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Battery SOC monitor from reliable source
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Simon:
You mean these people: https://elithion.com/battery_management_systems.php ?

All they want to replace is one of these: https://www.ebay.co.uk/itm/184379169102?itmmeta=01HQXTGTKE0J565GMS3XF42PA0&hash=item2aeddad94e:g:drIAAOSwqVdaqdrE&itmprp=enc%3AAQAIAAAA4BRSc7zNOeiqMpqomQwoPPyMezoCHVVP5GEiHkR8YK7gROTX16f2URUC0ahqoX4%2BqrLSHBmLJ%2BriqKWfu1hdcrJSsQYtne9jm06QMhMbGM4%2BKwMTPg%2FHcGJ23E%2BQ%2BS4iy%2Fm1Or1dDl8eQrUO%2FrOanJKC%2F5Jg5JKj0hH7qdQep%2B0ZVTF1D04Da8dLae0uRcUuY4UArFD9EG2UhYsdCruIKmN2%2B62sV%2BwVmiuyZhhKeFAzd5LUPOX5gtA6QeKt%2BRlzG%2F3QwFSq9hz%2FpTgP%2Bx%2BABzF%2FAWB934%2BYPHZ5hzeHGzMj%7Ctkp%3ABk9SR_Kpw7q_Yw
nctnico:
That one from Ebay looks like a piece of rubbish which is never going to work accurately / reliably (from the user's perspective). Even batteries with sophisticated coulomb counting hardware can (will) go wrong where it comes to determining SOC. I even had batteries reporting being empty while having plenty of charge left. As a countermeasure I had to implement a voltage based shutdown in the system I designed to avoid being fooled by the battery's BMS.
boB:
There is this tried and true Trimetric.

It comes with (or needs ?)  at shunt.  Most likely the  500A/50mV  one that is standard in the industry.

LiFePo4  will use a different charge efficiency setting than lead acid is all.  Usually higher efficiency like in the higher 90% range.

This one counts coulombs as is how SOC% should be done.

https://windandsolar.com/trimetric-battery-monitor/

nctnico:
The problem with Coulomb counting is that you are doing an integration which implies accumulating offset errors. Without resetting the accumulated error, the SOC will be wildly off after a while. A BMS typically resets the SOC at maximum and / or minimum charge voltage in order to reset the accumulated error. However, if the user never fully charges / discharges a battery, resetting the accumulated error won't happen. Nowadays I include battery voltage levels in order to verify the SOC reported by the battery. If the voltage & SOC don't make sense, I present an estimated SOC based on voltage to the user / system.
boB:

The SOC% monitor does have to reset to 100% when it knows it is there.

For flooded lead acid that was pretty easy.  After the charge cycle has finished its Absorb cycle and goes to Float, that is 100% if everything is set correctly.
Also return (ending) amps can help.

For Lithium it is a bit harder.   But LiFePo4 the voltage will rise to the same as Absorb as before but how long it stays there varies.  It might sit in Absorb for a while so that the BMS can balance the cells and THEN it is done with its charge.   OR, sometimes they go strait to Float.  Whatever Float means to that manufacturer.

Most of this depends on the BMS itself which sometimes gives you SOC%.  I assume that some times it is even accurate.

boB
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