How long it will take to recharge a battery is a question we are asked often, and the answer can vary widely. Many factors have to be taken into consideration, including the Ah rating of the battery, the depth of discharge, temperature and the output capacity of the charger. If the battery being recharged is still supplying parallel loads, this also needs to be accounted for. For example, if you are taking 10 amps out of the battery while recharging at 20 amps, you are only charging the battery at a net rate of 10 amps.
The type of charger used is also important. Any good quality charger should be capable of supplying at least a three-stage charge algorithm (Bulk, Absorption, Float) and have a charge output sized to the Ah rating of the battery. This should ideally be 20% of the Ah rating, so a 100Ah battery would require a 20A charger. When charging, any active parallel loads also need to be taken into account.
Effectively, a 100Ah battery with a 10 amp load present would require a charge rate of 30 amps to achieve the 20% recharge rating.
A lead-acid battery is about 80% charged at the beginning of the absorption period. The time to reach 80% can be calculated as Ah / I, where I is the charge current minus any load current and Ah is the amount of charge to be replaced. To reach a 100% state of charge (SOC), an absorption period of about 8 hours is required.
This is also where lithium batteries come into their own: they are 95% charged at the start of the absorption period and reach 100% state of charge after only 30 minutes of absorption.
When a battery approaches the end of its useful cycle life, or fails prematurely due to sulfation or corrosion, its capacity is considerably reduced and its internal resistance increases. A short or reduced bulk phase during recharging is an indication that the battery is at the end of its useful life.
If you would like to discuss any issues, or have concerns over your choice of battery type or charger, contact our help line on 0151 482 8970.