I really do not know how much this would top-up your battery. This would depend on your alternators rating. An electrical specialist would be able to tell you based on your alternator, vehicle and battery rating. I would say that 15mins would probably keep you going for another hour. More importantly, if your van is a diesel, is that it would warm the engine up which would mean when you came to re-start it, it would need less juice to do so.
A lot of whether this would work for you is dependant on what size your battery is and what amp rating your pumps are. If you have say 2x 5amp pumps then to run them for one hour would drain the battery of approximately 10ah of its capacity. If you have a 110ah battery in your vehicle then you could technically run them for 5 hours and still have half the capacity left in your battery which should be enough to start your van to get home. If you only have a 75ah battery then you would in all probability not be able to start your van at then end of a 5 hour pumping session. The best thing you can do is go to someone like Lucas and explain that you want to uprate your battery in your vehicle. They could then help you calculate which battery to upgrade to, that would best suit your loading.
If you could simply fit a 110AH battery then in all probability this would be more than enough. Check your vehicle to see what is the biggest battery you can fit in.
