# fun with 18650's



## Siwastaja (Aug 1, 2012)

No, no, no!

You DO NEED a cell level management for every cell to prevent them going overvoltage. Those LiCo cells can go to thermal runaway and explode if overcharged, this is much much more likely than with LiFePO4 cells.

Then, middle balancing based on voltage is very difficult because the voltage differential is small. Top-balance for full charge, partial discharge applications, or bottom-balance for full discharge, partial charge applications.

Also note that for a lead acid, you need to divide the Ah by 2 to get real usable Ah in a power application. Lead acid has poor full discharge cycle life combined with poor coulombic efficiency (less Ah out than Ah in) whereas lithium has practically 100% (Ah out = Ah in). Both have resistive losses (Uout < Uin).


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## Ziggythewiz (May 16, 2010)

They're called UltraFire for a reason, so for losing just 50% of capacity I'd say you got lucky.

Most of us would not even consider using 18650s or typical Lipos...the prismatic format is preferred and the LiFePO4 chemistry essential.

That said, I'm sure your little starter pack may have worked fine if you had gone 4 in series and maybe with a voltage regulator in case the bike couldn't handle the slight overvoltage.

Balancing is essential for all lithiums, cell monitoring is necessary for these lipos, and middle balancing doesn't exist.

I'd recommend repeating the experiment to get your feet wet with lithium using a small LiFePO4 pack. 4S2P with 8ah headways would make a great starter pack, which I know some people have done. I'm sure if you start right and do it proper it'd be a very rewarding lithium experience.


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## lithiumlogic (Aug 24, 2011)

Ultrafire batteries are known for been poor performers and having massively over stated capacity.

Also, your lithium cobalt cells are full at 4.2V. Above that electrolyte breakdown occurs and the cell looses capacity. The surplus charging energy goes into heat and above 200deg these cells go into thermal runaway, ie. start to burn with no further external input.

The cells these guys are using are lithium iron phosphate, which like lithium cobalt suffer electrolyte breakdown above 4.2V. However, these cells are fully charged at 3.4V, so there is considerable wiggle room at the top. They also have to be heated to over 400 before they start to thermally run away.


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