I want to have a 2nd battery to power a 1000w inverter (so I do not have to start the 2004 Honda Civic). How will this affect the charging system? Will both batteries be charged?

4 Answers 4


There are several things to consider in this setup. First is the types of batteries to use. Your typical car battery is intended for one main thing, starting the car. There are some other incidentals, but the main thing is providing power for starting.

To perform that duty it is doesn't need to be drawn down low and recharged frequently. But that's what the inverter will do. You will charge it (we'll get to that in a second) and then draw it down low using the inverter. The battery type you want for that is a deep cycle battery. The battery in most cars is not a deep cycle battery.

So you don't want the battery that powers the inverter to be the same type as your main car battery. Which may create an issue if you try to charge it with the one alternator. For one thing, the battery running the inverter will likely be a much larger capacity battery with a different battery chemistry. If it charges faster, trying to charge both at once may be more current than your alternator can handle. Especially if the inverter battery is very low, it will try to pull more current for charging. I don't think the different charging rates will cause other issues, but I would have to think about that one more. Mainly you would have to be sure that your alternator can source enough current to charge both when they are both low. Otherwise you will overheat and damage your alternator.

Some possible solutions:

  1. Separate the two systems completely by installing a second alternator. There are many ham radio operators who run high power amplifiers from their cars which requires multiple alternators, so there are probably kits available to do this. Use the second alternator to do nothing but charge the inverter battery.

  2. Insure your alternator is rated to source enough current to charge both batteries at once when they are both low. For this solution to prevent the inverter from drawing current from the car battery, install a large switch that removes the inverter battery from the car electrical system altogether and only powers the inverter. This way you can always start the car with the on board battery. I'm not sure what the current draw for starting will do to your deep cycle battery, so I would leave the inverter battery out of the line when starting the car. That mean having to throw the switch every time you start the car. Kind of painful.

  3. Completely separate the inverter from the car electrical system and charge the batteries at home. This is the system I use for a system I needed to put together. I have two very large AGM (Absorbent Glass Mat) deep cycle batteries to drive an inverter in the back of an SUV. The batteries are large and heavy and not easily carried about. So pulling them out to charge them can be painful. But, I get 18 hours of high current operation out of the inverter.

These are just some possible solutions. Just make sure that the alternator is capable of what you finally chose to do. Also be sure that the battery chemistry you pick is capable of what your asking of it. Some batteries don't like very short, VERY high current draw, but will do steady draw till the battery is nearly flat very well. Also be aware of what you are switching into and out of your cars electrical system. You don't want to cut off power to the car with the engine running while switches are between the poles for example.

  • So is it normal to have to run the car for the inverter to work?
    – Ronald Yu
    Dec 16, 2016 at 13:05
  • And please explain the technicalities of why the inverter works when the car is running (ie. inverter running off the charging system?)
    – Ronald Yu
    Dec 16, 2016 at 13:06
  • You don't need to have the car running to run the inverter. In the case of the separate system, it won't even be related. If you chose to run the inverter off the car's system, you will need to be sure you're not running the battery so far down that you can't start it again. All of this depends on how you wire it. There is no one answer to this. What might help is a detailed description of what the application for all this is going to be. (How do you plan to use this).
    – cdunn
    Jan 5, 2017 at 19:25

Well, there's different ways you can do this.

  • You can have the 2nd battery in the trunk (or wherever) and have your inverter directly connected to it. Make sure you have a kill switch so that your trunk battery only gets used when you want to use it. The 2nd battery will not be getting charged, since it isn't connected to the vehicle's electrical system. It would be up to you to charge it somehow.
  • You can have the 2nd battery connected to the vehicle's electrical system so it does get charged. You can still leave it in the trunk (many people relocated their 1 and only battery to the trunk to make room for other components). Your inverter will be powered by both batteries (make sure you leave yourself enough juice to start the car!) and both will get recharged when the vehicle starts.
  • You can have a more funky setup where only your 2nd battery will power your inverter when the car is off, but both will get recharged when the vehicle is on. The key would be to have your inverter connection to the main battery to only be active when the key is "on" or "acc" (like your radio and most of your accessories), but also have a direct connection to your 2nd battery. This will allow your inverter to run off your 2nd battery when the vehicle ignition is off, prevent your inverter from grabbing power from your main battery, but your 2nd battery would get charged when the vehicle is running.

Depending on which setup you pick, based on your preference and skill level and how much time you want to spend making this happen, #1 is super easy and quick, #2 is a bit more involved, #3 is slightly more involved still. You may need to learn about soldering and using diodes.


Aside from using a deep cycle battery for the inverter, you will want to isolate it from the primary battery for starting the car.

You could use a diode, but there will be a voltage drop. You could use a relay to connect that battery to the alternator, when the alternator is running, or a switch thrown by the operator when the alternator is running.

Probably the cleanest method is to use a relay/contactor which is located near the alternator, and have a fused run to battery #2. The relay can be controlled by an operator, and should be configured so that if the engine turns off, the relay drops out.


You can certainly use the alternator to charge 2 batteries if you want, but I'm not sure this is actually what you would need. You say you want it so you don't need to start the car to power an inverter, so I guess you have one of these 2 issues:

  • You don't have enough power to use your inverter for very long
  • You don't have power with the ignition off

For the first one, just use a higher capacity battery - this saves any sort of complicated wiring and finding a place to mount it etc.

For the second, use an always on connection to your battery (or at least not switched from the ignition)

I had similar requirements (driving a similar inverter and a large sound system for beach parties) and all I needed to do was get a heavy duty battery and a separate switched power line (and a large smoothing cap and some other accessories etc).

  • Not sure how down vote happened. I think I "butt voted." It wasn't my intention- hadn't even read the thread yet. If you exit I can withdraw the vote I think.
    – dlu
    Dec 15, 2016 at 23:55
  • 1
    Not to worry, I won't take it personally :-)
    – Rory Alsop
    Dec 15, 2016 at 23:56

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