Exactly:
Quote:
Originally Posted by Daox
So the main reason for doing things this way is to basically know when half your range is gone?
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It also lets me build one; lessons learned, and; build two. For example, I'll probably build the pack tabs with:
- tabs aluminum rivet nearest the cell - shortest distance for current flow
- insulation separator - somehow connected to one side of the riveted tabs to insulate from the next cell. My first thought is offset rivets so they won't short to the adjacent cell but I'd really like to find a rivet-like, insulating solution.
I'll always have the option to parallel the packs.
Quote:
Originally Posted by Daox
I just mention it because even though the Peukert effect is minimized with lithium it is still there. The benefits of running the cells in parallel is that you will probably gain a few % more range, have a bit more power (less voltage sag), and the life of the batteries will also be increased as well from only drawing 1/2 the current over their life.
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Sometimes we have to make tradeoffs:
- range anxiety - I don't trust the 'battery fuel gauge' techniques because of the flat discharge curve. We don't really know it is empty until the discharge voltage drops below 3.3-3.4V.
- 15A max load - the eBike motor controller is rated at a maximum of 15A and these are already 17-18Ahr cells. I'm already just under 1C load. But it is very likely I won't be at 15A due to the built-in, 25 mph speed limit. So I'm not going to exceed the 1C discharge rate in typical operation.
- mitigate thermal effects - we know the packs will get warmer in operation but I don't know if it will be a range limiting effect. Having two packs means I can switch to the 'cold one' if on a hot Alabama day the pack temperature begins to approach 60C.
- use all of the first pack energy - having a second pack means I can run the first to 'empty' or 2.8V/cell level knowing I have a way home (or to a recharge plug.)
BTW, I'm finding some sources including the video that suggest over 2C discharge may be a bad thing. But as a starter battery, the high discharge load will be brief. Still, I am finding some interesting, alternative LiON motorcycle batteries.
A wild thought for your starter battery pack . . . perhaps a string of ultra-caps might provide initial CCA current in the first "n" msec to reduce the peak discharge load on the cells?
Bob Wilson