Hi, I've done a couple of searches and read a number of Q&As on adding a battery for trailer power (and how to hook it up to charge off the vehicle, etc.). We recently got a new trailer and from what I have determined so far, the break-away battery is getting charged off the Vehicle's 12v system. The battery has a little charge status indicator on it and it shows "charging" whenever we're hooked up and the motor is running (tow vehicle is an Expedition).

So what I'm not clear on, is that if the folks that are adding in a large deep cycle battery for their trailer, are they replacing the little break-away battery? Otherwise, wouldn't the larger one keep trying to equalize with the smaller one, or worse, when the larger battery was drawn down from use while you have your trailer unhooked, the little guy would be trying to equalize the bigger battery upwards??

Can someone help this shade-tree electrician figure out how to add the big battery and have it charge (from the vehicle), but not hurt the break-away battery?

Thanks in advance...

 

There is no perfect way to do this, as your assumptions above are correct.
If you don't want to use the large deepcycle battery for all trailer functions, you should use a battery isolator to charge the breakaway battery to prevent the equalization problems. However, isolators have there own drawbacks. They typically have up to a 1 Volt drop across them, and won't fully charge the batteries.
There will be charging current and the batteries will function and charge, just not fully charge. A battery that is not fully charged will not only have a little less capcity, but will also have a shorter service life.
If you go with the isolator, fully charging the batteries with a battery charger after each use will help a lot.

 

That's right. I would just use the larger battery for all trailer functions.

 

Thanks, Ultraute and Bdox! I found that the break away box is called "engager" which I think is made by TAP. From what I can tell so far, they only offered one with the status lights like mine, and it had a charger inside. So my next question is: is a 12V to 12V "charger" really just an isolater? In which case I can add my second large deep cycle battery and all will be cool (I'd add it upstream of the charger so it "floats" on the vehicle's charge system)... I need to get some wirecutters and cut the zip ties off the "engager" box, to see if I can find something that looks like a charger inside...

Keep the critique/comments coming...

 

My bet is that it's just a diode/heatsink assembly so that the vehicle can charge the small battery but cannot draw from it. So if I'm right, yes, it is an isolator which would serve to keep any load on your new battery from drawing down the small battery.

 

Take the input charge wire off the breakaway box and see if you have 12 volts coming out of the breakaway box. I bet you do not, and that would confirm the breakaway box will not discharge backwards into your new larger trailer battery.

 

You guys are awesome -- thanks!!! I'll check to see if that's an isolator later this morning...

 

On second thought.... I don't even have to take my VOM down to check -- the lights in the trailer would work if the break-away battery was riding on the 12V system - right?! (I probably should'a thought of this myself..)

 

I guess you are talking about the interior lights in the trailer(for some reason I have open car trailer on the brain when you said trailer). If so, yes that's correct. Disconnect the plug to the truck, and your new aux battery, and see if the breakaway box will power the interior lighting in the trailer.

 

I'd sure like to understand this system a little better. I take it that the small battery is there to engage the trailer brakes in the event that the rig "breaks away?" Is that right?
If so there must be some kind of circuitry in that box to sense the situation.

How do it know? Does it sense motion?

Does it handle all braking functions or just the break-away function? Does it have an on/off switch?

 

Franklin2: Yes -- sorry I should have been clearer, it's a horse trailer with lights inside as well as flood lights outside for night-time loading/unloading the horses. We also camp and there are times you need to grab something from the trailer... So I checked and the lights don't work when disconnected.

Bdox: The break-away has a separate switch that is hooked via cable to the truck's bumper. If the hitch fails, the cable pulls the switch and the 12V from the small break-away battery is sent directly to all 4 brakes... The charge wire to the battery and the wire from the battery to the brake-away switch are two different wires...

 

The breakaway box is now required by law in my state. The first thought that comes to mind is more government people sticking their nose in and making more rules and more expense, but I guy I know just the other day had his trailer come loose for no explainable reason. Everything turned out ok, but the breakaway box would have kept the trailer from hitting the truck when the chains took up the slack.

One thing a guy told us after it happened that makes good sense. He always crosses his safety chains in an "x" underneath the front of the trailer tongue. He said if the trailer comes loose, it will fall down and catch in the "x" of the chains instead of hitting and digging into the ground.

This guy's tongue didn't hit the ground, but the trailer jack did. I could not believe it didn't bend the jack, but it did wear the bottom off till he got it stopped.

 

Yeah, they're required in CA too. In fact, this new trailer has mamoth chains and hooks with spring clips, something my trailer guy said is also now a state requirement.

The X cross is good advice. Been doing that ever since we ran a race car in the mid '90s and a fellow racer clued us into it -- makes a lot of sense. The only failure I've had on a trailer hook-up is the 2 rear bolts sheared on a 4 bolt frame mount receiver hitch -- luckily it caught on the spare tire and held there. Another reason to hook your break-away cable to your bumper and not the hitch... (of course I only use 6 bolt or more frame mounts now too!).

Thanks again for all your help on this....

 

Took me awhile to get the second battery installed -- turned out it was just the timing for my soldering gun to give up the ghost! Anyway.... As soon as I connected the final ground wire on the new deep cycle, the "charging" light comes on, on the break-away's status lights. It surprised me, but probably shouldn't have, since having the second battery is the same as having the Expy running and a charge current coming back to the trailer.

Is there anything else I chould be concerned with? I assume the break-away will just float on the deep cycle battery all the time now, and that's ok? (I could add a switch, but that means the wife has to remember to turn it on...)

Thanks as usual...

 

You could put a diode in the line from the Expy to the new battery to eliminate the feedback.