But overall I think I'll save money in the long run as compared to LAs, and I'm happier to destroy recycled LiFePOs than new ones, if that ends up being my fate. 

 

---

 

6 minutes ago, peterboat said:

Its not pointless ... their is no guarantee that cells will stay balanced even in the mid range

But why do you need the cells balanced in the mid range? What's the point of that? It's a genuine question, because my cells certainly are NOT balanced in the mid range - right now they are showing 3.340, 3.385, 3.358 amd 3.382. Which is a full 0.045V difference between the two out of balance cells.

 

And also just to point out, the cell that gets full first is Cell 3, not Cell 2 (the one currently showing the highest voltage). So balancing them here would not balance them at the top, in fact it would be counterproductive.

 

---

 

Alright so this conversation got me out into the rain to connect up my alternator, since we only got 420Wh of sun juice today. 

 

I started out with some throttle and saw 35 ants crawling into my LiFePOs. Running at tick over now I'm counting 27A. 

 

It should be noted that I'm using a very poor old 25mm^2 tinned copper conductor I had lying around to connect my LiFePOs to the alternator / LA. Partly because I'm not ready to use my juicy 70mm^2 cable for that purpose and partly because I think it might help limit the stress on the alternator. 

 

I'm checking the alternator temperature frequently. It's hard to point it at the internals but aiming through the grilles on the alternator the highest temperature I've managed to read is 53 degrees. On the metal housing it is showing 40 degrees. 

 

It has been going for about an hour now and the situation seems quite stable. So perhaps @Dr Bob is right and I don't need to worry about alternator overheating. At least not in winter (it's 15 degrees ambient today) and with a 25mm^2 conductor. My alternator is a rather pathetic 70A model so perhaps it's just not powerful enough to overheat??

 

I am tempted to run the engine now until my BMS trips the charging to check that that is all working as it should be, but for the sake of the environment I am rather going to wait for this weekend's cruise. I'll just set an audible alarm at 13.8V and keep checking the Victron app, and ask my wife to keep an eye on the BG-8S for the cell voltages.

[peterboat]

10 minutes ago, ivan&alice said:

 

---

 

But why do you need the cells balanced in the mid range? What's the point of that? It's a genuine question, because my cells certainly are NOT balanced in the mid range - right now they are showing 3.340, 3.385, 3.358 amd 3.382. Which is a full 0.045V difference between the two out of balance cells.

 

And also just to point out, the cell that gets full first is Cell 3, not Cell 2 (the one currently showing the highest voltage). So balancing them here would not balance them at the top, in fact it would be counterproductive.

 

---

 

 

 

I dont want my new batteries to develop bad habits so by keeping them balanced at all times, this is when charging, discharging or just in the middle its better for the bank. After I have finished charging they are bound to show some in balance the board will sort it out, I finish the journey the board balances the pack its what the EV makers do and if its good enough for them its good enough for me and they are cheap so why not?

Edited August 27, 2020 by peterboat

[jetzi]

1 minute ago, peterboat said:

I dont want my new batteries to develop bad habits so by keeping them balanced at all times, this is when charging, discharging or just in the middle its better for the bank. After I have finished charging they are bound to show some in balance the board will sort it out, I finish the journey the board balances the pack its what the EV makers do and if its good enough for them its good enough for me and they are cheap so why not?

"why not" isn't really good enough. As I see it, an active balancer would be counter productive, at least in my situation. My cells are currently showing 3.328V, 3.357V, 3.331V, 3.347V. An active balancer therefore would take some power out of cells 2 and 4 and give it to cell 1. However, when top balancing I found that actually it's my cell 3 (currently showing 2nd lowest voltage) that reaches fully charged (3.60V) first. So an active balancer working on the mid range would actually throw the cells more out. Unless I misunderstand the theory (very very possible).

 

I can imagine that there is some reason that the mid range being out of balance is a bad thing. Perhaps for example it would cause one cell to wear faster than the other. If anyone knows why then I would be very curious to hear because as above, my cells are always around 0.02V out. But slapping on 150 quids worth of kit that you may or may not need without knowing why is usually not a recipe for a robust system. 

[peterboat]

1 minute ago, ivan&alice said:

"why not" isn't really good enough. As I see it, an active balancer would be counter productive, at least in my situation. My cells are currently showing 3.328V, 3.357V, 3.331V, 3.347V. An active balancer therefore would take some power out of cells 2 and 4 and give it to cell 1. However, when top balancing I found that actually it's my cell 3 (currently showing 2nd lowest voltage) that reaches fully charged (3.60V) first. So an active balancer working on the mid range would actually throw the cells more out. Unless I misunderstand the theory (very very possible).

