# Connecting two or more leisure batteries? Read this!



## Firefox

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

This is a good article which I haven't seen linked before.

The basis is that if you connect two batteries in parallel, don't take the main feeds off just one battery. Feed and charge from the positive of one battery and the negative of the other. (batteries are still connected by thick cables +ve to +ve and -ve to -ve ). It also shows the importance of connecting batteries using thick cables 35mm2 plus (that's 350 Amps!) 

This is all so one battery does not take more load hence failing prematurely and dragging the other down. Just moving one take off point can make all the difference.


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## Canalsman

Useful link Vernon ...

Thanks 

Chris


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## Firefox

I thought you might like that  The rest of the site is about canal boat and marine battery applications.

He talks about how many people could be using the battery bank on a boat and the difficulties monitoring it properly. Usually on a motorhome it's easier for one person to keep tabs on the state of the batteries.


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## landyrubbertramp

yes thanks for this ff, i use methid 2 i didnt realise thier were better options. to be clear then is he saying option 3 is better but more difficult and re methid threee does the wires have to be the same lengh i,e all the posative cables etc ??? and last question ff how many batteries do you think it possible to connect i was thiming of getting 4 110 battieries for my 1000 watt inverter as it cuts out at around 11.5 volts instead of about 11 this way i wont put more load on the batteries with 4 batteries instead of 2 your thought welcome ff


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## Firefox

You can connect as many batteries as you want but the limitations become those of space and weight on a vehicle. With method 3, the cables need to be identical eg short battery to battery leads. You do need 2 more cables than with the first two.

I have got a 1000W inverter running off 2 x 110A batteries which to be honest is not really enough for the 700W applications. But those are rare and short term. Most of my appliances are less than 100W with the occasional drill or jigsaw 350W.

I would like 4 batteries... it's an extra 50kg. I have the weight but I don't want to take up more space, so I will probably stick with two.


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## landyrubbertramp

Firefox said:


> You can connect as many batteries as you want but the limitations become those of space and weight on a vehicle. With method 3, the cables need to be identical eg short battery to battery leads. You do need 2 more cables than with the first two.
> 
> I have got a 1000W inverter running off 2 x 110A batteries which to be honest is not really enough for the 700W applications. But those are rare and short term. Most of my appliances are less than 100W with the occasional drill or jigsaw 350W.
> 
> I would like 4 batteries... it's an extra 50kg. I have the weight but I don't want to take up more space, so I will probably stick with two.



thanks ff


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## Tony Lee

> 35mm2 plus (that's 350 Amps!)



I hope this misconception doesn't spread.


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## Firefox

Yes sorry it's less than that, about 150 A depending on temp allowed for cable and configuration.

A simple rule is approx Amps = 10 x mm2 for small cables up to about 5mm2 .  After that is starts to drop off due to less heat dissipation in thicker cables (volume to surface area). Eg  7 x mm2 for a 10mm2 cable and 5 x mm2 for a 30mm2 cable. It depends on cable configs though.

For most practical purposes we only need the smaller cables up to 5mm2 in many motorhome wiring situations.

Aside from the niceties, the point is you need a thicker cable than just for current carrying to cut down on resistance and voltage drop between batteries so they end up being loaded more nearly the same.


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## NicknClair

Hi Guys,

Surely if the leisure batteries are in the same spot, it makes far more sense to buy one single large leisure battery??? Connecting to one battery source means less cables, less resitance, more direct power. Large Batteries can be purchaced with an Ah of 220-270. I understand that in some cases this is a problem due to layouts etc, but the less cables the better. If I was forced into making a call, method's 3 and 4 would be favorable, as you also have 2 x single positive and negative connection points for inverter/main 12V power/solar panel connection that maintains battery balance. 

I have a 1.6kw inverter coupled direct to the leisure battery using 50mm cable, then using 25mm cable from the battery to a B2B charger then off to the engine battery positive terminal. The negitive connections are all going via a 200A shunt connected to a Battery Computer, which monitors current consumption and also tells me how many Ah's i've used. The inverter is normally used for Microwave and wife's hairdryer in short hits, with long use used for chargers, but this is mainly whilst we are on the move so to conserve energy and keep things toped up the moment we park.

Took a look at 2011's travel journal and only counted 2 x trips where hook up was involved and that was only because it was provided! Must be doing something right for how we use it


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## Firefox

Large batteries are heavy though. I have enough problems lifting an maneuvering a 110 Ah 25kg in a confined space to drop it in. I guess my battery compartment is quite tight, but I wouldn't want to lift a 50kg plus battery.


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## NicknClair

Firefox said:


> Large batteries are heavy though. I have enough problems lifting an maneuvering a 110 Ah 25kg in a confined space to drop it in. I guess my battery compartment is quite tight, but I wouldn't want to lift a 50kg plus battery.



Ok can kind of agree on the initial lifting in of a large battery, but by the time you have added a couple of batteries you will be near to/equal to weight. There is also less of a risk in damage to a battery, if there is no battery bank as failure of one battery can kill the rest of the bank. 
This happened last weekend to a customer's Autotrail, which the cold weather caused the bank to colapse. 2 out of the 3 batteries............ are in Battery heaven. Have now ordered one 230ah unit to be fitted, so only one connection, no additional wiring/resistance to worry about.


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## Firefox

True, but really getting them in is the big problem for me. Plus taking delivery and carrying them up stairs. 25kg is not nice, 50kg is a killer. I could fit in my van 4x 110Ah or 2x 220Ah with regard to weight, but no space, or not space want to use up with batteries, so I settled for 2x 110Ah


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