MPPT LiFePo4 settings - large PV capacity

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colin asked Jun 19, '20

MPPT LiFePo4 settings - large PV capacity

I have a number of new portable power systems, each with the same equipment:

- Victron Lithium Superpack 25.6V/50Ah
- BMV 712 Smart
- MPPT 100/20
- Solar PV, up to 3 x array of 400Wp each = total 1200Wp

The very large solar capacity is to cater for the very cloudiest days expected (no other charge sources available). This means that the vast majority of the time the system is going to have way more charge available than it needs. I am concerned about overcharging the Superpack and I have seen it recommended to not fully recharge Li-ion to maximise overall life.

1. From my lead-acid experience with the BMV, it likes to "synced" to 100% regularly to maintain accuracy. Not sure if this is the same with Li-ion? If so, then this conflicts with the recommendation to not regularly charge Li-ion to 100%

2. What would be your recommended settings for the MPPT in this case?
The default for the MPPT for Li-ion appears to be 28.4V, 2h absorption.

MPPT Controllers BMV Battery Monitor Lithium Battery SuperPack

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Answer 1 · 2020-06-19T12:22:34Z

solardude answered · Jun 19, 2020 at 12:22 PM

100% on the BMV can be whatever you want it to be. Charge it to your desired “full” voltage, then sync the BMV and tell it is 100%.

You shouldn’t have to sync nearly as often with lithium.

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colin commented · Jun 20, 2020 at 12:05 PM

Thanks, I hadn't considered that. It doesn't matter to the end user that the "100%" they're seeing doesn't strictly match the absolute capacity of the battery. But I guess it would also be a good idea to reduce the battery Ah setting in the BMV to match - so maybe use 40Ah instead of 50Ah?

I realise it's probably quite subjective, but any suggestion for MPPT settings (and by extension BMV sync setting) to reduce negative effects of overcharging the Superpacks?

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Answer 2 · 2020-06-20T12:18:50Z

dazey77 answered · Jun 20, 2020 at 12:18 PM

I think he was meaning that in the MPPT you can set your absorption voltage to be what you want.

Example I have a LiFePo4 battery and manufacturer specs for full capacity is a absorption of 14.6v. Victron standard LiFePo4 preset is 14.2v. I generaly use an apsorption of 14v for maximum cycle life. I have all the presets saved on my phone so I can charge to a higher level if I need.

The BMV I sync to the current 100% (I forget the exact number but 14v charge is 90 or 95% capacity but you just sync the BMV to it and it will show 100.

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colin commented · Jun 20, 2020 at 12:33 PM

Great, thank you, I will try that!

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