What Size Alternator Do I Need for 2 Batteries?

The best way to figure out what size alternator you need for your car is to measure the amps of your battery bank and then compare that to the maximum amperage of an alternator.

For example, say you have two batteries in a car and they have a combined rating of 100 amps. If your new alternator can handle up to 150 amps, then you should be good to go!

Will a Higher Amp Alternator Damage My Battery?


A higher amp alternator will not damage your battery, but it can overheat it if the alternator is plugged in and your engine is off. This is because a higher amp alternator will draw more power from the battery than a lower amp alternator, which can cause the alternator to overheat.

To avoid this problem, make sure you turn off the ignition when leaving your vehicle idling. This will cut power to your headlights and other electrical components so they don’t draw power from your battery while you’re not using them.

How much Should I Charge My Battery?

Generally, the best charging rate for your battery is 80% of its capacity. This means if you have a 100 amp-hour battery, it would be best to charge it at 80 amps.

However, this doesn’t mean that every time you charge your batteries at 80 percent of their capacity is the most efficient way to go about it.

Your goal should be to make sure that your batteries are always charged as fast as possible without overcharging them or undercharging them.

Is it Bad to Have an Alternator That’s Too Big?

If your alternator is too big, it will cause your vehicle to run inefficiently. The main reason for this is that a larger alternator will draw more current from the battery than needed.

This can cause you to have a dead battery at times or even drain your starter battery when the car isn’t running. You can also experience problems with electrical components such as lights and stereo systems.

You may be wondering what happens if you have an alternator that’s too small for your vehicle?

Well, in this case, your car won’t get enough power to start because there isn’t enough juice coming from the battery. In general though having an underpowered alternator is less reliable than having one that’s oversized due to them not being able to provide enough power when needed most like during cold weather driving conditions where most vehicles require extra cranking amps.


You need a 150 amp alternator. The size of your alternator is determined by the amount of amps it can put out, and the size of your battery bank. Your car uses 12 volts, so if you have two batteries that are each 100 amp hours (at 12 volts), then you need a 150 amp alternator.

Steven Hatman
Steven Hatman

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