I’ve written this page to help answer some of the many questions I have been asked after the original article “How To: Connect two batteries in parallel” was published. It covers some of the practical considerations to installing battery banks in caravans, motorhomes and vehicles.
A word of caution: Leisure batteries store an incredible amount of energy and if you short them out or connect them incorrectly then this energy can be released almost instantaneously with disastrous consequences. If you have any doubts about what you are doing then seek qualified professional help.
Connecting more than one additional battery
In the first part we went through the correct way to connect an additional battery…
A number of people asked for a drawing to show the correct way that more than one additional battery could be connected so that the voltage drops across all the batteries would be equalised and the batteries remained balanced in the bank.
Remote Battery Location
The next question often asked is how to connect a second battery if it can’t be located next to the original battery. Installations in some motorhomes and campervans only have space for one and the second battery needed to be located remotely at the rear of the vehicle or on the opposite side.
REMEMBER: When installing a second battery in a remote location it must be sealed off from the interior of the vehicle and the battery compartment vented external to the vehicle. There are battery boxes available that do this.
The existing set up will have all the live and neutral cables terminated near the existing leisure battery and it would take some effort to have to relocate all the earths for each individual circuit to the second battery location, there is a simple way of achieving it.
For the remote location, an additional cable is required. In the first drawing at the top of the page it shows the live cable coming from the second battery which is fine if the two batteries are adjacent. However with a remote battery this would require running two live cables through the vehicle and common sense tells us that it would be better if we only had to run one live cable. (drawing below)
In the drawing (above) it can be seen that now we have three cables running to the remote battery. Two battery interlink cables and a main earth cable. This has now reduced the amount of live cables running through the vehicle and allowed the original earth terminal block that the auxiliary services use.
A number of people asked about the sizes of the cables required to link the batteries together and raised the question could the cables linking the two batteries be smaller than the actual cables connecting the battery bank to the equipment. Their thought being as each battery was supplying 50% of the current the size could be reduced.
In the drawing above the battery bank is wired using 100 Amp cable (I’m rating the cable by current rather than cross-sectional area as it’s a bit easier to follow) From the battery bank there is a 100 Amp positive and a 100 Amp negative cable. The battery interlink cables are sized at 50 Amps as that is the current between the two. We’ll ignore any fuses for the moment – more about that shortly.
Well OK it could work, however if you were using an 1200 watt inverter that put a 100 Amp load on the battery bank and a cell in battery A failed so the battery effectively had little or no output, then Battery B would continue to supply the 100 Amp load put on it by the inverter… and the interlink cables are only rated for 50 Amps, therefore something’s going to get hot!
If you are going to demand 100 Amps from your battery bank… then use 100 Amp cable throughout the installation.
Calculate the maximum load you will ever put on your battery bank and size the cables to that load. Before finally selecting and settling on a cable size though it’s worth reading Understanding Cable and Cable Sizes as there might be a bit more involved.
Here is a rough guide to current capacity for good quality cable:-
Where to install fuses?
Convention is fuses are installed in the positive cable coming from the battery bank.
However if there is a short in the positive battery interlink cable to ground because of the longer cable run through the vehicle due to damage or chafing, then the main fuse will not protect anything in this event.
The sensible thing would be to install a fuse on the interlink cable as well… but at which end? The conventional location is as close to the battery as possible… but there are two batteries and if the short is at mid-point one battery will still be connected.
So we have to install fuses close to each battery. Now the problem comes with what size fuse.
Fuses are there to protect cables from overload and possible overheating and the general rule is the fuse rating should be equivalent to the current rating of the cable or lower.
Lets assume we have completed the installation in cable rated to 100 Amps. So we install fuses rated at 100 Amps at the points in the drawing above. Disaster strikes and we get a partial or intermittent short somewhere along our positive battery interlink cable to ground. Now at the point of the short, potentially 200 Amps could flow – 100 Amps from each battery and not blow the fuses. With 200 Amps we could be welding bits of metal together. Because it’s in the middle of the battery bank there also might not be any indication that there is a problem…… until it’s too late!
Is there a solution?
Well this is what I came up with:-
Installing a second fuse in the main neutral earth cable would protect the battery bank from any potential shorts down to the vehicle earth from the battery interlink cable. If there were a short the current would blow the fuse in the battery bank main earth cable.
IMPORTANT NOTE: There must be no connection between the battery bank and vehicle or other earths/grounds other than at the main earth terminal block for this to work!
For clarity I have removed the fuses from the drawing that were in the positive battery interlink cable. In practice it would be prudent to leave one in to protect against inadvertently shorting out while removing or installing batteries.
Installing a battery bank isolation switch
Another question that kept cropping up was based around “How can I install a battery isolator but still let the solar panels charge the batteries even when they are turned off?”
As long as the solar panel regulator is connected to the battery side of the isolator switch, then even if the isolator switch is open (the battery bank turned off) then the solar panel regulator will still be able to charge the battery bank.
Although the isolator and cable might be rated for 100 Amps and the battery bank fuse be 100 Amps, if you are connecting the solar panel isolator with 10 Amp rated cable, then the fuse between the battery bank and regulator needs to be sized accordingly… i.e 10 Amps (remember the fuse is there to protect the cable and must never be rated higher than the cable it is protecting)
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Understanding Cable and Cable Sizes – When undertaking an electrical project for your caravan or motorhome one key consideration is what type and size of cable to use. Selecting a cable that is too small for the task and you might risk melting the cable insulation or damaging equipment due to voltage drop.