Inquiry Regarding Battery Configuration

Batteries, Chargers, and Battery Management Systems.
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syam9494
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Inquiry Regarding Battery Configuration

Post by syam9494 » Dec 27, 2017 4:32 am

Hello, I'm Syam, from India, I'm working on an e-bike for daily commute to college. i have a BLDC Motor with 24V,350Watt, and Rating current of the motor is 20amps. i'm planning to do my battery pack with lithium ion cells, so i have salvaged lithium ion batteries from laptop batteries, rating of each cell is 3.6v-4.2v with 2500mah. i tried to download the data sheet of batteries, but unfortunately i didn't get any information. can you guide me to choose the right configuration for my battery pack. Below attaching pic of lithium ion cell for more info. awaiting your reply.Thank You.
Li-ion Cell.jpg

Joachim
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Re: Inquiry Regarding Battery Configuration

Post by Joachim » Dec 27, 2017 9:08 am

You need the max amps the controller takes and how much amps/c a singel cell can discharge
I think 7s 5p 6p battery wurks for the 350watt system,7s 25A Li-ion bms....25.2Volt 15Ah battery pack

dustNbone
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Re: Inquiry Regarding Battery Configuration

Post by dustNbone » Dec 27, 2017 10:44 am

Laptop cells are usually designed for fairly low current/high capacity.

For 2.5Ah cells like those I wouldn't want to push them much past 0.5C which is 1.25A per parallel cell. If you're pulling 20A continuous you'd want 16p (!) but if you're only peaking at 20A and running closer to 10A continuous then 8p should be OK.

For a 24V system you'll want 7 cells in series (3.6V x 7 = 25.2V), so your battery will have at least 56 cells in it (7s x 8p). That would net you a 24V 20Ah battery.

If you have enough cells available, you could add more parallel groups which would give you more range and put less stress on the batteries.

999zip999
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Re: Inquiry Regarding Battery Configuration

Post by 999zip999 » Dec 27, 2017 11:16 am

Here's a guy in India Made in India battery components he should be able to help you out. https://endless-sphere.com/forums/viewt ... 4#p1276942
They even have a how to make a battery book.
Low power cells need more in parallel.

syam9494
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Re: Inquiry Regarding Battery Configuration

Post by syam9494 » Dec 28, 2017 5:04 am

Hello DustNBone,

How can we find the continuous Discharge of a cell.

syam9494
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Re: Inquiry Regarding Battery Configuration

Post by syam9494 » Dec 28, 2017 6:36 am

dustNbone wrote:
Dec 27, 2017 10:44 am
Laptop cells are usually designed for fairly low current/high capacity.

For 2.5Ah cells like those I wouldn't want to push them much past 0.5C which is 1.25A per parallel cell. If you're pulling 20A continuous you'd want 16p (!) but if you're only peaking at 20A and running closer to 10A continuous then 8p should be OK.

For a 24V system you'll want 7 cells in series (3.6V x 7 = 25.2V), so your battery will have at least 56 cells in it (7s x 8p). That would net you a 24V 20Ah battery.

If you have enough cells available, you could add more parallel groups which would give you more range and put less stress on the batteries.
Hello DustNBone,

How can we find the continuous Discharge of a cell.

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dogman dan
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Re: Inquiry Regarding Battery Configuration

Post by dogman dan » Dec 28, 2017 8:51 am

As a guesstimate, in the past the best policy for reclaimed laptop cells has been to keep the continuous discharge rate below .5c max. They might handle more, but in general, those who obeyed this limit had success. In a laptop, the actual rate is way below .5c, so they don't put the high rate cells in them. They might be called 1c cells, but less than .5c is the rate you should aim for. And half that again, .25c, will work even better. So go big as you can, possibly by building one battery now, and another similar one later so you can run as low a discharge rate as possible later.

So your cells are supposed to be 2.5 amp hours ( 25000 mha). That was when they were new. Lets just assume they actually hold 2 ah on average now. That means .5c discharge rate would be one amp per cell.

20 amps controller, but in fact you wont pull 20 amps continuous, again, experience tells us that your setup will likely pull only about 10 amps max when up to full cruising speed.

So,, if you will pull about 10 amps continuous, you need to configure your cells in 10 p. ten cells or more in parallel. Then for 24v, I would suggest 7 cells in series, which would charge to 29.4v maximum. Likely 6 s would work too, but 7s 10 p is what I would shoot for. 70 cells. or if possible more, if you need longer range. This, assuming you get 2 ah per cell, would be a 24v 20 amp hour pack. That will be big enough for some decent range, more than 10 miles. Likely 15 miles, in fact. The above suggestion of 8p would be fine for new cells. But yours are not new, so you need to de rate your capacity in the calculation. Go with 10p or more. 10 p is still a relatively small pack anyway, for 24v.

Ideally, test every cell for internal resistance before assembly. Whatever the number is, and it might be poor, chuck out any cells that test a lot worse than the others.

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