Thread: Extender Lead for Camper Trailer

Originally Posted by Grizzly_Adams

Anyways was just wondering if it is worth purchasing a 6m extension lead - I have a SC80 in the vehicle setup for a 3 battery system and it works well..

I am booked in to get an SC80 in a couple of weeks and I think you may have answered a question that I emailed to Traxide(DriveSafe) this afternoon about how to keep the trailer battery charged when I set up for a week.

Just a quick question though: I thought the rear socket for the trailer would be a separate circuit than for the aux battery in the car. However your solution seems to rely on power being fed to the rear socket even when the engine is off. Does this mean that the two batteries are actually in parallel when the ignition is off and therefore the battery with the lower charge is recharged from the other until they are equal voltage? And therefore there is a further assumption that the aux battery is always charged more because the vehicle gets driven around during the day? With my very basic knowledge I would think this means the trailer battery would never get fully charged (not that it needs to), it would just meet the vehicle battery halfway approx. Whereas the inverter/charger option could actually fully charge the trailer battery even if at some cost to the aux battery, which is also okay especially if you plan a big drive for the next day.

However as always I wonder if I have missed the point somewhere along the line.

An answer to this may save DriveSafe having to deal with my even more convulated set of questions that I sent him this afternoon, which I am sure he would appreciate.

Hi Paul, first off, I left a message on your answering service but hopefully I’ll answer your questions here.

The first thing this is that just paralleling two batteries will not get the higher charged battery to charge the lower one till they have equal voltages.

If you parallel two batteries of deferent voltages, they will end up with equal voltages but it will be that of the lowest charged battery. In other words and put crudely, if you have one battery with a 12 volt charge and another with a 12.7 volt charge, after a period of time both batteries will have a 12 volt charge.

The reason for this is that all lead acid batteries require a chemical reaction to occur before the battery will absorb power but the reaction has to have at least 13.8 volts applied to battery before they will actually start to absorb the charge being applied. Any voltage below 13.8 will get the chemical reaction going in the battery, which in itself, will use energy but very little power is actually absorbed.

With the two batteries paralleled, because the low battery has 12 volts and the high battery is applying 12.7 volts to the low battery’s terminals, there will be a chemical reaction taking place in the low charged battery that will simply use up the higher charged battery’s voltage difference with no charge being absorbed by the lower charged battery.

Note, it doesn't matter if the two batteries are different sizes and/or types, the results are the same.

Now Paul, to your set up. As there will be two batteries to supply power to an inverter/charger set up when your arrive back at your camp site after each days drive, you should have plenty of stored power, depending on how low the batteries in the D3 were when you started and how long you drove for.

The biggest advantage for you if you use the inverter/charger set up is that you will be charging two batteries at the same time while you are driving each day, furthermore you won’t need to fully recharge the vehicle’s batteries, just replace the bulk of the charged, this means you will only need to get the batteries back up to about 85% to 90% charged.

This means about a two hour drive should get you enough replaced charge to run your inverter/charger set up each night and keep your trailer battery fairly well charged.

Cheers.

Thanks for the reply. I'll give you a call tomorrow but in case this is of interest to other people I'll pop another question in here.

I understand your comments about the inverter and given my critical need to not have the trailer battery get too flat i think the inverter is the way to go. Worst case is I flatten the aux battery but the SC80 would protect the main battery and the SC80 auto-cutoff would prevent damaging the aux battery by not completely flattening it. (For other people's sake, my email to DriveSafe highlighted that I have medical equipment ie CPAP running from the trailer battery so it is a higher priority than normal.) I predict that 99% of the time this would keep all my batteries close to fully charged assuming I do some driving most days, which I do.

Originally Posted by drivesafe

There is another way to charge the camper trailer battery and that is to connect a 12 to 240v inverter to the vehicle’s battery when you come back to the camp site.
Cheers

When you say connect to the vehicle battery, do you mean pop the bonnet and clip the cables directly to the battery? Or do you mean use the socket at the rear bumper that you would normally attach the trailer to?

Originally Posted by DiscoStew

When you say connect to the vehicle battery, do you mean pop the bonnet and clip the cables directly to the battery? Or do you mean use the socket at the rear bumper that you would normally attach the trailer to?

Hi Paul, if you have an Anderson plug fitted to the rear of your D3, make up an adapter with another Anderson plug and just connect up the inverter to the rear plug.

Cheers

So even though the Anderson plug at the rear is there for connecting the SC80 to the trailer battery while driving, it can also be used to draw power from the Aux battery to power the inverter?

Do I need to tell Motolek to do anything different or is that a standard configuration?

BTW I have a D2 not a D3 but I assume that is irrelevant to the advice.