Blog - How Long Does It Take To Charge A Dead Car Battery?

How Long Does It Take To Charge A Dead Car Battery? May 16, 2016

Understanding The Charge Demands of Your Battery

Author : Brittany Lin Team NOCO Leader

How long it takes to recharge a dead car battery depends on how discharged the battery is, and the current rating (i.e. amperage) of the battery charger. See our full line of battery chargers at NOCO Genius Multi-purpose Battery Chargers. The storage capacity of a lead-acid battery is rated in amp-hours (Ah). How many amp- hours that are drained (also known as, depth-of-discharge), represent the number of amp- hours that need to be replenished. For example, if a 100 amp-hour battery was drained 50% (or 50 amp-hours), you would need to return 50 amp-hours back into the battery. The amperage rating of the battery charger will help determine how fast the battery can be recharged. For example, if you had a 10-amp battery charger, it would take roughly 5- hours to recharge. Let’s explore this relationship of depth-of-discharge and recharge rates in more detail.
As explained above, determining the time to recharged a dead battery is simple, which is:

Now, this is a very simplified calculation, as it assumes the connected battery is delivering the same amperage from start to finish. In actual practice, most battery chargers, in particular smart battery chargers, utilize a multi-step charging process to replenish lost battery capacity.

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

To determine the depth-of-discharge (or residual capacity), you will need to measure the OCV using a voltmeter. For example, if the OCV measure 12.0-volts, then the depth-of- discharge is roughly 45%. To further illustrate this example, let’s say we have a group 24 AGM battery rated at 84 amp-hours. At 45% DOD, we have depleted roughly 54 amp- hours. If we were recharging the battery with a 15-amp battery charger, the recharge time would be roughly 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 an excellent guide to battery recharge times.

Sizing The Right Battery Charger

The easiest way to determine the appropriate battery charger is to use 7 - 10% of the rated amp-hour capacity of your battery. For example, if you had a 50 amp-hour rated battery, you would to select a battery charger with a peak amperage rating of 3.5-5.0 Amps.