 

I can imagine that there is some reason that the mid range being out of balance is a bad thing. Perhaps for example it would cause one cell to wear faster than the other. If anyone knows why then I would be very curious to hear because as above, my cells are always around 0.02V out. But slapping on 150 quids worth of kit that you may or may not need without knowing why is usually not a recipe for a robust system. 

It always works so it constantly balances just like a BMS, also as its 15S you would need one of it.

I have 225 cells making up my 3 banks that's 75 batteries per bank in a 5P 15S configuration I cant monitor them separately so I have installed 3 boards for 125 squids to do it for me. It's cheap when you consider how expensive the batteries were

[Dre]

56 minutes ago, ivan&alice said:

It does sound that this is mostly a manual process

The Sterling has a Lifepo4 setting plus I'm pretty sure it also has a custom setting if not happy with those parameters so it's fully automatic. I was getting 45/50 amps charge from the alternator to the LA bank originally so I went for the 60a b2b and it charges the lithiums at around 50 amps which is what I wanted. It also has the "half power" function that charges at around 30amps if I'm going on long cruises. 

57 minutes ago, ivan&alice said:

I think I probably have a similar power supply unit - it's advertised as 0-5A at 0-32V

Mine is similar to yours but it's dc to dc so you can hook it up directly to any battery. It's quite small and cheap but very good quality 

58 minutes ago, ivan&alice said:

Do you shut off the load at low temperature?

 I only mentioned the low temp protection as many bmss and most cheaper drop in lithium batteries don't have this function. On my bms you can set it to whatever you want (I set mine to 5 degrees C) and it will shut down the charging side of ever it goes below this temperature. The load side fets will still be on. That's in case the ambient temperature will drop below 0c (very unlikely as my batteries are inside) and you can irreversibly damage lifepo4 by charging below 0c.

58 minutes ago, ivan&alice said:

How do you "detect" that you need to dump some load here?

My cheap Chinese Hall effect (or you can get one with a shunt if you prefer) battery monitor has two programmable relays. They can be set to come on and off by voltage or state of charge. Say come on above 80% soc and turn off below 50% or whatever. At the moment I turn the mcb on manually (240v element) for a couple of hours when I need the hot water but I'll be connecting a relay to a SSR when I have the chance. 

59 minutes ago, ivan&alice said:

That does mean you're taking your cells to 3.55V each which is higher than I'd like to routinely go,

14.2v is the charge limit on the mppt (epever) that translates to slightly less at the battery terminals. Also remember that unlike your power supply the mppt provides quite a lot of current and the batteries seem to reach a higher voltage than they really are. Always check what the actual voltage is at the battery terminals and adjust the mppt settings until you get the voltage you want 

59 minutes ago, ivan&alice said:

Are you expecting to get a decent amount of power from solar in winter?

Well I have about 1kw so I expect to be using the sterling quite regularly. 

This is a useful little table of average daily Amps for any given month in UK from a 100w panel. 

 

Jan 4.68
Feb 9.36
Mar 16.29
Apr 25.71
May 34.85
Jun 35.42
Jul 34.57
Aug 28.19
Sep 19.34
Oct 10.85
Nov 5.60
Dec 3.54

 

I still don't know how much I use daily (as I frankly don't really care) so I'll be turning the engine one when needed ?

[peterboat]

5 minutes ago, Dre said:

The Sterling has a Lifepo4 setting plus I'm pretty sure it also has a custom setting if not happy with those parameters so it's fully automatic. I was getting 45/50 amps charge from the alternator to the LA bank originally so I went for the 60a b2b and it charges the lithiums at around 50 amps which is what I wanted. It also has the "half power" function that charges at around 30amps if I'm going on long cruises. 

Mine is similar to yours but it's dc to dc so you can hook it up directly to any battery. It's quite small and cheap but very good quality 

 I only mentioned the low temp protection as many bmss and most cheaper drop in lithium batteries don't have this function. On my bms you can set it to whatever you want (I set mine to 5 degrees C) and it will shut down the charging side of ever it goes below this temperature. The load side fets will still be on. That's in case the ambient temperature will drop below 0c (very unlikely as my batteries are inside) and you can irreversibly damage lifepo4 by charging below 0c.

