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I have a set of four panasonic 18650 3.7v 3400mAh batteries that I am using for a project. All four are wired so that the voltage is doubled (7.4v) and so that the amps are doubled (6800mAh). I've been checking the voltage with my multimeter frequently over the past couple weeks I've had them, and the voltage has dropped from 7.1v to 5.9v over that time. The thing is, I've hardly ever used the batteries in that time, only a couple times. I wouldn't expect these batteries to drain without hardly any use, especially since its 6.8 amps. I'm just trying to think of reasons it could have done this.. The best one I can think of is the fact that I accidentally shorted ground and voltage when measuring the batteries (the two traces are super close) and it resulted in a spark, though it still supplied voltage fine, even to this day. If anyone could either confirm this theory or give another, that'd be great.

Batteries wired to PCB with step down regulators attached to sides

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Batteries have very complex behaviour. (On top of that, playing around with lithium batteries can be extremely dangerous). There are many factors that influence what you're, measuring as the voltage:

  • battery condition. Most rechargeable batteries die when completely discharged
  • battery capacity (how do you know they're not fakes? Ie are they really 3.4 amp-hour batteries?)
  • charge state (how do you know they were fully charged)
  • load at time of measurement (was anything sinking or sourcing current to or from batteries while measuring voltage)

And in-between measurements: are you absolutely sure something isn't draining them? Some lithium batteries have internal circuitry that might be accountable. Plus that PCB you've soldered on, and anything else not in the picture.

If I had to bet, then I would put money on one or both of these being the problem:

  • batteries are duds/fakes. Fakes are flooding the market, with 100th (just 1%) of the stated capacity. This makes over-discharging easy to do accidentally, ruining it - not that it was any use in the first place.
  • battery was not fully charged
  • battery is being drained
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  • \$\begingroup\$ Definitely not fakes. These are the most reliable lithium batteries on the market and are known for actually meeting their amperage. Idk how anything could be draining them either as the only thing attached to the batteries are some step down regulators, which are separated by a switch. If it helps, the batteries were discharged for around 10 seconds which is rather long. Still my only explanation. \$\endgroup\$ – MH0517 Mar 29 '17 at 10:34
  • \$\begingroup\$ If you doubled the voltage and current, I'm assuming you have two batteries in series and two in parallel. I'm not saying you can't ever do it, but be aware that when putting batteries in parallel one parallel battery not holding its charge so well can be a load for the other. If you use two primary (non-rechargable) cells in parallel, one will pull the other down until they are both the same voltage- so always use two fresh ones. I'm not sure what can happen with rechargables. \$\endgroup\$ – Lance Beasley Mar 29 '17 at 12:32
  • \$\begingroup\$ @LanceBeasley what I would do is get one more battery to rule out factors logically, step by step. (Ie...firstly, use just one battery to enable ruling out problems of parallel/series connections. Then (obviously) be careful not to short or over-discharge them. Make sure it's properly charged.) \$\endgroup\$ – CL22 Mar 29 '17 at 14:23

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