Battery Equivalents and Replacements

How Long Does It Take to Charge a Car Battery?

Regardless of the car battery type or brand, from time to time, it may become necessary to recharge the car battery using the battery charger. Was the battery discharged due to the parasite load, multimedia devices, or for some other reason, this is not the topic of this article - however, if that happens several times over a short period of time, check the alternator and the rest of the electric system of your car.

When the battery is taken out of the car, the car should not, and most modern cars cannot be used - in that situation, many people wonder how long it takes to recharge the car battery ...

Updated: February 1, 2023.

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Car Batteries Short Intro

Car batteries are mostly lead-acid batteries used as starting/cranking batteries and for powering other electric loads while the engine is turned off.

Most lead-acid batteries are either wet/flooded or AGM (Absorbent Glass Mat) batteries, although some brands offer Gel-Cell SLA batteries as well.

Lithium-ion starting batteries are much lighter, but lithium batteries are not often used as starting batteries, but as deep cycle batteries in hybrid cars or electric-only cars, where their higher price is less important when compared with total weight savings.

Also, lithium deep cycle batteries, usually Lithium Iron Phosphate (LiFePO4) batteries, are often used as car audio batteries in systems where starting batteries (typically lead-acid batteries) must be isolated from the deep cycle battery.

Cars with internal combustion engines automatically recharge the battery and keep it (almost) fully charged all the time.

However, a car battery can become discharged for many reasons:

  • car audio amplifierthe car wasn't used for a longer period (a few months, for example) of time and parasite loads (security system, for example) eventually discharged the battery,
  • multimedia and other devices were used for a too long time while the engine was off (single battery car electric systems),
  • alternator or some other component of the onboard battery charging system failed, etc.

When something like this happens, perhaps the best course of action is to take the battery out of the car and recharge it using a dedicated smart lead-acid battery charger.

Note: some cars don't support battery removal except in the authorized mechanic shops (safety reasons) - before removing the car battery, check the Owner's Guide of your car to avoid any additional issues or call the authorized dealership and ask.

Also, some battery chargers that feature desulfation and similar charging modes should NOT be used to recharge the car battery while the battery is still connected to the car's electric system - spikes of high voltages used to break/remove deposits of lead sulfate can damage the car's electronics.

Charging Current vs. Battery Capacity

Car batteries usually can accept charge very well, especially modern lead-acid batteries used for stop&go eco-driving.

However, when the battery is charged at home, in order to keep things as safe as possible, don't recharge the battery with the currents that can recharge a fully discharged battery in less than 4-5 hours, and even that can be too short.

For example, if the battery features a nominal capacity of 60 Ah, a smart multi-mode, multi-type lead-acid battery charger with a maximum charging current of 12-15 Amps max can be used, although a 7-10 Amps battery charger is recommended to avoid stressing the battery too much.

15 Amps lead-acid battery charger can fully recharge 60 Ah battery discharged down to 80% DoD (a not recommended situation, but it may happen) in under 4 hours easily:

60 Ah * 0.8 = 48 Ah → 48 Ah / 15 Amps = 3.2 hours

However, smart battery chargers analyze the battery and adjust charging according to the battery condition, and in the end, they can even perform cells' equalization - that is why such battery chargers may require 4 hours, or maybe even more, to charge the battery with 48 Ah of current/charge required.

Similarly, smaller car starting batteries in the 45-50 Ah range can be quickly recharged using smaller battery chargers.

For example, if the Optima 8020-164 35 RedTop 12V 44Ah battery is discharged down to 80% DoD, 10 Amps smart battery charger can recharge it in under 4 hours, too:

44 Ah * 0.8 = 35.2 Ah → 35.2 Ah / 10 Amps = 3.52 hours

On the other hand, if You have a large 12V 100Ah battery that is discharged down to 80% DoD and You have 'only' 10 Amps battery charger, such a battery charger can easily recharge such a battery in under 9 (nine) hours:

100 Ah * 0.8 = 80 Ah → 80Ah / 10 Ah = 8 hours

Again, smart battery chargers don't recharge the battery the whole time with the same current - they are not 'Constant Current' chargers. That is why 10 Amps battery charger requires somewhat more than 8 hours to recharge the battery with the 80Ah of charge - usually, 9-10 hours, depending on the battery condition.

