Auto battery chargers range in price from fifteen dollars or so to hundreds of dollars but, like most things in life, you pretty much get what you pay for.
Automobile or car batteries are usually 12 volt rechargeable batteries containing a series of flat lead plates suspended in a solution of dilute sulphuric acid, or battery acid. They are commonly known as lead acid batteries, although some types use an acid gel instead of a liquid acid/water solution.
A cheap automotive battery charger costing around $15.50 can only charge your car’s 12 volt battery at a lower rate of charge. This is measured in Amperes (often shortened to just Amps). They are simple enough to use. You plug one end into your household mains, and you then have two rather thick cables, one red and one black, with spring-closed crocodile clips or bulldog clips at the end. You attach the black wire and clip to the (-) Negative battery terminal, and then connect the red wire and clip to the (+) Positive battery terminal. (In most modern vehicles, the Negative side is also connected to the car or truck’s metal chassis. This is sometimes called being earthed or grounded.)
The actual measured voltage of your car battery’ will depend on its state of charge and the ‘health’ of the battery. A fully-charged battery in good health will often measure 13.8 volts DC with the engine running. A battery with a low charge or in poor health will measure less than 12 volts.
A slow charging rate for a 12v battery would be around 2 Amps or less, which is sometimes referred to as a trickle charge. So while cheap and simple 12v battery charger product may save you a few dollars when you buy it, its inability to do a fast charge when you need it will cost you time. For while a more powerful charger (with more Amps) can charge a flat battery in one or two hours, a cheaper trickle-charger will take a whole day or even longer to do the same thing.
A fast charge for the same automobile battery would be 20 Amps or even higher. The higher the charging rate of your vehicle battery charger, the larger electrical transformer inside the battery charging unit needs to be — and the higher its price tag. But it also means it can charge your car battery faster.
The power transformer takes your AC mains voltage (usually 110V in the US or 220V in Europe) and steps it down to around 15 volts or so. The 15vAC voltage is then rectified and smoothed into a direct current (DC voltage), and this 15 volts DC current is what you connect to your flat battery with charger clips to charge it up so it is full of stored electric power to crank your engine over and get the car started for you. No more dead car.
The best model auto battery chargers will also have some built-in ‘smarts’ to sense when the battery has been fully charged. At this point they will stop trying to pump more current into the battery and change to maintenance mode, where they send the battery just enough electric current to keep it in a fully charged state. But they won’t over-charge the battery.
A cheap and simple car battery charger won’t have the circuitry to detect when your battery has reached its fullest capacity. So you have to keep an eye on the charging process yourself, and switch off the charger once the battery has been charged long enough. The manufacturer’s instructions usually state how long you should charge your auto battery; but if in doubt it is better to charge too little. That way you won’t ‘cook’ the battery and damage the lead plates inside through overheating.
Battery maintenance and safety
You need to be aware that there are some dangers from lead acid batteries.
The acid can spill on you and burn your skin or your clothes, and your eyes are especially vulnerable since any acid in your eyes could cause eye damage or even permanent blindness. So firstly, keep your face well away from the battery. Secondly, wear safety glasses or goggles to save your eyes from any splashed acid.
The chemical reaction when the battery is charging means that oxygen and hydrogen gases are produced by small bubbles that can sometimes be seen through the six filling caps on top of non-sealed lead acid batteries. The sealed ones are maintenance free, but the ones with removable caps on top are meant to be checked every few months and the liquid level should be topped up with distilled water when the levels get low enough to see the metal plates.
Hydrogen and oxygen together are an explosive mixture, and a spark or flame could cause a dangerous explosion if the gases are allowed to accumulate in a poorly-ventilated area. There will be no problem as long as the doors and/or windows are open in your home or garage, wherever you are charging your flat battery.
Also be sure to keep any screwdrivers or wrenches away from battery. Any stainless steel, copper tool, loose wire or other metal part could move or fall and cause a short circuit across the battery terminals. Even a momentary contact can cause a spark, but longer contact can cause the metal object to become red hot or melt. And there is a strong possibility of the shorted battery actually exploding.