LifePower4 Battery stack charge imbalance

[DIYrich]

what he said. Also voltage is fairly flat through most of the discharge range, so SOC can estimate in the middle. For charging purposes, the top and bottom voltage values are much more reliable and useful.

 

[RCinFLA]

if V is more reliable than SOC, why not just use V to determine battery level then?

Unloaded, rested open circuit voltage is more reliable.

Once you have cell current involved, the amount of cell terminal voltage slump due to overpotential cell voltage slump to migrate the lithium ions which is dependent on current level, temp of cell, and condition of cell, screws up any SoC correlation to cell terminal voltage.

SoC indicators are just amp-second counters of current going in and out of battery. It is like dead reckoning navigation. You need a known reference point for it to work. This is usually a full charge voltage condition. Errors accumulate in the amp-second count over time due primarily to inaccuracies in the current measurement.

The Columb (amp-seconds) counter needs to be given a fresh new reference point periodically to wash out the accumulated errors.

Second piece of info necessary is the expected AH of the battery so Columb count is subtracted from supposed total battery AH's to give % SoC result.

Most of the issues folks have complaining about their SoC indicator reading incorrect is due to not allowing the Columb counter to accomplish a full charge reset to wash out accumulated errors over a long period of time by giving battery a full charge with well-balanced cells.

In an attempt to make AH setting 'idiot proof' many BMS determine battery AH number automatically by measuring total amp-seconds from full charge voltage to full depletion voltage. I prefer entering in battery AH setting myself.

Many BMS's have really stupid way to reset Columb counter to battery 'full' condition. They require a BMS charging cell overvoltage trip and reset when the first cell trips an overvoltage charge shutdown. No accounting for balance condition between cells or the level of charging current being used when overvoltage trips.