Surprising performance from hobbyking batteries plus questions about paralleling

[ITman496]

So I'm making a big beast of a drone and I'm testing batteries for it.  It will fly with 4 16Ah 6S (22.2v nominal) packs in parallel.  Total flying watt-hours of 1420 (I thought).

I assumed that these batteries would be over-specced but.. I'm not so sure.  They performed SO well!

I discharged the batteries at slightly over what the calculated current would be for the pack at max takeoff weight.  It's a very efficient drone and would only draw probably around 100A @ 22.2v from the battery pack. Divide that by four so I am only testing one of the packs at a time, and that leads to 25A.

The test rig is a 5 gal water cooler tank filled with water and then around 15.5 ft of 26ga magnet wire.  I hooked it up to my very sketchy tester that I made in about an hour, and let her rip!











I had checked it with multiple very expensive instruments and meters, confirmed it was accurate to within 0.1A and 0.01V on all measurements.  But.. I still can't believe it.  I don't think my calculations are wrong either, but..  who knows.  I'll post the logs for anyone who wants to figure it out and check me.

For anyone curious, the test rig is a MCP3208 with TI REF3033 reference, Arduino pro mini, 6 dividers, and a 180A Attopilot shunt based current/volt sensor.  All junk I had laying around. Normally I like to make PCB's for these things and custom make it all but I only had an hour.

And yes, I did push the battery a LITTLE too hard at the end.  I went to the bathroom and came back and it was like 'OH WOW ITS LOW TURN IT OFF TURN IT OFF!!'   

But the battery recovered.  However it will not go into the flight pack after being damaged potentially, or at least mismatched from its siblings.  So it will go into the ground station power pack instead.

Finally, the other reason of this test was to figure out how to parallel these packs, if it was safe to parallel not only the power leads, but the balance leads.  The cells at some points do get a bit out of sync with each other, but I don't know how bad it would really be.  I'll have to just make another test rig with current sensors for each channel of balance lead, and connect them together, and then have a heavy discharge and see how much cross flow goes between the cells.  Does anyone have any experience paralleling lipos and how they act?

Also, interestingly, all the little bumps in the graph other then the big one, in regards to amperage, are caused for when we tapped on the container.  We noticed bubbles forming on the wires (starting to boil water?) and tapped it sometimes to see if they would come off. A few would, and I assume that cooled the wire down and caused resistance to drop, causing the fluctuations.

[Kilrah]

RC batteries are rarely if ever overspecced on capacity, since every charger out there calculates mAh they put in (and clever people use telemetry setups that count what gets drawn out) so anybody who tried to cheat on that would get caught and shot down in flames in no time. They'll rather underspec a bit and make you feel good about having more than written on the label so you buy again.

C rating though... is nothing but random marketing, and where the difference really is is durability / number of useful cycles a battery will let you do before performance drops or they start puffing dangerously. The Multistars don't seem to be too bad indeed.

I've typically been paralleling the balance leads but those batteries were bought at the same time, measured, matched, then charged and paralleled and never separated again. And I've never done it for types of loads where you run at high current for a long time and discharge the battery fully in one go.

[ITman496]

Glad to hear that it's doable though.  And I wouldn't say its a super heavy load.  It's not even 2C discharge.  I'm very pleased though.  This one battery heated the 5 gallons of water up 32F higher then ambient.  It's very impressive what the hobbyist can get nowadays.

I haven't seen people complain too much about the mulitistar high capacity line of batteries.  They surely are more durable then those GEB batteries that were only rated at 100 cycles.

I was thinking of putting PTCs in the balance lead paralleling harness so that way if something goes awry the ptc's can save my bacon and allow the batteries to slowly bleed into eachother.  Possibly through a bleeder resistor that normally gets shunted across with the PTC?