36 Volts LiOn battery profile parameters

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friaras asked Apr 21, '19

36 Volts LiOn battery profile parameters

Hello, I built a powerwall for emergencies and to run an AC unit, and other electric gadgets, it has a 5KW capacity or close to that. I use the Victron's 150/35 Charge Controller with BT adapter, it is working great. The question I have is about the profile, for my batteries, during charging the app shows 41.5 volts at around 6-12 amps, then tappers down, before going to float, when I look at the history the battery max voltage shows around 42.60 volts, when I check the voltage at the batteries it shows it 41.5 volts, what could be the cause for the app to show a higher voltage than the batteries show at the terminals? i would like to set the charging to 90% of the battery capacity which should be around 41-41.5 volts, so should I trust the app reading versus the battery terminal reading? is there a modification to the profile that will correct that? Thanks

battery charging charger

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Answer 1 · 2019-04-22T18:13:14Z

boekel answered · Apr 22, 2019 at 06:13 PM

do you measure at the terminals of the MPPT?
And what can cause this also is if you have temperature compensation selected, this should be 0

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Answer 2 · 2019-04-23T18:36:11Z

friaras answered · Apr 23, 2019 at 06:36 PM

Hello, thanks for the reply. I just checked the app, temp comp is set to off, the batteries (LiOn) 36 volts, are on the wall with plenty ventilation on a conditioned room 24/7. The MPPT shows 41.82 at the terminals, the watt meter with holo sensor unit shows 41.62 (those 2 are pretty close) according to the app history the float voltage hovers around 42.60 even though the app reads only 41. something most days, due to small daily power consumption(s) it will decrease slowly and honestly I have never seen anything above 41.something on the app during the day, which is OK because I don't want to charge above 41.5. There has been times, maybe 3 times in the last 6 months, where the app history will show overcharging sometimes as high as 44.55, which is not making sense because the batteries have BMS and the cut off is 41.5 so the voltage at the terminals and the watt meter are good IMO the discrepancy is with the app I believe or maybe there are other variables I'm forgetting to take in consideration. I use the same type and gauge wire (connectors are crimpped and soldered) and they are very short the longest one is approx. 12 in. with the exception of the inverter wires which are 18 in.

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Answer 3 · 2019-04-23T19:18:13Z

boekel answered · Apr 23, 2019 at 07:18 PM

There's your answer:

quote: the batteries have BMS and the cut off is 41.5V

So the BMS cuts off the battery, the MPPT voltage overshoots for a second and records this as highest voltage.

Please mention those details when asking questions... We cannot guess what you have installed...

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friaras commented · Apr 24, 2019 at 06:26 PM

Now that you mention the MPPT overshoot, is this normal? The profile is set for 42.60 absorption voltage, so I can see why the overshoot is possible while in absorption, once it goes to float (I setted the float voltage to 41.50 yesterday to make sure the batteries are charged to the voltage I prefer or closer to it, and it has remained pretty constant at 41.48 at the MPPT terminals and 42.28 at the battery terminals, most likely due to the losses but a lot better, so now that I understand that I'm feeling a lot safer. Thanks for the assist.

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boekel ♦ friaras commented · Apr 24, 2019 at 06:37 PM

MPPT overshoot:
The battery disconnects itself because one cell is too high, as the current has nowhere to go the voltage goes up for a very short time and the MPPT stops delivering.

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