Tim,
Charging via an inverter and a charger specifically designed for a leisure battery is not going to be quick, as the current is regulated to a level to protect the battery. But as discussed below there is more to this than charging rates.
My charger is a 4amp unit. Another well regarded unit, the CTEX 3600 [http://www.vertar.com/product.asp?P_ID=238] is a 3.6 amp device. The car alternator by comparison is likely to be at least 45 amps [Mine is 75]. This though has potential to damage a leisure battery.
Using the inverter/charger potentially can get the battery to 100% full and offers protection from both over current and over charging, gassing etc. The method you use is unregulated and the charging current can be both very high or low depending on the varying state of charge of your starter battery and the voltage drop of the wiring. Potentially with the massive current available you could damage your battery if is coupled when its charge state is low.
With the inverter/charger arrangement you can also take some power from the car battery whilst the car is parked as long as you exercise caution. Typically you can take up to 20% of a fully charged batteries power and still be confident of starting. I will give an example using my vehicles data together with conservative allowances. Battery is 110 AmpHr, say take just 10%; 11AmpHr. If I give the inverter/ charger a 75% overall efficiency I will need 5.33 amp to charge at 4 amps, therefore I can leave things set up for just over 2 hours to be within my targeted 10% depletion. I will replace in my battery 4 AmpHrs times the amount of hours running my car, plus another 8 AmpHrs if I leave it connected for the safe two hours. [When I go off for a 3 hour visit I risk not uncoupling as the engine will still be warm for the restart, so am charging for the whole time I am away plus two hours on my return. That is a half hour journey out and back is 0.5+3+0.5+2= 6hours at 4 amps ie a large 24 AmpHrs. Remember that on running the car you have the massive current available from the alternator to recharge the starter battery; this battery is designed to accept this high current and thus recovers quickly.
There are cheap 150-watt inverters. However if you are using the in-car socket these seem to be limited to 10 amps, [120 watts]. This suggests that you could consider say up to an 8amp charger, not that I think its needed.
A small colour TV takes about 3 to 4 amps so you would need to budget about 1.25 hours coupled to the car for each hour of goggle box use.
John
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Topic: charge leisure battery in car boot
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