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Disadvantages of low capacity cell?
#1
I've been offered some cells in the 1000mah to 1200mah range, at a price that has definitely peaked my interest. I'm thinking of using them to produce a 48v pack in a 13s80p configuration (think I got that the right way round) with one of the cheap Chinese Bluetooth BMS, as this is a stationary build size\weight isn't really an issue, and to 60a from the BMS will realistically only give me 2kw of power.

So firstly are there any disadvantages to using the lower capacity cells?

Secondly if I want to expand the setup, can I parallel in a second and third BMS etc. And if I can, do I need to use similar cells or do I just need to match the total capacity for each BMS?

Thanks.
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#2
There isn't a big disadvantage to using them, no. However, you just need a lot more of them to make something useful. Most here use 2200mAh and above. Using 1000mAh cells, you'd need at least twice as many to match in capacity, which is twice the physical size of the pack, as well.

Going with 80p would only yield you 80Ah, and at 48V (which is 14s, not 13s) would net you in a total of 4.160kWh. If you are indeed going with 13s, then you would get 3.840kWh.

Which chinesium bms are you looking at? It allowing you to get 2kW of power is quite a bit of available power. However, being realistic, it's probably going to only give you 40A max with 60A surge; and even the 40A continuous might be generous.

You can always parallel the outputs of each string (bms included) without issues. The problem comes when trying to parallel the multiple bms' with their balance leads.

Each pack in a string much match in capacity to keep things balanced. But each string can be different.

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#3
If they are eg. 2000mAh laptop cells that now test at just 1200mAh, then they won't have much useful live left.
If they are low capacity/high current 1300mAh powertool cells with now 1200mAh, then no problem.

Be careful when mixing low capacity (eg. 1000mAh) and high capacity cells (eg. 3000mAh). The high capacity cell will charge/discharge at about 3x the current, so you need to make sure that the cell+fuse can handle that .
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#4
I was looking at the AliExpress BMS https://www.aliexpress.com/item/32733414980.html I know the same ones are available from several sources. My figures for pack sizes were based on the batteries I have access too, so moving to 14s as recommended would reduce the parallel side of things slightly to keep the same total kWh.

I guess when I was suggesting parallel battery packs I was think something along the lines of 14s75p3s so each set of 14s75p is running at 48v on a BMS, and there are 3 BMS on the entire setup.

When you say that a 2000mAH battery now at 1200mah wont have much life left I guess this means it will be well on the way to 0mah, is there anyway to guesstimate how much life is left can it be nursed to a longer life by say keeping between 20% and 80% charge or restricting use even further. Obviously at the expense of kWh.
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#5
That's the BMS I use (the 60A version though), and also the same battery setup. Currently 3p of 14s100p, planning to upgrade to 4p of 14s100p in a month or so. Very happy with the device as well as the modular setup.

As with the mixing of cells of different capacities, you have to be careful when you parallel batteries of different capacities.
eg. if you have
battery#1 14s100p of 1000mAh cells
battery#2 14s100p of 2000mAh cells
then battery#2 will charge/discharge roughly 2x the current of battery#1. Ie the current will split 33% : 66%.


I don't think there are any public hard numbers, but generally the degradation of a cell is fairly stable/constant between 90% ~ 50%ish health, but after that starts to spiral down the drain much faster, and with an increasingly higher probability of sudden death.

Yes, staying between 80%~20% SoC helps, as well as charging/discharging at lower currents, and operating at moderate temperatures. But that all applies at any state of health.
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#6
So you have 3 of the BMS in parallel, and it all works okay?
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#7
Having bms in parallel works untill you overload it. Then they Will fail bad in worst case cause issues. Its far from recommended. A bms is There to protect. The extra cost doing it right is nothing...
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#8
(08-09-2019, 10:47 AM)daromer Wrote: Having bms in parallel works untill you overload it. Then they Will fail bad in worst case cause issues. Its far from recommended. A bms is There to protect. The extra cost doing it right is nothing...

So what is the correct way of doing it?
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#9
They work fine, but I run them very conservatively.
Chargers/inverters are software limited to 26A/4.05V/3.4V.
BMSs (60A type) are set to cut off at 30A/4.15V/3.0V. Balancing set to start at 3.9V, if voltages differs more than 0.015V.
Just to be safe, each battery has its own 30A fuse and a circuit breaker (used mainly when cells need replacing).

One issue is that once 1 BMS trips (or fuse blows), the other 2 has to share the extra burden, likely tripping them, too.
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#10
I guess you planned 4th BMS will ease the load considerably when doing maintenance etc.
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