The degree of drain and the battery charger's current rating, or amperage, determines how long it takes to recharge a dead motorcycle or car battery.
A battery's storage capacity is expressed in amp-hours (Ah). The quantity of amp-hours that need to be refilled is represented by the depth-of-discharge, or how many amp-hours are drained.
You would need to add 5 amp-hours back into a 10 amp-hour battery, for instance, if it had been depleted by 50%, or 5 amp-hours.
How quickly the battery can be recharged will be aided by the battery charger's amperage rating.
For instance, if you had a 1-amp battery charger, it would take approximately 5 hours to recharge a 10-amp battery that was at 50% discharged status. Another way of looking at this, is that you need to add 5 amps to this battery, to charge it to 100% and, in this example, your charger can add 1-amp of power, per hours. So 5-amps requires 5-hours of charge.
Let's take a closer look at the connection between recharge rates and depth-of-discharge.
Battery Amp-Hour Rating x Depth of Discharge = Used Amp-Hour Capacity / Charger Amp Rating
This equation, however, is extremely basic because it relies on the attached battery providing the same amount of current throughout. In reality, to restore lost battery capacity, the majority of battery chargers—especially smart battery chargers—use a multi-step charging procedure.
The depth-of-discharge, also known as residual capacity, must be ascertained by voltmeter readings of the Open Circuit Voltage (OCV). In the event where the OCV registers 12.0 volts, for instance, the depth-of-discharge is around 45%. Let's take an example where we have a group 24 AGM battery with an 42 amp-hour rating to better explain this. We have used about 27 amp-hours at 45% DOD.
The battery would take about 15 hours to fully recharge if we used a 3.5 amp battery charger, like the Provacell HyperCharge. Once more, this example does not account for the multi-stage charging process that the majority of smart battery chargers use, but it is still a great resource for battery recharge timings.
Appropriate Battery Charger Sizing
Using 7 to 10% of your battery's rated amp-hour capacity is the simplest method to choose the right battery charger. For instance, you would choose a battery charger with a peak amperage rating of 3.5–5.0 Amps if your battery was rated for 50 amp hours.
