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How to Charge Lead Acid Marine and RV Batteries in Parallel

Have you ever wondered why your batteries in parallel seem to fail sooner than you would expect? When asked how to charge batteries in parallel people commonly reply connect the positive to positive and negative to negative. Yep, electrically speaking that works. But what if you have an RV, for example, and need to add 3 or 4 or 8 batteries in parallel? Do you continue to add to the string in a linear fashion (Figure 1)? Or is there a better way to set up and charge your RV batteries in parallel? I’m willing to bet the vast majority of RV and marine battery owners have never considered or made the connection between early battery failure and improper setup. This How to Charge Batteries in Parallel tutorial is designed to move you from ignorance and insanity (doing the same things over and over and expecting different results) to a point of knowledge and confidence. To achieve this result we will share a framework of information and illustrate the improper and proper method(s) to achieve balanced charging.

Unbalanced Charging: It Is a Matter of Resistance

Common Method of Parallel ChargingBefore we move on we need to point out some assumptions we have made. We assume when you plan to connect your batteries in parallel, you are using the same type, age and size of batteries. For example you would not connect a deep cycle battery with a starting battery. Or connect 2 old batteries with 2 brand spanking new batteries. Or connect a group 24 with a group 27 and group 31 sized battery. Figure 1 is a representation of the setup many of you were taught and currently utilize to hook up your batteries and charger. This is sometimes referred to as stringing the batteries together; battery one connects to battery two which connects to battery three which connects to four and so on. To be clear this setup is NOT proper; it is unbalanced. Batteries paralleled together in this formation will result in premature failure and shortened lifespans. This is due to uneven resistance levels.

The first battery in this configuration is going to be worked harder and be charged faster then the one directly above. This pattern of faster charge and discharge times continues all the way down the string. Figure 2 for Unbalanced Charging demonstrates the charge effect using a 50A charger. Notice the drastic amperage decrease between the first battery and the last battery in the system. Theoretically, each battery would be receiving the same amount of current when charging, however, small yet measurable amounts of resistance between each battery connection makes this not the case. There is about a 50% difference between the first battery and the last battery! To say it another way, the battery closest to the charger gets about 36% of the available amperage while the last battery will receive about 18% of the available amperage. (Our friends at IOTA Engineering, the maker of the popular DLS and SDC charger converters used by many RV enthusiasts, created the images in this tutorial. Thanks guys!).

Unbalanced Charging Figure 2

“Balanced Charging” is a way of eliminating this problem by evenly distributing the resistance between the connections across all of the batteries, allowing you to reap the maximum potential of each battery, and ensuring that they all have a similar, lengthy lifespan.

Balanced Charging: The Correct Method to Charge Batteries in Parallel

Balanced Charging

Balanced Parallel ChargingTo achieve the criteria for Balanced Charging you simply need to start one of the charging leads from the opposite direction. In this example each battery will draw current through exactly three interconnecting leads. This is a far better method than what was demonstrated in Figure 1 and 2. In this configuration, the amperage drawn by the batteries looks symmetrical. If you were to split the system in half both sections would appear to be evenly distributed. There is no more than a 15% difference between the primary contact battery and the secondary battery. This method of balanced charging can be used on both even and odd numbered strings. However, the system is still not perfectly balanced.

Perfectly Balanced Charging

In addition to the need for a consistent number of interconnecting leads for each battery, the length (and wire gauge) of the battery leads should also be consistent to achieve Perfectly Balanced Charging. I will reiterate, that gauge of wire should remain the same throughout the entire system to maintain the same resistance level. Also, the image for perfectly balanced charging is a little misleading as the negative lead was routed below the battery bank so as to not cover up or confuse what is happening at the batteries. This gives the appearance of a longer negative wire length, when in actuality both the positive and negative wires are identical in length.
Perfectly Balanced Charging
This method of charging batteries in parallel will result in each battery drawing the same amount of current from the charger. It will maximize the lifespan of all your batteries as they will be charged and discharged evenly. This method of charging can be utilized when there is an even number of batteries (4, 6, 8, etc.)

To maximize the output performance you desire from your new Trojan, OPTIMA, LifeLine Sealed AGM batteries or any brand for that matter it is important to pay attention to how they are wired together in parallel. Rewiring to one of the balanced charging methods could save you hundreds if not thousands of dollars in battery replacement costs.

As an added bonus you should also consider including an on-board desulfator in your system. These inexpensive devices will help extend the working life of your new deep cycle batteries 2-3 times longer. They work by sending a pulse through the batteries at the known frequency that breaks up and dissolves sulfate crystals found on your battery plates. The combination of the proper charging of batteries in parallel along with an on-broad desulfator, such as the PP-12-L by Pulsetech, your RV and marine batteries will not become the bane of your existence. And you will have extra money to take your spouse out for a nice dinner...several times over!