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leadcork
12-15-2005, 12:24 AM
I have one Kokam 145 mAh 7.4 v 2cell pack and one E-tech 250 mAh 7.4 v 2 cell pack. My charger is a programmable AccuCycle Elite charger. I want to hook up my two commercial packs in parallel and charge both together as a 395 mAh, 2 cell, 7.4 v pack (my airplane almost negates being able to remove them and charge separately). My reason for hooking them together is I have no room in one direction for available larger commercial 7.4 v packs with higher capacity. I need the higher amperage capacity to give me higher voltage (close to 1.2 x rated capacity) to my motor for a longer time. Is there a problem with doing this?:confused:

U2Steve
12-20-2005, 05:45 PM
Oh, yeah. The charger can't know that there are different capacity cells in this setup. Just my opinion, perhaps, that you've got a fire waiting to happen...

Steve

Rugar
12-20-2005, 05:49 PM
I have one Kokam 145 mAh 7.4 v 2cell pack and one E-tech 250 mAh 7.4 v 2 cell pack. My charger is a programmable AccuCycle Elite charger. I want to hook up my two commercial packs in parallel and charge both together as a 395 mAh, 2 cell, 7.4 v pack (my airplane almost negates being able to remove them and charge separately). My reason for hooking them together is I have no room in one direction for available larger commercial 7.4 v packs with higher capacity. I need the higher amperage capacity to give me higher voltage (close to 1.2 x rated capacity) to my motor for a longer time. Is there a problem with doing this?:confused:

You should ask this in the batteries section so that one of the many experts can give you advise.

cyclops2
12-20-2005, 06:02 PM
You are really asking for a electrical fire and explosion.
At no time should different MAH. packs ever be put in series or parallel.
The smaller one could let go on the first couple of charges or discharges.
The smaller pack will run below the 3 volt level and be shot forever.
Use them somewhere else or buy another of the same of 1 of them.

slipstick
12-20-2005, 10:07 PM
People who have never tried it and who don't understand how batteries and charging works will tell you all sorts of worrying stories.

In fact because of the way Lipo batteries are charged there is no problem paralleling unequal capacity packs provided they are the same voltage. Lipos are very good at balancing out the available current among the packs. I've done it several times without any problems at all and there are plenty of guys over on RCGroups who do it regularly. Just make sure you keep the total charge and discharge currents within the normal limits for the combined pack.

It wouldn't be a good idea to do the same thing with Ni-based packs where the charger relies on small changes in voltage to terminate the charge but it's no problem with Lipos which use a completely different charging algorithm.

BTW Cyclops if 2 packs are connected in parallel it is not physically possible for them to have different voltages. There is no way one pack can run below 3V before the other.

Steve

Matt Kirsch
12-21-2005, 01:58 AM
Steve, I do believe you're right.

It is a fact that by being in parallel, the two packs cannot possibly have different voltages.

The only caveat I can think of, and I may just be chasing windmills here, is if the internal resistances are vastly out of proportion. Let's say you're coupling a 20C 340mAh pack to an 5C 1200mAh pack. Both have roughly the same internal resistance, so they'd be contributing roughly the same amount of current (assuming a 12A draw). Wouldn't the 340 pack discharge first? Yes, the voltages will balance, but it would be the 1200mAh pack cross-charging the 340, putting an even heavier load on the 1200mAh pack.

If both were 15C rated, the packs would contribute in proportion to their capacity. I can see where that would be okay. They'd discharge at the same %capacity per unit time rate.

cyclops2
12-21-2005, 02:47 AM
You are banking on the larger Mah cell always being able to carry the little guy. WRONG.
No guarentee HE will not open up before the small guy. When he opens, the little guy is on his own. Ready or not.
Is your 16-20C load designed for the little guy? NO.
Little guy dumps his guts in a hurry. End of ballgame.
At that size cells there is NO reason to juryrig.
At least use 2 equals and set the maximum C to the cell size.

slipstick
12-21-2005, 10:10 AM
The only caveat I can think of, and I may just be chasing windmills here, is if the internal resistances are vastly out of proportion. Let's say you're coupling a 20C 340mAh pack to an 5C 1200mAh pack. Both have roughly the same internal resistance, so they'd be contributing roughly the same amount of current (assuming a 12A draw). Wouldn't the 340 pack discharge first? Yes, the voltages will balance, but it would be the 1200mAh pack cross-charging the 340, putting an even heavier load on the 1200mAh pack.

If both were 15C rated, the packs would contribute in proportion to their capacity. I can see where that would be okay. They'd discharge at the same %capacity per unit time rate.
You're right of course Matt. It can get complicated working out how to rate the combined pack if the individual packs are very different.

But if pack A is good for say 5A and pack B is good for 10A the combined parallel pack will be good for around 15A. Because they are forced to stay at exactly the same voltage they will naturally share current. It doesn't matter if Pack A is a 1000mAh 5C pack or a 500mAh 10C pack it will still contribute it's 5A when required. However the fact that one pack will run out of capacity before the other does mean that the effective combined capacity will not be as high as you might think. When the smaller pack has run out of capacity the other pack will be asked to contribute more than it is able, the overall voltage will drop and you'll have finished even though the larger pack is not completely out of capacity.

I don't see any real danger there and I've never had any problems in real life but I've never paralleled 2 packs as wildly different as your example. I'd certainly recommend that anyone ensures that both packs have similar C ratings if they're going to play this game. That will keep the currents balanced more logically and will ensure that the combined pack behaves as you would expect a single pack of that capacity and C rating to behave.

Steve

Mike Parsons
12-22-2005, 03:50 PM
Moving to batteries.

RD Blakeslee
12-22-2005, 04:58 PM
http://www.rcgroups.com/forums/showthread.php?t=170524
http://www.rcgroups.com/forums/showthread.php?t=360273

leadcork
02-08-2006, 06:40 PM
Matt, Cyclops, & Slipstick, et al - Many thanks. For a while there, I thought I must have wasted all my high school and college physics classes! DC is still DC and voltage is still king... Analogy: Two cylindrical water towers, one 10 feet in diameter, the other 100 feet. Punch a hole in each five feet up from ground. Each delivers a stream of water that strikes the ground same distance from tower wall as the other....the 10 foot diam tower runs dry before the 100 foot diam tower. (voltage vs capacity) Thank you.