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richwest3 avatar image
richwest3 asked

BMV-717 Charge Efficiency Factor Setting

I have two 200AH, 12 volt no-name LiFePO4 batteries in parallel and a BMV-712 monitoring them. I have adjusted the shunt zero settings on installation. The synchronizations are happening as expected so those settings are at least close.

My issue is that if I don't charge to 100% SOC for a while, the BMV is accumulating error. The monitor says the system is at 100% quite some time before it reaches the cutoff voltage.

Each time it does reach 100%, I have been adjusting the Charge Efficiency Factor to try to get the BMV to track the batteries more closely. I started at 99% and have been dropping it 2% each time. I am now down to 89% and may have to go a little further.

I understood that LiFePO4 batteries were more efficient than this. Am I doing something wrong? I see that there are rather large balancing resistors on the internal BMS units - perhaps 2 watts. Could this inefficiency be due to balancing? The batteries are staying pretty cool - about 20C so I'm doubting that this is the issue.

I am charging the bank with about 50 amps, so a very low C rating of about 0.12C. My discharge currents are also relatively low. I have the Peukerts Exponent set to 1.05 as advised.

Thanks in advance for any help.

BMV Battery Monitor
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3 Answers
Paul B avatar image
Paul B answered ·

the batteries should not be balancing every day or if they are then only for 30 min at the top end of the charge cycle. you may have the balancing voltage to low ???? you will need to look at what the bms is doing with each cell and work out why.

set the charge voltage in the BMV to say at the absorption voltage and then the current threshold and the charged detection time

to suit what you are charging them to

when these three items are met then the soc will change to 100%

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richwest3 avatar image
richwest3 answered ·

Thank you so much for your reply but it doesn't really answer my question. What you say about balancing only being applied for a short time makes sense and because of this, I am thinking it's unlikely that this is the reason for the inefficiency I'm seeing. Also, if BMS settings were wrong and the power were being dissipated by the balancing resistors, the battery would be hotter. (I have no control over BMS settings - without replacing the BMS.)

So my question remains: Why do I have to set the Charge Efficiency Factor so low to make the BMV-712 match my battery SOC between syncs?

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JohnC avatar image
JohnC answered ·

Hi @richwest3

The Peukert can also influence this. You seem aware of where you stand with SOC, so maybe a little experimentation..

Reduce the Peukert to unity (1.0) and see what effect that has, keeping your load and charge habits consistent with what you've been doing.

There's even many who suggest unity for lipo's anyway, perhaps even the makers, who never seem to differentiate their capacity ratings between C rates, or even state a C rate..


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richwest3 avatar image richwest3 commented ·

Thanks @JohnC

I'll give it a try and report back here.

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richwest3 avatar image richwest3 richwest3 commented ·

I changed the Peukert's exponent from 1.05 to 1.00 as suggested by @JohnC and the SOC on the BMV-712 immediately changed from 61% to 55%. This surprised me. I guess it must be storing data (since the last sync?) and recalculating it when changes are made,

This is a change in the right direction but it'll be a few days before I get a chance to change to 100% SOC to really test it. Now I may have to change the efficiency factor back a higher number.

(Interestingly, changing the efficiency factor doesn't change the SOC on the meter instantly as changing the Peukert's Exponent does.)

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JohnC avatar image JohnC ♦ richwest3 commented ·

@richwest3

Yeh, it's good how it does that. Essentially cancelling the SOC 'bonus' it's applied because your loads are lower than C20.

You'll almost certainly need to raise Charge Efficiency to compensate, and you could get a clue there from the cumulative kWh history of the actual Charged and Discharged energy recorded - your overall round trip efficiency. Best read between periods of sync, but over time that possible deviation will diminish.

This is an addiction you know. Have fun.. :)


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richwest3 avatar image richwest3 richwest3 commented ·

Just for anyone's future reference, I changed the efficiency factor to 95% and have the Peukert's exponent set to 1.00 and the monitor is tracking the batteries very well now.

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