It's supposed to be a 48V system, isn't it? It seems they are using 3400mAh 18650 cells, 26 of them (13s2p). The Samsung 35E cells are 10,5A. I guess they chose 3400mAh over 3500/3600 to get the higher discharge current. So at 48V the current draw will be 5,2A to get 250W. At 42V it's 6A.The problem I see with the range extender is it will only supplement the main battery therefore will discharge at the same rate so when the extender is depleted the main battery will be depleted as well. Adding a fresh range extender with a fully charged voltage of around 42 vdc to a depleted battery of say 33vdc means that only the range extender will be supplying current to the motor which at only180whrs (5000mah @ 36vdc nominal) would mean a current rate of 7a would be required to produce the 250w of the motor. Efffeciency would go down quite a lot as well as life expectancy. Current 2170 battery cells are able to discharge at rates up to 10 amps but it is at a cost of useable capacity and charge cycles. It would be much better to use a higher capacity extender battery if you require more range than the 360+180 configuration can provide.
That said, I agree, there could be disadvantages to using these small extenders. It would be interesting to see the difference in range using the 3400mAh-10A-35mOhm cells compared to say the LG HG2 3000mAh-20A-20mOhm. Using the 3000mAh cells you get 140Wh, compared to 160Wh. But the higher current and lower internal resistance might more than make up for it...
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