FAQs: Car Battery Chargers, All Your Questions Answered

In its simplest form, a car battery charger draws 240 volts AC and converts it into a DC voltage, around 13 – 14 volts. It passes electric current through a battery, to transfer energy into the battery. The battery stores this energy and over time, becomes fully charged.

Regular under-charging will shorten battery life. Overcharging can severely damage a battery. Why? Because the battery converts the extra energy to heat and there’s a real chance your battery will explode.

Smart battery chargers continually monitor the condition of the battery. They vary the charge going into the battery by varying voltage and current accordingly.

A smart battery charger will never undercharge or overcharge a battery because it “knows” when the battery is fully charged.

A smart charger has an in-built microprocessor that monitors the battery as it’s being charged. This means the battery:

✔︎ Cannot be overcharged, and

✔︎ Will be fully charged to its optimal condition.


Good quality smart chargers will mostly have the following features:

✔︎ Automatically detect if the charge cables are incorrectly connected (reverse polarity),

✔︎ Spark protection,

✔︎ Short circuit protection,

✔︎ Over-current protection,

✔︎ Over-voltage protection,

✔︎ Low voltage protection, and

✔︎ Over-temperature protection.

Without mentioning brands (that’s an endless argument for which there’s no correct answer!), the best portable battery charger is a smart charger.

A smart charger will never undercharge or overcharge your battery.

Four other features to look for:

✔︎ If a battery is connected to the charger long-term, does the charger continually monitor battery state of charge and top it up as needed? This keeps the battery in optimum condition and will extend its life.

✔︎ Does the battery charger double as a 12v power supply? This is an invaluable troubleshooting tool.

✔︎ Can the battery charger charge LiFePO₄ lithium batteries? These are quite common and becoming more so as the price continues to drop.

✔︎ Does the charger indicate when the battery is faulty? It should give you a warning.

It depends on the current rating of the battery charger.

Say you have a 100Ah battery, 50% discharged. A 2A battery charger will take (50Ah/2A =) 25 hours to fully charge the battery.

A 10A charger will take (50Ah/10A =) 5 hours to fully charge the battery.

That’s a rough indication. Other factors affect charge time:

✔︎ Ambient temperature. A smart charger will decrease charging current as ambient temperature increases. This protects the battery from overheating.

✔︎ Condition of the battery. A smart battery charger will make every attempt to optimise the battery’s charge level. So it might take longer to finish off the charge, as it attempts to restore the battery to its optimal state.

Look for these features:

✔︎ Lightweight and easy to carry.

✔︎ Solidly made and durable.

✔︎ Gives you a fast charging time. Look for a high current rating, like 8 or 10A for a car battery.

✔︎ Easy to use. Just an on/off button and the charger does the rest.

✔︎ Automatically detects if the charge cables are incorrectly connected (reverse polarity), has spark protection, short circuit protection, over-current & over-voltage protection, low voltage protection and over-temperature protection.

Quality smart chargers will have all of these features at a minimum.

The answer is maybe, but don’t risk it. It depends on your battery charger and on the state of charge of the battery.

The safest option? Disconnect the battery charger before attempting to start your car. This way you’ll avoid any potential damage to the battery charger.

✔︎ Make sure the battery charger and the battery are compatible. Check the car owners manual for battery details.
✔︎ Connect the red clamp to the positive terminal.
✔︎ Connect the black clamp to the negative terminal.
✔︎ Plug battery charger into 240 volt wall socket and turn on.
✔︎ Turn battery charger on.
✔︎ Check it is charging okay and not showing any warning lights or alarms.
✔︎ When charging is complete, turn battery charger off.
✔︎ Unplug 240 volt power cord from wall socket.
✔︎ Disconnect black clamp from negative terminal.
✔︎ Disconnect red clamp from positive terminal.

It’s okay to charge your car’s battery while it’s still in the car and connected.

It depends on what you mean by “dead”. If the battery is flat because you left the headlights or stereo on, or you haven’t driven the car for ages… then yes you can.

However, if the battery fails internally it cannot be recharged. If you have a smart battery charger, it should let you know if the battery is faulty. For example, the REDARC SmartCharge battery charger has a flashing warning triangle to indicate battery failure.

Yes, if you use a smart battery charger. It constantly monitors the battery’s charge level and adjusts itself accordingly. Once the battery is fully charged, a smart charger will not overcharge the battery.

Trickle chargers slowly charge your battery. Some trickle chargers are designed  to be used for short periods, like overnight. Others are designed to keep your battery charged for long periods, like when you have a car in storage for example.

Many smart chargers have a trickle charge function. They can be left connected to the battery indefinitely and are smart enough to monitor the battery and decide when it needs topping up.

It depends on the type of battery charger. Simple car battery chargers will continue to charge the battery, even when it’s fully charged. This causes the battery to overheat and can lead to it swelling or even exploding.

A smart charger will constantly monitor the battery and adjust the charger accordingly. Most smart chargers can be left connected to a battery indefinitely without damaging the battery.

Car battery chargers slowly restore charge to a flat battery. They use low current to gradually charge the battery. This can take several hours.

Battery chargers runs through several charging stages, optimising the charge in the battery. When finished the battery will be fully charged (assuming the battery’s not faulty).

A portable car battery starter is simply a small charger you can easily pick up and carry with you.

A portable jump starter does not charge your flat battery. It simply provides the high level of current needed to start your car. The jump starter is designed to provide short bursts of very high current.

Note: If you want to know more about portable jump starters, go here.