Can I turn my 48v lithium battery into 72v with mixed cells?

[fatmarley]

I'd like to turn my DIY 48v battery into 72v, but I can't get the same cells for a decent price. Can I use a different brand but with similar specs? - The current cells are Samsung INR21700-30T 3000mAh 35A - I'm thinking of using EVE INR21700-40P 4000mAh 35A with 50A bursts.

Current battery is 13s7p, and I'd like it to be 20s7p.

Or is this a bad idea?

 

[docw009]

Off the top of my head, it seems like a poor approach, but let's put a 3AH and a 4AH cells in series, which is what you're doing.

If we drain 2AH of current from this combo, one cell will be discharged 50% and the other 67% in an ideal world, Unbalanced battery, But if we force 1 amps of charge current thru them for 2 hours, won't both cells be back to full charge?

The main argument I see against series connection of two different AH batteries is that the smaller one will hit LVC early, but there is a BMS for that, Then there is the question of whether they will get back into balance during charging, If they are good cells, don't see why they couldn't get close and if your BMS can balance, maybe it will work. Maybe I will test this with a 3S and a 10S.
.

 

[AnthonyC]

How are you doing the conversion, with 1 BMS or with 2? One probably entails new cells, BMS, case and mount (and controller and/or display?), for all that it might be worth investing in all-new so that the old cells don't limit performance. Two BMSs would need an entirely different approach. What's behind the change, 15% more speed? ( 15% = 72/48^0.33 )

 

[amberwolf]

I'd like to turn my DIY 48v battery into 72v, but I can't get the same cells for a decent price. Can I use a different brand but with similar specs? - The current cells are Samsung INR21700-30T 3000mAh 35A - I'm thinking of using EVE INR21700-40P 4000mAh 35A with 50A bursts.

Current battery is 13s7p, and I'd like it to be 20s7p.

Or is this a bad idea?

It'll always be imbalanced (cells of each section at different voltages because of their different capabilities and capacities), and you'll only get the performance and capacity of the worst cell.

You'll still have to buy a new BMS that can handle the entire pack's cells, and a new charger that can supply the enitre pack's voltage.

 

[fatmarley]

Thanks guys. I asked the same question on Facebook, and luckily the guy that runs Cell Supply gave me a good deal on some new Samsung INR21700-30T cells.
The bit I'm dreading is extending the pack. I was nervous as hell when I built it the first time. Obviously, I will make sure the voltages are the same before connecting anything - I don't want any fireworks

 

[docw009]

When you extend the battery, it's probably good practice to run it down to around 44V. That's about 30% capacity and is what new cells are shipped at, Of course, you should adjust to whatever voltage you find on those new 30T's. Will be easier to balance.

By the way, I did connect a 10S-5Ah in series with a 3S-12ah pack. Started at 42.0 and 12.6V. Ran it on a 3A load til the 10S shut off, The 10S was down to 3V/cell and the 3S at 3.9V/cell. These packs had common port balance BMS, so I recharged them in series with a 48V charger, and they came back to 42V and 12.6V. Tried it a few times. Always recharged.