Issues charging AGM battery

Tony,

At the end of your post you asked about upgrading cables to 6 B&S. This begs the question - how big (B&S, gauge, mm, whatever you can identify) is the cable presently supplying the charge kit and how long is it? Does the charge kit have its own wiring, or is it plugged into an accessory socket?

If it has its own wiring can you check for bad connections? Does it have a negative all the way back to the vehicle battery, or does it have a chassis earth? If the latter, check for a bad connection to chassis.

If it plugs into an accessory socket, same considerations - what size wiring, bad connections, how long is the cable run, etc.

If you have fitted an in-line fuse, check that for good connections. Sometimes a blade fuse holder at high current will develop a bad connection and stuff a working system.

Maybe the charger has karked it :-(

And if your wiring is smaller than 6B&S and you're looking to get 50 amps at 14.x volts at the battery, then I'd be going for at least 6B&S, larger if the cable run is long (it is a bus, right?). If you're going to use that cable as a built in jumper lead in case of a flat start battery then you'll need bigger cable again, ESPECIALLY if it is a long run between the batteries.

Tonydav,

The Thumper 50A charge kit should be adequate for your needs, assuming you are using suitable (original?) heavy duty cable between the "A" and "B" boxes to limit voltage drop.
The "A" box has a built-in isolator inside and will provide charging voltage to the remote battery, once it has detected the starting battery has reached an appropriate voltage level. This usually occurs within a minute or so of the engine starting and you should notice the indicator light illuminate to show the secondary circuit is active. The positive cable from the primary battery to box "A" should be fitted with an inline fuse. Check to determine if the fuse is OK.

The "B" box contains a fuse to protect the circuit from shorts, etc. It is located on the side of the box. Ensure the fuse hasn't blown.

If you are not reading around 14.2 volts at the battery when the engine in running and the secondary circuit is active, there is a definite problem with either the alternator, the cabling, or the Thumper charging kit.

As far as a low voltage cutout is concerned, the "normal" cutout level is around 11.6 volts, although some devices may have an adjustable level. 11.6 volts equals approximately 20% state of charge and is be the lowest level the battery should reach, for prolonged life.

Full voltage of the AGM battery (when no charge is being applied) is 12.8 volts.
10.5 volts is dead flat and the chances are the battery will be permanently adversely affected if drained too low, too often to this level.

The Redarc SBI12 is an isolator and is unlikely to improve the situation.

If you need the best solution for maintaining the remote battery in prime condition, consider a dc-dc charger, which will enable a full "smart" charging regime at the remote battery without the need for extra heavy duty cables.

I use both a Thumper charging kit (for my portable Thumper) and a Sidewinder Flyer with inbuilt charging capability (as my auxiliary battery supply) for the Engel in the back of the tub, plus a dc-dc charger (Ctek D250S Dual) for the camper.

Mate your reading of around 14 V was obviously without load to it. That way you don't see the voltage drop. Looks like 12.x V is the real voltage after the drop on your cables.

As per several posts above your cables are probably too thin, and the voltage drop just too much. The AGM likes high voltages for charging
CJ

The cabling in use is the standard cable supplied with the Thumper 50A kit. It's not as thick as 6B&S but still quite thick. I think it's 6mm as I recall reading that somewhere.

By "bus" I mean Grand Carnival, so not a hugely long run.

Main reason for the Redarc was so I had the switch in the cabin to link the batteries. I was also wondering if the Thumper kit was up to the job. Is there any disadvantage to fitting this with the 6B&S I have here? (Over the Thumper charger I mean).

I've manually set the cutout on the projector device at 12V to protect the battery more. It ranges from about 10.5V to 12V.

It sounds like I should look at the fuses as maybe something is wrong there.

I had considered a 12V-12V charger but a 30A version is about $250. Not a small investment. I already have a 10A version but this takes forever to even charge the 75Ahr Thumper.

My gut feel after reading these replies is something wrong with the charger. Will check the fuses.

Is it possible to charge the battery via both the alternator and my 12v-12v battery charger at the same time (assuming the wiring is correct)?

If the reading of 12.5 was taken with the fridge working ie with a load on the extra battery than I would assume it is ok.
Would need to take reading with no load on system.
On my sytem the voltage drops with a load and the more load the more drop.
As soon as you take of the load it goes back up to the charge rate.

Honky