Battery charging tutorial-Part 1
18 Apr 2022

Current battery charging technology relies on microprocessors (computer chips) to recharge; using 3 stages (or 2 or 4 stages) regulated charging. These are the "smart chargers", and quality units generally are not found in discount stores. The three stages or steps in lead/acid battery charging are bulk, absorption, and float. Qualification or equalization is sometimes considered another stage. A 2 stage unit will have bulk and float stages. It is important to use battery manufacturer's recommendations on charging procedures and voltages, or a quality microprocessor controlled charger to maintain battery capacity and service life.

The "smart chargers" are profiled with contemporary charging philosophy in mind, and also take information from the battery to provide maximum charge benefit with minimum observation. Some gel cell and AGM batteries may require special settings or chargers. Our units are selected for their suitability on the battery types they specify. Gel batteries generally require a specific charge profile and a gel specific or gel selectable or gel suitable charger is called for. The peak charging voltage for Gel batteries is 14.1 or 14.4 volts, which is lower than a wet or AGM type battery needs for a full charge. Exceeding this voltage in a Gel battery can cause bubbles in the electrolyte gel, and permanent damage.

Most battery manufacturers recommend sizing the charger at about 25% of the battery capacity (ah = amp hour capacity). Thus, a 100 ah battery would take about a 25 amp charger (or less). Larger chargers may be used to decrease charge time , but may decrease battery life. Smaller chargers are fine for long term floating, eg a 1 or 2 amp "smart charger" can be used for battery maintenance between higher amp cycle use. Some batteries specify 10% of capacity (.1 XC) as the charge rate, and while this doesn't hurt anything, a good microprocessor charger of the appropriate charge profile should be fine up to the 25% rate. You talk to different engineers, even at the same company, you get different answers.

Three Stage Battery Charging

The BULK stage involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger), and voltage increases. The properly sized charger will give the battery as much current as it will accept up to charger capacity ( 25% of battery capacity in amp hours), and not raise a wet battery over 125° F, or an AGM or GEL (valve regulated) battery over 100° F.

The ABSORPTION stage (the remaining 20%, approximately) has the charger holding the voltage at the charger's absorption voltage (between 14.1 VDC and 14.8 VDC, depending on charger set points) and decreasing the current until the battery is fully charged. Some charger manufacturers call this absorption stage an equalization stage. We don't agree with this use of the term. If the battery won't hold a charge, or the current does not drop after the expected recharge time, the battery may have some permanent sulphation .

The FLOAT stage is where the charge voltage is reduced to between 13.0 VDC and 13.8 VDC and held constant, while the current is reduced to less than 1% of battery capacity. This mode can be used to maintain a fully charged battery indefinitely.

Recharge time can be approximated by dividing the amp hours to be replaced by 90% of the rated output of the charger. For example, a 100 amp hour battery with a 10 % discharge would need 10 amps replaced. Using a 5 amp charger, we have 10 amp hours divided by 90% of 5 amps (.9x5) amps = 2.22 hour recharge time estimate. A deeply discharged battery deviates from this formula, requiring more time per amp to be replaced.

Recharge frequency recommendations vary from expert to expert. It appears that depth of discharge affects battery life more than frequency of recharge. For example, recharging when the equipment is not going to be used for a while (meal break or whatever), may keep the average depth of discharge above 50% for a service day. This basically applies to battery applications where the average depth of discharge falls below 50% in a day, and the battery can be fully recharged once during a 24 hour period.

If you have any requirements or any kind of query regarding the battery charger solutions, feel free to communicate with our dedicated team at any time at

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