A 12v charger won't charge a 12v battery. The voltage initially needs to be at 14V and a 500Ma output is modest to say the least. There is a very good article in this months Practical Caravan on chargers with CTEK and a RING charger showing good results. You really need a charger that is capable of at least 5 amp o/p as a minimum rising to meet the charging requirements of 110+ Amp leisure batteries and it really does need to be an "Intelligent" charger capable of reducing the charge rate with time and then discharge/recharge cycles. If the plates on your battery have already started to sulphate, it maybe already all too late to recover the battery. I have had an intelligent charger continually on a battery except in use for 6 years now and the battery is only now just starting to show signs of degradation.
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johnnymo9210,depending on the AH size of the battery,i.e 80AH 115AHetc,will decide on how long the battery takes to charge.I would suggest that anything over 12 days is excessive,but could be wrong.Trickle charging is slower and preserves charge and battery life for longer,but also be aware that excessive periods of charging if the battery is fully charged can be damaging to the battery.
------------- If the moon reflects the suns light,why does it not reflect the suns heat?
The latest Practical Caravan is doing a test on Chargers and suggest you get a copy very interesting. At the end of the day you get what you pay for and some of them are well over £100
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with a limited inventory