Simplest way to integrate another 300Ah LiFePO4 into existing bank

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chris99 asked Jun 11, '24

Simplest way to integrate another 300Ah LiFePO4 into existing bank

We want to expand our existing 2x300Ah LiFePO4 bank (2 years old) by adding another 300Ah battery from a different manufacturer. What is the simplest way to do this safely?

Details
On our boat we have 700W solar panels wiht Vitcion MPPT plus alternator with Balmar MC618 controller and a Victron 12/200 inverter (80A charger when on shore power) all running into two 300 Ah LiFePO4 batteries in parallel. Monitoring is via BT on the MPPT controllers and a SMArtSHunt on the bank. No other central BMS

In high summer we generate 300Ahday but use 400Ah with the aircon on. The idea of the extra battery is that we tank up on shore power when available and that will see us through 3 nights at anchor. Extra solar of alternator output is not an option. We could

1. Just parallel the third battery (not recommended)
2. Connect it via DC-DC charger - safe but we'd want it running in both directions (Victorn don;t make a bidirectional DC-DC charger thought there are some on the market from Sterling and Renogy.
3. Use a separate 230V-12V charger while on shore power to charge the third battery then a 9A DC-DC charger to trickle it into the main bank.

Thoughts or suggestions would be much appreciated!

leisure batteries

2 |3000

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jetlag answered · Jun 12 at 07:07 AM

I assume the battery voltage is the same and as you write both existing as well as the new bank are all LiFePO4 batteries. So in my opinion it is no problem to connect them just in parallel.

The "older" batteries might do not have the initial capacity any more, and also the internal resistance might be a little bit higher then it was two years ago. But this is no problem. This is just like paralleling two different capacity batteries. The charge/discharge currents will distribute through the batteries according their internal resistance and every battery will take the current it can take.

Same for discharge. The voltage across the banks will stay the same as of the other banks.

The only difference might be the limits set in the BMS. This might differ between the old and the new.

I did such a parallel arrangement already a few times with a 180Ah and a 42Ah battery and this works just fine. Also other, like Andy from the Offgrid Garage Australia is testing such constellations and has no worreis about that. So maybe you give it a try.

But before you test it, try to match the voltages as good as possible to reduce inrush currents when connecting them together.

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Answer 1 · 2024-06-12T08:50:38Z

jalle19 answered · Jun 12 at 08:50 AM

Go with option 1 i.e. just parallel the battery.

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