Learn how to charge a battery without a designated charger.
With technical knowledge, batteries can be charged manually with a power supply featuring user-adjustable voltage and current limiting. I stress manual because charging cannot be left unattended; charge termination is not automated. You need to observe the state-of-charge according to voltage and current behaviors. Lower the charge voltage or disconnect the charge when the battery is full. Because of difficulties in detecting full charge with nickel-based batteries, I recommend to charge only lead and lithium-based batteries manually.
Before connecting the battery, calculate the charge voltage according to the number of cells in series, and then set the desired voltage and current limit. To charge a 12-volt lead acid battery (six cells) to a voltage limit of 2.40V, set the voltage to 14.40V (6 x 2.40). Select the charge current according to battery size. For lead acid this is between 10 and 30 percent of the rated capacity. A 10Ah battery at 30 percent charges at about 3A. Starter batteries charge at lower currents, and an 80Ah pack would charge at about 10 percent of the rating, or 8A. Higher currents are possible.
Observe the battery temperature, voltage and current during charge. Charge only at ambient temperatures in a well-ventilated room. Once the battery is fully charged and the current has dropped to three percent of the rated Ah, the charge is completed. Disconnect the charge. High self-discharge (soft electrical short) may prevent the current from going to the anticipated low current level when fully charged. Disconnect the charge also when the current has bottomed out and cannot go lower. If you need float charge for operational readiness, lower the charge voltage to about 2.25V/cell.
You can also use the power supply to equalize a lead acid battery by setting the charge voltage 10 percent higher than recommended. The time in overcharge is critical and must be carefully observed. When using the power supply to perform equalizing, refer to the previous section entitled “Equalizing Charge.”
A power supply can also reverse sulfation but there is no guarantee of success. When applying a charge, a totally sulfated lead acid may draw very little current at first, and as the sulfation layer dissolves the current will gradually increase. If you must increase the charge voltage above the recommended level, set the current limiting to the lowest practical value and observe the battery voltage. If the battery does not accept a charge after 24 hours, restoration is unlikely. [BU-804, Sulfation]
Lithium-ion charges similarly to lead acid and you can use the power supply also but use extra caution. Set the voltage threshold to 4.20V/cell and make certain that none of the cells connected in series exceeds this voltage. (The protection circuit in a commercial pack does this.) Full charge is reached when the cell(s) reach 4.20V/cell voltage and the current drops to three percent of the rated current, or has bottomed out and cannot go down further. Once fully charged, disconnect the battery. Never allow a cell to dwell at 4.20V for more than a few hours. [BU-409, Charging Lithium-ion]
Please not that not all Li-ion batteries charge to 4.20V/cell. Lithium iron phosphate typically charges to 3.65V/cell and lithium-titanate to 2.85V/cell. Some high-energy cells may accept 4.30V/cell. It is important to observe these voltage limits. (See BU-205: Types of Lithium-ion.)
I do not recommend charging nickel-based batteries with a power supply. Full-charge detection is difficult to assess because the voltage signature varies with the applied charge current. If you must charge, use the temperature rise on a rapid charge as an indication for full charge. When charging at a low current, estimate the level of remaining charge and calculate the charge time. An empty 2Ah NiMH will charge in about three hours at 750–1,000mA. The trickle charge, also known as maintenance charge, must be reduced to 0.05C. [BU-407, Charging Nickel-cadmium; BU-408, Charging Nickel-metal-hydride]
Last updated 2015-03-28
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