Two 12V BMSs in series vs One 24V Charge Controller

JNCrain

Solar Enthusiast
Joined
May 30, 2021
Messages
9
Here’s what I’ve got:

  • Two SOK 100AH batteries in series for 24V
  • Epever Tracer4215BN 24V 40A charge controller w/MT50
  • Giandel 3000W pure sine wave 24V inverter
  • Four Rich Solar 100W panels in series (total of about 90V)

It’s time to program the Epever. I’ve done a lot of digging around, looking for the proper settings and have read the article “How to Find Happiness With LiFePO4 Batteries”, and everything I read says to program the charge controller as though it’s working with a single 24V battery. But one question is still vexing me.

Will there be any conflicts between the parameters of the individual 12V BMSs and the 24V settings of the charge controller (low and high voltage disconnects and reconnects, for example)? I know the importance of maintaining both batteries at the same SOC and can deal with that. My question has to do with the fact that each BMS has settings based on 12V while the CC will be set up with settings based on 24V. Is the fact that the BMSs are in series automatically make this a non-issue?

For completeness, I've attached a .pdf file showing settings for Epever 4215BN, including those recommended by SOK for the 12V 100AH battery, some additional 12V settings I've found, and the settings I *think* may work for my setup (they're basically the 12V settings doubled for 24V).

Any feedback/advice would be appreciated.

Thanks in advance!
 

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  • Epever4215BNsettings.pdf
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sparklehunt

New Member
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Aug 19, 2021
Messages
28
My thoughts are that the BMS is only there as an emergency protection. It does not know that the charge controller exists nor does have any interaction with it. The BMS protects against high V, low V, and hi and/or lo temperature. Simple.

With this understanding, I say program your charge controller to be even more conservative than BMS. Example: It’s threshold for high-voltage disconnect should be lower than the high-voltage disconnect of your BMS. Of course a charger's reconnect V would be even lower than the disconnect V, so you make sure to drain the battery a bit more before starting to charge it again.

I am confused about the low-voltage disconnect of a charger. If the voltage is dropping we would want to charge as much as we can?

Anywho what specific situations concern you?
 
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Mcgivor

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Joined
Jul 24, 2021
Messages
187
The charging voltage settings are, in my opinion, too high for cyclical use, 28V or 3.500V per cell is sufficient as there is little in the way of capacity gain above that. Normally equalization is disabled or duration set to zero minutes, I'm not familiar with the Epever SCC, the 180 min boost duration seems a little long, if under normal conditions there is little to no current flow after an hour, there is little point in holding the voltage high unnecessarily .

Assuming the low voltage disconnect is a fixed BMS value, you may want to include auxillary alarm or disconnect means set at around 24.8V to avoid going below the discharge knee.

Many manufacturers use extreme settings in their recommendations to meet their capacity claims, this can shorten cycle count expectancy, using more conservative values will diminish capacity available slightly but improve life expectancy, it's a tradeoff
 
Last edited:

JNCrain

Solar Enthusiast
Joined
May 30, 2021
Messages
9
The charging voltage settings are, in my opinion, too high for cyclical use, 28V or 3.500V per cell is sufficient as there is little in the way of capacity gain above that. Normally equalization is disabled or duration set to zero minutes, I'm not familiar with the Epever SCC, the 180 min boost duration seems a little long, if under normal conditions there is little to no current flow after an hour, there is little point in holding the voltage high unnecessarily .

Assuming the low voltage disconnect is a fixed BMS value, you may want to include auxillary alarm or disconnect means set at around 24.8V to avoid going below the discharge knee.

Many manufacturers use extreme settings in their recommendations to meet their capacity claims, this can shorten cycle count expectancy, using more conservative values will diminish capacity available slightly but improve life expectancy, it's a tradeoff
YES! Excellent points. Especially regarding manufacturers' extreme settings, often mimicked by their consumers. I've learned a lot since my original post, and I'll be posting my empirical experiences soon. Thanks, Mcgivor!
 
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