I have a 8.4V, 5000mAh NiMH hump pack running 2 PC cooling fans, each of which draws ~5A.

When I turned on the system (battery voltage ~9V), the fans ran fine for a moment, but then the voltage output from the battery begins dropping. After ~5 seconds, it stabilized at ~7V. After about 5-10 more seconds, I switched off the system and the battery voltage rose back up to 8.4V (not sure how fast, my analog voltmeter doesn't respond quickly). I did the same thing again - same result.

However, when I ran only 1 fan, the battery voltage remained near 8.4V.

What is happening to my pack? And how do I fix it?

Battery history: The battery is ~5 months old with < 50 charges on it. I have been using a smart charger that has an auto-cutoff. First ~10 charges were at 1C, but now I charge at 0.1C or less. It may have been overcharged a few times and at least once it got to ~7V or less.

  • \$\begingroup\$ Batteries have an internal resistance aka output impedance. \$\endgroup\$
    – PlasmaHH
    Commented Jan 7, 2017 at 22:16
  • \$\begingroup\$ @PlasmaHH: Internal resistance would show an immediate drop under a 10A load. I saw a drop over a ~5s period. Also, by that logic, I would have seen 1/2 the drop under a 5A load. I saw nothing of the kind: the voltage was >8V. \$\endgroup\$
    – dpdt
    Commented Jan 7, 2017 at 22:19
  • 1
    \$\begingroup\$ internal resistance is not constant and the result of various different chemical processes. \$\endgroup\$
    – PlasmaHH
    Commented Jan 7, 2017 at 22:21
  • \$\begingroup\$ @PlasmaHH How do I fix it? \$\endgroup\$
    – dpdt
    Commented Jan 7, 2017 at 22:35
  • 1
    \$\begingroup\$ You may have a generally weak pack (or even essentially fake specifications), or you may have one in which a particular cell has been weakened by overdischarge or overcharge. If you can measure individual cell voltages under load that may be quite informative. \$\endgroup\$ Commented Jan 7, 2017 at 23:10

1 Answer 1


Your discharge rate is too high.

You are drawing 10A from a 5000mAh battery. Most NiMh batteries are rated for discharge at a fraction of the mAh rating. The ones I used to work with were rated for discharge at .2 times the mAh rating. So, your battery pack shouldn't be discharged at more than 1A by the rules I used to work with.

When discharged at too high a rate, NiMh batteries go bad. The internal resistance increases everytime you discharge the battery too fast. That increase in internal resistance is permanent. Eventually, the internal resistance will be so high that you can't make effective use of the battery.

The battery still has its full capacity. If you slowly discharge a NiMh battery whose internal resistance has gone up due to mishandling, you will measure near its rated capacity. If you try to pull a higher current, though, the voltage will drop. This is the effect you are seeing.

The only fix that I know of is replacing the damaged cells

Back in the mid 1990s, Motorola brought out the Visor model portable two way radio. It was a fist sized radio capable of transmitting at 5Watts - the first radio that small to be that powerful.

The Visor was delivered with a 600mAh NiMh battery.

The company I worked for sold hundreds of the things. Everybody loved them - until the batteries started dieing.

Motorola replaced the 600mAh batteries with some "improved" batteries that delivered about 700mAh in the same physical size.

These died even faster than the originals.

We investigated, and found that typical battery analysers would give the "dead" batteries a clean bill of health. According to them, the batteries were fine.

The radios, however, would shut down and restart if the bad batteries were used.

I tested them with a rig I built out of a DAQ, LabView, and a current shunt, and found that the internal resistance of the batteries was ridiculously high. This made it impossible to draw high current from the bad batteries.

More investigation turned up the recommendation from the NiMh cell manufacturer to never discharge them at more than 0.2 of the rated capacity.

The Visor drew about 2A when transmitting at 5W. That's like 20 times the manufacturer's suggested maximum.

The solution was to use the optional high capacity battery for 5Watt operation, or just replace the small batteries when they died - most of our customers needed high power and small size, so they just budgeted for a LOT of replacements.

The company I worked for back then had good contact with Cadex. At our request, they added an internal resistance test for NiMh batteries to the C7000 so that we had a quick pass/fail for checking the batteries. That feature is still in the C7000 to this day.

I actually tested several batteries to destruction by simulating typical usage patterns. I ran the simulation at 5W with the 600mAh batteries, and they died. The same test at 1W didn't kill the 600mAh batteries. The larger batteries all survived the 5W tests.

  • \$\begingroup\$ Your claims here are rather unlikely to be correct. NiMH cells are routinely used in applications well above the "C" rate - they aren't quite as good at that as NiCd's were, but still far better than you allege. And both the capacity and the description as a "hump pack" suggest the OP has an RC Car pack, which is one of the higher discharge applications, far beyond your impression of what is possible. In contrast to what you claim, damage is usually not caused by rate of discharge, but rather by excessive discharge - particularly with a radio, that may get "left on" when it is "dead" \$\endgroup\$ Commented Jan 7, 2017 at 23:07
  • \$\begingroup\$ as the user doesn't realize that this is distinct from being off, unless the product is designed with battery-preserving shutdown circuits. Of course overcharging from an unsuitable internal or external charger is another potential source of damage. The main point is that what the OP is trying to do is not unreasonable in principle, but some detail of the implementation is wrong. \$\endgroup\$ Commented Jan 7, 2017 at 23:08
  • \$\begingroup\$ @ChrisStratton. The radios would shut off when the battery voltage was too low - deep discharge wasn't possible. A NiMh battery operated at.low disharge rates can handle 500 to 1000 charge/discharge cycles. Operating at high discharge rates reduces the number of cycles it takes before the battery is unusable. RC enthusiasts run their batteries hard. I don't expect them to get hundreds if cycles out of them. \$\endgroup\$
    – JRE
    Commented Jan 7, 2017 at 23:12
  • \$\begingroup\$ The cells RC enthusiasts use would have lasted a long time in your radios, baring design flaws in how they were managed. If you got cells actually only designed for low discharge that would be a different story - a problem of part selection. But the OP seems to have a pack marketed for RC use and to be using it in a comparable manner, so their expectations are not unreasonable. \$\endgroup\$ Commented Jan 7, 2017 at 23:17
  • \$\begingroup\$ No. @ChrisStratton is correct: I have a hobbyist battery pack for use in RC cars. 10A is an exceptionally low draw for it - most RC cars will draw much more (50+A). And hobbyists get many more cycles out of their batteries than I have. Downvoting for this reason (although this may be correct for different cells?). \$\endgroup\$
    – dpdt
    Commented Jan 8, 2017 at 0:25

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