Do I still need a tank circuit if I get a diode battery isolator?

**nobb** · 05-01-2007, 11:55 AM

I am not exactly sure how the diodes in a diode based battery isolator is arranged. I would like to add a second battery using a diode battery isolator. Here is a diagram of the connections of a diode isolator:

http://www.custombatterycables.com/b..._isolators.htm

My carpc and amplifiers will be wired in parallel with the "accessory" battery, and hence when the engine is off, everything will be run off the accessory battery (with the exception of my headunit, but that draws very little current). Now my question is, when I start the car, will the starter motor draw current from both the starter AND accessory battery? Or is current only drawn from the starter battery (if so, I can remove my tank circuit)?

If current will be drawn from the accessory battery during crank, then this would not work for me. The reason is that when I mean my carpc will be connected to the accessory battery in parallel, I mean that my tank circuit is connected in parallel too. This wont work because when I run my carpc off the battery, not only will my deep cycle (accessory) battery be run down, but my tank battery too (a 4aH SLA). I dont want to damage my tank circuit battery.

**Alanh** · 05-09-2007, 11:35 PM

The diode(s) conduct current in one direction only so a load on one battery doesn't discharge the other. Some have a switchable mode that allows current to flow back through the unit if you need to use the second battery to start your engine if the starting battery is discharged.

There are a lot of different units out there. The biggest thing to watch for is the voltage drop across the diodes (or other devices) which reduces the voltage to the battery when charging. Voltage drop means a possibly less than fully charged battery and also means more heat so a bigger heatsink is needed. Every isolator is going to give some drop, but some have a lot less than the approx. 0.7 volt drop of a regular old diode. That may not be a big deal for an accessory battery if you don't need to use the maximum available capacity.

Do your homework before deciding which to buy. There are a lot of units and more things to consider than I can cover or could explain adequately.

**Beehphy** · 05-16-2007, 06:32 PM

If you buy the "The Diode Battery Isolator" at the top of the page, it should do what you want. The car will start from the starting battery only and will not pull the acc. battery down

the alternator will charge both batteries, Starting with the one that is emptiest.

your starting bat will maintain full charge even if you drain the acc. bat. and the acc. bat. wont start your car.