Can heating your batteries make them work better? Can your cheapo batteries outperform the best cells money can buy? This test looks at the effect initial pack temperature has on the discharge curve. Safety is also considered, as final pack temperature is also measured.
I used the same setup and process as outlined in the Lipo review I did (part 1 and part 2). In short I used a multimeter, Watt meter, halogen bulbs, and a thermometers to record Voltage and Temperature at a discharge load of 17.5A.
I had noticed measurable differences in discharge curves with a temperature difference of just a few degrees. The Internal Resistance (IR) of the packs also changes with temperature. IR has a significant effect on pack performance, it is probably the single biggest indicator of performance.
To look at this more closely I used the Dinogy 30C pack. I would charge it up, then store for one hour. The temperatures tested were 73F, 81F, 91F, and 99F. Why those? It just happened to be that warm in the shade that day. I put the LiPo in my lipo safe bag and left it in the shade. I would check the temperature inside the bag with an instant read thermometer and the pack temperature with an IR thermometer. I only did one test at each temperature. It isn't the most reliable data, but good enough for a subjective test like this.
Effect of Temperature on Performance
How did temperature effect performance? You can clearly get a boost from your packs by heating them up. It looks to be, at least for this 30C pack, a point of diminished returns around 91F. HEating them up to 90F gives them a big boost in Voltage.
How does this compare to the best performing pack? If I graph the 76F data from the SMC pack alongside this data you can see the 30C Dinogy still does not compare, but heating it to 90F does close the gap.
Is it Safe?
I've seen some labels explicitly state not to warm your packs. However this is probably a word from the lawyers, not the engineers. I'll say that you should do this at your own risk. The data I got was not that bad. A bit odd perhaps. The 81F pack ended warmer than the 91F pack. This could be due to the higher IR created more heat from the discharge. It could also be something in the test process that repeated tests would not repeat. I'm not sure.
However even the 99F pack ended at just 134.8F. Certainly not good, but also not terrible. The 73F pack ended at 123.4, just 10F below the 99F pack.
Warming your packs looks like it could certainly give them a boost. However this is likely to have limited effect on better quality packs. Those with a lower initial IR won't see as dramatic an improvement. You will get some improvement, but not as much. In this case the IR dropped from 35mOhm down to just 18mOhm. Those with an IR of 18mOhm at 76F likely won't drop to 9mOhm at 99F, but a bit.
From this data it looks like heating packs to 90F is a good way to improve their performance while still staying on the safe side of heat issues. You will want to carefully monitor temperatures to make sure your batteries and loads don't create unsafe conditions (above 135F).