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Connect batteries in parallel with greater safety

First of all we want to make it clear that connecting two or more auxiliary batteries in parallel, although theoretically it may be the ideal option, for us it is neither the best alternative nor the most prudent. However, here are some tips and tricks so that, if you have no choice, you can do it as safely as possible 👍

In this entry we will focus on the connection of two, or more, gel, AGM or lead acid auxiliary batteries, lithium batteries are an exception and work in a different way. We will also not refer to the parallel connection between the main battery and the auxiliary battery.

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How to connect two or more batteries in series?

The ideal way to connect two power sources is in series. Maybe you are thinking the same thing we are thinking: if this is so, why do most vanners connect them in parallel and not in series? 🤔 What’s the problem? Let’s start at the beginning:

To connect batteries in series you will have to connect the positive pole of one to the negative pole of the next in the way shown in the schematic:

Conexión de baterías en serie
Connecting batteries in series

The “problem” is that when connecting two batteries in series their voltages are added, not their capacities. For example, in the image of the previous diagram there are 3 batteries of 100Ah to 12V. The resulting set will be of 36V (12V + 12V + 12V + 12V) and 100Ah, since its capacity is maintained.

How to connect two or more batteries in parallel?

To connect two, or more, batteries in parallel you will have to join all the positive poles on one side and all the negative poles on the other. In the following diagram you can see the most common way of parallel connection, in which both outputs, positive and negative, start from the first battery

Conexión de baterías en paralelo
Most common connection of batteries in parallel

What are the problems of connecting batteries in parallel?

The connection in this way originates a series of problems that promote an accelerated deterioration of the batteries, with the consequent reduction of their useful life. Next we explain you, briefly, which are the problems that cause this premature aging.

Internal resistance of the batteries

The first problem of this configuration is that, although the batteries are “equal” (same brand, model, series, capacity, with the same time of use) they will never be identical, reason why unbalance currents will be created that will cause that they will yield load between them to equalize their voltages. These continuous charges and discharges will produce a premature aging of the batteries.

If to all this we add that many van owners what they do is the following:

  1. I install, at first, a single 12V battery.
  2. does it fall short? I buy another one the same (or not) and I connect them in parallel to add their capacities.

On having done this the problem is aggravated, since the internal resistance of the batteries increases with the number of loads and discharges, reason why already it is not only that the batteries are not identical, it is that they are not even similar, reason why the currents of unbalance will be much greater.

Resistance of the cables and connection elements

On the other hand we have the problem that generates the resistance presented by the cables. The conductors, generally made of copper, do not offer a perfect conduction of electricity, they offer a resistance that increases as their section decreases and their length increases.

If you look at the previous image, the current of the battery of the right, will find a smaller resistance in the wiring since it will have to make a smaller route, so it will be the battery that, in principle, more cycles of load and discharge suffers, reason why theoretically it will be the first one in exhausting its useful life.

A possible solution to this problem is to connect the batteries so that the current has to make the same route, as much during the load as during the discharge, independently of the battery to which it goes or of which it leaves. For it would be necessary to make arrive the positive cable to the battery of an end and the negative one to the battery of the other end.

Conexión correcta de baterias en paralelo
Correct connection of batteries in parallel

To load batteries in parallel

The last problem of which we will speak happens when trying to load batteries in parallel. In this case the charger or the regulator will not receive a correct voltage reading. A correct voltage reading is essential to be able to perform the step charging properly. This is because the batteries will not all be at the same charge level. So it is likely that some batteries will be overcharged and others will never reach 100%, which means that they will not be charging optimally and will reduce the life of the batteries.

Conclusions on the connection of batteries in parallel

After asking people who understand the subject and looking for a lot of information in the net, we came to the conclusion that it was better to buy a battery of greater capacity of first. We, as we needed to have enough autonomy we decided for one of Ultracell of gel of 316Ah.

Other quite better options that to install two batteries in parallel, would be the following ones:

  • To connect the batteries of 12V in series and to make the installation to 24V.
  • To connect the batteries of 12V in series and to add a converter that passes from 24V to 12V.
  • To install two 6V batteries connected in series.

With this post we have wanted to transmit you that you must know and think a lot about the pros and cons before connecting several batteries in parallel. Our opinion is that it should be done only as a last option.

We hope our article has been useful for you. If you have any other questions, do not hesitate to leave them in the comments, we are sure we can solve them together.