My cheap Chinese Hall effect (or you can get one with a shunt if you prefer) battery monitor has two programmable relays. They can be set to come on and off by voltage or state of charge. Say come on above 80% soc and turn off below 50% or whatever. At the moment I turn the mcb on manually (240v element) for a couple of hours when I need the hot water but I'll be connecting a relay to a SSR when I have the chance. 

14.2v is the charge limit on the mppt (epever) that translates to slightly less at the battery terminals. Also remember that unlike your power supply the mppt provides quite a lot of current and the batteries seem to reach a higher voltage than they really are. Always check what the actual voltage is at the battery terminals and adjust the mppt settings until you get the voltage you want 

Well I have about 1kw so I expect to be using the sterling quite regularly. 

This is a useful little table of average daily Amps for any given month in UK from a 100w panel. 

 

Jan 4.68
Feb 9.36
Mar 16.29
Apr 25.71
May 34.85
Jun 35.42
Jul 34.57
Aug 28.19
Sep 19.34
Oct 10.85
Nov 5.60
Dec 3.54

 

I still don't know how much I use daily (as I frankly don't really care) so I'll be turning the engine one when needed ?

I keep my batteries inside under the rear step next to the central heating pipes, like you my excess solar is used for heating hot water via a 3kw immersion heater works well so well that occasionally it turns on the central heating pump to get rid of excess heat!

[Dre]

16 minutes ago, peterboat said:

I keep my batteries inside under the rear step

I keep mine under the pillow! 

[Dre]

17 minutes ago, peterboat said:

3kw immersion heater

THREE KILOWATT!!!!!! 

 

Is your Tesla parked on the stern?? 

 

Oh wait... you have 15s right? 

[peterboat]

13 minutes ago, Dre said:

THREE KILOWATT!!!!!! 

 

Is your Tesla parked on the stern?? 

 

Oh wait... you have 15s right? 

4.6kw of solar I have 30 x 36 volts valence batteries for the electric drive motor and 6 x 12 volt valence batteries for leisure use.

So when not moving drive solar heats hot water, in the winter all solar can be switched to domestic use. Works for me anyway,  at the moment all solar is switched to drive and whispergen is keeping me warm and leisure batteries charged.

I set up the system to be self sufficient years ago and have modified it as something better comes along 

[Dr Bob]

15 hours ago, ivan&alice said:

But why do you need the cells balanced in the mid range? What's the point of that? It's a genuine question, because my cells certainly are NOT balanced in the mid range - right now they are showing 3.340, 3.385, 3.358 amd 3.382. Which is a full 0.045V difference between the two out of balance cells.

That is balanced ok. Exactly like mine. Typically 0.03V in mid range. One of the laggard cells becomes one of the high cells on getting into the knee therefore the delta gets smaller as the charge goes in then gets bigger as the laggard cell gets to the winning line!

Dont worry. That is fine.

[Dr Bob]

15 hours ago, ivan&alice said:

It has been going for about an hour now and the situation seems quite stable. So perhaps @Dr Bob is right and I don't need to worry about alternator overheating. At least not in winter (it's 15 degrees ambient today) and with a 25mm^2 conductor. My alternator is a rather pathetic 70A model so perhaps it's just not powerful enough to overheat??

I've not read all the latest posts but will jump in on this comment. I think your alternator is good to 90deg. If you want to charge up when parked then just run the alternator connected directly and turn it off after a set number of minutes where you know the alternator doesn't overheat. If cruising all day then the b2b may be the option. I will reply later on the b2b as it's not all good news and interested to get DREs input on how he is finding his. 

[Dr Bob]

16 hours ago, ivan&alice said:

"why not" isn't really good enough. As I see it, an active balancer would be counter productive, at least in my situation. My cells are currently showing 3.328V, 3.357V, 3.331V, 3.347V. An active balancer therefore would take some power out of cells 2 and 4 and give it to cell 1. However, when top balancing I found that actually it's my cell 3 (currently showing 2nd lowest voltage) that reaches fully charged (3.60V) first. So an active balancer working on the mid range would actually throw the cells more out. Unless I misunderstand the theory (very very possible).

My set up is exactly the same. It would be impossible to balance using these mid range voltages unless a BMS measured the inbalance when up in the knee, ie measured the voltages when the first cell hit 3.55V and then worked out how much charge to transfer from cells to cells and then did this when off charge and back in the mid range.

