Inverter low voltage high current 5-12v in 13.8v out 30 amps

[corsymons]

I am looking for a design to achieve the following. I have an off road camping trailer with a second deep cycle battery installed. The second battery never reaches full charge because of the voltage drop from the alternator in front to the battery in the rear of trailer To charge the second battery in the trailer to its full capacity from the tow vehicle I use a 12v to 220v inverter and then a 12v battery charger. Very inefficient and bulky. Where can I find information to assist in designing a 5-12volt(dc) input to 13.8v(dc) output voltage booster/inverter that can deliver 30 amps.

 

[vk6kro]

If you take the worst case, convering 5 volts to 13.8 volts at 30 amps would require an input current of 82.8 amps even if you could do it at 100 % efficiency

30 amps * 13.8 volts / 5 volts = 82.8 amps..

Realistically, it would probably take 100 amps at 5 volts.

However, you probably don't really need 30 amps.
How about 6 amps?
This would charge a 40 amp hour battery in about 7 hours, so it would be ready to use after a day of driving.
Also the 5 volt figure is a bit optimistic. That would be a very dead 12 volt battery. How about 10 volts?

There are booster circuits available on Internet which could probably do this.
Here is one that can convert 10 volts up to 12 volts to 32 volts

http://www.ebay.com.au/itm/DC-10-32V-12-35V-Step-up-Boost-Power-Supply-Module-/120757538824?pt=AU_B_I_Electrical_Test_Equipment&hash=item1c1db5d008

As they say on the above site, though:
But please be reminded that the output voltage cannot be lower than input voltage.

There are probably plenty of them if you would like to do a search.

 

[pantaz]

How much voltage drop are you experiencing? It really shouldn't be an issue with proper size wiring.
Are you using a battery isolator between the trailer and the tow vehicle?