How do I charge a car battery with a battery charger?

Quick answer

Use a smart (multi-stage) charger set to the mode that matches your battery: standard for flooded and EFB batteries, AGM for AGM, and lithium only for a lithium battery. With the charger unplugged, connect the red clamp to the battery’s positive post and the black clamp to a bare-metal earth point on the chassis or engine (or the point your owner’s manual specifies), using the negative post only if there’s no suitable earth point. Then plug in and switch on. Charge in a well-ventilated place away from sparks and flames, because charging gives off hydrogen. Never charge a battery that is frozen, swollen, cracked or leaking. When it’s finished, switch off and unplug first, then remove black and then red.

Is this a DIY job?

• Difficulty: Easy
• Time: 10 minutes to set up; charging takes from a couple of hours to a day depending on the charger and how flat the battery is
• Tools: smart battery charger, safety glasses, gloves, spanner (only if removing the battery)
• Skills: following the charger’s instructions and keeping positive and negative straight

Charging a 12V car battery is a safe job if you use a smart charger and follow the basics. Get help if the battery is damaged, if your car’s manual says the battery must only be charged at a workshop, or if it’s a hybrid or EV and you’re unsure which battery you’re dealing with.

Choose the right charger and mode

• Smart (multi-stage) chargers check the battery, adjust the current as it fills and then drop to a maintenance (float) mode. Most can be left connected safely. NRMA and RAC WA both point people towards this type.
• Simple trickle chargers have no electronics to check the battery and usually no overcharge protection. You have to watch them and switch them off yourself.
• Match the mode to the battery. Look at the battery label for “AGM”, “EFB” or “lithium”. EFB (enhanced flooded battery) is a tougher version of the standard flooded battery, built to cope with the constant restarts in stop-start vehicles. Flooded, EFB and AGM are all lead-acid batteries, and many chargers handle them on one setting: one Australian battery maker’s charger, for example, uses a single AGM/flooded/EFB setting. Others have separate AGM or “start-stop” modes, so check the charger manual and pick the setting it lists for your battery type.
• Lithium batteries need a lithium mode. Chargers built for lead-acid batteries can damage a lithium battery. Equally, don’t use a lithium setting on a lead-acid battery unless the charger’s manual says it’s suitable. Only use a charger the battery maker approves.
• Size the charger to the battery. A common guide is a charge current of about one tenth of the battery’s amp-hour (Ah) rating, for example about 6 A for a 60 Ah battery. Smart chargers set this themselves.
• Avoid “recovery” or high-voltage modes on a battery that’s still in the car. Some chargers have a reconditioning mode that goes above normal charging voltage. Battery makers advise disconnecting or removing the battery before using anything above 14.8 V.