Ivan, as above, you are in the right ball park and if you terminate charge at 13.8 or 13.9V at a charge current less than 40A (so your highest cell is around 3.5V) you should be around the 80% mark. These voltages could vary from system to system. MP has always taught us that you can only terminate knowing you are full when the tail current decays to 4-5% of capacity. I see that each time I charge using the battery charger but what I am also seeing is that once one of the cells gets into the knee then it is likely 90% full ish, therefore if you are normally terminating at 80% (by far the best) then cut the charge when one cell gets to 3.45V or 3.5V......hence the recommendation of 13.8V. You really dont need to go higher.

If you are however charging at 80A or more, I find that you reach that state of charge earlier by say 0.1V so you would leave it to 13.9V to get to that stage of charge......but why? You arent far off the 80%.

You only need to worry about top balance every 6 months as you take your system up to 100% to re-sync the BMV.

[Dr Bob]

14 hours ago, Dre said:

The Sterling has a Lifepo4 setting plus I'm pretty sure it also has a custom setting if not happy with those parameters so it's fully automatic. I was getting 45/50 amps charge from the alternator to the LA bank originally so I went for the 60a b2b and it charges the lithiums at around 50 amps which is what I wanted. It also has the "half power" function that charges at around 30amps if I'm going on long cruises. 

I'm a month into using the Sterling BtoB so still learning how to use them.

My alternator is  240A so I bought 2 * 60A BtoBs wired in parallel. Firstly, advice to anyone buying them, dont get one of the reconditioned units! They say in the adverts they are the same as the new ones but they are not. The cooling capabilities are much reduced so they overheat. I returned 2 and got 2 new units which cool much better (very good service from Sterling though).

Couple of issues.

Firstly these units take circa 55A input from the alternator each but only put out 45A into the batteries. The rest is heat! There is a lot of waste heat but the issue is that you are loosing 10A that the alternator is making which would be far better into the batteries. Its fine for me as my alternator is huge and I can afford to loose the 20 A (from the 2 btob's) and the alternator doesnt overheat. If you are on a 70A alternator you will loose some of the engergy from the alternator.

Second issue is how the BtoB controls. The voltage the unit goes from Bulk to 'condition' via absorbtion seems to be at a voltage that the btob makes up itself. I have put a voltage sense wire on mine but the voltage shown on the unit by the right hand side LEDs seems to have a mind of its own and doesnt seem so consistent over a varying output current range. This makes it a bit difficult when trying to get up to 100%, as when you get into the tail current phase and the amps fall, its a bit hit and miss when the btob drops its charge. The LiFePo4 setting seems too high so I am using a lower voltage now of 14.0V (so it goes out of bulk at about 70% charge) and leave it in absorbtion for an hour (all at 80A charge), using the custom setting. I think the btob can be set to match any system but it takes a bit of playing round with it. Typical of Sterling kit though, one of the units can decide to go to half power if the wind changes to the north so the voltage at the Li's reduces, so an impending move from bulk to conditioning is delayed!

One benefit I am finding with them though is my 'hybrid' system is using power from the LA's when I get the Li's down to 30-40% so I need to get the LA's fro 90 something% to 100% each day (ie put 30-40Ahrs back into the LA's). With the BtoBs, the alternator puts 14.4V back into the LAs and charges them in an hour or so whilst charging the Li's so I end up with say 160Ahrs going into the Li's and 40A going back in the LAs in less than a couple of hours, so that is 110Ahrs each hour rather than 90Ahrs. That is fine for me.

Overall I think the BtoB's are the best idea for my set up with a big alternator and LA's in need of charge but not sure I would use them on a small alternator and a single dump LA battery. I would charge direct from the alternator using one of Nick's controllers.

I am still learning!

[nicknorman]

Just on the B2B current in and out, don’t forget that the voltages in and out will be different. So you need to consider the power in and power out. Probably the B2B is pulling the alternator voltage down in order to get adequate current out of it, so whilst the current out may be less, the voltage out will be greater.

[Dr Bob]

31 minutes ago, nicknorman said:

Just on the B2B current in and out, don’t forget that the voltages in and out will be different. So you need to consider the power in and power out. Probably the B2B is pulling the alternator voltage down in order to get adequate current out of it, so whilst the current out may be less, the voltage out will be greater.

You are of course right but thinking about it, the voltage goes down from 14.4V (normal without the B2Bs) to 14.2V. The overiding view though is that there is so much heat coming out of the BtoBs! In the winter, heating the back of the boat is going to be no  problem! This heat is just lost power from the alternator so we are wasting 10A on the 60A btob.