Note: if the battery has issues and the battery charger must go through desulfation mode and, in the end, it also goes through equalization mode, the charging cycle itself can be even longer than 9-10 hours. If that happens, the best thing to do is to check the battery charger status and let it do its task - most probably smart battery charger just saved You the battery and prolonged its operating life ...

Such multi-stage battery chargers are relatively cheap due to the advances in electronics and technology in general and can come in very handy.

For charging smaller AGM, Gel-Cell, wet/flooded lead-acid batteries that are used in motorcycles, ATVs, snowmobiles, power generators, and similar, battery chargers in 1-5 Amps (sometimes even more) are used, depending on the actual battery size.

Charging The Car Battery While Driving

When the car battery voltage drops down to 12.0-12.2 volts, depending on the temperature and exact chemistry, the battery is discharged down to 40-60% DoD.

The battery can still crank the engine, but there is a great chance that the engine's control electronics automatically have turned off the Start&Stop system - when that happens, the driver receives a warning on the dashboard.

Many people wonder how long one has to drive to recharge the engine - if the battery successfully cranked the engine, but there is a Start&Stop system warning on the dashboard, one has to drive at least 30-40 minutes to fully or almost fully recharge the car's battery.

Note: to recharge the battery faster, don't keep the engine idle (usually around or even below 700-800 RPM) - drive around and let the alternator charge the battery faster.

car alternator

Charging Almost Completely Discharged Car Battery

Sometimes car's battery gets discharged down to almost 0 (zero) volts, and in such condition, many jump starters and battery chargers cannot detect the battery and cannot help the user jump-start the car (jump starters) or recharge the battery (battery chargers).

Note: discharging the lead-acid battery that low can damage the battery easily, but if the battery is revived quickly, one can restore it probably to its 90-95% nominal capacity, sometimes even slightly more. But, it should be avoided as much as possible.

But, if that happens, there are several options, including:

  • get a jump starter that allows the user to override the safety features of the jump starter and force a jump start - again, this is NOT a situation that should be happening on a regular basis, but it can be a lifesaver. After jump-starting a car, let the car recharge the battery by driving it around for at least 60-120 minutes.
  • get a car battery charger that can revive and recharge highly discharged batteries. When such a mode is activated, the battery charger pumps a current of certain amperage into the battery regardless of how low the voltage is and, after some time, measures the battery's voltage - these cycles can be repeated several times until the battery's voltage reaches a certain threshold, after which the battery is charged using standard charging modes.
  • jump-start a car with a bad battery using another car with a good battery: turn on the engine of the second car, connect the batteries (positive terminals first), and rev up the engine of the second car at least a little bit for a few minutes, but not more than that - in such setup, second car's alternator provides plenty of currents and it may get damaged due to the overheating! But, this will build at least some "surface" charge in the bad battery (actually, the battery is not bad, it is good, but it is "only" very, very discharged) and most probably let the user jump-start the first car engine - now, drive the car with bad battery for at least 60-120 minutes in order to recharge the battery (almost) completely.

Note: if the car battery was discharged down to almost 0 (zero) volts and after jump-starting a car or recharging the battery using a multi-stage battery charger, the battery appears good, do yourself a favor and take the car to the car shop so that the battery and car's electric system can be checked - better safe than sorry.

charging car battery

Car Battery Charging Frequently Asked Questions (FAQ)

Here are some of the most common Frequently Asked Questions (FAQ) about car battery charging:

Does idling a car charge the battery?

Yes, but at a very low rate - when driving around, it takes 30-90 minutes to recharge the battery, while the engine on idle sometimes requires hours to recharge the battery.

