Well, automatic car battery chargers basically take household current 120 volt AC current and convert that down to a 12-volt DC current used for charging vehicle batteries. Also, the units that are stated auto battery chargers have a circuit of regulating device. It ensures the battery only charged until it is full and then comes on as necessary to keep the battery fully maintained.
How it does this is by taking 110 volts AC and it converts it down through a step-down transformer down to a lower voltage. Usually somewhere between 24 and 28 volts AC that goes through a full wave rectifier or a half wave rectifier which removes almost all or some of the AC sine wave. Then converting into a full DC or partially halfway of DC voltage used for charging automotive batteries. This is the basic principle on which almost all the intelligent car battery charger works.
Car Battery Charging Steps (Lead Acid Battery):
How do you get a car battery to be fully charged after it has been discharged?
In this part, we are going to concentrate on lead-acid batteries. Because each different type of battery has slightly different charging stages.
Lead acid battery chargers range from very simple to very complex but they generally follow three main stages of charging. The same is true for solar charge controllers which are basically just battery chargers that use input voltage from the Sun.
There are technically four stages of battery charging;
- Bulk Stage (constant current).
- Absorption (constant voltage).
- Float (constant voltage).
- Equalization (low current/high voltage).
1. Bulk Stage:
The bulk stage is about 80% of the recharge. During this phase, the charger holds the current constant but the voltage increases. There is a lot of different opinions and philosophies on what formula should use to calculate the current for this stage. The manufacturer will give you that specification but whatever current is used it should not take the battery over 125 degrees Fahrenheit if it is a flooded battery or 100 degrees Fahrenheit if it is an AGM or gel battery.
The next stage is absorption, where the charger holds the voltage constant but slowly tapers down the current. Again the best voltage for the absorption point will be provided to you by the manufacturer. This stage completes the remaining 20% of the actual charging of the battery.
The third stage is the float charging stage which does not actually charge the battery but just more of maintains it. The charger or charge controller provides a lesser voltage than in the absorption phase and only about 1% of the current that is used to charge the batteries. You could run this stage forever if you wanted to and basically, it would just keep the battery topped off.
Equalization is necessary every once in a while to maintain the health of the battery. In this stage or process, the charger will provide a constant voltage that is much higher than the other stages. This is done to equalize the specific gravity inside each cell of the battery and to de-sulfate the metal plates.
Speaking of specific gravity ideally we would base all of these stages of charging off of specific gravity measured by a hydrometer. The specific gravity of the acid inside a battery is the truest way to tell the state of charge.
However, hydrometers can be hard to use and are only accurate down to five thousandths. So most of us will use voltage instead which gives us a close approximation. Most battery manufacturers will provide charts to show you the different voltages of the different stages of charge.
In reality, most of us are going to rely upon a microprocessor controlled battery charger with minimal input except for basically the type of battery chemistry we are using usually these controllers will manage these stages for us. But it is still a good idea to be able to program them with the exact specifications the manufacturer gives you for the longest battery life.
Car battery chargers have different current ratings. In order to pick the right one for your particular battery, there are a couple of different formulas we can use.
Take the capacity rating of your battery and amp hours and then divide it by the C-rate or charging rate provided by your manufacturer.