How Long Does It Take To Charge A Car Battery With A Trickle Charger?

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Keeping your car battery charged isn’t much of an issue on your daily driver, but it is critical if you keep a classic car or high performance car for occasional weekend driving.

When a car battery is not regularly recharged by running the car, it must be recharged from an external power source to keep it in good working condition.

Otherwise, the battery will continuously lose charge over time, causing the cells to deteriorate and the battery to lose all capacitance. In other words, your car battery dies and becomes useless.

Rather than having to buy a new battery for your secondary car every couple years, you can keep your battery in good condition with regular charging cycles using a car battery charger.

Let’s take a look at the types of battery chargers available, and why trickle chargers are best for long term storage.

Fast Chargers For Car Batteries

Any automotive battery charger that draws 20 amps or more is considered a fast charger. These significantly decrease the time it takes to recharge your car’s battery when the voltage has dropped too low, but there are a few drawbacks that come with that reduced charging time.

First, quick chargers use a significant amount of electricity and can create excess heat, potentially damaging your car battery and drastically reducing the service life. Second, they use a tremendous amount of electricity, and it’s  got a tendency to spike your utility bill a bit if you use one regularly.

Finally, when you use a high amperage charger to quick charge your car battery, there is a significant risk of overcharging. Over time, repeated overcharging and fast charging will shorten the service life of your battery.

Trickle Chargers For Car Batteries

This is the preferred method for keeping your car battery topped off if you don’t drive that car or vehicle except for a few days a year. RV owners, ATV riders, motorcycle owners and other seasonal vehicle operators prefer trickle chargers for many reasons, but everyone’s primary reason is it doesn’t kill your battery’s service life at an accelerated rate.

Trickle chargers can take as long as 48 hours to charge a car battery to full capacity, mostly because the average trickle charger only uses about 1-2 amps. The advantage of the slower charging rate is it doesn’t run the risk of overcharging the battery and it keeps it from overheating. You can leave a battery on a trickle charge for days or months at a time and your battery will stay in prime condition to fire up and go for a drive.

Calculating Charge Time

If your car battery has discharged to the point that you can’t get your ride to start, you will need to calculate the charge time. This involves reading the spec sheet and doing some simple math if you have a multimeter handy. You can also buy an automotive battery tester to determine how much your battery discharged and the estimated charge time, but battery testers aren’t always available. If you don’t have access to a car battery tester, you can use the following:

Reserve Minutes x 0.6= Reserve Capacity in Amp Hours

100 minute reserve capacity battery x 0.6 = 60 amp hours reserve capacity

Test Open Circuit Voltage (one probe touching a terminal of a disconnected battery)

12.2 voltage and below = 50 percent or less capacity

60 amp hour battery at 50 percent charge = 30 amp hours

Calculate for Battery Resistance by adding 20 percent to the difference between current charge and capacity: 30 amp hours x 1.2 (adding 20%) = 36 amp hours

Take the amp hours to a full charge and divide it by the charging rate of your car battery charger: 36 amp hours/10 amp hour charging rate= 3.6 hours to recharge your battery to 100 percent.

For a trickle charger that pushes only 2 amp hour charge rate, you would need 18 hours to reach a full charge. At a 1 amp hour charge, you’d need the full 36 hours to safely charge the battery back to 100 percent. However, trickle chargers don’t pose the same risks as using a slow or fast charger to recharge your car battery.

So the ultimate answer to the question in the introduction is this: define the reserve capacity of your battery in amp hours (0.6 x Reserve Minutes=Amp Hours), then divide by the amp hour recharging rate of your trickle charger. The lower the amperage of the charger, the lower the charge remaining in the battery and the higher the amp hours of your battery, the longer it will take to recharge the battery.

You can anticipate for most car batteries that are completely dead that you will spend anywhere from 30-60 hours using a 1 amp hour trickle charger, and half that time using a 2 amp trickle charger.