[nicknorman]

7 minutes ago, Dr Bob said:

You are of course right but thinking about it, the voltage goes down from 14.4V (normal without the B2Bs) to 14.2V. The overiding view though is that there is so much heat coming out of the BtoBs! In the winter, heating the back of the boat is going to be no  problem! This heat is just lost power from the alternator so we are wasting 10A on the 60A btob.

Seems surprisingly inefficient. With modern power electronics it should be easy to achieve without much inefficiency / heat loss. Perhaps the design is quite old.

[Dr Bob]

16 minutes ago, nicknorman said:

Seems surprisingly inefficient. With modern power electronics it should be easy to achieve without much inefficiency / heat loss. Perhaps the design is quite old.

....don't forget it is Sterling!

[WotEver]

Interesting review of used Valence batteries here. $150 US each with built-in BMS and 90% capacity left!
 

 

[WotEver]

Oh... just realised this is a year old... they’re over double that price now. Still, it shows that used Valence are a good option. 

[nicknorman]

39 minutes ago, WotEver said:

Oh... just realised this is a year old... they’re over double that price now. Still, it shows that used Valence are a good option. 

They are well regarded by a few on here. On a general point, one thing I’m not clear about is how such batteries age, in other words, is there a steady and gradual loss of capacity, or are they fine for ages and then suddenly crash. 80% capacity is considered end of life, but what happens after that?

[peterboat]

21 minutes ago, nicknorman said:

They are well regarded by a few on here. On a general point, one thing I’m not clear about is how such batteries age, in other words, is there a steady and gradual loss of capacity, or are they fine for ages and then suddenly crash. 80% capacity is considered end of life, but what happens after that?

Like my ex electric bus batteries you keep on using them! For me it doesn't matter really they will do 10 hours of cruising still, I don't think they're harder to charge and they fit my needs of cheap boating 

[WotEver]

29 minutes ago, nicknorman said:

They are well regarded by a few on here. On a general point, one thing I’m not clear about is how such batteries age, in other words, is there a steady and gradual loss of capacity, or are they fine for ages and then suddenly crash. 80% capacity is considered end of life, but what happens after that?

It would appear from the first graph on the link below that ageing is pretty linear, certainly down to below 70%. I’ve not read anything that would suggest that trace would change shape drastically although I’ve never seen a Lithium tracked down to say 20% capacity. 

https://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

[MoominPapa]

A quick report on developments in Moomin-BMS land. I've adopted a new balancing strategy, which seems to be working well for my use. The idea is to overcome the problem that cell voltages don't really give you much information about balance except if extremes of charge and discharge. For a boat system, extreme discharge is a rare event, and I want to top balance anyway, so I chose full charge as the reference point. When the system does charge termination using the usual criteria (V > threshold and I < threshold) it notes the individual cell voltages _at_that_point_ and subsequently applies balancing discharges to cells which are proportional to the voltage difference, so the lowest cell gets no discharge, the other three get discharge which is proportional to the difference between their voltage and the lowest cell's voltage. The constant of proportionality is low: typical discharge is around 1 Ah. 

 

This scheme does have problems in general: it won't work at all if you never or rarely get to charge termination, of course, and whilst it does sort-of adapt to the frequency of charge termination (if that happens more infrequently, the accumulated imbalance will be greater, so an individual session of balancing will do more work) that's not ideal. However in my case it's working well. The balance is now very good, without me having to do manual interventions every couple of months. As the main aim now is to make the system completely automatic and able to work reliably and safely without its creator fussing over it, this is probably the final stage.

 

1) Automatic battery protection in all circumstances - check.

2) Accurate SoC percentage reading always accurate - check.

3) No maintenance activity needed - check.

 

MP.

 

[Dr Bob]

1 hour ago, MoominPapa said:

The balance is now very good, without me having to do manual interventions every couple of months.

That sounds really good. Well done!

How were you doing the manual interventions before?

[jetzi]

On 27/08/2020 at 20:31, Dre said:

This is a useful little table of average daily Amps for any given month in UK from a 100w panel. 

Jun 35.42

Dec 3.54

Is this amp-hours? If so, theoretically my 1.5kW of panels would produce 53Ah per day in December (0.6kWh @12V) which is enough for me if I don't run my inverter.

