Parallel car batteries

Thread Starter

ApacheKid

Joined Jan 12, 2015
440
What considerations are there when we want to combine several car batteries, placing them in parallel sounds obvious but even a small difference in relative voltages might cause large currents to flow between the batteries, is this correct or am I safe to do this?
 

Dodgydave

Joined Jun 22, 2012
10,088
Yes they can be put in parallel to give a bigger capacity, as long as they are all the same Ah..otherwise then you get the largest Ah battery charging the smaller ones.
 

nsaspook

Joined Aug 27, 2009
8,908
What considerations are there when we want to combine several car batteries, placing them in parallel sounds obvious but even a small difference in relative voltages might cause large currents to flow between the batteries, is this correct or am I safe to do this?
It's much better to go with a higher voltage serial connection. At high currents the very small differences in connection resistances combined with different internal resistance of parallel batteries will imbalance a parallel bank fairly quickly on several discharge/charge cycles. In general the battery bank circulation currents will be small once voltage differences stabilize. If this is a standby power application it's less critical but a daily cycle RE applications will murder the batteries.
 

dl324

Joined Mar 30, 2015
13,542
If these batteries aren't new or in the same relative condition, I wouldn't put them in parallel. High power systems that put them in parallel use high quality batteries that all start out in about the same condition. If this is a hodgepodge of batteries, it'll likely be more trouble than it's worth.

If you want to do it right, do some research on solar battery banks. The batteries are expensive because their performance is critical.
 

nsaspook

Joined Aug 27, 2009
8,908
Thanks lads, well in this case I was wondering if I could parallel two or more batteries as the source for this inverter, that's my real objective.
You could but just think about how much 12vdc current it takes for 5000W. I use 4/0 cables battery cable on my 12vdc 2000W inverter system to minimize imbalance effect but it still requires sequenced charging to equalize parallel banks after several discharge/charge cycles. I built an entire system just to manage it automatically.
 

nsaspook

Joined Aug 27, 2009
8,908
When jump starting a car that's exactly what happens - two batteries connected in parallel. It doesn't matter if they are the same or not.
That's true for a one shot or mainly idle duty like a power fail UPS. For continuous cycle RE duty it's critical that batteries be balanced and properly matched for daily service.
 

AlbertHall

Joined Jun 4, 2014
11,610
If you have a boat or a caravan with separate batteries for starting the engine and running stuff like lights or fridge all there is is two diodes so the lights etc can't flatten the engine starting battery. They are charged in parallel just the same.
 

nsaspook

Joined Aug 27, 2009
8,908
If you have a boat or a caravan with separate batteries for starting the engine and running stuff like lights or fridge all there is is two diodes so the lights etc can't flatten the engine starting battery. They are charged in parallel just the same.
Charged the same but NOT discharged the same. It's the repeated complete parallel cycle that murders parallel banks.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
440
My goal is simply to double the time that I can run the inverter, I certainly won't be pulling 5 KW either, perhaps just a two/three hundred.

It's no huge pain to just swap a discharged (or semi-discharged) battery and replace it with the other one, but avoiding that would be nice.

The batteries that I buy will be brand new and identical, probably some kind of RV (as they're known in the US) batteries.
 

nsaspook

Joined Aug 27, 2009
8,908
If they are simply connected in parallel then they will both be the same voltage as they discharge (or charge).
As you well know voltage is not the determining factor for State Of Charge. Batteries are not electrical charge storing devices, it's a chemical reaction process that generates voltage from charge separation as needed to balance the chemical reaction.
 

dl324

Joined Mar 30, 2015
13,542
They WILL be exactly the same voltage.
Only in theory. Cell conditions won't be exactly the same and the batteries will have different voltages when charging or discharging; even when connected in parallel. The interconnect will take up the difference. If that can't compensate for the difference, one battery will charge the other until they equalize.
 

nsaspook

Joined Aug 27, 2009
8,908
My goal is simply to double the time that I can run the inverter, I certainly won't be pulling 5 KW either, perhaps just a two/three hundred.

It's no huge pain to just swap a discharged (or semi-discharged) battery and replace it with the other one, but avoiding that would be nice.

The batteries that I buy will be brand new and identical, probably some kind of RV (as they're known in the US) batteries.
Buy a 1,2,both switch like this so you can isolate batteries for easy swapping and charging of each battery.
 

gramps

Joined Dec 8, 2014
86
12 volt batteries in parallel? It's done all the time. In boats, in semi trucks, even in your heavy duty diesel pickup trucks. It's very common to do so in order to increase available capacity......Exactly what the OP wants to accomplish.
 
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