Outer Limits 4x4 Board

i have a ctek dc to dc dual battery controller to charge my auxiliary battery and i would like to add a third battery and potentially run my accessories at 24v so that my fridge and led lights last longer when camping..

so heres my idea. obviously i have my main battery and when the motor runs the alternator kicks in which starts up the charger charging the auxiliary battery, but i would like to add a third battery under the tray linked to my auxiliary battery through two changeover relays so that when the charger kicks in the batteries are in parallel at 12vdc, but as soon as the charger stops the relays switch to running the batteries in series at 24vdc and i was hoping to use the 12v output cable from the charger/battery controller as the trip cable, obviously the problem i can see is that i would need to stop the voltage from reaching the relays when the charger has stopped running, hence the diode?

am i stupid? am i missing something? apart from the diode, i think it should all work?

thanks in advance for your time

You don't need to run at 24 volt to extent battery life, just run two 12 volt batteries in parallel on your auxiliary system. Charging times will double though, so unless you do a lot of driving between camp spots you might have issues

oldmate wrote:You don't need to run at 24 volt to extent battery life, just run two 12 volt batteries in parallel on your auxiliary system. Charging times will double though, so unless you do a lot of driving between camp spots you might have issues

sorry.. correct me if i'm wrong


but the way i thought it worked was by having my two batteries in series i double the voltage.. which means my accessories run at half the amperage they used to and (this part i'm not too sure on) i thought that by having the two batteries in series i would be doubling my AH capacity of the battery bank effectively making my fridge run 4 times as long?

have i gone wrong somewhere in there?

cheers

andrew91 wrote:
but the way i thought it worked was by having my two batteries in series i double the voltage.. which means my accessories run at half the amperage they used to and (this part i'm not too sure on) i thought that by having the two batteries in series i would be doubling my AH capacity of the battery bank effectively making my fridge run 4 times as long?

You're half right., the current through the lights will be half at 24volts, compared to 12volts.

However assuming the fridge is 12/24volt compatible, the power output of the motor will not change. So in order to maintain the same power, the resistance of the motor must be higher for a 24 volt system. When that is the case the current at the batteries in a 24 volt setup is the same as it would be with a 12 volt system, and you get the same amount of life out of the batteries.

If you double the voltage, you could fry some components in your caravan eg any resistance type (as opposed to LED) lights. Also if you have a 3 way fridge (240V/LPG/12V), the 12V element is a resistor and hence it may be cooked. They are not cheap. How do you get 24V from a 12 V source anyway, other than via a Redarc or similar DC - DC charger. You are better off setting your batteries in parallel.

It could be done, I see your point. BUT I'd think it would be more trouble than it was worth, all you need it for one sticky relay and you pump 24v straight back into your 12v system and melt everything leaving you stranded in bumfuckidaho. More trouble than its worth.

Another vote for paralell.