How long does it take to charge a car battery with a 12-volt charger?

This depends on the battery Depth of Discharge percentage (%DoD), battery capacity and condition, battery charger charging current, charging modes, and similar.

For example, if the 60Ah battery is discharged down to 50% DoD, a 10 Amps advanced battery charger can recharge it in ~4 hours.

However, if the battery is not in the best condition and sulfation is detected, and also the cells need equalization, this charging can last 5-6 hours, maybe even more...

Can you leave a car battery charger on too long?

Old battery chargers have the tendency to overcharge the battery if left too long - after recharging wet/flooded lead-acid batteries, it is always recommended to check the level of the electrolytes and to add distilled water if required.

AGM and Gel-Cell lead-acid batteries should be recharged with advanced multi-stage lead-acid battery chargers that monitor battery condition and switch automatically between modes, preventing overcharging the battery and often maintaining the battery for a longer period of time.

How do you know when a car battery is fully charged?

Check the battery voltage or check the status of the battery charger.

What should a 12-volt battery read when fully charged?

Depending on the lead-acid battery chemistry, a fully charged lead-acid battery should read between 12.6 and 12.8V when the engine is turned off.

Can you jumpstart a completely dead battery?

Yes, a car with a completely dead battery can be jumpstarted, but be aware that your alternator will work very heavily first 30-60 minutes.

Can a completely dead battery be recharged?

If there are no short-circuited cells, then yes, a completely dead lead-acid battery can be fully recharged up to 90-95+% of the battery's nominal capacity and reserve capacity (RC).

Just be sure to use advanced battery chargers to charge such batteries or raise the battery voltage by other means (connecting another car for a few minutes, for example).

Will leaving something plugged into a cigarette lighter drain the battery?

It depends on the car's electric system - some cars disconnect the cigarette lighter when the key/card is out of the ignition, allowing the user to leave devices connected to the cigarette lighter port.

If the cigarette lighter port is NOT disconnected when the key/card is out of the ignition, then connecting devices to this port may discharge the car battery.

How long can you leave a car battery charging? Does leaving a charger plugged in damage the car battery?

Advanced battery chargers with battery selection and temperature probes also feature a maintenance mode - after fully recharging the battery, they switch from charging to maintaining the battery and can be left connected to the battery for a very long time.

Old-style battery chargers can overcharge the battery, causing the wet/flooded batteries to lose water and AGM/Gel-Cell batteries to swell and, in the end, to lose the water via safety valves.

What is a trickle charger?

The trickle charger sends a constant but low current into the battery, keeping it fully recharged over a longer period of time.

The current is small enough not to cause issues with the battery in the long run.

After trickle charging wet/flooded lead-acid batteries, it is highly recommended to check the electrolyte level and, if required, to add some distilled water.

car battery fluid w600px

Is 30 minutes enough to charge a car battery?

30 minutes is usually enough to recharge a good car battery that was, for some reason, discharged low enough to prevent it from cranking the engine - but the dashboard and onboard electronics worked well.

Note: after 30 minutes, the battery probably won't be fully recharged, but it will be charged enough to crank the engine on its own.

However, if the battery is completely dead, 30 minutes are usually not enough.

And if the battery is completely dead, it is recommended to recharge it using an advanced battery charger that will try to desulfate and balance cells.

car in the wood w300pxLong Story Short: Car battery charging times depend on the battery capacity, depth of discharge (DoD), battery charger charging current, battery charger type, etc.

When looking for a suitable battery charger for your car battery, go for a smart AGM battery charger that also supports the charging of other lead-acid battery types (wet/flooded, Gel-Cell, Enhanced wet/flooded), but also if possible, that supports the charging of lithium-ion batteries as well.

Since most cars come with 50-100 Ah lead-acid batteries, car battery chargers in the 5-12 Amps range are perhaps the best choice for 'quick charging' and avoiding additional load on the batteries.