Charging an aux battery in a trailer?

**drivesafe** · 25th June 2007, 08:13 AM

Originally Posted by Teks

The easiest solution is to use Pin 7 on a standard trailer socket as a charge output - assuming you have the common type round 7 pin type socket (or use a an unused pin in the flat type socket). When wiring up the back of pin 7 on the socket use the thickest cable you can lay your hands on (ditto to the trailer battery) and wire the other end to the start battery. This way the trailer battery will get a charge when you drive. It would be prudent to add a switch in the cable as a simple isolator to prevent current backflow when you turn the engine off or otherwise if you are handy with tools, use a relay for automatic switching via the ignition.

Any other way (i.e. using electronic isolators) will cost a lot more and despite the manufacturer's claims, suffer from an insertion voltage drop (0.5-1.0V) which when added to the voltage drop of the cable (>0.5V) will more than likely prevent the trailer battery from getting a full charge.
Total voltage drop between charge source and load using electronic isolators (alternator-battery) could well be around 1.0-1.5V which when deducted from the maximum charge voltage of say 14.6 leaves you with 13.1-13.6V. The battery will need at least 13.8V to charge but upwards of 14.5-14.65V would be better. If you or any reader of this post discovers a voltage of less than 13.8V at the trailer battery, all is not lost as this can be overcome by use of “black box” electronics in the form of a DC-DC Boost converter.

If you need more info, PM me.

Hi Teks, I think your getting mixed up with older type Diode Separators as there is a fair voltage drop through them but I use relays in my dual battery controllers and there is next to no voltage drop through these, certainly not enough the measure.

Although correct about the 13.8 volts being the minimum voltage required to charge a battery properly, the analogy that vehicles produce voltages upwards of 14.5 or more is a long way from the average vehicle voltage.

Most vehicles have an operating voltage of 14 to 14.2 volts, a few D2as, RRSs and D3s have higher voltages but even my MkIII RR has a maximum voltage of only 14.3 volts, so planning on getting anything more than 14.2 volts from most vehicles is just not going to happen.

As for these DC to DC converters, if you use these devices you must know and understand how they are going to charge your battery and far more importantly to be able to get a real advantage out of these devices, how long you will need to drive.

In MOST cases, you are far better off both from a cost point of view and from a use prospective, to fit an additional battery rather than buy a DC to DC converter.

Cheers.

**jik22** · 25th June 2007, 08:36 AM

Originally Posted by drivesafe

So best to use the correct device for the job at hand.

Cheers

Thanks...just wanted to confirm my thinking, as I had serious doubts that the trailer socket was up to it, or that it would allow thick enough cable to be used.

It's not my 4x4 that's getting this, and it's doubtful the owner will spend the money to do it properly (Especially when he sees the price of the cable) but at least my advice will have been accurate.

**drivesafe** · 25th June 2007, 08:39 AM

Originally Posted by jik22

but at least my advice will have been accurate.

And it also sets you up to say “I told you so” at a later date.

Cheers

**jik22** · 25th June 2007, 09:00 AM

Originally Posted by drivesafe

And it also sets you up to say “I told you so” at a later date.

Cheers

If only that were true when it comes to family, as in this case!

**modman** · 25th June 2007, 01:22 PM

i can vouch for the abr stuff, i also have shares in this bridge for sale

like mentioned, you need >10mm2 to charge battery in trailer properly. 16 mm2 is the largest that fits into 50 a anderson plug. voltage drop comes from resistance, in 12 v thats usually diameter of wire/cable and length of run. resistance will cause heat (which increases VD again)
pretty good pics on traxide site and not expensive, unless you wish to pay someone like arb to do it

david
sorry this was a late post. you can get the cable cheaper if you are prepared to hunt around at scrap metal merchants or approach a cable wholesaler. they will sell smaller runs to the public way cheaper than supercrap or auto elec's. (no offence drivesafe)

**Teks** · 25th June 2007, 09:04 PM

Originally Posted by Redback

Yep as said above, don't use your trailer plug.

Baz.

Oh ye of little faith!

Anderson plugs are not the "be all" of connectors.

I have used the trailer socket on many cars and 4WDs over the last 20yrs to charge batteries in campers, caravans, boat trailers etc etc. The socket can easily cope with energised current. If it didn't, please explain how you are able to illuminate the tail lights and side lights of a caravan for example?

As for the cable to the socket and from the plug I would use 6mm sq cable and if this didn't fit the plug/socket simply splice in a short bit of 2-3mm sq cable. At the end of the day you are not going to get a gobfull of charge current over say 6-7m of cable anyway! At best the current will be around 10-30A at best which is OK anyway.

So, to those who doubt me - what do you think the maximum current is in a trailer socket? 5 amps, 10 amps, maybe 15 amps or more??

Back to you.

**Discobunny** · 25th June 2007, 09:12 PM

Breathe Drivesafe, Breathe!! Count to ten and back awy from the keyboard, not worth it.

Aahhhh stuff it, I'm gunna get me a beer and chips and watch this

**Disco300Tdi** · 25th June 2007, 09:25 PM

Originally Posted by Discobunny

Breathe Drivesafe, Breathe!! Count to ten and back awy from the keyboard, not worth it.

Aahhhh stuff it, I'm gunna get me a beer and chips and watch this

Grab me a beer or 2, this could be a re-run again.....

And we always know the ending.....

**ladas** · 25th June 2007, 09:29 PM

Originally Posted by Discobunny

Breathe Drivesafe, Breathe!! Count to ten and back awy from the keyboard, not worth it.

Aahhhh stuff it, I'm gunna get me a beer and chips and watch this

I have a cold six pack here - ready and waiting, this could be fun

**jik22** · 25th June 2007, 09:32 PM

Originally Posted by 84RR

Grab me a beer or 2, this could be a re-run again.....

And we always know the ending.....

All right, can someone let me in on the joke, or point me to a previous thread I'm presumably missing?

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