serious power

Hi,
Is there a problem with using 2 different capacity batteries in parallel to drive our motors, say 4400's and 750's.
This would allow one have light packs to make weight at comps and or to fly as light as possible in calm conditions and to 'boost' the capacity for practice and or in strong winds.

Brian

Malcolm H

Brian,

At first glance I would say no this is fine. After all the first generation Thunderpower Prolites were 5s 4ps and se of my old ones were still flying (someone else's) F3A model after hundreds of cycles!

However the problem I do see is that your hypothetical 4400 and 750 cells might well have very different IRs and that under high discharge currents might well not discharge at the same rate. Having said that the fact that they are connected in parallel means that the voltage when not drawing current will be the same and so they may very well survive this treatment. My guess is that the smaller capacity battery would be the one to suffer and so the experiment wouldn't be that costly.

Malcolm

serious power

Hi Malcolm,
I'll take that happily - it would be a case of 'gaining in the bends that what I'd loose in the straights' etc.
I'm thinking to tee into the main battery cable on the battery side of the battery to Esc connection. This way a 'poor' main connection would not squeeze the wee pack.

Brian

can773

My Feedback: (1)

Although I have never tried, from what I have read in order to make different capacities in parallel work reasonably well you should limit your draw to that of the smaller packs rating otherwise its going to get abused by the bigger pack. So when they are fairly close, say a 4000 and a 4400 then it would likely work quite well for a long time, but my guess in this case is it will work for a couple of flights and due to the significant difference in size that 750 will be toast in a few flights.

There is probably a lot more discussion about this on RCG!

Personally what I do is have two 5000's for the majority of my flying (practice and probably 80% of contest flights) and have one 4400 pack if I want to get light. With the faster chargers of today one pack is more than sufficient during a contest anyway.

Food for thought. Like Malcolm said, cheap experiment, just be careful you don't light the 750 on fire in your plane

Chad

serious power

'just be careful you don't light the 750 on fire in your plane '

Hi Chad,
- I'll try not to !
Ohm's law would say it should be ok - provided there is not an issue with the 'main' pack or it's connection.
If I try it I will take care and build up the test level slowly.

Most bipes are not legal with 5000's or bigger - therefore 4400's or less are req'd - then these are easy to hurt.

Brian

can773

My Feedback: (1)

Not sure about other bipes, but my Citrin is well under weight with up to 5400's, since its one of the more popular bipes I would say most are easily legal . I actually find I prefer the heavier packs most of the time.

Chad

serious power

Hi Chad,
You're correct - I should have said 'a lot' not 'most'.
However I'm also talking in the context of running a contra and these bipes are going to get a bit bigger - weight allowing.

I think that ,assuming that the two packs work, the biggest risk would be putting in two packs with a different charge state. That could get messy very quickly .

Brian

Malcolm H

Brian,

You would parallel charge them as well so no danger in putting them together in different charge states. I have been parallel charging 4 x 5000mAHr 10s packs at 20A for the last couple of seasons with no problems whatsoever.

Malcolm

serious power

Hi Malcolm,
Good thinking
I assume you parallel balance as well !!?
Should be ok with the different packs !??

Brian

Malcolm H

Yes parallel balance charge every time. Had to make a balance harness as have never found a 10s paraboard but prefer the harness concept anyway as I'm less likely to connect it wrongly!

M

serious power

Hi Malcolm,
I've never really fancied that idea - but for no particular reason.
So you are (while charging) connecting ,in parallel, packs in slightly to quite different charge states !??
I think I will try this - if only just as a test - won't be for 8 or 10 weeks though.

Brian

Malcolm H

Not really Brian, my flight to flight consumption is pretty consistent so batteries end up very close voltage wise. For the first few charges I was pretty paranoid and used a voltage checker to put the packs in ascending order of voltage and connected them that way. Then I realised that the maximum voltage difference I was seeing was less than 0.5V so I stopped bothering and now just randomly connect them. Oh and I always connect the main leads first so that they carry most of the current, then the balance leads.

Malcolm