DC/DC Charger for both Tow Vehicle and Trailer

[TheRoadLessTaken]

I'm in the very early planning stages of a build - Tow vehicle will have a 100ah LifePo and the trailer will have a larger LifePo battery bank.

Is it possible to use a single DC/DC charger to charge both sets of LifePo batteries? My thoughts were to to wire the DC/DC charger (30A) output to a 40A 14VDC relay - wire the NC side to the tow vehicle LifePo and the NO side to the trailer LifePo and use a simple on/off switch to control the relay... or some other way to accomplish this

If it is possible, would it be advisable to get the isolated version of the DC/DC charger? I'm guessing when the trailer is hitched they would share a common ground (assuming both grounds ultimately make it to the frame) - but I might also want to charge the trailer when it's not hitched (ie parked side by side).

And in line w/ the immortal words of Jeff Goldblum in Jurassic Park; " YEAH, BUT YOUR SCIENTISTS WERE SO PREOCCUPIED WITH WHETHER OR NOT THEY COULD THAT THEY DIDN’T STOP TO THINK IF THEY SHOULD "... If this is possible, are there any reasons I still shouldn't?

Thanks in advance

 

[4000lbsOfGoat]

Sure, anything is *possible*

What is the 100ah battery in the tow vehicle going to be used for? What size is the bank in the trailer?

but I might also want to charge the trailer when it's not hitched (ie parked side by side)

You want to run the truck to charge the trailer batteries while it's parked? That's unusual...

 

[jonyjoe101]

It should work, I have 2 battery banks in my van a 220ah lifep04 and a 312ah li-ion battery. I use a 5 pin 12 volt automotive relay to switch from the main (NC) lifepo4 to the aux (NO) li-ion. The switch between battery banks happens quick enough that it doesnt affect the charger as it changes from different voltages. The charger I'm using is a solar controller which are very sensitive to losing power and need to be reset, but the relay switches fast enough that in 2 years never had problems, but there are ways to compensate if the switch isnt fast enough for your charger by using a small capaciter.

 

[TheRoadLessTaken]

Sure, anything is *possible*

What is the 100ah battery in the tow vehicle going to be used for? What size is the bank in the trailer?


You want to run the truck to charge the trailer batteries while it's parked? That's unusual...

The 100ah tow vehicle battery will be running lights, comms, etc within the cab. Would also be utilized when camping without the trailer on harder trails.

The size of the trailer battery bank has yet to be determined but would be of the same battery chemistry and voltage. And my loose understanding is that you can still get some charge to the batteries at idle - the question was mostly geared towards the grounding and if I needed to be hitched up.

 

[TheRoadLessTaken]

It should work, I have 2 battery banks in my van a 220ah lifep04 and a 312ah li-ion battery. I use a 5 pin 12 volt automotive relay to switch from the main (NC) lifepo4 to the aux (NO) li-ion. The switch between battery banks happens quick enough that it doesnt affect the charger as it changes from different voltages. The charger I'm using is a solar controller which are very sensitive to losing power and need to be reset, but the relay switches fast enough that in 2 years never had problems, but there are ways to compensate if the switch isnt fast enough for your charger by using a small capaciter.

I appreciate the response JonyJoe - I was hoping to get some responses about how a charger might react when it switches

 

[emulous74]

Redarc makes a low powered dc/dc trailer charger I believe it's 12A, but hooks into the 7 pin trailering wire. I would suggest running two in that if you want to solar or shore charge your trailer when it's not connected to your tow vehicle the second dc/dc charger would make all of that easy.

 

[4000lbsOfGoat]

Redarc makes a low powered dc/dc trailer charger I believe it's 12A, but hooks into the 7 pin trailering wire. I would suggest running two in that if you want to solar or shore charge your trailer when it's not connected to your tow vehicle the second dc/dc charger would make all of that easy.

Yes, the Redarc BCDC 1212 runs right through the standard 7 pin and works really well on a pair of 100AH batteries. Really simple and no fuss.

However, it does not work as a shore charger or solar charge controller. You still need separate devices for those applications.

 

[TheRoadLessTaken]

Redarc makes a low powered dc/dc trailer charger I believe it's 12A, but hooks into the 7 pin trailering wire. I would suggest running two in that if you want to solar or shore charge your trailer when it's not connected to your tow vehicle the second dc/dc charger would make all of that easy.

Throwing a charger in the trailer is definitely on the menu if the single charger plan can't/shouldn't be done. Just trying to minimize components/failure points where I can.

 

[TheRoadLessTaken]

Unless their cost is substantially different, I dont see much reason to favor one or the other.
But one scenario for non isolated involves the negative connection for trailer marker and tail lamps.
If a failure of the charging negative conductor occurs, charging current will try to flow thru the lighting negative conductor. That conductor/connector probably wont carry 30+ amps very long.
(In real life some current would flow thru the drawbar. But that current path is intermittent at best. Should never be included.)
In this case, best scenario is eliminate lighting negative conductor to rely on the charging negative conductor to carry all negative current from the trailer.
But..
Eliminating the lighting negative as described means lighting wont work if the trailer is towed by a non charger equipped car.

The rough price difference is around $60 on a quick search (more for isolated).

And I appreciate the feedback on grounding regarding the hitch connection - I wasn't too sure if you could treat that metal contact as joining both the vehicle and trailer into one common ground. So it sounds like; to be safe; I should go isolated (and it can't hurt as far as I can figure).