Oct 07, 2015, 02:47 AM Registered User Canada, BC, Surrey Joined Feb 2011 604 Posts Question Mixing Batteries of Various mAh (same Voltage) Hi Guys, I have a multi=rotor that is currently using a 4S 5000mAh battery, and want to increase my flight time. Unfortunately, I can only add another 2600mAh (max) due to weight and size limitations. This will be done by placing 2x 4S 1300mAh batteries in parallel with the 5000mAh pack. Without getting into the complicated, "why are you doing it that way" discussion, let's just assume that the aforementioned is the only alternative currently available. Here are my questions: (1) When I discharge the batteries under load, how will the batteries be discharged? For example, if total amp draw is 30amps, will it draw 10amps from each battery simultaneously? Or is it 30amps from all batteries at the same time? (I'm confused ) (2) I want all 3 batteries to be discharged to the same voltage (or at least within a reasonable tolerance), is this possible? (3) Here's what I'm afraid of: That by connecting all 3 batteries (1x 5000mAh and 2x 1300mAh) in parallel that the 1300mAh will get discharged to zero or close to it, and there will still be plenty of battery left on my larger (5000mAh) pack. Is this likely to happen?
 Oct 07, 2015, 03:05 AM Registered User Staffs, UK Joined Nov 2003 11,723 Posts Think about it. Batteries connected in parallel can't possibly be at different voltages. You can't have different voltages on the ends on a short piece of thick wire . This causes them to automatically balance the load. What happens is that batteries in parallel share current according to their capacity (approximately). So if you connect a 5000mAh in parallel with a 2500mAh then 2/3 of current comes from the 5000mAh and 1/3 from the 2500mAh and they're empty at the same time. In your case you have a ratio across the 3 batteries of roughly 4:1:1 so at 30A you'll see about 20A from the 5000mAh and roughly 5A from each 1300mAh. Should be fine. Steve
Oct 07, 2015, 03:15 AM
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Quote:
 Originally Posted by slipstick Think about it. Batteries connected in parallel can't possibly be at different voltages. You can't have different voltages on the ends on a short piece of thick wire . This causes them to automatically balance the load. What happens is that batteries in parallel share current according to their capacity (approximately). So if you connect a 5000mAh in parallel with a 2500mAh then 2/3 of current comes from the 5000mAh and 1/3 from the 2500mAh and they're empty at the same time. In your case you have a ratio across the 3 batteries of roughly 4:1:1 so at 30A you'll see about 20A from the 5000mAh and roughly 5A from each 1300mAh. Should be fine. Steve
Thank you for your reply, Steve. I'm glad I checked cause I was kinda hesitant to put my bird in the sky without being sure. Thank you kindly
 Oct 07, 2015, 03:44 AM Registered User Edmonton, Alberta, Canada Joined Oct 2008 316 Posts Not recommended. What slipstick says makes sense, but in reality what dictates how much charge is drawn from either battery isn't battery capacity. It's things like internal resistance and other not-so-obvious things where the delination isn't so clear cut. For example, can you really assume that internal resistance is directly proportional to the battery capacity? Even if it is, can you assume that both batteries have been charged to exactly the same proportion of their full capacity? What's more likely to happen is that one battery discharges faster in proportion to the other, causing the fuller, higher voltage battery to dump charge into the weaker battery. This can cause weird, undesirable things to happen. The most obvious of which is additional heating and the fact that charging a battery from another battery (which is basically what is happening) is not 100% efficient so you lose energy.
Oct 07, 2015, 03:49 AM
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 Originally Posted by DKNguyen Not recommended. What slipstick says makes sense, but in reality what dictates how much charge is drawn from either battery isn't battery capacity. It's things like internal resistance and other not-so-obvious things where the delination isn't so clear cut. For example, can you really assume that internal resistance is directly proportional to the battery capacity? Even if it is, can you assume that both batteries have been charged to exactly the same proportion of their full capacity? What's more likely to happen is that one battery discharges faster in proportion to the other, causing the fuller, higher voltage battery to dump charge into the weaker battery. This can cause weird, undesirable things to happen. The most obvious of which is additional heating and the fact that charging a battery from another battery (which is basically what is happening) is not 100% efficient so you lose energy.
thank you for the information.
