Parallel car batteries

nsaspook

Joined Aug 27, 2009
8,904
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.
Sure, it's done all the time for intermittent duty with matched batteries. What I'm saying is that even originally matched batteries will become unmatched unless charge/discharge cycles and interconnects are given careful consideration for repeated duty.
 

gramps

Joined Dec 8, 2014
86
That doesn't mean it's an ideal solution. It has some problems that can be worked around. Ignoring them will eventually kill weak batteries and decrease available current.
That's true, but start with new batteries of equal quality, that would literally take years. What you have with several 12 volt lead acid batteries in parallel is essentially one battery made up of any number of single 2 volt cells connected in a series/parallel configuration. Like I said, it's a very common and done successfully all the time.
 

dl324

Joined Mar 30, 2015
13,525
That's true, but start with new batteries of equal quality, that would literally take years.
Not as many as you'd like. Lately, I haven't been able to get anywhere near 84 months from a battery with an 84 month warranty. And that's just a single 12V battery. Six cells in series is also a compromise. One dead and the battery is useless.
 

gramps

Joined Dec 8, 2014
86
Not wanting to get in a pissing contest here, I just want to help the OP make a decision. Paralleling two (or more) 12 volt automotive batteries will help him achieve his goal. The ideal solution would be something like a 4D or 8D battery, but that would have other disadvantages.
 

nsaspook

Joined Aug 27, 2009
8,904
Not wanting to get in a pissing contest here, I just want to help the OP make a decision. Paralleling two (or more) 12 volt automotive batteries will help him achieve his goal. The ideal solution would be something like a 4D or 8D battery, but that would have other disadvantages.
It's not a contest as I've designed the software and hardware in energy monitor systems for paralleling batteries and operated (and logged performance data) them for decades.

Having a proper isolation switch like I linked above will help the OP manage a simple two battery bank by allowing for easy separate charge/discharge cycles.
 

AlbertHall

Joined Jun 4, 2014
11,606
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.
Perzackly.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
424
These kinds of batteries will without doubt have a very very low internal resistance, so even a 0.5 v relative difference in voltage could lead to large currents flowing between the batteries when no load is present, I need to understand the consequences of that.
 

AlbertHall

Joined Jun 4, 2014
11,606
These kinds of batteries will without doubt have a very very low internal resistance, so even a 0.5 v relative difference in voltage could lead to large currents flowing between the batteries when no load is present, I need to understand the consequences of that.
So when your car battery is flat and you connect jumper cables to a fully charged battery in another car, does anything bad happen? No it doesn't.
 

nsaspook

Joined Aug 27, 2009
8,904
So when your car battery is flat and you connect jumper cables to a fully charged battery in another car, does anything bad happen? No it doesn't.
Sometimes the flat battery explodes.
050930.gif

Detonation of excess hydrogen liberated by electrolysis of the water found in lead-acid car batteries is the cause due to high current disassociation the water molecule from excess energy not absorbed by the normal lead-acid redox.
 
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SamR

Joined Mar 19, 2019
3,610
Was pretty much eliminated with the "No-Maintenance" sealed batteries. Even when they didn't explode they could squirt acid into your face.
 

AlbertHall

Joined Jun 4, 2014
11,606
That single battery exploded while it was conncted to a charger just like all the ones in cars. A battery exploding in car while it is being driven and being charged by the alternator?
 

nsaspook

Joined Aug 27, 2009
8,904
That single battery exploded while it was conncted to a charger just like all the ones in cars. A battery exploding in car while it is being driven and being charged by the alternator?
The bottom line is this. When you engineer energy banks using batteries, there are specific configuration and charging/discharge recommendations for each application. The generic car application is not valid for renewable energy or even UPS applications. The prudent engineer reads the literature and designs systems using those recommendations.
 

Reloadron

Joined Jan 15, 2015
6,211
If I wanted 5 KW I would be seriously thinking about an inverter fed by 24 or 48 volts. Then worry about parallel. %.0 KW @ 12 VDC is about 417 Amps and that does not even consider any inefficiency. Even at half load you will need some large lead acid batteries. Yes, with considerations mentioned you can parallel batteries but you gonna need a whole bunch of batteries. Even my smaller 1500 Watt UPS units use two 12 volt batteries in series for 24 VDC.

Ron
 

nsaspook

Joined Aug 27, 2009
8,904
If I wanted 5 KW I would be seriously thinking about an inverter fed by 24 or 48 volts. Then worry about parallel. %.0 KW @ 12 VDC is about 417 Amps and that does not even consider any inefficiency. Even at half load you will need some large lead acid batteries. Yes, with considerations mentioned you can parallel batteries but you gonna need a whole bunch of batteries. Even my smaller 1500 Watt UPS units use two 12 volt batteries in series for 24 VDC.

Ron
Agree: https://forum.allaboutcircuits.com/threads/parallel-car-batteries.168080/post-1489280
 

DbLoud120

Joined May 26, 2014
82
I drove a Kenworth truck for 10 years with 3 batteries hard-wired in parallel, for a 12 volt system. They were replaced only once in that time.
 
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