Hi
BEB,
As i suspect a car alternator only ok for replacing whats used by starting.
If you fit a proper dual battery management system, that includes an automatic battery charge switching devise, your cars alternator is more than capable of charging an extra battery or even batteries. My car has a dual battery system installed and I normally charge the main (starting) battery and the second battery and occasionally, on trips, charge a third as well, all automatically and all without hassle.
A dual battery management system generally works like this:
After starting your car the system will automatically charge your main battery - the battery that started the car - until its voltage is around 13.6 volts (mine is a 12V system, 24V systems are available), then the system will charge the secondary battery until it is also at 13.6V at which point the system will maintain charge in both batteries until the engine is switched off.
The system, if installed properly, will always separate the main battery from the secondary battery during the discharge phase. The main will be left undischarged and ready to do its only job - start your car - the secondary will be discharged by the accessory loads that you place on it, such as the light you mentioned, or in the case of one large motor home I fitted out in 2003, the secondaries will power your laptop, fridge, microwave, lights, fans and air conditioning system!
To achieve this, make sure that when you install the secondary battery, the wiring for the starter motor remains on the main battery and the rest of the wiring and subsequent wiring goes on the secondary.
This will prevent you being stuck in a lonely place with a discharged main, as well as a discharged secondary.
All this is from my own experience as manager of Battery World here in my home town. (Mods, this is not an advert, as the business was sold after my departure and closed down sometime later.)
SLI batteries (Starting, Lighting & Ignition) do not like to be discharged to below about 80% of their full charge, roughly equivalent to the drain caused by starting the car two or three times in quick succession. If they are drained below this their useful service life is diminished dramatically,
every time this happens, firstly, following a deep discharge of an SLI battery, you will never get it fully charged to 100% of the charge state before the discharge, secondly, SLI batteries suffer dramatically from sulphation when discharged. Simply put sulphation causes the lead to flake off the plates and collect in the mud racks in the bottom of the battery where it does more harm than good.
For a camping application you need a
Deep Cycle battery. Deep cycle batteries have thicker plates and slightly different internal wiring, the thicker plates give the battery more 'voltage holding power' and resist sulphation much longer than SLI batteries.
The best type of deep cycle battery has only ONE plate per cell, this plate is about 6 feet long and rolled into a spiral, with fiberglass between the roll to prevent shorting but allow electrolyte flow. Spiral batteries resist vibration extremely well and have more lead for a given volume that plate rack batteries.
We never had a single battery present for a warranty claim with Optima batteries.
Just another disclaimer for the mods, I no longer have anything to sell, I left the industry in November 2006 and am now labouring for a mate of mine who owns a landscaping company while I get through my ab initio flying training.
This started out as quick reply and turned into an essay, but I hope it helps.
Cheers,
ABX
