Paralleling new packs and old

[lazarus2405]

Greetings folks,

I've got a new battery built of 20C Turnigy LiPos, 20s10Ah. My old battery is a 21s 9ah pack of LiMn e-molis, which are now five years old. While their capacity has held up, they have an awfully high internal resistance, which is one reason I replaced them (the other being weight). I thought to myself, for longer trips, I could always bring the old batteries along and swap midway. Tonight, I wondered about the wisdom of chopping down the emoli pack to 20s and running them in parallel.

The pack voltages would be pretty much the same - I'd charge both to 4.15v/cell and use a LVC of 3.65v/cell. The old pack has an internal resistance of ~0.7Ohm, and the new one I've measured at close to 0.069Ohm, roughly 1/10th. My reasoning is that under high load (acceleration), the LiPos would provide >90% of the current, but when the load falls off the cells would have to reach a voltage equilibrium. Effectively the LiMns would charge the LiPos.

Can anyone see a problem with this setup? Will it perform the way I expect? How about safety and longevity issues?

Thanks

 

[Kin]

I would try a few searches on google, including the term endless-sphere (or be very fancy and search "site:endless-sphere.com __terms___"). I believe this kind of paralleling between similar discharge curves has been contemplated before. I'm not saying your thread isn't good- I think the situations vary, but it might give you a better background idea.

On another point, though, I wanted to ask you about the 4.15v. Just curious- have you always been charging the LiMn that much? I was under the impression they're more keen on a 4.1v charge.

 

[lazarus2405]

Thanks for the gentle reminder to search first - I seem to forget that. The best info I found was actually an active thread, viewtopic.php?f=14&t=36324. Interesting. The discussion is all around whether the Konions would self-balance the LiPos, and not whether it would work in general. Encouraging.

As far as the 4.15v cutoff, I never really thought about it. The cells were from the Milwaukee V28 system. When I started out in the old days, I just let the Milwukee chargers and BMSs do their thing. I remember that hot off the chargers my CA showed a total voltage of 87.0v-87.1v. The datasheet specc'd 4.2v, so I assumed 4.15 was pretty optimal. The vast majority of their cycles were done with the Milwaukee chargers. When I stripped out the BMS and used a RC charger for a short period, I still cut them off at 4.15v.

If you're asking for lifecycle data, I just don't have it. The cells saw a pretty low number of cycles and then sat in a basement for a few years. Age has done more to these cells than any charging regime.