Yes with either the charging control or a separate split relay you will still need to periodically charge the battery. This is because the alternator is trickle charging the second battery and the amount of charge will be based on how long and how fast the alternator is running.
Put another way your pump is drawing between 3 - 7amps an hour an hours driving will probably replace 1 - 2 amps.
Spruce has posted a very detailed reply on the dynamics of a split charge relay
http://www.cleanitup.co.uk/smf/index.php?topic=130038.0
And absolutely ensure you have a fuse in line on the two lives but do not exceed the recommended maximum fuse rating.
Since this post Ian refers to, I have a volt and amp meter combination unit installed in the van and have noted these points in our visual monitoring.
If my pump draws 4 amps per hour and I work for 4 hours pumping water, I have used 16 amps from my battery. In my case I have a Citroen Relay with a 150 amp alternator. At 3/4 charged my 110 amp Numax leisure battery will only accept 8 amps from the alternator. (I have the charging circuit protected with a 15 amp fuse which has never blown in 4 years.) If I drove for an hour that day (charging is all about time taken not distance travelled) then I would only put 8 amps back into the battery. If I drove for 3 hours the chances are that my leisure battery still wouldn't be fully charged as the fuller the battery gets, the lower the charge rate it will accept.
It doesn't make any difference whether the engine is idling or at 3000 rpm on the motorway, the battery is still charged at the same rate.
My son in law has a small Transit Connect L220 LWB and his charging statistics are similar to mine with his 85amp Numax leisure battery. He travels 9 miles to work and back each day, so he spends about an hour 'on the road'. Once a month he uses his van for a 48 mile round trip to visit his parents and washes their windows at the same time.
Each time I have checked his leisure battery it is about 75% charged. He is supposed to take it out and bench charge it every week, but I know he hasn’t done it for a few months now. He parks in the street/public car park so he can’t run a cable out to the van. One day it’s going to catch him out. It’s the same old thing though; the flatter the battery gets the more charge it will accept. His cable is also protected by a 15 amp fuse as is mine and it hasn’t blown yet either, telling me than the battery has never been charged higher than 15 amps.
We have a seperate SCR fitted which works independently of the controller. The latest one fitted the SIL's Connect van is an intelligent unit which is more simple to fit.
At most I work 10 minutes drive away for my customers, so I have to supplementary charge my leisure battery which I do every 2 to 3 days.
