Is It Safe to Add an Auxiliary Battery?

When and How to Add Extra Automotive Battery Capacity

safe to install auxiliary battery
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Although a battery that won’t hold a charge is a clear indicator that it’s time for a replacement, you probably have some kind of issue that needs to be dealt with before you start worrying about installing an auxiliary battery. It’s perfectly safe to install a second battery, provided that you follow the right procedures for wiring and battery placement), but it won’t necessarily fix your problem.

If you run a lot of electronics when your car is off (i.e. high-performance audio, camping, tailgating entertainment, etc), then installing a high capacity battery or a second battery might be the end of it. If not, then you’ll want to check for a parasitic drain (and fix it) before you do anything else.​​

Why is the Battery Going Dead?

Before you replace your battery, let alone install an auxiliary battery, it’s important to make sure that there isn’t a parasitic drain in the system. This can be accomplished with a test light, but a good ammeter will provide you with more precise results. is fairly straightforward, but it’s important to remember that certain components (i.e. the ECU, the keep alive circuit in your head unit, etc) will tend to draw a small amount of current, which is normal. You can also run into situations where it seems like a drain is present, but it’s just a relay that’s unable to energize and close.

If a drain is present, then you’ll want to fix it before you do anything else. That may be the end of your problem right there, although your battery may already be toast from all of those times that it went dead and you needed a jump start. If the problem has been going on long enough, you may even find that your alternator’s operational lifespan has diminished due to the extra load that your constantly dead battery has placed on it.

Adding an Auxiliary Battery for High-Performance Audio

If you have a high-performance audio system that you enter in competitions, or you just like to use it when your car isn’t running, then you may want to add a second battery. This is perfectly safe, although it’s important to follow wiring and installation best practices. The second battery should be wired in parallel with the original battery, and most car audio competition experts will suggest that you buy “matched” batteries instead of wiring a high-performance battery into a configuration that includes an existing battery that’s already old and tired.

The battery cables should be the thickest gauge you can reasonably use, and you need to be really careful if you place the second battery inside the passenger compartment of your vehicle. Since batteries can and do explode, the battery should either be placed in the engine compartment, the trunk, or inside a solidly built speaker box if it has to be inside the passenger compartment. Of course, you’ll typically want to locate it as close as possible to your amplifier.

In some cases, you’ll be better off with a single, high capacity battery than two lower capacity batteries wired in series.

You may also be better off with a stiffening cap located close to your amplifier. If you have a problem with your headlights dimming when your music is turned up, then a capacitor will usually do the trick. However, more reserve capacity in your battery (or batteries) is what you’re typically looking for if you’re entering your system in competitions.

Adding a Second Battery for Camping or Tailgating

The other main reason to add a second battery is if you spend a lot of time tailgating or dry camping. In those cases, you’ll typically want to install one or more deep cycle batteries. Unlike regular car batteries, deep cycle batteries are designed to run down into a state of “deep discharge” without being damaged.

That means you can use your electronic devices all you want without any fear of damaging your battery.

If you do add a second battery for either camping or tailgating, the battery should still be wired in parallel with your original battery. However, you may want to install one or more switches that will allow you to isolate the batteries depending on whether you’re driving or parked. When you’re parked, you’ll want to have it set up so that you only draw power from the deep cycle battery, and when your engine is running, you’ll want to have to option to isolate the deep cycle battery from the charging system. Recreational vehicles are all wired like this with “house” and “chassis” batteries, but you can DIY the same type of setup yourself if you know what you’re doing.

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