In August I harvested 2kWh/day on average (versus 28.19Ah @ 12V = 340Wh per 100W PV, *1500W PV = 5.1kWh predicted by the table) BUT I'm generally limited by the capacity of my battery (i.e. how much I used since the last charge and using while the sun is shining) so in effect this is more a measure of my usage than my generation capacity. So, if the table is accurate and the sun is consistent enough, 3x my PV capacity ought to tide me through the winter without dropping my usage? I have roof space to double my capacity, also I could do a lot to improve the angling of the panels. I think I'll see how the winter goes first, thanks for the table though!

On 27/08/2020 at 20:40, peterboat said:

occasionally it turns on the central heating pump to get rid of excess heat!

I love that this is an automatic process, well done for setting this up. I hope I get there one day. Though perhaps an aircon dump load might be more appropriate than immersion if you're maxing out your batteries in summer! Imagine an automatic heat pump dump load that goes one way or the other depending on the ambient temperature inside the boat?!

 

On 28/08/2020 at 10:08, Dr Bob said:

That is balanced ok. Exactly like mine. Typically 0.03V in mid range. One of the laggard cells becomes one of the high cells on getting into the knee therefore the delta gets smaller as the charge goes in then gets bigger as the laggard cell gets to the winning line!

Dont worry. That is fine.

I'm not worried, I'm alright to live with the status quo until I get a chance to rebalance in a few months time. I'm more wondering what @peterboat's motivation is for mid balancing his cells, and if there is something I don't know.

 

On 28/08/2020 at 11:19, Dr Bob said:

hence the recommendation of 13.8V. You really dont need to go higher.

If you are however charging at 80A or more, I find that you reach that state of charge earlier by say 0.1V so you would leave it to 13.9V to get to that stage of charge......but why? You arent far off the 80%.

Totally agree with you, and no intention to go higher. BUT there is a certain pride and joy you can get from a well tuned system, and an extra layer of safety. Even if I never intend to take the battery to 14.6V (3.65V*4) it would be nice to know that it could. For example, what if my charge termination fails during a cruise and I have a hole in my hospital silencer drowning out the alarms etc etc and the alternator ends up "floating" the batteries at 14.4V (where it's currently regulated)? It would be pretty satisfying to know that such a failure, while it would take the battery well over 80%, would not actually overcharge any one cell!

 

 

On 28/08/2020 at 11:51, Dr Bob said:

Overall I think the BtoB's are the best idea for my set up with a big alternator and LA's in need of charge but not sure I would use them on a small alternator and a single dump LA battery. I would charge direct from the alternator using one of Nick's controllers.

I'm very glad the BtoBs are working for you folk with big alternators, but I'm rather glad I didn't bother. My alternator has been comfortably charging my batteries with no problems (temperature of the outer case never exceeds 50 degrees celsius, though I am yet to try it in hot weather). I only get 30A in from my alternator, not sure why so low, it could be the thin 25mm^2 tinned copper cable I intentionally used.

 

2 hours ago, peterboat said:

Like my ex electric bus batteries you keep on using them! For me it doesn't matter really they will do 10 hours of cruising still, I don't think they're harder to charge and they fit my needs of cheap boating 

Peter have you posted the story of how you came to have an electric powered boat? I would love you to point me to it if you have . Did you have it custom built? Modify a diesel boat? Or buy it that way? I would love to have a custom built electric narrowboat one day, perhaps with extra fold-out PV to compensate for the lack of roof space!

 

1 hour ago, MoominPapa said:

A quick report on developments in Moomin-BMS land. I've adopted a new balancing strategy, which seems to be working well for my use. The idea is to overcome the problem that cell voltages don't really give you much information about balance except if extremes of charge and discharge. For a boat system, extreme discharge is a rare event, and I want to top balance anyway, so I chose full charge as the reference point. When the system does charge termination using the usual criteria (V > threshold and I < threshold) it notes the individual cell voltages _at_that_point_ and subsequently applies balancing discharges to cells which are proportional to the voltage difference

This is very interesting and relevant to the conversation we were having at the top of this page. This surely only works if the voltage at that point is a good predictor of SoC for the cells, or if you are charging your cells to quite a high SoC. I find that the cell that climbs up to 3.5+V first is not the cell at the highest voltage in the mid range. It might just be that my cells have rather different capacities, though?

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I'm now fiddling with an arduino to try to get into electronics, hardware is not an area of expertise for me but I am a computer programmer so I have that going for me at least. My first aim is to create a basic data logger that I can log the voltages of each cell over a long period of time and hopefully draw some insights into the health of the cells. But eventually I'd love to get to the point that I can create a MoominBMS of my own..