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Link about why you can't mix new and old batteries.

This guy, who is an engineer is building battery packs out of cells he scavenged.

I can't add an extra cell to my ebike because my batteries are 4 month old (mine are SLA though had 100 cycles on them.)

a busy cat
(source: playstation.com)

JRE
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alcoholic
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    …that's nice, but what is your question? –  Jan 13 '16 at 01:44
  • The last time you asked, you said the store was only about 2km away. If you just walked you'd be done and home by now. Or trade in that e-junk for a mountain bike with decent low gears for the hill, and a trailer for the beer. –  Jan 13 '16 at 11:53

2 Answers2

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If you put 1 old battery in parallel with 1 new battery (of the same voltage), it will work fine. Similarly, if you always add batteries in parallel to a series of batteries, it'll be fine.

The problem arises when you try and place old and new batteries in series. The new one tends to drive the weak/old one backwards. That can be hazardous to your health. Many batteries will leak fluid or become unstable after they've taken an inverse cycle. Once they unstable, when charging, they might explode or vent.

horta
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  • so I am fine in parallel, because the plan was for parallel in fact I might add 2. I have a 48 volt e-bike charger and it's a smart charger, can I still use it to charge my 6 battery pack? – alcoholic Jan 13 '16 at 02:11
  • can i add batteries in parallel that are lithium to my sla pack? I am going to blow something up aren't I? – alcoholic Jan 13 '16 at 02:15
  • The answer to your first comment is "yes that's fine". The answer to your second q is, "no definitely not". Don't do it. They both have very different charging profiles and putting the two together will throw one of them off. Never mix battery types. – horta Jan 13 '16 at 02:18
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Whether new or old, all the cells in a series pack should have matched capacities and internal resistances. If they don't then the 'weakest' cell will drop its voltage first, and could become over-discharged before the total pack voltage reaches cutoff.

Even new cells from the same batch have slightly differing capacities, so battery manufacturers may test the cells and 'match' them by capacity. When you mix cells from different packs they could have quite different capacities, and then your pack will continually go out of balance in use.

Older cells tend to have higher resistance and/or lower capacity. Mixing new and old cells increases the likelihood of the pack becoming unbalanced in use. And like a chain the whole pack will only be as strong as its 'weakest link', limiting performance to that of the tiredest cell.

Nicad and NiMH cells need up to 10 charge/discharge cycles to reach full capacity. If you don't do this then the new cells in the pack could have lower capacity and get over-charged and/or over-discharged in use. If damaged the new cells will lose even more capacity until they die.

Bruce Abbott
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