 Oct 07, 2015, 08:30 AM Registered User United States, TX, Katy Joined Jul 2013 165 Posts I am certainly not an expert, but... my approach based on what training in electrical theory that I do have leads me down this path... DKNguyen's point about internal resistance and the real world drain from each battery under load conditions being perhaps different than ideal is certainly something to consider. The way I look at it is that I am quite comfortable putting two batteries of different mah capacity (but same voltage of course) in parallel *IF* the C rating on each of the batteries is large enough that they could individually supply the max current that I'd expect to see under load conditions. What I would not be comfortable doing is putting two batteries in parallel into a system that lets say needs to draw 50 amps and each of my batteries only have a C rating that they would deliver 30 amps (together they would be theoretically 60 amps which is more than the needed 50... but individually they'd fall short). Certainly if the internal resistance is much higher in one battery than the other there may be unequal draw on the batteries under load and then recharging from one battery to the other when load is removed. And... that certainly wouldn't be the most efficient setup, but in my view I'd live with that inefficiency and still be relatively comfortable that the setup was safe if their C ratings were individually high enough. That's my \$0.02 worth... (and if my thinking is flawed or missing something, I'd love to hear more technical explanation about how I should be looking at it)
 Oct 07, 2015, 08:46 AM Space Coast USA Space Coast Joined Oct 2000 22,136 Posts You can even use packs with similar IR's to further reduce any pack incompatibilities. .
 Oct 07, 2015, 12:01 PM Registered User Staffs, UK Joined Nov 2003 11,723 Posts I guess you guys are keener on theory than I am. If you've done any tests to confirm your theoretical problems I'd be interested to see the results. I've only done a few tests a while ago and then been using mismatched packs in parallel (for years). Not had any problems yet, but perhaps I'm just lucky . Steve
 Oct 07, 2015, 12:10 PM Registered User United States, CA, South Lake Tahoe Joined Sep 2006 639 Posts A couple of short test flights should be able to resolve this issue if you have the patience to test each battery after landing. Start out with a flight of the same duration as you are getting with the larger battery and work up in steps. If you are flying your quad for time duration its unlikely that you will draw burst currents of sufficient magnitude to damage the batteries.
 Oct 07, 2015, 01:28 PM Registered User So. Cal. Joined Oct 2004 9,088 Posts I've been using packs of varying capacity, discharge rating (C-rating), and internal resistance in parallel for many many years without any issue whatsoever. Of course, they're always of the same cell count and are in the same state-of-charge prior to connecting in parallel. One simply needs to ensure that the discharge rate of the aggregate pack does not exceed the maximum allowable of the least capable pack. e.g. - If using a 15C pack in parallel with a 60C pack, make sure that the total discharge rate is no greater than 15C continuous in order to prevent excessive heating of the 15C pack. Yes, there will be some uneven current sharing if the packs do not have identical characteristics but my experience and measurements demonstrate these effects to be benign if one simply observes and obeys the maximum discharge rate of the least capable pack. Mark
Oct 07, 2015, 11:46 PM
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Quote:
 Originally Posted by Flyswamper I am certainly not an expert, but... my approach based on what training in electrical theory that I do have leads me down this path... DKNguyen's point about internal resistance and the real world drain from each battery under load conditions being perhaps different than ideal is certainly something to consider. The way I look at it is that I am quite comfortable putting two batteries of different mah capacity (but same voltage of course) in parallel *IF* the C rating on each of the batteries is large enough that they could individually supply the max current that I'd expect to see under load conditions. What I would not be comfortable doing is putting two batteries in parallel into a system that lets say needs to draw 50 amps and each of my batteries only have a C rating that they would deliver 30 amps (together they would be theoretically 60 amps which is more than the needed 50... but individually they'd fall short). Certainly if the internal resistance is much higher in one battery than the other there may be unequal draw on the batteries under load and then recharging from one battery to the other when load is removed. And... that certainly wouldn't be the most efficient setup, but in my view I'd live with that inefficiency and still be relatively comfortable that the setup was safe if their C ratings were individually high enough. That's my \$0.02 worth... (and if my thinking is flawed or missing something, I'd love to hear more technical explanation about how I should be looking at it)
@flyswamper - Thank you for your input. You bring to light a very important point in the "C" rating. I think I'll need to bump it up a little bit just to be on the safe side... but so far, what you're saying seems to make perfectly good logical sense to me. Thanks again
Oct 07, 2015, 11:47 PM
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 Originally Posted by hoppy You can even use packs with similar IR's to further reduce any pack incompatibilities. .
Thank you hoppy
Oct 07, 2015, 11:49 PM
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Quote:
 Originally Posted by slipstick I guess you guys are keener on theory than I am. If you've done any tests to confirm your theoretical problems I'd be interested to see the results. I've only done a few tests a while ago and then been using mismatched packs in parallel (for years). Not had any problems yet, but perhaps I'm just lucky . Steve

@Slipstick - Thank you for sharing your experience with us. If I may ask, what kind of discrepancy (if any) were you getting on the various mismatched packs at the end of your flights? Like, were they within an acceptable range. (I know "acceptable" is loosely defined here but I think you get what I mean )
Oct 07, 2015, 11:50 PM
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