Brake battery charged with house battery

jim65wagon

TundraBird1
Our trailer chassis came with electric brakes and the little control box that operates the breakaway safety switch through a small 12v battery. Would there be any issues with just tying the house batteries for the teardrop into the same charge line? Or should the small battery be deleted and just use the house batteries to operate the breakaway switch?
 

elmo_4_vt

Explorer
Not sure if it's correct, but I plan on running the break-away switch from the house battery on mine. If you can isolate the two batteries in the charging circuit, like say from two separate shore power 120VAC chargers, it would be fine, but I don't want to hook them up in parallel in any way because the resistances of the two batteries will certainly be different, and will then cause the batteries to discharge each other slowly. That's my plan anyway. I couldn't think of a down side to using the house battery other than possibly running the house battery down while camping, but once you hook back up to the truck, you'd be putting at least some charge back on it that should be enough to operate the brakes in the even of a disconnect.

On my other trailers, I just hook up the break away battery once or twice a year to a battery charger when it's convenient to make sure their charged and I've never seen the voltage go under 11VDC, so there is not parasitic draw on them even when their mostly used on trucks with no charging circuit.

On another note, the magnets in the brakes have a pretty good resistance, so you can pull the break away switch for short periods if you need to have a the tires locked up (like if you're disconnected on a hill) and not run a house battery down, just make sure that you don't have it connected to the tow vehicle if you do... My prodigy brake controller manual said that back feeding any type of voltage into the controller will cause a short condition and smoke it.

Don

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Redline

Likes to Drive and Ride
The break-away set-up on my old (1978) travel trailer does not have a separate battery to power it. I don't know if this is how most big RV trailers are set-up these days, I would think so as two battery systems would cost more, but a trip to a local RV lot should clarify what's 'standard' these days.
 

dwh

Tail-End Charlie
but I don't want to hook them up in parallel in any way because the resistances of the two batteries will certainly be different, and will then cause the batteries to discharge each other slowly.

Not exactly. It will cause whichever battery has a lower voltage to drain the battery with the higher voltage - until the voltage of both batteries is equal. Once the batteries achieve equal voltage (resistance), then no current flows either way.

If one battery has a higher self-discharge rate, then as it self-discharges, current will flow from the other battery to keep them equal. So, they will ultimately both drain together - but they don't "drain each other".

The main problem with tying a larger and smaller battery into a full-time battery bank, is that one of them is going to do more work - both charging and discharging - than the other, and will wear out sooner.


There is no problem tying different size/type/whatever batteries while they are being charged with a constant voltage type charger (such an an automotive/marine alternator/voltage regulator setup). I'd say that might be a good place for a lowbuck diode-type battery isolator. If the brake battery is always half a volt low - well, so what?

EDIT: On second thought - no, not a good idea. If the house bank ever gets charged with a multi-stage charger, then the diode isolator is going to transfer that to the little brake battery, and it'll get boiled.

Probably have to rig a split-charge relay instead, but the problem would be switching it on only when the ignition is on...
 
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orangeTJ

Explorer
The break-away set-up on my old (1978) travel trailer does not have a separate battery to power it. I don't know if this is how most big RV trailers are set-up these days, .

Hi, it's correct - RVs (towables) tie the breakaway device to the main house battery bank, there is no second battery dedicated for the breakaway device.

I'd just delete the breakaway battery and tie the breakaway switch to the main battery.
 

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