How long to charge a car battery, you asked. Lots of people want to know how long it will take to charge a flat car battery, and if there was a simple answer I would give it to you. There are flat batteries and there are dead batteries. A completely dead battery cannot even be charged again, but a partially-dead battery may be revived — at least for a while — if you have a really big and powerful battery charger, such as the large ones the mechanics wheel out when you’re at the local garage. So there are different levels of discharge in a flat or dead battery.
To make matters more complex, there are also many different types and sizes of vehicle batteries – even for 12 volt car batteries. A battery with a larger capacity will take longer to charge because it can store more electricity inside. The amount stored in a battery is often rated in Ampere-Hours (amp-hours), which is usually abbreviated to A/H or just AH. A 12 volt auto battery that is marked 200 AH is supposed to be able to supply 12 volts at 1 Ampere for 200 hours, at 20 Amps for 20 hours or at 200 Amps for just one hour. That’s the theory at least. In practice, the battery is unlikely to able to supply that much power for so long, but it is a standard people use to compare different battery sizes. You will note that in this case the three examples are all for batteries rated at 12 volts DC. (Batteries are always DC or direct current. However they can power electronic devices such as a DC to AC inverter, which then produces an AC current as its finished product.)
How long to charge car battery? Well, just how long is a piece of string?
Just as there are different batteries, there are also a variety of car battery chargers. I am not talking about brand-names here, I am talking about design types, in car batteries and in car battery chargers or automobile battery chargers if you will. They all differ in price, their power throughput (in Amperes) and the complexity — and therefore the cost — of their electronic circuitry.
The amps a battery charger can handle depends mainly on the size of the ac transformer it uses. The transformer takes the 110v or 220v ac mains power and drops it down to around 15 volts or thereabouts, so the electronic circuit can change it to a dc current and use that to ‘fill up’ the battery with an electrical charge.
The cheapest circuits have no timer and leave it up to you to disconnect the battery before it can get over-charged. Some of them have a very slow rate of charge that is so low that you could almost leave it on permanently without damaging the battery. A great example of this is a solar car battery charger. This is a solar panel you can leave on your dashboard to catch the sun during the day. It only ever puts out a few hundred milliamps (one-thousandths of one amp) into the car battery, so it is a trickle-charger and can be left there, like forever, and never overcharge the car battery. The solar charger connects to the battery with a set of clamps, or sometimes through the cigarette lighter socket. (Not all cars will let you use the lighter socket, since most disconnect it from the battery when the vehicle’s ignition is switched off.)
The next kind of battery charger to consider is mains operated, and usually has a switch where you can choose how many amps you want to charge your battery at. They may have a switch with a high/low setting or one that chooses either, for example, 1 amp or 4 amps. And again, if you leave this connected for too long, the overcharging will damage your car battery.
The better types of car battery chargers have measuring circuits inside them that calculate how much charge your vehicle battery needs, and for how long. They monitor the voltage (in volts) and the current (in amps) and are designed to detect when the battery has been fully-charged. The smart battery charger then switches over to a trickle-charge, which sends just enough electricity into your auto battery to maintain that full charge all the time. So, if you like, you can leave the charger on permanently — or at least until you want to start up the engine and maybe drive the vehicle away.
There are charging-time tables which can help you work out how long it will take to charge your car battery. However, you will need to know the reserve capacity rating (RC) of your vehicle battery. Or, in the case of some motorcycles or ride-on-mowers, their batteries are rated in ampere hours (AH), and you will need to know that figure.
| C H A R G I N G T I M E
Time To Charge Fully Discharged Battery |
||||||
| Battery Type | Battery Rating | Charger Output Rating (Amps) | ||||
| 2 | 6 | 10 | 12 | 30 | ||
| Cars/Trucks etc. | RC | Time to charge your battery (hours) | ||||
| 315 CCA | 60 | 20 | 7 | 4 | 3 | 1 |
| 450 CCA | 70 | 23 | 8 | 5 | 4 | 2 |
| 550 CCA | 85 | NR | 9 | 6 | 5 | 2 |
| 850 CCA | 140 | NR | 16 | 9 | 8 | 3 |
| 1050 CCA | 165 | NR | 18 | 11 | 9 | 4 |
| Note change in ratings below to AH | AH | Time to charger your battery (hours) | ||||
| small Motorcycle Garden Tractor |
12 | 7 | NR | NR | NR | NR |
| large Motorcycle Garden Tractor |
32 | 18 | 6 | 4 | 3 | NR |
| Deep Cycle | 55 | NR | 10 | 6 | 5 | NR |
| Deep Cycle | 80 | NR | 15 | 9 | 7 | 3 |
| Deep Cycle | 105 | NR | 20 | 12 | 10 | 4 |
| NR = Not Recommended for this size battery. | ||||||
Charging times for your battery might be different from these. If your battery is only half discharged, then you will only need half the time to charge it. It is also possible that some older batteries may not accept a charge at all and will heat up on charging. So if the battery gets hot (above 125 degrees F) or if acid comes out of vent caps, stop charging it right away. Take the car battery to an auto electrician and have it tested by a professional.
So, how long to charge a car battery? Just as long as it needs, and no more!