Step by step

1. Check the battery first. Safety glasses and gloves on. If the case is swollen, bulging, cracked or leaking, or it smells of rotten eggs, don’t charge it.
2. Never charge a frozen battery. In alpine areas, a battery that has frozen can explode when charged. Let it thaw and have it checked.
3. Pick a ventilated spot. Charging lead-acid batteries gives off hydrogen, which is explosive. Work outdoors or in an open garage, away from flames, cigarettes, grinders and anything that sparks. Keep vent caps or vent holes clear.
4. Check your owner’s manual. Many modern cars say exactly where to connect a charger, and some have remote jump-start points in the engine bay. Note any warnings about battery sensors, stop-start or battery location (some are under a seat or in the boot).
5. Switch the car off. Ignition off, key out, and lights, radio and other accessories off.
6. Decide: in the car or out. With a smart charger you can usually charge the battery in the car. If you need to remove it, disconnect the negative (−) terminal first, then the positive (+), and lift the battery out upright. (see How do I replace a car battery?)
7. Make sure the charger is unplugged and switched off at the wall.
8. Connect the red clamp to the positive (+) post.
9. Connect the black clamp. If the battery is out of the car, connect it to the negative (−) post. If it’s in the car, follow the owner’s manual. Charger makers’ instructions generally say to use a bare, unpainted metal earth point on the body or engine, away from the battery and fuel lines, and to use the negative post only if there’s no suitable earth point. On cars with a battery sensor on the negative terminal, the earth point matters more: the charge has to pass through the sensor so the car’s battery management system can see it.
10. Select the battery type (standard, AGM, lithium) and the charge rate if the charger asks.
11. Plug in and switch on. Check the charger shows it has started and the polarity is right.
12. Check on it now and then. If the battery gets hot (NRMA gives about 50°C as the point to stop), bubbles violently or smells, switch off at the wall and stop.
13. Wait for the “full” or maintenance light. A smart charger moves to float mode when it’s done. Battery makers suggest not leaving a battery on a charger for more than 24 hours unless the charger is designed to maintain it.
14. Switch off and unplug the charger at the wall.
15. Remove the black clamp first, then the red.
16. Refit the battery if you removed it. Hold it down firmly, then connect the positive (+) first and the negative (−) last. Reset the clock, radio and windows if needed.
17. Test after it has rested. For a true reading, let the battery sit for several hours (overnight is best) and measure the voltage. About 12.6 V means fully charged. If it won’t hold charge, have it tested. (see How do I test my car battery?)

How long does it take?

It depends on how flat the battery is, its size and the charger’s current. RACV says charging at home can take anywhere from 2 to 24 hours depending on the charger. NRMA says a trickle charger can take from a few hours to more than a day on a flat battery. A rough estimate is the amp-hours you need to put back divided by the charger’s amps, plus extra time for the final stages, when a smart charger slows down.

Driving is not a good substitute for a charger. RAC WA notes that recharging a flat battery fully by driving can take up to eight hours of continuous driving, and NRMA warns that jump-starting and driving won’t fully recharge a completely flat battery.

Specs table

Charging voltages vary with temperature and battery design, so a smart charger with temperature compensation and the right battery mode does this for you.

What you’ll need

• smart battery charger (multi-stage, with modes for your battery type)
• lithium battery charger (only if you have a lithium battery)
• battery maintainer (for cars that sit for long periods)
• battery terminal brush
• safety glasses and nitrile gloves

When to call an auto electrician

• The battery is swollen, cracked, leaking, frozen or smells of rotten eggs.
• It won’t take a charge, or it goes flat again within days. There may be a drain or charging fault. (see Why does my car battery keep going flat?)
• Your manual says the battery must be charged or replaced by a dealer or workshop, or the battery needs registering after replacement.
• The car is a hybrid or EV. The 12V battery can usually be charged, but the high-voltage battery and orange cables are professional-only.
• You’re not sure which charger mode suits your battery.

Related questions

Q: Do I need to disconnect the battery to charge it? A: Usually not with a smart charger: charging in the car is fine if you follow the owner’s manual for where to connect. If you do disconnect it, take the negative terminal off first and put it back on last.

Q: Can I charge an AGM battery with a normal charger? A: Use a charger with an AGM mode or one the maker says suits AGM. The wrong charging profile can undercharge or overcharge an AGM battery and shorten its life.

Q: Can I leave a smart charger connected all the time? A: Most smart chargers drop to a maintenance mode when the battery is full and can stay connected, which suits cars that sit for weeks. Simple trickle chargers without this feature should be switched off when the battery is full.

Q: Is it safe to charge a car battery in the garage? A: Yes, if there’s good airflow and nothing nearby that sparks or burns. Charging lead-acid batteries gives off hydrogen, which can explode if it builds up.

Q: Will jump-starting my car charge the battery? A: No. A jump start only gets the engine going. A smart charger is the proper way to bring a flat battery back to full charge. (see How do I jump start a car safely?)

