Two battery charging

domptg

New Member
Aug 20, 2022
9
Hello. We have the Starcraft tent trailer battery and little lawn tractor battery. I wonder if I can charge them both in parallel on battery maintainer. Please see the pictures
 

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neighbormike

Super Active Member
Gold Supporting Member
Oct 6, 2012
4,090
WI
Hopefully you don’t use the two for the camper in parallel… and no, you should not charge those two in parallel.
 

davido

Super Active Member
Jul 17, 2014
1,407
This is more important when connected in serial.

In parallel voltage dictates the flow of current. Because more current can be drawn from the larger capacity battery before voltage dips, the actual discharge percentage of the larger and smaller battery will happen in unity. So at 100% charge, both batteries will be at 13.6v. Apply a 10A load and the smaller battery's voltage will want to dip more quickly. That's actually good, because it means the larger battery will carry the majority of the current flow. If you have a 100AH battery, and a 10AH battery, 90% of the load will come from the 100AH battery, and 10% from the 10AH battery. That means both of them will get to 50% charge at about the same time.

Recharging works similarly; the smaller battery will tend to rise in voltage more quickly, so more charge current will go into the larger battery.

*WITHIN REASON* the batteries don't need 100% identical capacity. I wouldn't go with a Group 24 and a lawn mower 12v battery, though. That's probably too big of a gap. But a Group 24 and a Group 27 in parallel should be fine.

What *IS* important is the condition of the batteries. A battery in poor condition will never reach 100% charge, and thus will constantly be at a lower voltage than the good battery. Therefore the battery in bad condition will draw down the good battery until the voltages balance. If a end-of-life battery can only reach 13.3v, that battery will draw current out of the 13.6v battery until the 13.6v battery drops to 13.3 also. In this case the bad battery is actually sucking some capacity out of the good one.
 

domptg

New Member
Aug 20, 2022
9
So is no other way to charge them together. Best idea is to buy second maintainer - just set it for the winter and forget it. Thank you all !
 

Lug_Nut

Active Member
May 29, 2016
309
Mt. Wachusett area, MA
Thank you, davido.
The smaller battery and a medium battery in parallel are functionally the same as one larger battery. One charger is all that is needed provided the charger output is sufficient for the combined capacity.
 

Hilldweller

Super Active Member
Mar 2, 2021
1,057
Hog Waller, GA
T....

What *IS* important is the condition of the batteries. A battery in poor condition will never reach 100% charge, and thus will constantly be at a lower voltage than the good battery. Therefore the battery in bad condition will draw down the good battery until the voltages balance. If a end-of-life battery can only reach 13.3v, that battery will draw current out of the 13.6v battery until the 13.6v battery drops to 13.3 also. In this case the bad battery is actually sucking some capacity out of the good one.
That's what popped into my head first after looking at the pics. One battery looks like it was rode hard and the was rode harder...
 




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