You are going to hate me but the answer is almost never.
If you just take the numbers you get
13w = almost 1 amp
75 amp batt full flat
so 75 hours of sunlight.
you should not take the bat to more than 20% discharged for long bat life or to 80% discharged for a short bat life.
Also charging a bat is very ineficient so for every amp you use you need to input 1.5 amps. Add in the self discharge rate of the bat & you need even more. A 13 watt panel will keep you full battery full whilst NOT in use. Count on 5-8 hours of power in summer & 1-2 in winter. So in summer you might get 5 – 8 amps from the panel resulting in 3 to 5 amps into the batt per DAY. A better bet is to work out how much power you actualy use daily / weekly whilst in the van. Then calc the bat size needed & the charging source & size needed. To give you some idea a standard kettle would need 230 HOURS of the 13watt panel charging a batt to run for ONE hour.
Will you be able to charge from site power, the tow car or a genny?
We are looking at going of grid soon & will need a 2kw (2000 watts) solar panel to provide the needed charge rate to supply less than 1kw of daily power year round. Just for the short summer we can get away with a 400watt pannel as there is more sunlight hours.
I am looking to buy a solar panel to charge the leisure battery in my camper when away touring.
There seems to be a lot of them on the market – in particular, I have been looking at a Topray 13w briefcase style one. Does anybody have any experience with these and do I need a charge monitor to prevent over charge of the battery. Also, does anybody know how long this would take to charge a 75 aH battery from flat as I cannot find an ampage output rating for it.
Thanks for the help – in honesty while stationary, the only things that run off the battery are a couple of lights, the radio and a couple of cigarette sockets for charging stuff occasionally. Everthing else is gas powered (except the spark for the fridge and heater to keep them lit). It is very rare I manage to drain the battery very low, as it charges from the dynamo while in motion. However, I prefer not to run the engine for long periods when stationary, so as not to burn petrol unnessarily or annoy other people with the noise. Also the dynamo produces very little charge at low revs.
I guess this would help maintain the battery charge if it is connected whenever stationary.
I guess it also answers the question that I would not need a charge monitor as it seems highly unlikely that I will ever over charge the battery from a panel of this size.