Current limiter

[hantto]

I'm setting up an aditional battery in my car for all the misc systems that i have set up. I don't want to use the main battery, because then I would run the risc to deplete it and not be able to start my vechile again (as i like to listen to music while the engine is off among other things). I have inserted a relay between the batteries in order to keep them from equalizing their charge level when the engine is off. The relay will be closed when the engine is running and open when the engine is off. Now when you have the basic idea of my setup (se picture), let's look at my questions.

To keep the main battery from draining to the accessory battery when the relay is closed (engine on) I tought that it might be a good idea to put a current limiter between the batteries so that the accessory battery wouldn't steal all the charge current available. Is this nessesary? or is the alternor capable of charging both batteries, even if their voltage levels are different? Is a limiter needed?

A charging current of about 10-20A to the accessory battery would be suitable (it's a 60AH battery) to charge the battery during driving. Is the charging current too high (in aspect to the battery's life length and alternor's poweroutput capabilites?)? With what current are the batteries in cars nominally charged?

And here comes another problem: HOW would I limit the current without loads of powerloss. I calculated about 50W of dissipation with a simple resistor current limiter. Is there any suitable circuits out there? recommend one?

All this to keep the main battery as charged as possible the whole time.

Thank you for your time.

 

[hantto]

I got an idea, is it possible?

To have a circuit to monitor the voltage level at tha main battery (opamp), if the voltage is over, say, 14V the relay would be on, otherwise off. If the accessory battery draws the voltage down under 14V it would immediately turn off the realy, and the voltage would again rise to over 14 and then energize the relay again, and carry on like this in a loop. Wouldn't this be a sort of current limiting and watching that the main battery doesn't get discharged into the accessory battery. And at the same time this would be engine on/off indicator because the voltage only goes to 14V then the engine is on.

Is this cind of pulsing charge method unhealthy to the battery?

And it would be a good idea to but a diode in between and change the relay to a high current fet?

 

[gerty]

You should be ok with just the relay..The charging system should be able to keep up, providing you don't kill both batteries. But that's where the relay isolation keeps that from happening.. We have 2 Rescue Squad truck that have electric winches on them and are wired with a relay as you describe and work fine.. 8)

 

[hantto]

Thanks for the info!

Does anyone else have an oppinion about this?

 

[Oznog]

I prefer the solid state diode battery isolator myself.

The relay alone should work fine.

Sadly, there are plenty of alternators out there that are not well suited for charging a partially discharged batteries. They'll have the capacity to charge it while driving around, but may provide little more than the car needs to run when turned at idle RPM. More of a problem is they're not very reliable when used for delivering high currents. Case in point, Motorcraft 2G alternators used in older Fords. They didn't even put an output stud bolt on it, just used a plastic housing with 3 flag connectors. Yes you plug it in. They can, and will, physically melt, if the crappy internals inside the alternator don't fall apart first.

If you've got a 2G, Motorcraft later made the 3G which is only slightly larger and has exceptional performance (160 amps, I believe, and 80 at idle alone) and reliability. Slight modifications to the bracket may be necessary to make room for the larger body diameter. Mounting holes are the same.

 

[chemelec]

Just a Relay is Fine, That all I use. Your second battery should be a Deep Cycle type. They stand up better for deep discharges.
If you want to slightly reduce the Full Charge to the second battery, Put a Heavy Duty Diode (50 or 60 Amp @ 50 Volts) in the Charge lead.

Some Dual charge systems, just use Two Diodes, One to each battery from the Alternator. This Stops discharge from one battery to the other.