bradya_johnson

Hello all,

I have an 11.1 V 1800 mAh battery for an airplane that is reaching the end of its life. As a replacement, I have two other batteries just lying around that Im considering connecting in parallel as a replacement. The replacement batteries are both Electrifly 15C, one is an 11.1V 650 mAh and the other is an 11.1V 1250 mAh.

My question is this: Can you run 2 batteries with the same V, but different mAh in parallel?

The motor Im using is pulling 9 amps max. If the batteries are in parallel, will the motor try to pull the same current from each battery, or will it pull an average: more from the bigger battery and less from the smaller battery?

Running the batteries like this seems rather logical to me, but something in the back of my head tells me that this could be dangerous.

Any help would be appreciated.

Thanks,

Brady

guver

My Feedback: (325)

Hi, and welcome to RCU. This topic gets debated over and over. Yours is unique because of the significant difference in capacity. One thing that it has going for it is that is not being operated at or near the max current capability. All things aside, I think it'll work just fine.

I can leave it to the others to list the reasons it may not be good. One thing is for sure the voltages will always be equal. Here's one question that may help to proccess this.

Is the larger of the two batteries sufficient to do the job?

bradya_johnson

guver,

Thanks for the welcoming

The larger battery (1250 mah) would indeed do the job, but the flight time would be alot shorter... about 1/3 of the time shorter, right? Another problem is that this is for an older Great Wings Slow Stick, that was originally intended for a brushed NiMh setup that I converted to a brushless lipo setup which is alot lighter. The first few flights I used a smaller battery and had to add lead to compensate (center of gravity balancing) for the heavier battery, etc, but later decided to fill up the lead weight with battery weight, and more flying time. So, if I go with just one of the batteries, then I'll have to add extra weight again.

I could just go and buy another 1800 mah battery, but thought Id try to use these other batteries that are just sitting around collecting dust.

A funny story about the Slow Stick: I used to take it to work and fly at an empty field nearby over my lunch break. One day I was charging the battery while it was still attached to the plane and I had the hood up with the plane on top of the motor. A friend of mine came running in and as a joke, said "come quick, there's a huge mosquito on your car sucking from your battery..."

Brady

guver

My Feedback: (325)

Yep, I agree. The two batts will work fine. The smaller one simply adds to the capacity and detracts from the load of the "main" one.

The average current will be proportional from the two batts of course. The beginning and ending (or spikes and non currents) may not quite be proportional. If we had a 20C batt that we were pulling 20C from may not be good if we adde a 10C smaller battery to since it would also pull the 10C battery at something between 10 and 20 C.

bradya_johnson

I just thought of something: Is there a possibility that I could "over-drain" the smaller battery? The ESC will cut the power once it sees its input voltage drop too low, but when the smaller battery drops to its lowest limit, the overall voltage will probably still be above the ESC cutoff.

"Over-draining" or dropping below the min voltage is bad for lipos, right?

Brady

guver

My Feedback: (325)

Overdraining is bad for them, and they would both have to be overdrained at same time since the voltages must be the same because of the parallel connection. Scroll down to the 7th pic here to illustrate why the voltage remains same. http://www.rcgroups.com/forums/showthread.php?t=932319

I'm curious to hear some more feedback especially if it's from the other side of the debate. []