High Current vs Low Current
How do different charge/discharge rates affect the lifespan of NiMH batteries when all other conditions are kept the same?
We know the lower the charge current the better for the battery. But will halving the charge rate double the cycle count? And will doubling the load halve the battery’s life span?
I could not find any quantitative data on this topic so I decided to run my own experiment to find out. I took some VARTA 2100 cells and started cycling them with low (500 mA) and high (1500 mA) charge/discharge currents to see the difference.
Test object - I picked VARTA 2100 because
- they have the worst cycle counts from all batteries I’ve tested so far. This means the test won’t take that long (only 200 cycles or so)
- they are relatively cheap - the test won’t cost me that much
Test conditions - Except the charge/discharge currents all other conditions are identical
- 0dV charge termination
- 0.9V discharge cut-off
- internal resistance measured every 10 cycles in charged state
Results
| Capacity | Cycles to 80% | Cycles to 64% | |
|---|---|---|---|
| 0515 | 1998 | 180 (117%) | 228 |
| 1505 | 2011 | 182 (118%) | 206 |
| 1515 | 2024 | 154 (100%) | 199 |
Conclusion
To be honest I am puzzled with the results. I was expecting the 0515 and 1505 cells to perform much better than the 1515 cell. Capacicy-wise they all are similar, cycle-wise the 0515 and 1505 did only 17-18% better. I was expecting the internal impedance of the 1515 cell to rise the quickest, but it ended up between the two other cells. That’s strange…
Looking at the graph I don’t like the shape of the curves. Did I mess up something during testing? I don’t think so. But to be sure I will run a second set of 0515, 1505, 1515 cells again and for sake of curiosity I will add a 0505 cell (which will run for ever)