Wiring two batteries in a parallel format - by Glenn Moyle

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Often used in caravans, solar setups and to a lesser extent in motor vehicles, the parallel connection of batteries provides a simple means of doubling the available current in a given situation.

eg: 2 x 100 amp batteries wired in parallel will provide 200 amps

BUT is it as simple as that?

Well, not quite!

To create a safe and viable parallel battery pack, a couple of criteria need to be met.

Let’s list the requirement and then discuss each item individually:

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1 - The Positive and Negative Terminals on each battery must be connected together with the matching terminal on the next paralleled battery

2 - The shortest possible interconnect cable of equal length and diameter must be used

3 - Batteries of equal age, size, resistance/state of health and capacity must be used in this application

4 - Sufficient charger capacity to satisfy the larger battery pack during the recharge process should be considered

5 - Recognition of the types of batteries this wiring format will suit

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1 - A parallel connection means that the batteries output voltage remains the same ie 12 volts but the battery current reserve is doubled, assuming all requirements are met. This is achieved by wiring positive to positive and negative to negative on the batteries themselves. Note, when drawing power from, or sending charge voltage to this style of battery pack, the positive cable which goes to the load is connected to one battery, while the negative cable which goes to the load is taken from the second battery. This wiring arrangement has the effect of making the two batteries act as one and current draw and current charge is equal on both batteries at all times.

2 - The gauge and length of cable and terminals fitted to each of the cables will play an important role in the overall performance, but also in the safety of your parallel battery setup, the performance is a direct result of the wire guage being an appropriate size to carry the current required to operate the devices and load you plan to connect. The terminals will provide the connection point to each battery and should be tight and clean, as should the battery terminals themselves. These terminals should also be capable of appropriate / reliable termination with the gauge of cable you are using.

Finally, the wire gauge itself should be the same for every jumper cable and connection cable being used, this guarantees that all cabling can handle the same current level and there is no heating of cables under high load due to size variation.

(Thinner cables will present a higher resistance under load, so will heat up more at the same length than a thicker cable might)

3 - It is extremely important that the batteries used are identical both electrically and physically. This will mean the batteries operate successfully in unison as one larger unit.

The resistance of each battery is most important as it will impact the rate at which an individual battery charges and discharges, so whenidentical units are wired in parallel, the two batteries will truly act as one. Battery current size is also paramount. Connecting a 100 amp battery to a 120 amp battery will not yield best results as the smaller battery will dictate the overall performance of the package. Equally, if one of the two batteries is down on performance and a fresh new battery of the same current rating is added in parallel, the lesser battery will instantly draw down the new battery and the overall package will operate as two of the lesser batteries in terms of performance. If you are considering a parallel battery package it would be advantageous to start with two batteries of the same size, current rating, age and even batch number if possible, that way you are assured of the desired outcome.

4 - Charging your parallel battery pack should be considered as well, because a charger suitable for a single battery will charge a parallel pack at approximately half the rate or twice the time depending on your point of view. Using a 100 amp AGM battery as an example, these batteries generally charge between a low level of 10 amps to a higher level of charge at 20 amps per battery. Once a parallel setup is completed, a charger capable of 20 amps is now charging each battery at approximately 10 amps which is generally considered a little on the low side. Replacing your charger to achieve the most appropriate supply level is not mandatory but it will bring back the charging time line and parameters to a level you are accustomed.

5 - Finally it should be noted that not all battery types perform the same way in the parallel format. The parallel format suits AGM and Gel batteries and will also work with wet cell batteries as found in motor vehicles, however it will not work the same way with Lithium batteries which have a BMS system on board, these batteries will be limited by that onboard BMS system meaning that a parallel setup of 2 x 100 amp BMS equipped Lithium batteries, will still only supply 100 amps output, as the BMS units will control the current output to match the BMS parameters. Ultimately the Lithium supply will last longer than AGM or Gel batteries due to the larger depth of discharge that a lithium battery can provide, but the current delivery itself will not exceed the capacity of the Lithium’s onboard BMS

So, if a parallel battery setup has been in the back of your mind, there is nothing to stop you from having the benefits available to you. Simply use the correct products to set it up and enjoy the peace of mind offered by the larger battery capacity you will have available to you!