Idiot's guide to a self build battery

[Stinsy]

As cool as these super-fancy BMSs are, you only really need a balancer that shifts enough amps. The inverter takes care of charge voltage and low-voltage cutoff, and that’s all you need.

In a perfect world I would agree with you, especially as I like to keep things as simple as possible - (KISS principle). You are correct that if the cells are perfectly matched for capacity, for internal resistance AND have been carefully top balanced then a simple balancer should keep them all "inline" and max and min voltage limits on the inverter should be all you need. BUT as we all know, or should know, the world is not perfect and "cheap" DIY LF280K cells from various sources are also not perfect. Capacity and more importantly internal resistance varies. Yes you can match them as best you can, but most people need 16 cells, so order 16 cells to make you 48V pack, so need to use all 16 - matched or not - and how many have the equipment to measure all the parameters accurately? Who knows how the specs change with age? When I was taught "electronics" I soon found out there is a world of difference between the "perfect" op amp that all the theory and formulae are based on and the practical implementation with requirements for offset's, frequency, temperature compensation etc.

Another issue is how accurate is the voltage measuring on the inverter? especially when looking at the end of 2m long cables with 100A flowing through them?

What is your definition of a good balancer? how much current should it be able to divert? in reality if things are going wrong then it needs to "shift" the full charge current - potentially 100A ! If a cell does go "faulty" how would you ever know? wait for it to start bulging or smoke escaping?. Your balancer may be shunting as much current as it can but once the cell hits 3.65 V or what ever charging needs to stop. (or at the other end if discharging, 2.5V then discharge needs to stop).

As I read somewhere on here if battery manufacturers could get away without a BMS I am sure they would to save cost.

Each to their own, but having "played" and observed my own stack (Ok somewhat obsessively 'cos I am like that ) over the last few weeks I am convinced that I need a) a balancer with a decent balance current capability, b) a decent BMS (not sure the Daly quite cuts it but it's not bad apart from the lack of easy inverter COMS) and c) Ideally inverter COMS - if nothing else it gives you a ready reading of S.O.C. so you can guestimate how much / long to charge for over night (after gazing in the crystal ball to see how much sunshine will happen the following day )

As I have quite a few £k invested in the LF280k stack I intend to look after it and in my own opinion a hundred or so £ on a decent BMS (and balancer) should be viewed as an investment/insurance and not a cost.

Interestingly if you look at commercial applications they’re not as concerned as you are. Eg IIRC the LEAF BMS only balances at 0.5A, Pylontech batteries don’t balance at all unless they’re charged to 100% and even then only balances at 0.05A. Whereas the simple “made-in-China” balancing boards shift up to 6A.

[ALAN/ALAN D]

Back in the near Dinosaur Days in a Light House. The balancing was done by walking past the big glass jars and looking at the amount of bubbles in each jar.
If you saw Lots of bubbles in one jar the next test was to put your hand on the Bubbly jar and the other hand on a not so Bubbly jar. If the Bubbly jar was hotter the next step was to have a Cup of Tea and Ponder what to Doooo next. Some times it was just smoke coming out of the battery charger.

[marshman]

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[marshman]

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[Tinbum]

Pylontech batteries don’t balance at all unless they’re charged to 100%

That's not correct, they balance above 3.36v and 30ma cell difference. Any setup will only balance near the top voltages as that's when the cell imbalance will show.

[Stinsy]

Pylontech batteries don’t balance at all unless they’re charged to 100%

That's not correct, they balance above 3.36v and 30ma cell difference. Any setup will only balance near the top voltages as that's when the cell imbalance will show.

3.36V is 100%!

[Tinbum]

Pylontech batteries don’t balance at all unless they’re charged to 100%

That's not correct, they balance above 3.36v and 30ma cell difference. Any setup will only balance near the top voltages as that's when the cell imbalance will show.

3.36V is 100%!

Mine never reach 100% at 50.4v

52.5v to 53.2v is the charging voltage for a Pylontech.

[Stinsy]

That's not correct, they balance above 3.36v and 30ma cell difference. Any setup will only balance near the top voltages as that's when the cell imbalance will show.

3.36V is 100%!

Mine never reach 100% at 50.4v

52.5v to 53.2v is the charging voltage for a Pylontech.

Victron have written about this, they recommend ignoring Pylontech and setting a charing voltage no higher than 52.4V, they also say to ignore the battery reporting that charging sticks in the 90s this is Pylontech being very aggressive with their batteries at the top end.

[Tinbum]

Victron have written about this, they recommend ignoring Pylontech and setting a charing voltage no higher than 52.4V, they also say to ignore the battery reporting that charging sticks in the 90s this is Pylontech being very aggressive with their batteries at the top end.

Yes I know what they have said. I have mine set to charge up to 52.5v except for once a week where they go to 53.2v. I did try the 52.4v that they recommended but I found it slightly too low. I also didn't want Pylontech to be able to come back and say that I hadn't charged to their settings in case of a warranty claim.

Re the charging sticking. This I find interesting, if you stay within the 89-100% SOC it doesn't always seem to happen.

[Tinbum]

I know what you mean- I think our washing machine may be having an Arduino implant soon as it's driving my wife mad. It literally has a mind of its own.

It stopped working all together today so I had a look. Removed the PCB- looked ok, put it back. No better. As it had been intermittent decided it may be mechanical so gave the relays on the PCB a tap with the handle of the screwdriver. Now working perfectly.