How can I charge my trailer battery, off of my vehicle battery charging system?

[Woods]

Makes sense. My concern is when the trailer battery is low on power. Does the same apply?

Lets lay it out, my truck for instance puts 14.4V out at the alternator when rev'd up (best case scenario, it's 14.1V at idle). Also lets say under 13V getting to your battery and there won't be any motive force to push amps in (probably need a higher number than that but we'll call this the worst case differential). So that is only a 10% voltage drop before nothing is driving amps into the battery, on a 12ga wire at 20' that is about 20amps. In reality you likely need a higher voltage at the battery to actually push some amps, lets say 13.7V instead of a 14.4V alt system. That means you can only have 5% voltage drop before there isn't any force pushing amps in, which on a 20' long 12ga wire is about 10-11amps... If you need 14V then it would only be 6amps when the truck is rev'd and only about 2 amps at idle.

I can't speak for the exact voltage numbers needed to push amps into the battery but you get the point, the system is self limiting. Usually people have the reverse problem in that they're using too small of a wire to keep the voltage up to push the amps they desire.

Make sense?

 

[pods8]

Makes sense. My concern is when the trailer battery is low on power. Does the same apply?

Yes, in the above I was just using examples of differential voltage numbers to make a point and also didn't factor in battery resistance (which increases as a battery is depleted). The link below previously posted by DWH that talks about battery resistance & initial surge concerns. So you've got battery resistance in combination with line losses in your 12ga wire which drops the available voltage at the aux. battery all self limiting your system. http://www.smartgauge.co.uk/nosurge2.html

I don't think you have anything to worry about personally, the reality is you may get less charge into your battery at times than you expect.

Edit: But to be clear you'd still want some sort of isolation/switch between the batteries whether its this product (an automatic charge relay) or something else like a solenoid, diode, charger, etc. So if you're happy with the relay and wiring package by all means go for it, I just don't think the 10amp limiting aspect really comes into play in your scenario much based on wire length/size.

 

[red_5]

battery

this is perfect- am ordering asap - thanks for posting.

Haven't been on here much lately. Glad that was of some help.

I've been using the TOAD charger thingy going on 2 years now on my rig and it's been working great. I've had 0 battery issues. Went 2 weeks without shore power on one trip.

:ylsmoke: