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How low of mah should I go when building an 18650 pack

Stucco

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Oct 4, 2021
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So I am almost done mah testing my reclaimed 18650 cells and starting to think about building the 21s?p battery pack for my electric cycle.

One question that I had is how low on mah should I go for building my battery? The majority of my cells are 2000mah - 2999 mah. And I have a good amount of 3000mah+ cells. But I also have quite a few 1000-1999 mah cells.

So how low should I go when building this? Do I omit all the 1k mah? Do I start at 1500mah? Or are 1000 mah putting out enough oomph for my pack.

I know I will have to put them all into the battery pack builder, but am I bringing down the average by having lower mah cells?
 
I would keep your cells within a 200mah range. Especially if you have cells that are supposed to be 3000mah. And only have 1000mah capacity left. The 1000mah have degraded to far and give you problems in your pack.

The closer you can keep your cells mah the better your pack will be.
 
Any cell that has degraded more than 20% of its rated capacity will likely be on the rising steeply part of the IR/life cycle curve. Once it hits 40% degradation you’ve pretty much guaranteed that DC IR has dramatically increased and the cells continuous discharge rating can no longer be determined just by reading the datasheet. Because DC IR doesn’t have one standardised method everyone agrees upon you’d have to pick a DC IR testing method and apply it to all cells to get anything close to a consistent result. No slot style tester is going to cut it.

I see no point in wasting time on these cells prior to pack building and certainly you will be wasting time in the future rebuilding the pack when some of these cells short internally.

Use cells over 2000mAh currently and only if they are at 80% or higher of their rated capacity. It doesn’t hurt to find a slot style tester you feel gives half decent results and set a high IR figure that disqualifies cells that passed the capacity standard. These cells (if you find any) are going to be your self dischargers which some people weed out by letting fully charged cells sit for X weeks and then choosing a voltage drop cutoff. I just voltage tested 400’ish LG MJ1’s I have in storage at 3.65-3.72V. After 1 year I don’t see really any noticeable self discharging. Maybe 10-20mV? None of them have fallen below 3.65V and there are an even number spread out in that voltage range which is what I remember when I put them away a year ago. So when I see people keeping cells that have dropped 100mV in 4 weeks (granted it is from 100% SOC) I’m kinda like ?
 
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