dual battery switching system

I recently put a dual battery setup in my 00 Ram 1500, and utilized a starter relay to be able to electronically (through a toggle switch) control the connection of the batteries. Unfortunately, the starter relay isnt normally designed to stay latched for a long period of time, so the relay burned out within about a week or two. So now I am looking for something that will work better (needs high amperage so that I can use the second battery as a jump start if needed) Also I would like to find a way to make the relay automatically close if the second battery drops below a certain voltage, then turn back off when it is charged. Not sure how I can do that, but it would be great if someone could point me in the right direction. Thanks KennyKenny

Owner / Technician

KKD Garage LLC

Albany, NY 12205

I used a NAPA PHI54140 for my dual battery setup. It is rated for continuous duty, but they don't list the amperage rating. If I were you I would not waste the time messing around with detecting the voltage mainly due to the surge of current when the battery is brought back into the system with a lower voltage. Mine is setup so that the relay is triggered off of the ignition wire, this way the relay connects up the second battery when the ignition is in either START or RUN but off in ACC. This allows me to listen to the radio and do other things off the main battery while leaving the second one for backup, it also allows for the usage of the second battery for cranking and the second battery is constantly charged whenever the vehicle is running. Lastly due to the relay coil actually being powered by the primary battery, should you run the primary battery dead the relay will not close. To get started you will have to "jump" yourself. Really all you need to do is just get power from the secondary battery + terminal to the + terminal on the relay. I have done this many times and all it requires is a screwdriver or some other chunk of metal. Just be careful when working with bare metal around battery terminals.

u can use a starter solenoid or two starter solenoids

tvcomputerwiz wrote: u can use a starter solenoid or two starter solenoids

No you cannot, they are not rated for being on continuously. That is why you need a continuous duty solenoid.

is there a way to hook it up both through the ignition (which will come off the primary battery) as well as have a switch off the second battery so I dont have to use a screwdriver if I run the second battery dead? The only problem I can forsee is that it may try to use the smaller guage ignition/switch wires to start the truck. I was thinking about using a diode to isolate them, but wasnt sure if that would work properly. ThanksKenny

Owner / Technician

KKD Garage LLC

Albany, NY 12205

offroadzj wrote: is there a way to hook it up both through the ignition (which will come off the primary battery) as well as have a switch off the second battery so I dont have to use a screwdriver if I run the second battery dead? The only problem I can forsee is that it may try to use the smaller guage ignition/switch wires to start the truck. I was thinking about using a diode to isolate them, but wasnt sure if that would work properly. Thanks

I've never had any trouble with that, technically there should be a surge of power from your shorted screw driver through your ignition harness and on to the dead battery for a split second while the relay closes its contacts. However after the contacts close there should be no damaging amount of current flowing through your ignition wires. If for some reason the relay failed to close due to its failure than yes there is a small possibility that you could burn up your wiring harness. If you are really worried about this you could either use a small gauge wire to do the shorting or even have a fuse connected inline. If I had to guess I would say that there is a surge of around a max of 30A which is well within the current carrying capacities of you ignition system. Also if you are using a small gauge wire to go from the ignition to the relay coil than that wire will easily burn up before you cause any damage to you ignition system. You could and even probably should put a fuse inline between the ignition and the coil on the relay. The closer to where you tie to the ignition wire the better, obviously. For those who care I did a test to see how much current the NAPA solenoid draws to keep closed. I believe it was around 0.75A so not horribly much. Your radio probably draws 5-10 times that through the ignition.