I have cheap solar charge controller left over from another project, and a deep cycle marine battery. Is there any reason I couldn't / shouldn't run an ignition switched feed to the input side of the controller to give me a simple dual battery set up for my rig? The controller doesn't care where the input comes from as long as it is with in operating specs right? It seems to easy so I figure I must be missing something.
Dumb Idea?
- Thread starter Ferball
- Start date
Peter_n_Margaret
Adventurer
Run the ignition switched feed straight to the aux battery. It is the right voltage already.
Cheers,
Peter
OKA196 motorhome
Cheers,
Peter
OKA196 motorhome
Thanks for the info. I figured the charge controller would limit the current to the aux battery, but I was alright with that. Had not considered the current protection issue. Thanks.
The theory makes sense, but why does every one use isolators and all the other fancy hardware?
Peter_n_Margaret
Adventurer
Everyone doesn't.
I use a solenoid and charge the house batteries direct from the alternator. That way I can slam a lot of power into the aux batteries very fast and let the solar top them off.
Cheap, efficient and effective.
If the cable run from the crank to the aux is very long and the voltage drop is excessive, then there is an argument for a DC-DC charge system to boost the voltage, but otherwise nothing is required.
Cheers,
Peter
OKA196 motorhome
Martinjmpr
Wiffleball Batter
Well, the purpose of the isolator is to prevent your aux battery from drawing your starter battery down to the point where it will no longer start your vehicle.
If you're talking about running something directly from the starter battery or alternator to an aux battery, you have to consider how many amps are potentially being put through that route and how long the cable is going to be because that will determine what gauge of wire you'll need to run. Putting too many amps through a wire that's too small is asking for a fire.
That was kind of my reasoning for using the charge controller my thought was it would limit the amp draw to keep me out of trouble. I don't draw a lot of power off my house battery just a few lights and charging small electronics, I just want to keep it topped off, so I don't need a heavy duty charging system.
Martinjmpr
Wiffleball Batter
But you'd still go directly to the battery?
I don't know enough about charge controllers to be sure but I know most charge controllers have a specific amp rating, 20A or 30A or so on. As long as your wiring is heavy enough to account for the maximum amp draw I'd guess you'd be OK but I'll let others chime in to be sure.
Now that I think about it, I've connected other relatively low-draw items directly to the battery before (off road lights and a HAM radio, specifically) and I don't recall that I had any issues with them. Obviously you'd want it fused on the battery end at whatever the maximum amp rating of the controller. My only question is whether, for example, a 20A controller will draw a constant 20A from a source like a battery.
Now that I think about it, what you are doing is basically using a solar controller as a DC-DC charger.
Martinjmpr
Wiffleball Batter
What are the specs of the solar controller you want to use? We could then look it up and see what its limitations were.
It doesn't really work like this. The vast majority of cheaper PWM charge controllers don't measure OR control current. They control based on voltage. When the charge controller is in bulk mode, the 'switch' in side the controller is fully on and the controller acts like a wire. Once they reach absorb and float, the controller starts controlling the voltage to keep it at the absorb and float voltages.
What will most likely happen is that if your house battery is discharged, when you start the car the charge controller will turn fully on (bulk mode) a bunch of current will flow through the charge controller and you will burn out the mosfet switch inside. Alternatively, if you battery is mostly charged, the alternator voltage will not be enough above the battery voltage for the charge controller to turn on, and nothing will happen. Final scenario is that you will discover that your charge controller is 'positive ground' and actually PWMs the negative side, and nothing at will happen, not matter what.
You can use a solar charge controller as a DC-DC charger, but you need to use specific MPPT charge controllers and add a boost converter to boost the input to the charge controller up to > 17V or so. I do this with my Victron MPPT to charge a lithium battery.
What will most likely happen is that if your house battery is discharged, when you start the car the charge controller will turn fully on (bulk mode) a bunch of current will flow through the charge controller and you will burn out the mosfet switch inside. Alternatively, if you battery is mostly charged, the alternator voltage will not be enough above the battery voltage for the charge controller to turn on, and nothing will happen. Final scenario is that you will discover that your charge controller is 'positive ground' and actually PWMs the negative side, and nothing at will happen, not matter what.
You can use a solar charge controller as a DC-DC charger, but you need to use specific MPPT charge controllers and add a boost converter to boost the input to the charge controller up to > 17V or so. I do this with my Victron MPPT to charge a lithium battery.