I have a NiCd 2000mAh 7.2V battery that drives two motors. If I run the motors for 30 seconds, the voltage reduces from 7.60 to 5V. Once motors are stopped, it takes a minute to come back to 7.50V at most. What's happening? Is there a problem or is this fine?

Guys sorry I forgot to mention one thing, I designed my own charging Circuit and I think there's not enough current output from the charger. Can that be a possible reason ? What happens when you charge a battery with low current ?

  • \$\begingroup\$ The effect is called polarization. \$\endgroup\$ – Spehro Pefhany Jun 18 '14 at 20:08
  • \$\begingroup\$ link to the motors will probably help. \$\endgroup\$ – Brian Drummond Jun 18 '14 at 21:30

You're probably sucking too much current out of the battery pack too quickly. If you can dig up a data sheet for the battery there will likely be a rating for continuous draw and one for peak impulse draw. Find those numbers, and try to stay under the continuous draw figure. you might need more batteries in parallel.

  • \$\begingroup\$ I dread to think how much current is being drawn if the battery pack is nicad... \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 18 '14 at 20:37
  • \$\begingroup\$ @IgnacioVazquez-Abrams, certainly tens of amps. \$\endgroup\$ – Scott Seidman Jun 18 '14 at 20:40

You really shouldn't be drawing that much current from that size of battery. You're causing a local depletion of the reactants at the plates of the battery, and it takes time for diffusion through the electrolyte to replace them. This kind of abuse raises the internal impedance and causes excessive heating from the I2R losses.

If you can get your hands on liquid-electrolyte NiCads, they're much better at handling high peak loads. There was a time when such batteries were used to start jet engines, but they're rare now.

Would it be possible to design your system to use ultracaps to handle the high peak loads?

  • \$\begingroup\$ Or use a more modern chemistry like high-discharge NiMH and Li-ion. \$\endgroup\$ – user36129 Jun 19 '14 at 7:07

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