The amount of time a battery will last will depend on how much current per hour you are using and on what type of system - pump -hot - cold water- a pump controller.
How long the battery will last will depend on the current draw of your system (amps per hour)
as a general rule a WFP system is going to draw between 2 and 9amps an hour. It may be worth checking all your connections and ensuring the cables are as short as possible. This would help minimise current drop through the cables.
A split relay is a good idea as it will help trickle charge the leisure battery.
The charging capacity is controlled by the alternator which is generating current.
The amp per hour charge rate is dependant on a number of factors for example the length of time the engine/alternator are running and how quickly they run.
Most alternators are designed to trickle charge a battery so the longer and faster they run the higher the charge rate. (Up to the maximum capacity of the alternator and battery) There is also a restriction based on the maximum capacity of the alternator and battery in any system plus any drop off through the wiring due to resistance.
The ampage per hour rate will drop off as the battery nears full charge, the batteries capacity to take amps drops as the voltage capacity nears its maximum. The only way of physically pushing higher current into the battery would be to increase the Voltage capacity and increase the maximum generating capacity of the Alternator.
Ohms Law http://en.wikipedia.org/wiki/Ohm%27s_law
Hi Ian
You are absolutely right. I would also like to add the following.
I did a quick search on the internet and found a solar panel 15cm x 2cm x 41cm which has an output of 15w. I guess this output is measured under the most ideal conditions - eg. in the Australian desert with the panel at exactly the right angle to catch the sun's rays perfectly.
15 w is 1.25 amp hours. Lets give them the benifit of the doubt and say that the panel would produce 1.25 amp h at battery charging volts (13.8 - 14.1 volts.)
As Ian says your pump will be drawing 2 - 9 amps. I believe ours will draw around 3 - 4 amps per hour even on a controller. So before you start, your solar panel is already only producing a quarter of the pumps useage in ideal conditions. In the real world that panel will probably only produce half an amp, so you are even worse off.
As an illustration, fill your bath with cold water and then pull the plug out to empty it. But you must keep the bath full by adding water with a cup from another container.
The full bath would be your fully charged battery; emptying the water will be your pump and trying to keep the bath full with a cup with water is your solar panel.
This solar panel is advertised for £75.00 plus postage. This solar panel would be fine to keep a fully charged leisure battery fully charged over winter when the mobile home is parked up and not being used, but not much use for anything else.
We use split charge relays which help to keep the batteries charged, but we don't do enough mileage to really make a difference. My son in law works 2 days a week and drives 18 miles home and back that night. He has an 85amph Humax Leisure battery on a split charge relay on his Transit Connect. When he leaves for home his battery is about 50% charged and when he gets back the following morning its about 75% charged. We still have to 'bench charge' all our batteries every few days to keep them fully charged - and it works this way for each of our vans. My first 110amp leisure battery lasted 3 & 1/2 years and my sons first leisure battery lasted 4 years. My replacement is about 18 months old and his is a year old - works perfectly. My son in laws battery is 4 months old so hasn't got any credability yet.
We initially tried to run my son in law's pump off his van battery, but it just went flat and won't start his van - hence the reason why he now has a separate leisure battery. BTW - we all have Shurflo pumps and Varistreams.
Spruce
