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Fully charge battery even though SOC says 100%


Stanley
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I have this problem that has been mentioned here before, where some Lithium batteries' BMS report incorrect SOC values.

Now I know why this happens, but the latest one I have had a problem with is a Freedom Won battery.

The SOC has been reaching 100% every day (according to the BMS), but if you look at the battery voltage the highest and lowest values have been decreasing daily.

In the past, this has caused the reported SOC to suddenly drop from ~80% to 8% on this particular battery (probably because of the cell voltage hitting some lower limit) 

Anyway, Freedom Won has said that we need to make sure that the battery is fully charged at least once every 2 weeks.

The problem I have is, how do you decide if the battery has been fully charged if you can't rely on the SOC reading?

I was thinking of maybe checking that the actual voltage has reached the 'full' voltage, but this is not a fixed value like in Pylontech. The BMS in the Freedom Won adjusts the maximum voltage to control charging and also just reaching the voltage isn't enough, it needs to be held there for some time to allow for cell balancing.

So I was wondering what other people do to solve this problem.

I'm also a bit annoyed that the battery guys want us to jump through hoops because they can't make an intelligent BMS.

Surely the BMS can tell that the battery hasn't reached it's 'full' voltage for a while and slowly decrease the reported SOC.

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9 minutes ago, Stanley said:

In the past, this has caused the reported SOC to suddenly drop from ~80% to 8% on this particular battery (probably because of the cell voltage hitting some lower limit) 

@plonkster, and myself had a great discussion earlier this week on why this happens. I hope his got the time to share it here... 

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5 minutes ago, Jaco de Jongh said:

@plonkster, and myself had a great discussion earlier this week on why this happens. I hope his got the time to share it here... 

Yeah, I don't actually think the FreedomWon battery has that exact problem. At least, they told me they don't... 🙂

BYD and BlueNova does have this problem. The current sensor is not very accurate low down. For example, with a BYD battery it cannot measure accurately below 2A. Per module, so if you have two modules (as minimum recommended), that's 4A or 200W. Any load below 200W... the battery doesn't even know it is being discharged, so the SOC drifts. Then suddenly a cell voltage drops out, and the BMS has to make a drastic change to its SOC estimate.

I'm told FreedomWon has a more accurate current measurement. They use the Orion BMS, so you could just look into that, whether it uses a shunt or a hall effect sensor. So quite probably something else is causing it overestimate its SOC.

The fact that the per cell voltages are also drifting indicates something else to be wrong. Unfortunately the only way to fix it that I know is to cycle it several times and see if the cell voltages converge to some ideal. And if not, it becomes a warranty claim.

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40 minutes ago, plonkster said:

I'm told FreedomWon has a more accurate current measurement. They use the Orion BMS, so you could just look into that, whether it uses a shunt or a hall effect sensor. So quite probably something else is causing it overestimate its SOC.

I've been looking at a lot of BMS's in the past few days, if I'm not getting it confused with another product offering. Orion uses a two hall effect sensors in one package. One of them is dedicated to being accurate at small currents, which then switches to the other when the currents are higher.

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