question

boater avatar image
boater asked

Using SOC to control charging with Multiplus

What I have:

  • Multiplus 2 12/3000
  • Non Victron LiFePO4 bank
  • Non Victron battery monitor which sends SOC via NMEA2000

What I want to do is utilize the SOC information to control how the batteries are charged. Ideally, I could have the charger switch to 'OFF' when the batteries achieve a certain, programmable, SOC level. This is done with iPhone often times--when your battery gets to 80% SOC, the phone will often times stop charging it (depending on how you use the phone). I want to do something similar--have the Multiplus stop charging at, say, 80% SOC by default. Ideally this threshold would be something easy to change so that when I occasionally want to take advantage of the full capacity of the batteries, I can charge to 100% SOC. This will maximize battery life.

I suppose there are two main questions here:

  • Can the Multiplus interface with this non-Victron NMEA2000 SOC meter?
  • Regardless of that answer, if there is a compatible SOC meter on the Multiplus network (say a compatible Victron monitor), can that SOC information be used to stop or even impact the charger output? If so, how can it impact it?

I have seen the impact low SOC can have via the Multiplus, I'm wondering if anything can be done via Multiplus when a 'high' SOC is achieved.

Multiplus-IIbattery chargingSOC
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5 Answers
kevgermany avatar image
kevgermany answered ·

There's more than one type of lithium battery. Cell phone batteries are different to LiFePo4 (LFP).

LFP batteries are much more tolerant to full charging, and need regular full charges to balance the cells internally, if this is not done you'll find that the battery doesn't deliver full capacity.

The Victron battery monitors such as the BMV712 have a relay which can switch on state of charge. But this relay is not designed for heavy loads. You may be able to use this to control the charge function, but no idea how on my side. Note that these monitors also need to see a regular full battery charge to synchronise and work correctly.


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nickdb avatar image
nickdb answered ·

If you intend building a bank out of iPhone batteries, then limiting full charge may be a good idea. If not, none of the commercial/supported batteries want, nor require, this. There are more factors that affect a battery’s life.

If you are diy’ing with cells bought online, rather limit their upper and lower charge/discharge limits in the bms.

SOC is a derived metric which has it’s uses and where accuracy can vary.

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boater avatar image
boater answered ·

LFP batteries last longer if you do not leave them fully charged for long periods. Leaving them fully charged stresses the cells and reduces their capacity. I do not need them fully charged often, and currently have to manually manage the charger. I'm trying to avoid that.

If anyone has information that could help answer that question, I would appreciate it. The information about the BMV712 being able to control a relay seems hokey but is interesting and on the right track, so thanks for that.

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Alexander Bartash avatar image
Alexander Bartash answered ·

I'd like to do the same but there seems to be no easy way to do it. Ideally I'd like to have

  • min/max SOC setting in ESS
  • another setting which lets me choose how often I'd like to override that setting automatically and let the batteries fully charge to balance them, e.g. once a week/month/quarter
    • additionally it should do that full charge using sun energy whenever it becomes available enough to do that
    • if at the end of the set period there was no enough sun to do full charge then do it from grid


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bartrs avatar image
bartrs answered ·

https://community.victronenergy.com/questions/99777/control-mains-power-supply-to-multiplus.html

I had the same question and sorted it with a Cerbo and the info in above link.
Now start and stop loading at desired SOC.

1 comment
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boater avatar image boater commented ·
Very interesting...can you show the programming steps you took?

Thanks

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