How to Connect Your Battery and Alternator

Updated: May 1

Alternator Battery Connection

If you are looking to get an off-grid lifestyle without the off-grid price, an alternator battery connection might be the best option for you. A lot of people who are looking to live on the road seek the off-grid lifestyle because of the freedom and the ability to camp without hook-ups. When most people think about going off-grid, the first thing that comes to mind is solar and wind energy. The issue with these systems is that they can be expensive and working with them is usually outside the average persons skill set. When I first started my bus conversion, I didn’t know anything about battery banks or solar connections. The black hole of understanding solar is a deep one for many. This is why I suggest starting your off-grid lifestyle with an alternator-battery connection; it is cheaper and, most of the time, it is easier to deal with.


This article is not intended to say that wind and solar energy aren’t great options for many of us on the road. Nor is this article saying that there are not restrictions to having an alternator-battery connection. I personally have a Solar System, inverter, and a Battery Bank, along with a 30-amp shore power plug-in, back up gas generator, and the alternator-battery connection. The goal for everyone should be to diversify the ways they can generate power. However, for many, it will take time to learn new skills and acquire the required finances. This is why I suggest starting with a simple alternator battery connection. You can always add another alternative power source later.

How does an Alternator-Battery Connection Work?

When you start any vehicle, the battery engages the starter to turn and crank the engine. Once the engine is started it runs on its own and the starter turns off. Every time you do this cycle, the batteries lose power. This is why when people crank their engine a hundred times to try to get a vehicle started, the battery eventually dies. In order to keep the batteries from dying over time, it is important to have a way to recharge the battery system in vehicles.

Luckily, every vehicle has a recharging system built into the engine. It is known as the alternator. After you start your vehicle and the engine is idling, the alternator engages. The alternator spins off the engines serpentine belt and recharges the vehicle’s batteries as well as run the electrical systems in your vehicle. such as: radio, lights, etc. As long as the alternator is working, you will have a fully charged battery every time you park your vehicle at night.

The key is to harness this power into a separate battery. When the alternator fully re-charges your vehicle battery, it will stop charging your batteries but continue to run the vehicle. The batteries are only used to start a vehicle not run a vehicle. If you were to connect a separate battery bank to your vehicle battery you would be able to charge them off the alternator as well. This can be an effective way to capture the power you are already generating while driving.

How It's Done

The idea of this hook-up sounds great, but many people may not know how to do it. It is not as hard as you might think. But before you start, you will need to make sure your voltage matches across all systems. The basic idea is that your vehicle battery will be connected to an auxiliary battery bank. Since your vehicle battery is running 12v, your auxiliary battery bank will also need to run 12v. In my case, I have 3 12v batteries used for starting the bus and 4 6v deep cell AGM batteries in the rear of the bus. My 4 6v batteries are run in series paralell outputting a 12v charge.

**For example: If you run 4 12v batteries in series parallel (giving them a 24v charge), you will not be able to connect the two systems. Likewise, if your battery bank is 6v, then you will not be able to connect them. 12v=12v.


- 1 x – Relay battery Isolator

- 2 x – Breaker

- Various battery connectors

- 4 gauge wire (what ever length you need)

- Wire Protector Loom

Step 1:

The first thing that you will want to do is disconnect the bus batteries from the terminals. Remove the negative first, then the positive; this way you will be safe and you will not accidently shock your electrical system while working.

*Remember the axillary batteries also carry a charge, so you will want to make sure they are disconnected as well.

Step 2:

The second step is to run the wire from one battery bank to the other. This wire need to be size properly to overcome the voltage drop that happens over long distances. This can be difficult to determine depending on how far apart your battery banks are and if you have access through the engine firewall or subfloor of your vehicle. The wire should be placed inside the wire protective sleeve. This will prevent the wire from rubbing up against any metal and eventually shorting out.

**The wire should not be connecting to either battery yet - just laid in place.

Step 3:

Once the wire is in place, you now can begin to attach the two systems together. The first part is to install the two breakers. The breaker should be sized to the wire used. (I used 4 gauge wire and a 75 amp breaker) One should be placed as close to the vehicle’s batteries as possible. The other should be next to the axillary batteries. The breaker will protect the two systems in case of a short. Also, these breakers will stop the wire from sending power if the wire would break anywhere between the batteries.

Step 4:

Now that the wire is laid in place and your two fuses are wired in, you can begin installing the Relay Battery Isolator. This part will take a little extra time because you need to wire the isolator into the ignition of the vehicle as well. The reason why this needs to be done is because you will not want to always be sharing the battery power. You only want the batteries to work as one unit when the alternator is running. The two systems will act in isolation when the vehicle is off, ultimately preventing either system from draining the other.

Step 5:

The last step is to connect the wire to the two battery banks and test your new system. But, before you connect the batteries, you should check all you connections and make sure that you do not have any loose connections. Once you’re all safe, connect the batteries and start the vehicle. If the system is working properly, you should be generating power into your secondary battery bank.

Negatives of The Battery Alternator Connection

This is a great option to begin your off-grid lifestyle. However, it does have a few drawbacks and things to consider. The battery alternator is great if you are moving and constantly recharging, but if you are planning on parking for a week at a single location, you will most likely run your batteries down. Every battery bank setup will have a different number of amp/hours it can run before losing power.

For example, my bus has about 480 amp/hrs at 12v (48 amp/hr 120v). Therefore, my bus could run without recharging for around 2 days. This is also assuming a constant draw of power for 48 hours, but I think you get the point. You will have to consider the size of your battery bank for longer stays. Also, note that the bigger the batter bank, the longer the engine alternator will take to recharge them, resulting in needing the engine to run longer.

Other Off-Grid Power Options

If you add a battery alternator connection and later decided you want more power, it is totally possible to do so. I have met many people on the road who gain energy from many different mediums. Solar and wind energy are some of the most popular. These systems can easily be tied together with the alternator connection, making it possible to be driving down the highway while charging from both the sun and the engine. I have a solar system on my roof, which charges my battery while I am parked. Also, I have a full 30amp transfer switch that allows me to plug into an on-grid system if I need to.


The most important thing when deciding to plan an off-grid energy package for your vehicle is to figure out your usage and needs. Once you know how much energy you will use on a typical day, you will be better suited to adjust your energy inputs and outputs. The battery alternator might not solve all the power issues you might run into, but it is a cheaper way to get started and give you the opportunity to later add to your energy inputs as you go. Ultimately, saving you some serious money in the beginning of your off-grid life.

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