Connecting batteries in parallel (1 Viewer)

Aug 10, 2012
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While I agree with the sentiments of the article below does it really make any difference ?
using 25mm sq. copper cable is there any noticeable volt drop over 300mm jumper?

98AD67A7-308F-478B-B5AD-5F96C4C62659.jpeg
 
Feb 22, 2011
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I think that explanation is clear enough and correct 👍
I suppose it's like it's taking power from the line of least resistance, so battery B does less work in first diagram.
Imagine it was water flowing in and out and batteries were water tanks, battery / tank B would for the most part be a dead leg.
Or is connection in series better ?
 
Last edited:

Hoovie

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While I agree with the sentiments of the article below does it really make any difference ?
using 25mm sq. copper cable is there any noticeable volt drop over 300mm jumper?

View attachment 712130
Yes, it will, and especially if you tend to use higher loads.
If you have two batteries, connect like example 2 rather than example 1.
 
Jun 16, 2014
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On my Autotrail Savannah it has (had) 2 separate cables to connect 2 batteries in the hab area. I studied this type of diagram after running with 2 batteries connected as per fig 1 for some years. Finally decided to ditch one set of cables and connect with just one as per fig 2.

Not noticed any difference other than it is more difficult to remove for over winter storage. :unsure:

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Lenny HB

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Two batteries close together the marginal difference is virtually unnoticeable but with more than two batteries it is worth the extra effort to connect them correctly. My three batteries are connected correctly.
 
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bubble63
Aug 10, 2012
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This is what’s causing comments
I understand electronics and I can’t see an issue, been so close together as Lenny says

C37C5F0A-7172-4D57-A5AC-C1EF768D46D6.jpeg

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Apr 3, 2018
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IMV Figaro is correct with the water anology.
In my distant past the RN decided to teach us mechanical chappies electrics.
However the one bit of advice we received was to treat electric systems as plumbing
So in first photo water/electric is taken out of first tank/battery and the second one will equalize levels out eventually..
In second photo the water/electric is allowed to flow through and therefore out of both tanks/battery's..
Obviously as in plumbing the cable size between both battery's will have an effect.
But as stated... for our needs both are suitable but if going to do it why not take time to do properly.👍
 
Sep 17, 2017
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I thought it was a failure detection issue? If you're running all the connections off the primary battery, then just piggyback the other batteries off of it, if one of those piggyback links fails, the first you know about it is a hot and dead primary battery.
 

Hoovie

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IMV Figaro is correct with the water anology.
In my distant past the RN decided to teach us mechanical chappies electrics.
However the one bit of advice we received was to treat electric systems as plumbing
So in first photo water/electric is taken out of first tank/battery and the second one will equalize levels out eventually..
In second photo the water/electric is allowed to flow through and therefore out of both tanks/battery's..
Obviously as in plumbing the cable size between both battery's will have an effect.
But as stated... for our needs both are suitable but if going to do it why not take time to do properly.👍
The levels will equalize out eventually, that is correct. but there are two things to consider ....

1) The primary battery (as what you have here is not a battery bank, but a primary and secondary battery) will provide the greater power, and depending on the usage, it will get lower and more frequently than the secondary battery, and the effect will be it will age and degrade sooner than the secondary. So the voltages will equalize, but the useage patterns will not.
2) On large loads, where the voltage drops will be greater due to the higher current, the primary battery will do even more and the effect will be greater.
The water analogy is good, but it doesn't cover wear and tear :)

(I used the balancing vessel system when I added a second fuel tank to my diesel heater - easy to just connect a secondary tank as the levels will equalize out, but when when I was filling them, if I filled one, the other would not come up to the same level for quite a while - I had to fill each in turn or else would have to fill just the one VERY slowly. No need to have a big pipe between the two though as the draw from the heater was so small).

Now when you are talking very small distances for battery cable interconnects, you may well also be talking very small differences, but that does not mean there is not a difference.
I did a little test with a pair of batteries around 6 years ago - one setup configured as a Primary/Secondary. and then another with them configured with balanced cabling. Load was a 12V Waeco CRX50 Compressor Fridge and timed how long batteries would take to go from fully charged down to 12.0V. The balanced cable setup lasted nearly 12 hours longer. I didn't expect the difference to be that great, but that is what the test gave me.

End of the day, you can configure them either way and you will get power out the batteries, but as the saying goes ... "If a job is worth doing, it's worth doing properly" - so why not just do it properly if it is a new install.

You will often find brand new motorhomes and campervans battery pairs wired in the Primary/Secondary way, but that is not because it doesn't matter, but because the cabling is setup for a single battery and when the second battery is added, either the original leads won't stretch to the second battery, or the fitter doesn't know to balance the cabling.

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Last edited:

kevenh

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🤔but neither battery knows which is the primary or which is the secondary.
Plus there’s wire directly connecting them.
I’d expect negligible difference but I’m looking at the diagram from an electronics career background rather than an electrical one :rofl:

I.e. functionally I see
C9FCC6FB-77A3-4581-84F7-CBB2790A2129.jpeg
 

Hoovie

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🤔but neither battery knows which is the primary or which is the secondary.
Plus there’s wire directly connecting them.
I’d expect negligible difference but I’m looking at the diagram from an electronics career background rather than an electrical one :rofl:
The thing with electronics is the currents are very low and the voltages are very low.
But if you are looking at it from a electronics career background, you would also know that cabling is not just about direct connections but about the length of the cabling which can effect the speed of the signal and having things balanced (which is why, for example, a Cray Supercomputer was circular in its construction as that was the most optimum design layout for the speed required. You might be a bit young to have come across those possibly? I recall sitting on one once whilst waiting to see my customer, thinking it was just a circular bench until I was told I was sitting on the computer that did all the medium range weather forecasting :) )
Maybe coming from an Electronics AND Electrical career background, I can see both aspects? ;)
 
Nov 2, 2022
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'Functionally' it is the same but with 200A flowing the minute differences in the resistance of the cables will create a slight difference in voltage drop. This is balanced out using the correct connection procedure shown in post #1 and using equal length and size cables between the batteries.

