Deep cycle Battery question

At that voltage you'd have to think it was had it. It's possible that there is something amiss with the Redarc.

What voltage is at the second battery terminals when the engine is running.

mono2, to answer your questions:

If you don't need the second battery for the next nine months, don't put it inside the vehicle as yet.

It'll have a much easier life if you put it in a cool spot and do some maintenance charging every three months or so (in case it's going to be a new deep cycle AGM VRLA battery). Flooded batteries self discharge faster and need more regular maintenance charging to prevent them from getting sulfated.

Since you don't know yet, why the one battery has died, it would be good if you could put any battery in its spot and have it test charged by the redarc.
Make sure the boost voltage doesn't exceed 14.4V/14.7V (for flooded/AGM). The final float voltage should be around 13.7V for both.
Note that for every 10 degrees above 25, these voltages need to be reduced by approximately 0.25V.
That's why, for in situ charging in hot environments, temperature compensation becomes almost mandatory.
Does your redarc have an external temperature sensor which can be attached to the battery?

Also make sure the redarc reduces the boost voltage to the float voltage level with the fridge connected.
Some multistage chargers get confused because they can't tell battery charge currents from load currents.
The result would be dry out of your deep cycle battery because it'll be sitting on 14.7V for prolonged periods of time.
Top of the range chargers prevent this from happening by applying adjustable time limits to the boost stage, thus effectively overriding any misinterpretation of the results from the chargers internal current sensor.
Note that I was using the 14.7V as an example as I don't know your alternator/redarc max charge voltage output.
But it's still relevant even if your redarc only puts out say 14V, as this voltage can lead to the same effect over longer periods, especially in the summer months, and if the charger lacks temperature compensation.

Hope this does make some sense to you.

Best regards, batteryvalue

Does he mean he has a REDARC isolator or a Redarc charger.

Why I ask is that he mentions charging the new one with a CTEK

I think he has a misinterpretation of technology here.

Hi Mono

I think you are referring to the Redarc SBi12 isolator.

These like most isolators simply parallel the batteries when the engine is running and the cars alternator and regulator control the charge. The problem is that the Redarcs do burn their contacts and then the aux battery suffers a voltage drop from the isolator.

You will need to replace the battery as it sounds dead and beyond recovery, then do a load test on the Redarc.

To do this test discharge the new battery slightly with your fridge or other accessory. Then start the car. The Redarc should kick in and the red LED should come on. Now measure the voltage on each side of the Redarc and they should be very close, no more than 0.2V on a discharged battery.

If this test is good then perhaps the battery was just old and not well maintained, if not then you need a new Redarc.

I would also suggest you check all you connections and earths, especially those that have now been covered in acid.

Clean the acid off all paintwork and metal parts. Hot water and bicarb is good for this.

Regards

Derek from ABR