If you look for size 26650 batteries on Ebay or Amazon, there are a lot of possibilities that show up, some with amazing listed capacities. Readers of flashlight or vaporizer forums are aware that most of these are fakes, but an occasional buyer of lithium batteries may not. Here at OpenROV, for the most part we don’t recommend any of the batteries that are commonly available on Amazon or Ebay, and urge extreme caution if you decide to use such a cell.
Recently a customer showed us a cell that he had purchased on Ebay - an UltraFire 6000mAh Lithium-NMC cell. This cell is quite commonly advertised:
The only rub is that 6000 mAh is pretty much beyond the state-of-the-art right now for size 26650 lithium batteries. For our recent exploration of the SS Tahoe, we were using 5000 mAh cells that we have been evaluating and are so far quite happy with. Could it be that somebody makes a more powerful cell? I warned the buyer that this was likely not the case.
I put the Ultrafire cell on the battery tester at OpenROV, and here’s the result:
The Ultrafire cell, when discharged at 0.25C (1.5 amps- a typical average current draw for the ROV) only provided 2.1 amp-hours- barely 35% of the rated capacity of the cell. Far from being an amazing power source for the ROV, a set of these cells would probably only run the ROV for maybe an hour and a half.
As a final comment, note that the Ultrafire cells include a protection board to cut them off at high discharge currents, and so they probably wouldn’t work in the ROV at high throttle settings anyway. This is the same issue that we had with the original TrustFire cells that were used on OpenROV back at model 2.5. The current LiFePO4 cells that we sell on our store have a much lower internal resistance, and handle large current spikes with ease.