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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 05-19-2011, 02:03 AM   #1
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If I have a overweight plane or want a longer flying time. I know it is ok to wire two identical packs in parallel. What about two 1300 mah packs one is 20c and the other 25c? What about two packs that are 1000mah 25c from different manufactures. What about a 1800mah and a 1300mah both 25c together. Anyone have any ideas on what would happen when the smaller pack gets at cutoff on the esc.
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Old 05-19-2011, 02:07 AM   #2
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Originally Posted by flyyy View Post
If I have a overweight plane or want a longer flying time. I know it is ok to wire two identical packs in parallel. What about two 1300 mah packs one is 20c and the other 25c? What about two packs that are 1000mah 25c from different manufactures. What about a 1800mah and a 1300mah both 25c together. Anyone have any ideas on what would happen when the smaller pack gets at cutoff on the esc.
Others will comment on parallel connecting a 20C and 25C battery. (I think it should work ok??)

But running any Lipo pack to the ESC LVC cut off is usually not a good idea. It's hard on the Lipo cells, especially if you have a battery pack with more than perhaps 4 series Lipo cells.

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Old 05-19-2011, 02:21 AM   #3
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I Think it will be ok to hook 2 - 1300mah packs together with a slight difference in C Rating 20C and 25C, that should not be a problem.
I dont see any thing wrong with hooking 2 lipos that are the same mah and C rating and from different Mfg. as long as both lipos are in good Condition.
I would not hook up 2 lipos of different mah ratings such as a 1800 and a 1300 together, that might present a problem. Just My 2 cents worth, Take care, Chellie

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Old 05-19-2011, 02:33 AM   #4
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The only critical thing when connecting packs in paralel is that they be the same voltage - ie - two 3S packs or two 4S packs or two 2S packs etc are ok to connect in parallel.

The C rating isnt that big a factor. Each pack will tend to contribute its fair share for the most part.

You can get into some trouble if one pack has a much lower Ir value than the other. Say one hi quality, fairly new pack together with an older low quality or lower C pack.

The newer or better pack wil likely have a lower Ir (internal resistance) and it will contribute more amps than the hi Ir pack. This can put more strain on that pack and cause it to carry more than its share of the load and possibly even over discharge it or over heat it to some degree.

For that reason, its best to keep the packs close in age and size and close in C rating, but more importantly, close in Ir.

Unless your using a higher end charger you wont know the Ir.

To play it safe when you ARE using mis-matched packs, dont discharge as much out of the packs - maybe 50-60% instead of the normal 70-80% - to be safe AND keep the max amp draw well under the combined total for each pack - maybe 75% of the total until your sure how they will handle being run together.

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Old 05-19-2011, 03:34 AM   #5
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Originally Posted by Larry3215 View Post
You can get into some trouble if one pack has a much lower Ir value than the other. Say one hi quality, fairly new pack together with an older low quality or lower C pack.
Good point:
Guess to be on the safe side, if you have two 3000 mah packs, one with a 30 "C" rating, and a second with a "20" C rating, the maximum current you should pull would be that of the weakest link, the 20 C battery.

As Larry points out, what could happen, under a 90 Amp load (30C times 3 Amp Hrs) is the 30 C battery goes flat first, then you've got a 90 Amp load on your 20 C battery, overloading it at 30C.

It would be quite interesting to actually check what happens in the real world with a AC/DC clamp on ammeter such as the Sears Craftsman #82369 meter, available for about $60.00. (Don't try to insert a separate series type ammeter into one battery on a parallel battery setup. The series connection of the ammeter will add a lot of resistance, throwing off your readings.)

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Old 05-19-2011, 10:30 AM   #6
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Mismatched packs in parallel are fine. As Larry says they tend to share the current according to their capabilities. Although Ir is important it's a mistake to think of it as a fixed value. In general as a pack get emptier the effective Ir increases and the amount of current it contributes decreases so both packs end up empty at effectively the same time.

In the real world the doomsday scenario of one pack refusing to contribute any current leaving the other to handle the full load simply never happens. If one pack really did get very close to empty it would drag the overall voltage down and so reduce the output current.

This has been tested extensively by many people and reports published on several forums. I thought I'd even seen some here but I can't find any now.

But it's still a good idea to treat batteries fairly gently...whether they're in parallel or not .

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Old 05-19-2011, 11:04 AM   #7
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Actually Steve, RDBlaksley and I were among the first to do those very tests.

In a severley mis-matched pairing, the week pack can indeed be over discharged and stops contributing its "fair share" early on under heavy loads.

At light loads its no problem at all.

I was originally of the same opinion you are, but the tests RD and I conducted clearly showed the weaker packs going below a safe level while the better pack stayed at a higher voltage.

Another contributor to this is how good your connecters and Y harness are. Hi resistance connections can aggravate the problem.

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Old 05-19-2011, 11:10 AM   #8
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I should point out that the mis-match needs to be fairly severe for that to happen AND you need to run the packs down fairly hard.

One other point - those tests also showed that its really not worth it to pair badly mis-matched packs. The week pack doesnt give you enough extra power, time or amp load handling ability to be worth the extra weight.

So, you can probably get away with it - but its not worth it

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Old 05-19-2011, 02:46 PM   #9
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Thanks for the knowledge everyone. The reason I asked was that I have read somewhere that to maximize your usable battery inventory that if you need a bigger battery to connect two in parallel. Then if you need smaller batteries you have an ample supply; However, If your not paranoid about every gram of weight it appears that the best buy per mah in batteries is the bigger the better. Just an example. [3] 3s 1000mah@$9.50 vs. [1} 3s 3000mah@ $21.00.
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