Hi Tam North
I have a 110 amp leisure battery powering my system with a split charge relay.
I have this in a Citroen Relay SWB van with a 150amp alternator.
The reason why we have to charge our leisure batteries regularily is that we mostly don't drive enough miles to put back into the battery what we take out.
Each of my Shurflo pumps draws about 3.4amps with the digital controller setting we use. The battery will accept a recharge of about 10 amps at best, but usually between about 5 - 7.5 amps with a fairly full battery - remember a battery nearing a fully charged state will accept an ever reducing charge until the battery is fully charged.
So if we travel 10 minutes to and from the day's work, a 20 minute charge of 7.5amps (2.5amps) won't come anywhere near replacing the 13.5 amps we have used assuming the pump runs 4 hours.
The split charge relay wiring from the main van battery is protected by a 15 amp fuse which has never blown in the 3 years this system has been in this van. This means that the amps drawn by the leisure battery has never been more that 15 amps even when the battery was flat. If it had then the fuse would have blown.
My son in law has a Ford Transit Connect LWB with the latest Varistream digital controller and he drives around trip of about 30 miles a day and he now manages keep his 85amp leisure battery pretty much fully charged travelling this distance with his split charge relay. (The new digital Varistream draws current even when the controller is switched off, and its only since we have put an isolator switch on the supply to the Varistream that we have been able to achieve this. We estimate that the draw on the battery was about 25 amps over a week.)
Now I'm not sure about a standard starter battery, but I believe it will accept a faster charge than a leisure battery. But again if your split charge relay is protected by a 15 amp fuse as is recommended, then the alternator will not be charging the aux battery higher than that if the fuse has never blown. It would be interesting to put a volt meter across your battery to determine exactly your battery's state of charge. Your controllers will still be working fine with a half charged battery, but won't indefinitely if you are taking out more than you are putting back - so it's all down to amount of time your engine is running.
I have found that my battery will still be charged at the exact same rate whether the engine is reving at 3000 rpm or just idling at traffic lights.
Spruce