battery shunt and SOC drift

[Anže Košir]

I might be missing something in the settings. but after a couple when the battery doesnt reach full charge, the displayed SOC  gets quite optimistic (showing a number higher than the actual SOC).

It shows as either a shut off of the system before it show 0%, or it keeps showing 100% for a while before the full battery voltage is reached and the SOC resets.

is there an option to configure round trip efficiency/losses to compensate for that? or any other mechanism?

it doesn't really bother me, as i've implemented a separate SOC display through home assistant which does take battery round trip efficiency into account (alongside temperature compensation and internal resistance). But i would still prefer to have a more accurate showing on the SBMS0 itself.

best regards, 
Anže

[Dacian Todea (electrodacus)]

Anže,

There is no setting for round trip efficiency in the SBMS.  I can implement that in a future update.

For typical solar energy storage applications using LiFePO4 the round trip efficiency is around 99% or higher.

In normal cases where you discharge say battery down to 40% over night and have a full charge the next day the SOC discrepancy will be less than 1%.

If you have a full week with not a full charge it will also likely mean that you where not able to use 60% of battery capacity every day maybe just an average of 20 or 30% with those partial charge cycles so maybe the SOC will be OFF by 2 or 3% and will get corrected when a full charge is made.

Of course if you operate for many weeks without a full charge for some reason you can get to significant SOC discrepancy.  On the other hand if I implement the round trip efficiency and that value is introduced incorrectly it can create as large or even larger discrepancy.

With my own two SBMS units I never noticed more than 2 to 3% SOC discrepancy over the past few years. I only have periods in winter where there may be a few days up to around a week where I do not get a full charge.

You can get a way larger discrepancy if you have a bad cell connection or you charge with more than 0.2C since in this cases the cell due to internal resistance or extra resistance of a bad connection can get to the 3.55V limit way before cell is at actual 100% SOC. So you can get even a 20 or 30% SOC discrepancy in this conditions in just a single day. Same will happen if you set the over voltage limit to  some lower value than default 3.55V.