Battery Voltage

If your Redarc device requires 13.2V to commence charge to your battery in the caravan then I should think that it is a Redarc SBI12 isolator, not a "charger". There is a significant difference.

Certainly the Redarc SBI12 requires 13.2 volts in order for it to connect the alternator source to the caravan battery and will disconnect when the supply voltage falls to 12.7 volts. If the voltage at the cranking battery (which is the available alternator voltage) is only reaching 12.8 with the motor running then there is little margin to engage the Redarc. However all may not be as it seems. Firstly, you may be measuring the voltage at idle speed where the alternator is not achieving its full output. secondly, are you sure that you voltmeter is reading acurately?

Try measuring the voltage with increased engine revs.
If you want reassurance that the caravan battery is being charged, apply a voltmeter to the caravan battery terminals and have someone start the engine and raise the speed a bit above idle. you should observe an increase in the voltage at the caravan battery terminals.

A much better arrangement is to use a Redarc BCDC1220 dc-to-dc charger at your caravan battery which will charge from as low as 9 volts supply from your vehicle supply. It also overcomes the voltage drop that you experience in the cabling from the alternator to the caravan. Click here.

For an explanation of how alternators work click here.

I have one of these and it raised voltage by 0.6 volt and everything works a treat


http://www.ebay.com.au/itm/Alternator-Voltage-Booster-Dual-battery-systems-/320867723182?pt=AU_Car_Parts_Accessories&hash=item4ab534e7ae

Greenant

Dave.
What is happening with your alternator is the starting battery is rising as it charges, as soon as the Redarc connects it is holding the alternator down. You can test this by disconnecting the van battery and see if the starting battery rises to 14.2. If it does you are OK. Eric

To get a real voltage reading, run your vehicle for 10 minutes, then measure the voltage at the battery terminals with the engine doing 2000rpm. That will eliminate factors (eg glow plugs, flat battery, low idle speed, high underbonnet temp) that keep the voltage down.

Many thanks to the people who replied, I think the problem is a faulty alternator which doesn't have sufficient output voltage to initiate the charging cycle on the Redarc charger.
Allan B: The charger I a using a Redarc BCDC 1220 and after talking to Redarc they explained how they work in much more detail than with what comes with the product, so I believe now all my problems go back to low output voltage from the tow car alternator.
Again thanks very much
Puttingdave