Duel battery charging

Well considering the current that would flow between the two batteries on charging alone, that would have to be some BIG SUCKER of a diode

Sounds easy doesn't it? Not so. Diodes have a voltage drop through them which increases with increased current flow. If your alternator is putting out something like 14.2volts, The diodes will drop this to approx 13.2v and your batteries will not be charged. The only way to counter this is to modify your alternator to put out an increased voltage to compensate i.e. 15.2volts. It can be done.
Ian

In theory this will work ok !

General Purpose & Rectifier diodes have a 0.7v voltage drop.

Germanium diodes have 0.2v voltage drop.

Not sure what will happen if you discharge the second battery, maybe the primary will still feed power into the second battery? The diode will only stop the second battery from discharging into the primary battery.

Have a look at www.surepower.com. It looks like their 'isolator' (as opposed to their 'seperator' - which I have incidentally) works like this.

I have no formal auto electrical training, but my only guess at a downside would be the batteries are independent when charging and discharging? So both batteries would charge at their own rate by taking what they need from the alternator? The main starting battery would not get any priority. Whether I'm accurate or not, I really don't know - and I don't know if that would have any real world issues either.

Pete

If you want to get techo try this site. It explains everything much better than I can.
www.smartgauge.co.uk/facts.html and click on the truth link.
Ian

Build one youself check out
www.oatleyelectronics.com $19.00 plus P&H about 3hours to build
Works Great

Sorry Steve

Kit No. K227

I used one of these successfully on my old Mav (GQ). Used a pair of 100A stud diodes (from the spares box) with a suitable heat sink. Worked well.
It's true that you will drop up to 1 volt across the diodes, but the Nissan/mav alternator monitors its battery voltage right at the battery, so any volts dropped across the diode is made up for in the regulator, hence it charged the battery at the correct voltage. QED.
Now, as suggested above, if the main battery is fully charged it will only slowly charge the aux battery; this is not really the case, as the discharged aux battery will pull the volts from the alternator down further, bringing the main battery volts down with it. Hence the alternator will correct the voltage and feed more to the aux battery - think about it!
BUT, another problem is the commutation noise caused by the diodes (it's not pure DC, but pulsed of current from the alternator). In my case, the diode combiner generated some radio interference, which I fixed with a 0.1uf capacitor across each of the diodes.
If you're making your own combiner, cheap diodes are the 50A press-fit diodes as used in alternators. But you'll need a big enough heat sink to take the heat away from the diodes. 1v at 50A is 50watts dissipation. Another consideration is that if you fix the diodes directly to the heat sink, then you will need to electrically isolate the heat sink from the vehicle body.
You can get high current Shottky diodes which have much lower volt drop (about 0.2-0.4v at the rated current). A possible source of these cheaply is from an old computer power supply, where they are used for the +5v 30A output. These diodes are capable of about 30A continuous, tho on their own are a bit light-on.

For a switching type of combiner, Oatley electronics have a neat kit which is probably a better proposition link if you want a relay-type battery combiner. Costs $19, and uses a latching 80A relay.
Gerry

Steve,

Steve, TRYING AGAIN, I had such a diode unit. It was a Hella unit that had bolt type connections on a sealed aluminium heat sink case. If you checked around Hella may still supply them. From memory they handled about 60amps per battery. I had it installed in my Triton but as mentioned above you loose 0.7volts to the batteries. This was not too much of an issue day to day driving until you camped for a weekend and the sub battery didnt last as long as expected as it wasn't getting full charge. I removed it and installed a solenoid for about $60. I put a timer on the solenoid of about 45 seconds and that allowed the starting of the car without have the chance of the solenoid switching on while you where starting as this could put a few hundred amps thru a 60 amp rated solenoid. You could do without the timer because if you connect the activate wire to the ignition correctly , it should switch off when starting the car. There now a very noticable difference in the life of the battery when camping.