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vonkanehoffen avatar image
vonkanehoffen asked

BMV-712 settings for 100Ah LiFePO4 battery?

Guys I'm totally lost trying to configure my BMV-712 Smart. It keeps resetting to 100% SOC and only ever getting down to ~60% when my battery is completely drained to the point where it can't even power the BMV itself. Can any of you clever people take pity on me and suggest some settings please?

My system (it's a camper van):

- This 100AH LiFePO4 battery: https://ultramax.co.uk/pub/media/sebwite/productdownloads//s/l/slaumxli100-12-tech-xam-v1d21.pdf
- BMV-712 Smart

- MultiPlus 12 / 500 / 20

- SmartSolar MPPT 100 / 30

Thanks!

BMV Battery Monitor
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matt1309 avatar image matt1309 commented ·
Hi @vonkanehoffen


I've got a smart shunt (and generic lifepo4) rather than bmv and the brand you describe but imagine they're similar settings options in bmv vs smart shunt and i'll tweak mine for what you need.

Battery capacity: 100ah

Charged Voltage: 14.6 (This is from spec sheet although I set mine personally much lower at 3.55v per cell so 14.2v but this is what manufacturer has told you so you should probably stick to it. You could lower it but it's likely dependent on what BMS the battery is using and at what level balancing turns on, if you're not sure just stick with what manufacturer has told you to use).

Discharge floor: what the min you want battery to go to (mine is 0).

Tail current: Used for how minimum amount of current going into the battery in absorption phase before going into float phase. (low percentage maybe 2%).


Peukert exponent (1.01 to 1.03 I'd go for).


Charge efficiency factor 99%


Your SoC might jump to 100 if voltage of 14.6 is hit (max voltage), however once it's calibrated it should track relatively well with the above settings.


The other thing to check if the multiplus settings. If you have the battery settings set incorrectly in there it might be that that's causing the system to shutdown rather than the battery actually being dead/the monitor settings being the issue.

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vonkanehoffen avatar image vonkanehoffen matt1309 commented ·
Ah thanks so much! I've still not tried adding the solar charger back in, but with charging via Multiplus things are looking much better!
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2 Answers
snoobler avatar image
snoobler answered ·

When charging with solar, charged voltage is 0.2V below absorption voltage.

Most LFP batteries spec full charge at 3.65V/cell (14.6V) @ 0.05C tail current (5%), I'd set it to 6%.





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vonkanehoffen avatar image vonkanehoffen commented ·
Ah good to know. Thanks! So what does that mean for my settings? I have "Charged Voltage" set to 14.6V currently, while I'm testing without the solar charger, and it seems to be working ok. Do you mean set it to 14.4V instead when I add the solar charger back in? .....I don't see anything labelled absorption voltage in the settings.
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snoobler avatar image snoobler vonkanehoffen commented ·

"charged voltage" = voltage the battery must reach

"Your absorption voltage" = what you have your charger set to.

so, yes, charged is 14.4V if you have your charger set to 14.6V absorption.


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kevgermany avatar image
kevgermany answered ·

Another thing with solar charging is that periods of low charge rate are common and can cause a false sync to 100%. To avoid this, increase charge detection time.

And contra the other comments, you may need to decrease tail current, not increase it. It's made worse by the limited amount of panels you can get on the roof of a camper, under tree/shade parking, winter conditions. As well as the effects of cloud.

It's worth investigating what's happening. Check the actual charge current in good sun, and the conditions you're experiencing. Also check the voltages. There's reasonable reporting in the 712. If it's winter, all bets are off.

I'm on lead carbon, so may not translate as well, but 14V, 2% tail and 15 mins charge detection works well on solar for me. 4% tail gave false syncs, as did the default detection time. That's with 600W solar, 400Ah of battery. (It's a 24V system, but I've converted to 12V for you).

But... Due to differences in systems, there's no right answer, you tune the settings to match the system. It's why they're there.

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snoobler avatar image snoobler commented ·

Since these are LFP batteries, the absorption period is so short, tail current barely matters.

The surface charge of LFP burns of VERY quickly to below 13.8V, so false syncs are pretty rare in low PV conditions.

Additionally, when charging with lower current (limited amount of panels on a camper roof), by the time you hit 14.4+, the battery is so close to full it doesn't matter.

Sure, a false sync is a concern, but with LFP, if they happen, you're almost always up in the 95%+ SoC... I'm not sweating a few %. :)

If you're charging at a 0.5C rate, then the false sync error may be greater, but you're still going to be at a high true SoC.

If one is pursuing a more conservative charge to 3.45V/cell, then I agree 100%. You need a very low tail current.

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kevgermany avatar image kevgermany ♦♦ snoobler commented ·
Just updated to add my settings, before seeing your comment. I was wishing I could add something LFP specific based on the setup in my boat. Then remembered that the BMV isn't there, everything comes from the BMS.
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snoobler avatar image snoobler kevgermany ♦♦ commented ·
Try putting a BMV on an NMC battery (3.7V lithium). Never charge to 100% and periodically manually reset BOTH the BMS and the BMV due to drift. :)
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