Xmas Lifepo4 Battery SoC Drift - - - - 2022
its that time of year again - Christmas & New Year,
a excellent time for me to hang my boots up and chillout for a couple o weeks.
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That means a great opportunity for some testing.
one of the tests i done over Xmas was the old -
Charging the starter battery with a Mobile EHU battey pack trick,
which you can see over here -->
[Guide] Battery Maintainer (for Starter battery) -- How I Done It --
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during this test i disconnected my "Ablemail starter battery maintainer" -
[Guide] Battery Maintainer (for Starter battery) -- How I Done It --
which had the effect of dropping the load on my Leisure Battery ( LB ) to a very low current.
so what happend?
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well look at this:
The Battery
internal BMS after 3 weeks shows 99% SOC - sweet right , the battery is full and ready to go.
well - not so fast,
i also have a Victron battery shunt connected to the LB, . . . this shows
66% SOC after 3weeks
...............
so what's going on?
and what reading is correct !!!
well believe it or not
the Victron is correct and the battery BMS is wrong !!
(This happens on all lithium battery's, I've witnessed it on all my POWEROAD, Renogy & Valance Lifepo4 batterys I've owned. - that use a BMS where the load draw is under the BMS min rated detectable current, which is normally below 500mA or even 1000mA in the case of the POWEROAD)
whats going on here is the loads on the LB are 0.48A or 480mA, which is BELOW the 500mA that the internal battery BMS can detect.
at the same time the internal battery BMS will show 0A being drawn.
in fact we have drawn 92.3Ah worth of power,
drawing the battery down to 66% SOC.
.....
here is the 15day BM2 voltage logger plot . . . which shows the VA graph which is the LB.
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so what now? -- what do i do,
i call this
BMS drift or SOC drift, and as you can see relates to a drft in SOC from what the internal battery BMS will tell you and what is actually happening in the battery.
the drift will get bigger the longer the battery is left in this state . . . .
infact you can easily drain the battery completely flat like this (which i have done before).
the battery will show its fully charged . . . untill the voltage starts dropping off, which may cause trhe BMS to trigger some other Voltage related SOC charge reading.
but ultimately you will end up with a flat battery even though it shows full, or 50%, or whatever.
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so what the fix?
well that's easy - just fully charge the battery again. to the real 100%, don't rely on the BMS readout . . . just keep charging until the current drops down to 0 (or bellow 1A with a standard lifepo4 charge profile )
I've tested this before with a Victron IP22 & IP65 chargers as these monitor the Ah capacity that's charged back into the battery.
you will see that even if the battery BMS shows 99% SOC. . . . .
the charger will but back 60% SOC into the battery.
its called TOP -BALLANCING, well that rather refers to balancing the individual cells within the battery pack to balance them selves out,
so its more
TOP BALLANCE BMS DRIFT CORRECTION - lol.
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this is where im at today.
my Roamer 230SB battery is showing 99% SOC,
but i know that the Victron Smart Battery Shunt reading of 60%
so i need to recharge back to the real 100%. . .
in this case 40% of 230Ah is 92Ah according to quick maths.
im taking the van out now,
and I've reconnected the Renogy DC-DC charger . . .
and reconnected the AMT12-2
and know that i need to put about 92Ah back into the battery,
Ill set the Renogy to a 30A charge limiter, so that should be around 3hrs worth of engine run time to recharge.
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ill update this later.
) mega or midi fuses..........it don't matter which type of van you have a combi a camper what ever.....there always gonna be a whole host of other stuff Zave
