I'm overthinking an easy question :(

Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
OK so here's my dilemma. I have several of these 3.7v 6000mah rechargeable batteries left over from a separate project. I want to connect them up in series so that I can use 7 of them as 1 battery. My problem is that I'm worried that when I go to charge it (with a 25v 1.5amp charger) it will charge it unevenly or even damage the batteries somehow. So my question is would I be OK with doing this or would I have to dismantle the battery and charge each individual cell every time. I was reading up online but no one has really come to a consensus. Especially since the fact that the battery pack I am replacing with my home made one is made up of similar cells joined together in series. There is probably some sort of framework or a small chip which keeps the cells balance but regardless it should be doable I think, I hope. Thanks,
 

dl324

Joined Mar 30, 2015
13,333
Welcome to AAC!
My problem is that I'm worried that when I go to charge it (with a 25v 1.5amp charger) it will charge it unevenly or even damage the batteries somehow.
You concerns are justified.

Even if you start off with matched cells, they'll eventually become unmatched. When that happens, the weakest cell will become depleted first and be driven into reverse polarity by the stronger series cells. That means the death of most batteries as I'm not aware of any chemistries that tolerate voltage reversal.

Before the weakest cell dies, it will charge the fastest and spend more time in overcharge which will speed it's demise.
So my question is would I be OK with doing this or would I have to dismantle the battery and charge each individual cell every time.
The batteries you're using have a specific charging profile. It's unlikely that you'll find a charger designed for charging multiple batteries in series. If you don't follow the correct charging profile, you'll likely decrease battery lifetime.

Operating batteries in series or parallel is a compromise.

Did you check batteryuniversity.com? They have a wealth of battery information.
 

MrSoftware

Joined Oct 29, 2013
2,015
Connect them in series, but also add wires to every cell so you can monitor each cell individually. Charge them in series, but when one or more cells get higher than the rest, stop the series charging and use the little wires to drain the higher cells back down to match the lower cells, then resume series charging. It is my understanding that this is how some LiPo balance chargers work. Or I guess you could just charge each cell individually with the little wires.
 
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Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
OK so it's looking like what I aimed to do initially won't really work so that's a bit of a bummer. I'll probably end up making some sort of housing that will let me easily pop the batteries in and out of a case so I can individually charge them, it's a little cumbersome but it's looking like my best bet. What I am still curious about though is how the actual battery packs work? Since you can see the individual cells in the battery pack wired up in series why don't they become unbalanced? There is probably a small chip or something that keeps the cells balanced, do you think I could make one of those chips or just put one on my pack?

Thanks for the quick replies by the way, really appreciate it!
 

dl324

Joined Mar 30, 2015
13,333
What I am still curious about though is how the actual battery packs work? Since you can see the individual cells in the battery pack wired up in series why don't they become unbalanced?
The manufacturers select matched cells to make the packs. Over time, the cells become unmatched and deterioration of the pack accelerates.
There is probably a small chip or something that keeps the cells balanced, do you think I could make one of those chips or just put one on my pack?
In battery packs with paralleled cells, there are often fuses that will prevent a shorted cell from shorting the batteries that are still good. This results in lower runtime, but no smoke.

For series cells, the battery pack is end of life when one of the cells "dies".
 

Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
is there any chip i could purchase which would keep the cells balanced while charging? to just drain any battery that was exceeding the other calls voltage or something?
 

AnalogKid

Joined Aug 1, 2013
9,414
Thanks to electric cars, Linear Technology and maybe Maxim make battery stack management chips that take most of the effort out of what you are trying to do.

ak
 

BobTPH

Joined Jun 5, 2013
3,837
You can get chargers for various numbers of LiPo cells in series. They will have a connection to each battery, and they work by limiting the voltage across each cell. When a cell becomes fully charged (4.2V) it simply shunts the charge current across that cell.

bob
 

Thread Starter

1quickquestion1

Joined Oct 13, 2017
22
I have a lipo balance charger but mine only goes up-to 6 cells, and the battery has 7. I would still need to make the balance lead as well. I talking more along the lines of a power distribution board, which would balance the cells.
 

strantor

Joined Oct 3, 2010
5,677
I've taken apart dewalt 18v and 20v (same thing) batteries and chargers and found that they don't charge each and every cell individually but they do have some taps in the middle between cells (maybe every 3rd cell, I don't remember) which the charger and probably the cordless tool (I'm guessing) uses to monitor individual cell groups for balance. (Guessing again) when one of those groups becomes unbalanced because of a bad cell in that group, the charger will no longer charge that battery pack. It will just blink and not charge <- that part isn't a guess; I've taken apart several blinky packs and found only 1 or 2 bad ones and the rest good.
 
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