We are stocking a portable power pack for USB charging. We tested them when they first arrived and found no issues but after 3 months we tested some of the remaining stock and found about 50% of them were dead. I took a few apart and found the cells were 2.5v (+/-.1v) There are two lithium-ion cells connected in parallel.

I charged a few of them back up a little and tested them every few days. They self discharge back down to 2.5v and then stop.

I believe this is because of poorly balanced cells and at 2.5v the stronger battery no longer has enough energy to overcome the internal resistance to charge the weaker battery so rapid self-discharge stops.

Our manufacturer is claiming it was a fault in the circuitry and they changed a capacitor to solve the problem. I do not believe this because it seems like any circuitry problem would keep discharging the cells.

Am I correct in how two unmatched lithium-ion cells will behave in parallel? Will they self discharge to a point and then stop?

Also what are the potential dangers of this? Is there increased risk of fire from charging batteries that are not well matched?

Is this an obvious case of mismatched cells or could it possibly be a circuitry issue?


  • \$\begingroup\$ I would ask the manufacturer for the relevant part of the circuit where a cap would fix this. That sounds fishy. Sounds unmatched to me. Manufacturer name? \$\endgroup\$
    – ACD
    Commented Oct 21, 2014 at 16:57
  • \$\begingroup\$ It's a small MFG. A name would mean nothing. I'm trying to understand how poorly matched Lithium Ion batteries would behave. If they just charge each other until the voltage is equal then what is the point of checking voltage, internal resistance, etc.. and matching cells? What if you do not pick good matched cells? What happens? \$\endgroup\$
    – BenRefurb
    Commented Oct 23, 2014 at 14:17

1 Answer 1


Batteries in parallel will "charge" each other until they are the same voltage. You could think of it as each cell being a bucket of water. If you put a pipe connecting all the buckets from the bottom, (analogous to batteries in parallel) all the buckets will equalize to the same level of water. This wont harm anything if your batteries are all close to the same amp rating (+/- 200ma or so) I suppose if one of your cells is bad and "leaking" energy the that could be causing your issue by constantly discharging all the other cells in your parallel bank...

All in all, batteries in parallel balance themselves automatically. Where you need things to be actively balanced is with cells in series or packs of parallel batteries in series with other packs. That active balancing is done with a balancing charger or a circuit in the pack. As long as all the packs connected in series are the same summed capacity.

Hope that helps!

  • \$\begingroup\$ Well, I need to understand what is going on then. We have two power packs. One uses a single battery the other uses two batteries in parallel. The single battery unit has no issues after 4-5 months. The dual battery units are aprox 50% dead. All 2.5v (+/- 0.1v) \$\endgroup\$
    – BenRefurb
    Commented Oct 23, 2014 at 14:14
  • \$\begingroup\$ Quote: "if one of your cells is bad and "leaking" energy the that could be causing your issue by constantly discharging all the other cells in your parallel bank..." \$\endgroup\$
    – Alex
    Commented Jan 2, 2019 at 12:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.