Pylontechs + DIY battery.. unexpected behaviour

[Oldgreybeard]

Personally i'd give it a month and see how it plays out first but a balance is worth doing.

I'd definitely have give the new batteries and the pylontechs the opportunity to go through a few dozen cycles before taking action.

So would I. Been said here a few times now, but they do seem to take a fair time to sort themselves out. I suspect it's down to their built in balancing current being pretty low.

[nowty]

@Caesium, ironically your DIY cell pack using your active cell balancer will all remain within 5mV.

[Caesium]

Personally i'd give it a month and see how it plays out first but a balance is worth doing.

I'd definitely have give the new batteries and the pylontechs the opportunity to go through a few dozen cycles before taking action.

So would I. Been said here a few times now, but they do seem to take a fair time to sort themselves out. I suspect it's down to their built in balancing current being pretty low.

Yeah all fair points. I guess my urge to tinker is too strong

That said though I don't think adding new LF280s has caused this particular unit to have a cell so far out of whack, I think this has been happening for some time. Agree this might be worth a warranty claim, I've emailed to see what Pylontech say.. its 3 years old and I'm pretty sure I registered it on their website when I bought it.

As for the rest of the stack, I think thats settling down a lot more now, so I'll probably try changing the inverter back to knowing it has Pylontech batteries (without this slightly dodgy unit for now) and see how that goes separately.

[Caesium]

@Caesium, ironically your DIY cell pack using your active cell balancer will all remain within 5mV.

Yeah I've been checking that with a multimeter every day.. every single cell is within 2mV so far, happy with that.

Fighting the urge to order another 15...

[Caesium]

Just as a quick update, I've switched back to using the Pylontech BMS talking to the inverter again for the last two nights and no longer see any of the current see-sawing previously reported. I guess they just needed time to balance.

Using the Pylontech BMS is a nicer solution because the inverter pumps a good 3kW in almost right up to the end, so they reach target voltage faster. Using lead-acid mode has a much more gradual ramp down in current as the target voltage is reached.

So its feasible that if the pack were large enough (it might be if I order another 15!) you can get more charge into them by keeping the amps higher for longer when you're on a 5 hour deadline.

On the other hand I don't want to overwhelm the balancer on the DIY pack by keeping so many amps flowing in when some cells are probably getting near 3.65v. Trying to get some more monitoring sorted out for individual DIY cell monitoring, but slow boats from China are slow..

[Stinsy]

I see how attractive it'd be to have access to real-time voltages for each cell in the pack and data-logs thereof. But do you have the time to process and analyse this data?

[Caesium]

I see how attractive it'd be to have access to real-time voltages for each cell in the pack and data-logs thereof. But do you have the time to process and analyse this data?

Depends what you men by analysis My intention is just occasionally glancing at it to see if any rise/fall significantly faster/slower than the mean. That's probably the closest I've got to identifying which cells are letting me down.

As per other thread though, the balancer will be working hard for this not to be the case, so I've also added a switch to let me turn that off temporarily which I'm hoping will expose any potential issues.

[Oldgreybeard]

When I've built battery packs in the past I've just tested every cell before putting the pack together and written the measured capacity on each cell. That way you only ever really need to check one cell, the one with the lowest capacity, and set the whole pack capacity at a multiple of that value. My experience is that the normal range of capacity variation with very good cells is about 2%.

Almost always reputable manufacturers set the advertised capacity as being right at the lower capacity end of the tolerance range (lots of dubious sellers in far-off lands don't do this!). As long as the lowest capacity cell triggers the end of charge then you know nothing is really changing. The chances of the lowest capacity cell in the group being the one to fail isn't that high so if the pack starts to limit charge based on another cell then that's a good indication that there is a problem developing.

To make this easy, on packs I've built I put LEDs in the balancers that turn on whenever the shunt turns on. That way a glance at the pack showed whether the pattern was normal or not (normal being the lowest capacity cell turning its balancer on first every time).

[Stinsy]

I see how attractive it'd be to have access to real-time voltages for each cell in the pack and data-logs thereof. But do you have the time to process and analyse this data?

Depends what you men by analysis My intention is just occasionally glancing at it to see if any rise/fall significantly faster/slower than the mean. That's probably the closest I've got to identifying which cells are letting me down.

As per other thread though, the balancer will be working hard for this not to be the case, so I've also added a switch to let me turn that off temporarily which I'm hoping will expose any potential issues.

Great! I’d be tempted to get sucked into a rabbit hole of drawing graphs and monitoring voltages minute-by-minute!