How long does it take to charge a dead car battery?

The time it takes to recharge a dead car battery depends on the battery’s degree of discharge and the current rating (i.e., operating current) of the battery charger. View the full range of battery chargers at NOCO Genius Multi-Purpose Battery Chargers. The storage capacity of a lead-acid battery is measured in ampere-hours (Ah). The number of ampere-hours that have been drawn (also known as the depth of discharge), represents the number of ampere-hours that need to be replenished. For example, if a 100-ampere-hour battery has been drawn 50% (or 50 ampere-hours), you should return to the 50-ampere-hour battery. The battery charger’s operating current rating helps determine how quickly the battery can be charged. For example, if you had a 10-ampere battery charger, it would take approximately 5 hours to complete the recharge. Let's explore this relationship of depth of discharge rates and recharge rates in more detail.

As explained above, determining the recharge time of a dead battery is simple, namely:

Battery Amp-hours x Depth of Discharge = Capacity Amp-hours consumed / Charger operating current rating

Of course, this is a very simplified calculation, as it assumes that the connected battery delivers the same amperage from start to finish. In practice, most battery chargers, especially smart battery chargers, use a multi-step charging process to replenish the lost battery capacity.

Another way to determine the depth of discharge of batteries is to understand the relationship between the open circuit voltage (OCV) and the residual capacity of the battery. The graph below illustrates this relationship:

To determine the depth of discharge (or remaining capacity), you will need to measure the OCV using a voltmeter. For example, if the OCV is measured at 12.0 Volts, then the depth of discharge is approximately 45%. To further illustrate this example, let’s assume we have a Class 24 AGM (Absorbent Glass Mat) battery rated at 84 amp hours. At a 45% depth of discharge (DOD), we have exhausted approximately 54 amp hours. If we were to charge the battery with a 15 amp battery charger, the recharge time would be approximately 3.6 hours. Again, this example does not take into account the multi-step charging approach of most smart battery chargers, but it does serve as a great guide to battery recharge times.

Choosing the right battery charger capacity

The easiest way to find the right battery charger is to use 7-10% of the amp hours of your battery's rated capacity. For example, for a 50 amp hour battery, you should choose a battery charger with a maximum current rating of 3.5-5.0 amps.