Opinion on max battery SOC and management (Solis/Pylontech)

[grahamsc18]

Hi,

Curious to on general views regarding max charging on battery health over time - specifically a pylontech.

I have a Solis 3KW and 5x US2000c batteries - during most of the summer solar was easily charging to 100% and holding it there until later in the day. Now in the winter I'm using Octopus Go and charging up overnight which only peaks at 100% for a short period.

There seems to be a lot of mixed views out there which are no different to EVs regarding having batteries at 100% for pro longed periods - I guess its not at 100% for a long time but wondered if there is headroom or BMS intelligence built in so its not actually hitting 100% thus no real issue, or is better to restrict the upper SOC to under 100?

If the latter is there a way to even do this on Solis - you can set the floor SOC but I don't see any obvious settings for Max.

Thanks in advance!
Sean

 

[shavermcspud]

The pylontech have a stated usable capacity which is less than the actual cell raw capacity, this is there for longevity. I really wouldn't worry about them being at 100% for what in reality is a few hours at most.

 

[SeaGal]

Curious to on general views regarding max charging on battery health over time - specifically a pylontech.

I don't have Pylontech, but have a DIY 14.3kWh battery using it with my Solis. Apart from Pylontech's unfortunate choice of only using 15cells, rather than the more common 16 cells, the underlying battery chemistry (LiFePO4) is the same.

There seems to be a lot of mixed views out there which are no different to EVs regarding having batteries at 100% for pro longed periods - I guess its not at 100% for a long time but wondered if there is headroom or BMS intelligence built in so its not actually hitting 100% thus no real issue, or is better to restrict the upper SOC to under 100?

'better' in what way? I am guessing you mean better for battery longevity.

IMHO, what is important is:-

a) what voltage the cell voltages reach (or more specifically, what the highest cell goes to). Obviously 3.65V is the absolute max, but depending on BMS settings, the reported 100% may be 3.4V, 3.45V, 3.5V etc.

b) how does the BMS re-calibrate itself (back to 100%) as coulomb counting over time will drift from actual SOC.

c) at what voltage does the BMS start balancing cells and how long are those cells being charged (slowly) to give the balancer time to do its job.

I guess you'd need to connect to the Pylontech's admin console to see that depth of info.

In my case (with custom software between battery BMS and my Solis), I charge to 3.45V, the BMS will reset itself to 100% when 3.45V is reached. Balancing starts at 3.4V with > 15mV difference between cells. To give the cells time to balance, charge current is reduced in several steps as 100% is reached. I presume that Pylontech do similar as others with Pylontechs have reported it takes a long time to get from 98% to 100% reported SOC.

IMHO, it is better to charge to what the BMS has been configured to report as 100%, rather than a lower value, assuming that 100% is not higher than say 3.5 or 3.55V. If you don't do that then...
1) you are not giving the BMS time to do its job of balancing the cells and
2) you are potentially wasting money, by not having enough energy stored until the next cheap rate arrives. Better to get the most you can out of your batteries.

If the latter is there a way to even do this on Solis - you can set the floor SOC but I don't see any obvious settings for Max.

I use the Solis 'user defined' settings, which has an "overcharge-SOC" setting, which I set to 100%. However, assuming you have set the battery type to PylonLV, then it will (quite rightly) be the Pylon's BMS that will tell the Solis what to do.