As a general rule, however, you should avoid allowing any battery to run flat. The spare battery should be fitted into a sturdy bracket installed by a professional auto-electrician. When you switch off your vehicle at the end of the day's driving, the spare battery is isolated from the main battery. This will prevent your fridge, if you have forgotten to switch it off, from drawing all the power from both the spare and main batteries and it s best if this is done automatically when you switch off the vehicle. Manual switches are available, but I do not recommend them, purely because you could easily forget to switch off the battery.
Air pump An air pump has a multitude of uses, apart from the obvious one of inflating tyres.
It can be used for blowing open clogged fuel lines, inflating rubber ducks, blowing out dust from inside the car and even inflating air mattresses. Like the spare battery, the air pump can also be mounted in the engine compartment of your vehicle. There are several types of air pumps on the market. I don't recommend those which operate from the car cigarette lighter as they are noisy, overheat quickly, and tend to seize up in hot weather. The more expensive electrical unit, if installed correctly, works much better.