Chapter Three - Charging Batteries

Last issue, we explored the various types of batteries that you might encounter for powering the lights and other loads on your house bus or truck, or in a stationary renewable energy system. In this article, the wonders of charging those batteries will be presented.

I frequently tell people that batteries are a kind of life form, an electrical pet which requires husbandry to keep happy. They don't mind equitable work, and it is their pleasure to serve you for many years. Forget to feed or water them, freeze or roast them, neglect and mistreat them and they will crawl into a corner to shrivel up and die.

This writing is about the feeding part.

Your batteries should be recharged as soon as possible after use. Although they will continue to provide power until their capacity is reached, best battery life expectancy is realized by immediate and full recharging.

Several methods of charging are common: from the power grid, solar (and other renewable sources) and the engine in your vehicle.

Grid powered chargers are available in many configurations and ampacities, and may include manual or automatic controls for current, voltage and possibly may include multiple stage charging.

Solar charge controllers are likewise available in a multitude of types. Properly installed, they will maintain your batteries at full capacity when sufficient sun is available.

Charging from a vehicle's generator or alternator is the trickiest of all, as you will see in a future article

Multiple stage charging is the best and most economical way to fill up batteries. Three stages are usually preferred, bulk, absorption, and float.

Bulk charging is done to return 75% of the energy removed from the battery quickly. Amperage is kept as high as the battery and charging source will allow, usually equal to a value of 10-20% of the batteries ampere-hour capacity ( a 220 ampere-hour battery would be charged at 22 to 44 amps, expressed as C10 or C5. The equation for determining this value is Capacity divided by charging current determines time to charge fully [C value] 220/5=44=C5) When the battery voltage reaches approximately 2.41 volts per cell (14.5 volts for a 12 volt battery), absorption stage is initiated.

In absorption stage, the current into the battery is limited to hold the terminal voltage at a pre-set value just below it's gassing voltage, usually 14.5-14.6 volts, for a specified period of time, from half an hour to several hours, depending on the specifications of the battery manufacturer. During this time, the battery is allowed to slowly absorb approximately 20% of the electricity being returned in the charging process. At the end of the given time, the float stage is initiated.

Float charging is a condition in which the battery is held at a specified voltage that is well below it's gassing point, but high enough that the remaining 5% of capacity can be returned. Typical float voltage for a 12 volt lead acid battery might be 13.1 volts. A battery can be left on float charge indefinitely without damage or excessive water loss.

Then there is the issue of occasional over-charging of deep cycle batteries, which is required for good battery health. This is also called an "equalizing charge", and it is applied once a month or so to help even out the state of charge between all of the series-connected cells in a battery. The regulator or charge controller is set to allow the battery's terminal voltage to reach 15+ volts for a period of several hours. This will cause furious gas production in the cells, helping to strip away any soft sulfate deposits, and de-stratifying the electrolyte. Care must be taken to allow the pressure inside the cells to dissipate, usually by removing the cell caps. This generally leads to electrolyte being spattered onto the top of the battery, which must be cleaned up after the equalizing process. The hydrogen gas produced is highly explosive, and all care should be taken to prevent it's being ignited.

The state of charge of any lead-acid battery can be determined by the use of a battery hydrometer, which indicates the specific gravity of each cell in a battery bank. Personally, I prefer modern digital electronic instruments to mucking about with corrosive acids. When the batteries are operating properly, the readouts from the instruments is more than adequate to indicate the state of charge. I save the hydrometer reading for once-or-twice a year maintenance and for locating a sagging cell when performance drops off due to age.