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Sep 17, 2017
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Personally I can't see it making much difference. The voltage drop, even at high loads, will be a few millivolts, so less than 1%. I bet two batteries off the same production line have more variation than that. And the vast majority of the time, loads will be much lower, so it'll make even less difference.
 

Hoovie

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Cabling 3 and 4 batteries are more fun. This was my bank of 4 Batteries in the last camper
RR712_1K7TfQi5nt93UrIu3JPmP_q_DLTw6Q87Td3VJhm8QQJ5pV7XA5AT1MLwhGJPI7LZ8BNu-qkYtydi4n3IiyCQtoW2Bnk4aLM7cOkbKyxImLS6NQ7vXTziu38-DVZtL0Iakpv1ve3XE1PbFkWfUVHWQN-st9gaMCrBraqCCWMpe9fYzN4KyLqm98VUKJ75vHbnliHU27KxAX9CPbcvX6h-Iz5vFRAqNkrzIFu1-Z0vsEFHoRBshrpvdC99n52vjNKFlU8mS6B7T1wYOmyL4lGqhyOClfsAB8lWdkrYL4rL7nMeCHhvrG5JxCDX7zQ43djl2g9rxKq3IZ5epu6USXTWewQqbtIBTkGxExN0XD8rgDYgHXE9JySNgApzG9D8YFQFg8o79u7XHKsFthwcwH7favSHpyENkmBeBV8BgBMZZVqc5u_PvHFJQ-iC3lhceXmnXnJO7qFNNGrXNlRizzamNJoSkAM2jcXZ-qKDRAmlMWwQrkEe_btSENMhk7Xk0OEyC1YpV-4qR8gEPX8zhEdnhkbHiwDzDtKtbEfDf9gqF9GMyhOGumjriVy0F6ccZD3fZImcW-aYiJ7T0TUJjkK-jwwO_nyt8-_ZdxldFXSfLF6i6qKpfn-Wbom1KrbcLJXxDk9Lfgm3EhYLYMebQVSdVyPBOpkUK8yxqaq2aj2YgRgEiG81dgZixQfB3Awf2iJ0I5wrK9QAV4Z8PS4k8Ml3UlXgAk761tfDbHT0grEVhMc80_01QBQt3GV-7rq8nwQwlPkHO-SZZmUYUInkiydsBJHypW0_w5gPuDLpFhR8eKwnLagEqdjSf2SfXG4dRIMeFASFgiz5ros5N45WIZCvnm6079IHRJF-KMBPLVkbznJmc5xcm6c87oPG8RMsWqz3B9ye4uDwSkpTquRNSkcFqfShNNgHHGlu6S-MssOxZ0jnURiCh7kzD3wDoMRLkyDHUfk1xGwXGB0gQC2w=w690-h919-no

You can have unequal length cables if done in a certain way - the above is paralleling two pairs of paralleled batteries :)

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Feb 27, 2011
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Technically he is correct.

Practically, in most cases providing you use thick enough interlink cables there will be no difference in a 2 battery system.

There will only be a difference between the 2 methods if there is enough resistance in the interlink cables to cause problems. If the batteries are next to each other and you use a decent interconnect then the resistance will be negligible.

If however, your batteries are not next to each other and your interlink cable is more than says 50cm then you should do it the way he suggests.

Also worth noting, if you are drawing a high current then it may be worth doing it the proper way. So if you have say a 3KW inverter, it won't hurt to do it the way he suggests. This is due to losses being I²R The current being squared then multiplied by the resistance.
 
Feb 27, 2011
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I am just amused at how many reasons and justifications are being given for cabling up incorrectly :)

Why not just do it right and be done with it!!
Because it is easier to do it the wrong way and makes practically no difference for most installs.

Why create work for yourself if there is no benefit.

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Jul 24, 2020
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Interesting discussion. When we took delivery of out catamaran, we had 8 115 amp hour Sonnenschein Gel batteries wired like the first diagram. That's just the way Catana did in 2001. We lived aboard full time for almost 8 years sailing the Med, the Red Sea and the Indian Ocean as far as Singapore. We had a 3000 watt inverter which was used regularly and two 12 volt refrigeration systems. When we sold the boat in Malaysia, the buyer, an American, and the surveyor went over all the electrical systems and pronounced them in good working order. Sure, the batteries were not 100% but still accepting and holding a charge and still functioning well. About 2 years after we sold it, we got an email from the owner telling us that he finally had to replace the batteries. So almost 10 years of heavy use.
Whats my point? There is a right way to wire batteries, no question. But in real world usage, at least in my personal experience, I don't think it made a heck of a lot of difference.
 
Feb 19, 2020
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The voltage drop is so incredibly small it will make no difference. The 2 batteries will just act as one. There's an online calculator for voltage drop. It really is so insignificant over that distance, cable size and current draw. Thus was based on 10 amp draw

Screenshot_20230201-231234_Chrome.jpg

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