NiMH Charge and Energy Efficiency

Inspired by HKJ’s NiMH Battery Charging article and after doing some research on internet I learned that

I also learned that there are two chemical reactions happening during charging. The primary charge reaction is desirable, but from approx 80% SoC a parasitic side-reaction starts competing with the primary reaction and becomes dominant in the overcharge region. This parasitic reaction is responsible for oxygen evolution, nickel electrode swelling, warms up the cell and increases its pressure. In overcharge state it causes a slight voltage drop. The effect of this parasitic reaction is mostly reversible by discharging the cell.

But enough of theory, let’s do some tests…

Test Method

The test is performed on SkyRC MC3000 for its accuracy and Panasonic Eneloop for its consistency. Both charge and discharge currents are 1.5 A.

  1. initialize the cell with two charge-discharge cycles, leave the cell discharged (our 0% SoC)
  2. charge a certain amount of capacity (start with 100mAh) into the cell, note down how much charge and energy went into the cell
  3. discharge the cell to 0% SoC, measure how much charge and energy was extracted from the cell
  4. calculate the increments, graph the results
  5. keep increasing the charge capacity in 100mAh increments and keep repeating Steps 2-4 until feasable

Charge Efficiency

NiMH Charge Efficiency

Energy Efficiency

NiMH Energy Efficiency


General Notes

Eneloop Notes

AA Cycler