Diode selection to stop multiple battery back feed.

On a boat with a starting battery and an accessory battery. During starting the accessory battery need to keep the accessories running, and when not starting, the starting battery and accessory batteries needs to be connected to allow charging and power for accessories. I would like to place a diode between batteries to facilitate this. What diode should I use? View attachment battery.bmp

you need a diode rated for the max current and say 100V well heatsunk, my company actually sells these to industry for similar reasons.

There are battery isolators on the market as you probably already know that are specifically for this. But if you can do the same thing with a simple diode why waste the money right. Wish I'd thought of that back when I had my boats.

Hi,

The problem with diodes is*
* voltage drop...causing the slave battery not to fully charge
* the waste of power --> generated heat.

The diode solution will work.
But an "ideal diode" circuit with current limit feature is the better solution.

Klaus

Due to the low voltages and high currents involved, at a minimum a Schottky-type diode should be employed.

But a far better solution is what Klaus proposed: an "ideal diode", which is a Mosfet driven by a sensing IC.

Linear Technologies among others, sells those ICs.

https://www.linear.com/products/PowerPath_Controllers_*_Ideal_Diodes

The reason for the isolation is so your accessories don't drain your starter battery. I'm thinking the diode should be facing the other direction. Maybe place your alternator wires on accessory battery and current would flow to starter battery to keep it built up. Also in the off chance your alternator develops a short it won't drain your starter battery. Not and expert by any means but that's the way it appears to me.

A real diode is far more rugged to overcurrent events than a driven mosfet, hence why they are used in industry, easily get 2kA capability in a real diode.

I may have misunderstood what you are trying to accomplish. Is the problem that when you crank the engine it causes some accessories to shutoff? In this case my idea wouldn't work for you. What I suggested is to give you a little piece of mind that you don't drain both your batteries. No worse feeling when there some bad weather approaching and the motor won't start. I've worked on the water a long time and have been in some hairy situations before. I was on anchor one morning and a huge waterspout dropped down about a mile away and was heading straight towards me. The float on my bilge pump had hung up and drained my battery overnight. It passed about 100 yards off my stern luckily and not a direct hit. After that I installed another battery just for starting the engine and powering my vhf and gps. I also installed a perko battery selector switch so I could start from the other battery if need be. Maybe you don't venture out into open water so it may not be of much concern to you. But always remember Murphy's law.

BTW: waterspouts don't always come in ones. I have a picture I took in chandelier sound. There were 4 in a straight line about 1 mile apart. Pretty awesome site.