Thursday, September 6, 2012

Keeping Your Engine Battery Juiced - Without The Engine!

My Main Electrical Panel
    My electrical system is convoluted. There, I've said it. I have some VERY weird issues between my start battery and my house batteries. They don't always get along. That's an ongoing project. The real question here is, "What do you do to keep your start battery charged when you AREN'T going to run the engine?" If you have shore power or a decent solar charging system than you can "steal" some amps from the house charging system when the batteries are charged or charging and route them to your start battery.


A Common "Charge Thief"
Easy. Some enterprising folks have designed an manufactured devices that do just that. They range in price from under $100 to well over $500. The expensive ones have some bells and whistles, but the simple one works for my application. It comes with all the hardware to set it up either hardwired or with alligator clips. I would be hard-wiring mine to the engine start battery.

The Knife switch Disconnect
In my setup I have a "knife" switch that disconnects the start battery from everything. That is, until I figure out why it won't get along with the house batteries under certain conditions.

So, on to connecting the "Amp Thief."

You will be connecting 3 wires.Yes, you read that right ONLY three wires! One is the ground. This connects to the chassis or any ground point you can find (even a battery negative post.) The next 2 are for power. The first one connects to the engine start battery positive and the second one connects to the house battery positive. These wires can also be connected to any convenient place you can access positive wires from both batteries. Your battery isolator or perhaps the solenoid disconnect switches. If you are unsure have someone install it, or run the wires to the batteries directly to avoid any issues. Make sure you use heavy enough gauge wire if you need more than what is supplied by the manufacturer of the charger.

Engine Battery with Blue Wire Attached
Locate your engine start battery, or a convenient place to connect to both the engine start battery and the house battery systems. This could be under the "hood," in the rear engine bay, in a compartment or even inside your RV. Once located, you will connect one of the positive leads from the charger (the BLUE one!) to the positive post of the battery.

Make sure you clean any corrosion from the terminals to make a good electrical contact. Yes, I should have cleaned mine MUCH better. I'm replacing the batteries this year, at the end of the season so....

Next, locate your house battery(ies.) these could be almost anyplace. I've seen them in separate compartments, under the hood, inside the RV on a vented box, hanging name it, I've seen it. Anyway...find yours! Once there, connect the other positive wire (the YELLOW one!) to the positive post. If you have MANY batteries, it's a good idea to connect this to the same battery that feeds your RV's electrical needs. It's always better to deplete the power evenly across all the batteries than to favor one.
Charger Mounted and Wired

Green LED Means Maintaining
The way my RV is wired, all house Negative loads feed through the Battery Monitor's Shunt. This allows me to tally all the power used and put back into the batteries so I have an idea of where they stand, power-wise, at any given time. At the moment, my batteries are tied together, so negative is negative and everything feeds through the shunt. When I finally work out my battery oddities, I will move the charge thief's negative ground to the shunt itself to let the battery monitor keep track of it's draw as well. But that's a winter project.....stay tuned!

Mount the charger anywhere that is convenient. It's basically waterproof and shocks don't bother it either. I mounted mine to a convenient vertical support in the engine bay with plastic "zip" ties. You can mount it with the included self tapping screws as well. I made sure to mount it in such a way so that I could see the three LEDs that indicate what it's doing at any given time.

Green is Maintaining, Yellow means charging and Red is a Reverse Polarity problem. Likely, you've got a positive to a negative or vice versa. Check the wiring!

So far, it is working very well. The Solar Charging System charges the batteries, the charge thief charges the engine start battery and everyone's happy. No dead batteries, no "Battery Boost Switch" needed. Also great for long term storage, if you have a source of power! A very elegant solution to a general problem.

Be Seeing You...Down The Road,

Rich "The Wanderman"


  1. Interesting technology, definitely useful for long term storage when the motor is being run regularly :).

    1. Matt,
      If you have shore power or a solar charging system or even an auto-start generator system this works great. Especially if you have Deep Cycle charging profiles for your house batteries and a regular engine start battery. Much nicer for the engine battery!

      Rich "The Wanderman"

    2. my house batteries and engine batteries are side by side in same compartment, I took a 18" 10 gauge wire and put 2 gator clamps on it and now I have a jumper cable for starting the engine when it sets for a while and won't start.

    3. Discoduck,
      That's a great way to eliminate or add a "battery boost" feature to your RV. This setup is more for maintaining your engine battery, rather than assisting the starting of the engine from the house system.

      The wiring for this would be REALLY easy in your case, since the batteries are right next to each other!

      BTW: LOVE your name! I even have the song on 45....

      Rich "The Wanderman"

    4. My Areo has solar and the previous owner put it in storage for me (it's in NM and I am in VT) and she said she hooked the solar to trickle charge the start battery. I will be curious if it starts after sitting for 3 months. I should be out there mid Sept. Coming right up!


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