Sources

1. NRMA, “Car maintenance: charging your car battery” – smart vs simple trickle chargers; a few hours to more than a day; connect before switching on and switch off before disconnecting; flammable gases; never charge a frozen battery; stop if around 50°C. https://www.mynrma.com.au/open-road/advice-and-how-to/car-maintenance/charging-your-car-battery (checked October 2026)
2. RAC WA, “Can flat car batteries be charged?” – hydrogen and oxygen gases need ventilation; don’t charge a cracked battery; smart multi-stage chargers; up to eight hours of continuous driving to fully recharge. https://rac.com.au/horizons/drive/recharge-flat-battery (checked October 2026)
3. NRMA, “Common car battery myths” – jump-starting and driving won’t fully recharge a completely flat battery. https://www.mynrma.com.au/open-road/advice-and-how-to/car-maintenance/car-battery-myths (checked October 2026)
4. RACV, “Car battery replacement” – home charging 2 to 24 hours depending on the charger; battery registration on some vehicles. https://racv.com.au/on-the-road/roadside-assistance/car-battery-replacement.html (checked October 2026)
5. RACV, “Car battery guide” – 12.6 V fully charged (rested). https://www.racv.com.au/royalauto/transport/cars/car-battery-guide-how-to-know-if-it-needs-replacing.html (checked October 2026)
6. Banner Batteries, “How to: correctly charging the AGM, EFB and SLI car battery” – 14.8 V maximum charging voltage at 25°C; stop if acid exceeds 55°C; little charge accepted below 0°C; charge current about one tenth of capacity; disconnect before modes above 14.8 V; ventilation. https://www.bannerbatterien.com/en/Battery-knowledge/1-HOW-TO-Correctly-charging-the-AGM-EFB-SLI-car-battery (checked October 2026)
7. Banner Batteries, “Battery sensor – application and function” – sensor at the negative terminal; charging current must pass through the sensor; black clamp to bare-metal earth. https://www.bannerbatterien.com/en/Battery-knowledge/20-Battery-sensor-%E2%80%93-application-and-function (checked October 2026)
8. VARTA, “How to charge car batteries at home” – ventilation in enclosed spaces (explosive hydrogen); charger mode depends on battery type; no more than 24 hours on the charger. https://www.varta-automotive.com/knowledge/articles/article-details/charging-car-batteries (checked October 2026)
9. Century Batteries, “How to charge a lead-acid car battery” – charge rate about 10% of Ah; connection and disconnection order; check for bulging or damage first (official guide; see for model-specific steps). https://www.centurybatteries.com.au/battery-hub/expert-advice/how-to-charge-a-lead-acid-car-battery (checked October 2026)
10. Century Batteries, “How to maintain your lithium deep cycle battery” – use a charger with a dedicated lithium mode; lead-acid chargers can damage lithium batteries. https://www.centurybatteries.com.au/battery-hub/expert-advice/how-to-maintain-your-lithium-deep-cycle-battery (checked October 2026)
11. CTEK, “Charging an AGM battery: settings, safety and best practice” – incorrect charging profile can undercharge or overcharge an AGM battery (official guide). https://ctek.com/en-us/blogs/ctek-magazine/charging-an-agm-battery-settings-safety-and-best-practice (checked October 2026)
12. Century Batteries, CC1212-XLi battery charger owner’s manual – single charge setting for AGM, flooded and EFB batteries (14.4 V absorption, 13.6 V float). Also: on negatively earthed vehicles, red clamp to battery positive first, then black clamp to the vehicle’s chassis, away from fuel lines. https://www.centurybatteries.com.au/media/tapbtybn/cc1212-xli_century_owners_manual_0123_r12.pdf (checked October 2026)
13. CTEK, “How to connect a car battery charger” and Australian FAQ – connect the black clamp to unpainted metal on the chassis away from the battery, negative post only as a fallback; on vehicles with a battery management system, connect to the earth point rather than the negative post. https://www.ctek.com/au/faq (checked October 2026)