Batteries in Parallel - is there a right right and wrong way to do it.

Honestly Ken this has been covered so many times it could be committed to memory.

Try doing a search. A little time spent by you searching will save a lot of time for many others holding your hand.

The answer to your initial question is YES

Just connect them in parallel any old way and draw power from them any old way and it will be fine.... There is a right way of doing it but the improvements would be minimal.... I sure there would be other issues in the system thats more critical.

In a high current situation doing it right is more important.

Seriously too many people worry too much about the .001% 'ers and get confused on stuff they have no idea about and cause bigger problems.

I'm sure this will turn into a discussion with input from all the engineers out there saying this is what happens and this could happen.

Think KISS.

Ken, some of this was discussed in thread 88757 a couple of days ago, however.....

Q1. It is quite OK to connect batteries of the same "chemistry" in parallel even if of differing capacity. If batteries are connected in series they need to be not only of the same chemistry but also of the same ampere hour rating otherwise the smaller will get both overcharged and over-discharged.

Q2. This fancy concept of "taking the negative feed from one battery and the positive feed from the other battery" is an absolute nonsense proposed by those who know little but seek to be seen as very wise. The idea is based on balancing the wiring impedance to each battery, but if adequate cabling has been used then it is of negligible consequence. Your "major battery supplier" as usual knows little of electrical fundamentals.

I hate using the search function on this site, does it really work properly

Ken , was is it you want to do ?

Are you looking for 12V or 24V or simply double up your amp hours

The old imported South African GQ patrol had a 24V system it must have been 2 x 12 volt batteries, many trucks run 2 x 12 volts , Kimberly camper-trailers run 10 or so 12 volt batteries for it's 12 volt power supply

Hi Ken,

You have asked for some technical discussion here, so at the risk of going against the grain here, I believe there can be merit in taking the +ve and -ve supply from different batteries!

But... that’s only if the cables joining the two batteries are grossly undersized, or if there is a significant distance between the two batteries. For most, there is no real world benefit of doing so if appropriate sized cables between the batteries have been used and/or the cable run is short. However, there are circumstances where it is beneficial to do so. Some camper trailers have the batteries mounted behind the wheel arches on opposite sides so there is a long wiring run between the batteries and the charger for one battery.

As an example to show why there is benefit; - assume a load of 30A, a charge rate of 40A and a cable distance between the two batteries of 5m using only 6mm (4.5mm2) automotive cable to join the two batteries (undersized!). Under discharge conditions, both batteries should supply an equal current of 15A, but the second battery would have a voltage drop 0.6V, so in reality the first battery would supply more of the current (the exact amount depends on several things and the batteries internal resistance, but will be in the order of a few amps).

Now, under charge conditions of 40A, (using say a Redarc bcdc1240...) the voltage drop is now 0.8V if they were receiving an equal 20A charge, but the second battery would receive a few less amps at a LOWER charge voltage. Now the first battery actually needs more charge as it has been doing more of the work, but over time the second battery may become undercharged due to the lower charge voltage.

If the wiring had have been such that the +ve was taken from one battery and the –ve taken from the other, the voltage drop would be equal and both batteries would be charged/discharged at the same rate.

The above is not a typical situation but also not unrealistic either. But it does demonstrate why there can be technical merit to wire the batteries as such. It can be particularly relevant where many batteries are used as the different batteries may age at a different rate. But if the effort has been made to wire correctly, then appropriate sized cables would also have likely been used, so almost makes it a moot point.

So to answer your original question, yes there is some technical merit but no, it’s unlikely to make any difference for the vast majority of users.

Cheers

Captain

Ken,
You haven't stated whether you are paralleling the cranking batteries or the aux batteries, (or both!)

Many vehicle manufacturers connect cranking batteries in parallel so its not all bad! You double the cca's, and reduce the stress on each battery so they last longer. 100series TD and the late 3.0 Jackaroo TDs are examples.

The big issue is that if one battery dies (drops a cell etc), then it will flatten the good battery with it. If you're talking cranking batteries then you can be left on an isolated bush track with not enough electrons to power your HF radio or satphone, leaving you up the creek. So if you are a solo traveller it is better to isolate the cranking battery. And if you are paralleling batteries its good to have batteries of identical age and type, so the likelihood of one failing is reduced.