Hi guy just bought a new 11o amp hour leisure battery 6 weeks old and its volts drop when under load when its fully charged.( from 12 volt to 6.4) The battery supplier says we need to state when buying its for window cleaning. What difference does the use make if its a motor home a caravan or should it last few days. Am I right or wrong.Thanks in advance. I trust you guys as one knowing your oats. 
This is the offending battery http://www.ebay.co.uk/itm/171073055139?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
It would appear that since Numax started the leisure battery's warranty being nul and void if used in the window cleaning industry, other manufacturers have followed suit.
I have no idea why we are singled out as users.
As Diamond has noted, a good supplier should be able to test the performance of a fully charged battery under a known load and determine if the battery isn't performing to standard.
As I've mentioned in another thread, we have had good service from Numax leisure batteries as have also had good service from Orange batteries (in name and colour) sold from Roper's caravan world.
Although we have split charge relays on both my son and my vans, we rarely let the battery charge drop below 75% which means they are supplementary charged every second night depending on the day's usage.
We have found that our pumps draw about 4.5 amps for every pumping hour, so we would typically use about 15 amps each a day doing residential. If my son is working with me using his back pack the amps are around 6 amps as he uses the pump to trickle fill his 25 liter plastic container. This is about the same amount of current used by an average motorhome user per day (estimated at around 33 amps), but my guess that our daily use of the battery will highlight any weakness very quickly.
Fully charge your battery, switch on the pump with the brush connected and watch what happens to the magic eye. If it goes from green to black quickly then the battery is suspect. It either means that the battery is flat and that the battery charger is faulty, or the battery hasn't got any capacity left and needs replacing.
Theoretically the 110 amp leisure battery should last us 3 days, but it isn't good to let the battery discharge to that amount.
KentKleen swears by Banner batteries. He uses car batteries and they have worked well for him and his team. We have found that leisure batteries only accept a trickle charge where car (starter) batteries will recharge more quickly. I'm tempted to try a Banner battery next time, but will probably end up replacing my leisure battery with another Numax when the time comes.