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Old 01-16-2015, 02:32 PM   #1
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1979 31' Excella 500
Charlevoix , Michigan
Join Date: Aug 2014
Posts: 340
Charging AS from TV - Inline Battery Charger to boost Voltage Drop?

As I get deeper into the renovation of my '79 Excella 500, I keep coming up with more electrical questions. Fortunately these forums seem to be chalk full of people who either made the same mistakes I'm about to make myself, or managed to avoid them in the first place. I'm hoping to fall into the ladder category by asking this question... Here's the background.

I want to be able to charge my AS batteries (4 x 6v in series-parallel) from my Tow Vehicle (TV) while I'm driving. My batteries are by the axles in the AS, making them about 35 feet from the TV alternator. My guess is that unless I run some really thick wire (4/0 AWG?) the voltage drop from the TV alternator to the battery bank would be too low to actually charge the batteries to full (although even with the voltage drop, the existing 12-14 AWG wiring could probably get the batteries to 75%). My assumption seems to be backed-up by others who have tried the same thing (see this thread over at

In order to boost the voltage from the TV to charge the AS's batteries to 100%, I am thinking about using a "12v to 12v" or "in vehicle" battery charger (see the PST-BC1212-15 or the Promariner Digital Mobile Charge40 or the Redarc BCDC1240).

My idea is to put one of these in my AS connected between the TV 12v (input) and battery bank (to which the charger would output) which I think will allow me to charge the AS batteries to 100% while driving. Having a unit like this installed would also take care of the issue that a direct connection from the TV alternator to the AS batteries could OVERCHARGE the batteries since the alternator wouldn't stop sending power once the batteries are full (SIDE NOTE: if you've Google'd to find this post and are looking for a simple way to cut tow vehicle power to your camper trailer when the camper's batteries are full, Precision Circuits "Battery Isolation Managers", aka BIMs, will do the trick).

So, here are my questions...
  1. Has anyone done this before? If so, I'd love to hear about an issues you ran into so I can avoid them
  2. Since I already have an Inverter/Charger combo unit in the AS, do I need to have a "charging source" switch to choose between the Inverter/Charger and this "12v to 12v" charger (although I can not imagine a scenario in which both would be on at the same time)?
Any guidance would be appreciated; thank you!

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Old 01-17-2015, 03:53 AM   #2
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2011 34' Classic
Westchester Cty.NY , / Miami FL
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this thread by jammer is what you're looking for.

you might also consider a solar setup lewster has many threads posted suggesting just using the trailer solar panels to charge the batteries and not use the umbilical line at all.

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Old 01-20-2015, 12:20 PM   #3
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1979 31' Excella 500
Charlevoix , Michigan
Join Date: Aug 2014
Posts: 340
I wanted to let everyone know what I eventually decided to do in case someone Google's this topic and arrives at this post.

I spoke with multiple battery charger manufactures and electrical supply companies to find out that there are only really a few businesses that make DC to DC chargers that would work in my scenario. The 3 I mentioned in my 1st post seem to be the most popular, although there are probably more if you dig deeper into the Marine and Boating industries (they are used to charge "house" batteries from separately wired "starter" batteries).

By and large the "right" way to do this seems to be to just run a 12v wire from the Tow Vehicle to the AS battery bank DIRECTLY (without any sort of in-line charger) that is sufficient gauge (roughly 10AWG in my case) so you don't have significant voltage drop. Running the charge wire directly from the TV to the AS puts the onus on the vehicle's alternator to detect the load on the entire 12v system (which now includes the AS) and supply charging voltage accordingly.

In my case, I was worried about the alternator over-charging the AS batteries (a problem I thought a 4 stage DC to DC charger would rectify) however from my understanding over-charging would only happen if the alternator was defective (bad voltage regulator).

Another issue I was worried about was that the AS batteries would never fully charge from the TV due to the voltage drop from the TV to the AS. This is a problem rectified by using the proper gauge wire: thicker wire = less voltage drop = ability to fully charge AS batteries from TV. In my case, I want to use the existing 12v wiring on the pigtail from the TV, which is probably 10 AWG, and then use 10 AWG wiring in the camper to charge the battery bank. Depending on how many AMPs the TV's alternator is producing I can expect some voltage drop (maybe 3-6%) but hopefully not enough that it wouldn't charge the AS batteries; still not sure if they'll ever become fully charged in my scenario...

Airstream nor any of the other fifth-wheel or camper manufactures I researched use any sort of DC to DC charger in-line between the AS and the TV, which leads me to think that some engineer somewhere already figured this out and went the K.I.S.S. route on this one.

Thanks to all those that helped me arrive at my solution: no DC to DC charger needed, just run thicker wires and let the TV alternator do it's job.
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