Hi Simon
There is a thread about this that we are still awaiting a reply from Grippa.
I contacted Numax battery manufacturers and they advise that a leisure battery should not be charged at anything more than 10% of the rated amperage of the battery. You run a special battery which is very voltage sensitive, so this is a little different.
The way I see it is that the 30 amp unit is way big enough. Grippa say that their BS100 Dual smart relay with its electronics will increase the boost rate of a vehicle's alternator and you will not need to supplementary charge your leisure battery unless your driving is just down the road, but I have my doubts about this.
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Just to clarify the above, our relay does not increase the charge power of the alternator. Rather it allows more of the charge from the alternator to get through to the battery. For example a 40amp alternator would typically charge at 38amps maximum. Our relay will allow that charge to get through to the system battery, whereas a 5 amp relay for example would majorily restrict that charge. Our relay allows the maximum amount of amperage when the battery requires it. The charging amount is solely controlled by the alternator.
The reason why we do not get flat batteries on our system is because if the battery should be run very low, our relay allows full charge from the alternator (to a maximum of 100amps).
If you have a specific question you are always welcome to call us on 0800 098 8407.