John in CR said:
I have no commute, and this would be a blast around errands bike where the rides are in short spurts...a few miles of high performance and then the bike is parked for 20-30min or a couple of hours, not like many of you guys with a 10 or 20 mile ride with a number of hard accelerations mixed in. For the pretty rare long rides I'd either use another bike or go easy on the throttle with this one.
I've got the 4p of A123 just sitting there aging that I'm finally ready to put to use, but it makes for a smaller capacity pack than I'd like. Yes I could make a pack with just them for a short range blaster bike, but I also have these unused 10ah cells gathering dust that I'd never use other than in this manner because they are low power for my needs. They're rectangular cells to they pack nicely.
Charging the A123 from the cheapies during resting might work by just paralleling. It won't be optimal, as charge rate will taper off and become exponentially slow towards the end, and practically it may be that only a small amount of charge transfers. However why not try an experiment? Take a fully charged cheapie, and a discharged 4p A123. Connect together with an Ampere meter. Read amperes at 0min, 5 min, 10min, 20min, 40min, 80min. the unequal spacing of readings is to account for the exponential decline in current, so the total of 6 readings of the Amp meter give us about the same accuracy as if we had read it every 5 min (uniform sampling) for a tedious 30+ total readings. (This is called adaptive sampling, and we will next do "adaptive quadrature". It just saved 30/6 = 5 times of work. If you paid college tuition and didn't learn this demand a refund!).
Now calculate amperehours charged to the A123 as follows:
Amin = 2.5*A0min + 5*A5min + 7.5*A10min + 15*A20min + 30*A40min + 60*80min + 60*160min
Ah = Amin/60
This gives you the number of Ah charged into the A123 after 160min
If you want the Ah charged after 3 h you do:
Amin = 2.5*A0min + 5*A5min + 7.5*A10min + 15*A20min + 30*A40min + 60*80min + 80*160min
Ah3h = Amin/60
2h:
Amin = 2.5*A0min + 5*A5min + 7.5*A10min + 15*A20min + 30*A40min + 60*80min
Ah2h = Amin/60
1h:
Amin = 2.5*A0min + 5*A5min + 7.5*A10min + 15*A20min + 30*A40min
Ah1h = Amin/60
With similar capacity cells and similar chemistry, at best 50% of the charge will transfer, so the charge is equalized between the paralleled cells. I don't know how long that will take though, and the final charge transfer may be lower than 50% due to hysteresis in the system.
John in CR said:
jag said:
To make this approach work, one has to build two packs, the lower Ri one with a higher nominal voltage, then connect them together with a charge controller.
I have to pick up some charge controllers for some solar stuff anyway, so I really like this idea a lot. I could even combine dissimilar packs, both chemistry and voltage, as well as fix the current drawn from the lower power cells. I love the flexibility of this approach. Kfong came up with a nifty little LVC switch that would be perfect to protect the charging pack from over discharge. I just need to decide on a relatively wide input range and top of charge exact voltage for the solar charge controllers I need for my projects.
That way I run A123's as my pack, and toss any old pack on the bike as my range extender that bulk charges my A123's at 10A both on the fly and while stopped. I'm liking this the more I think of the possibilities.
Thanks,
John
First note misprint " the lower Ri one with a higher nominal voltage" should be "the higher Ri one with a higher nominal voltage" (edited and fixed in original post).
You need also to protect any lower voltage battery bank from overcharge. This is normally not a problem with a wall charger as they are CC/CV, and the final rest voltage is the right final charge voltage for the batteries. If you find a solar charge controller with the correct voltage for the A123 pack you can use it for the approach I mentioned above. It will need to handle the max current experienced when connecting the packs together, or (perhaps more practical) implement a current limiting to a reasonable max current.