Like most things there is a right way and a wrong way of doing it and one that I receive emails about is how to connect two batteries in parallel and get even more people finding the site by asking the question on Google. So here is a quick “How To” guide with some explanations on the right way and the wrong way.
Most people who want to connect two batteries together are trying to expand the battery capacity of their existing setup. One thing to remember, if you are going to install a second battery, you are going to have to start with two new, identical batteries. Same make, Amp hour (Ah) rating and if you can the same manufacture date. Connecting batteries with different Ah ratings or from different manufactures is not a good idea as it can/will lead to one battery trying to charge the other (due to the internal forward and reverse resistance being different) and can lead to all sorts of other problems.
So now we have two new identical batteries how do we go about connecting the two batteries together? Well the most obvious is to simply connect a new battery to the original battery using some cable and new battery terminal clamps like this….
OK, that will work. Hang on though lets have a closer look.
Battery A is the primary battery and Battery B is our newly installed secondary battery. Now when we put the whole system on load… starting the engine for instance, there will be a slight voltage drop across the two cables linking the two batteries together… lets say it’s 0.5 volts on each link or bridge cable, that’s one volt in total. So Battery A will always supply more power than Battery B as battery B’s apparent voltage is always less than Battery A because of the volt drop. Now over a period of time, Battery A will always be ‘used’ slightly more than Battery B so at some point Battery A will have cycled more and be ‘aging’ faster than Battery B and it will eventually lead to problems and will need replacing. But remember what I said earlier, you should always use batteries from the same manufacturer with the same rating and manufacture date… well that still applies and now you will be replacing Battery B that is probably still working OK.
The other side of this is charging. We will still have the voltage drop – although it will be smaller as the charging current is a lot less than the cranking current for starting your engine, lets call it 1/4 volt (0.25 volts) on each cable, so now the engine is running and the batteries are charging .. but Battery A gets the full charging voltage – 14.0 volts for example but Battery B only gets 13.0 volts, so not really enough. So now we are in a situation where Battery A is doing more work and Battery B is not getting charged properly. So what’s the solution?
Well, this isn’t going to cost you much….. just connect them slightly differently!
By taking the electrical feed from the second battery we now even up ‘the pack’ electrically. When we start our engine now, Battery A has no volt drop to earth and 0.5 volts to the output…. and Battery B has 0.5 volt drop to earth and no volt drop to the output. So simply by moving one connection we have evened out the volt drop to both batteries. Now the same happens in reverse when charging. Battery A has a volt drop on the +Ve side and Battery B has a volt drop on the -Ve side. So we are now charging both batteries identically.
A few notes on installing a second battery
As I said earlier, always use identical batteries - same make, same Ah rating and same manufacture date if possible.
Before installing the batteries, take the time to charge them up individually using a smart charger so that when you link them together they will both have identical charges – you will ensure that you get most out of your batteries this way.
Always try to use identical size cable or larger as the existing cables connecting your old battery.
Always try to keep the link or bridge cables the same length and route them through the same hole in any bodywork. This will stop the chance of Eddy currents in bodywork.
NEVER connect both batteries earth connections to the chassis of the vehicle and rely on the body of the vehicle as the electrical path . Only ever link them to each other and have ONE connection the chassis or bodywork.
All installations are different. It may be that instead of moving the positive lead to the new battery (B) is is better to leave that where it is and move the negative (earth) to the new battery instead. Just remember one lead from one battery, the other lead from the other battery. Take time to plan before committing to anything.
NEVER cut a few of the strands off the cable to try to get two heavy-duty cables in a battery terminal designed for one cable. There are specialist battery terminals designed to take two cables out there… check out the on-line specialist stores.
As you are probably doing this because you need more capacity, it is worth checking the existing heavy-duty cables… it would be a good time to think about if the existing stuff needs upgrading too.
If you want to install a second battery in your 4 x 4 as you are installing a winch, this is not the way to do it. I’ll cover that in a future “How To”
There is a huge amount of energy stored in these batteries, if you short one out it will go BANG… if you connect two together the wrong way… it still goes BANG but a lot LOUDER! Seriously, if you have any doubts, get an Auto Electrician to do it. Getting it wrong will seriously hurt you.
If you found this useful, I’d appreciate it if you would click on the rating star do-hicky at the top of the page, thanks.
I have attached a PDF drawing (Below) of the above drawing that you can down load and print out with the wiring diagrams above.