We do not give the 'van much use in the cold winter months. I like to give it a run every month and keep an eye on the state of the batteries. This involves opening the door to take a look at the Solar Panel Voltage/Current Indicator. Leading up to the shortest days the sun has had some spells and I have charged NiCad cells from the Solar supply. I remember on one occasion that the Voltage was only 12.5 on a dull day so I declined the use of 12 Volts to charge the NiCad's.
The other day I checked and the Indicator was dead. One of the Leisure Batteries was flat. It is amazing how quickly the Alarm and other sensors including the Solar Panel Indicator, (yes it takes power all the time in order to show you that it is not producing any), runs down an 80AH battery but remember the demand is 24/7.

I have two coupled Leisure batteries, the other one was switched out via the Isolator Switch, I don't remember when I did that. Closing the Isolator would not be the correct thing to do in this situation, what is needed is a balancing charge. Step forward the BUSE.
As a reminder the BUSE is my invention I solder a Halogen 12 Volt BUlb across an old blown FUse to produce a valuable piece of test gear.

I removed the 20 Amp fuse that links in the flat battery and replaced it with a BUSE, the bulb lit bright so proving that the battery was at least still able to take a charge. The BUSE by way of its Positive Temperature Coefficient of Resistance limits the current to less than an Amp. After a few hours I turned on the Hymer charger then after a few more hours swapped back the 20 Amp fuse. After three days the batteries were fully charged and the Solar Controller was applying 14.2 Volts pulsed conditioning charge as the sun was shining and supplying around 0.6 Amp.

Ssh don't tell anyone it looks as is if I might have got away with it. I can't praise the BUSE too much without it it would have been a struggle to rig up a surge limiter, just closing the switch could have damaged both batteries, rigging the charger to charge just the flattened battery would also have been awkward.

If the lack of Voltage had been due to a short the BUSE would have been used for fault finding, inserted instead of a blown fuse, if a short is still present then the worst thing that can happen is that the bulb lights.
The other use of the BUSE is to supply a balancing charge to the Vehicle battery from the Leisure batteries. This is often done by Camping Car owners by fitting a fuse which will blow if left in circuit when the engine is started. If this fuse, often specified by Motor Home journalists, is replaced by a BUSE then the it can remain in line, once again the worst thing that can happen is for the BUSE to light.