The Ideal Battery Charger

Figure 9-2 Graphic summary of battery discharging and charging cycles.

The Ideal Battery Charger

Battery charging is the reverse of discharging, but the rate at which you do it is critical in determining its lifetime. The basic rule is: Charge it as soon as it’s empty, and fill it all the way up. The charging rate rule is: Charge it slower at the beginning and end of the charging cycle (below 20 percent and above 90 percent).

When a lead-acid battery is either almost empty or almost full, its ability to store energy is reduced due to changes in the cell’s internal resistance. Attempting to charge

Figure 9-3 Ideal and actual lead-acid battery charging characteristics.

it too rapidly during these periods causes gassing and increased heating within the battery, greatly reducing its life. Ideally, you limit battery current during the first 90 percent of the charging cycle and limit battery voltage during the last 10 percent of the charging cycle. Either method by itself doesn’t do the job. (Figure 9-3 shows why.)

The graph at the upper right in Figure 9-3 shows constant voltage charging in the ideal case. The constant voltage is usually set at the level where gassing causes a decrease in current flow through the battery with time as the battery charges. Unfortunately, with no restrictions on current, this method allows far too much current to flow into an empty battery. Feeding 100 amps or more of charging current to a fully discharged battery can damage it or severely reduce its life.

Let’s look at the ideal approach during all four state-of-charge phases—-0 to 20 percent, 20 to 90 percent, 90 to 100 percent, and above 100 percent. Figure 9-3 shows the results.

Charging Between 0 and 20 Percent

The first 20 percent of a fully discharged battery’s charging cycle is a critical phase and you want to treat it gently. You learned in Chapter 8 that all batteries have a standardized 20-hour capacity rating. Every battery is rated to deliver 100 percent of its rated capacity at the C/20 rate. During the first 20 percent of the charging cycle, you ideally want to

charge a battery at no more than this constant-current C/20 rate. To determine the first 20 percent charging current, Charging Current 5 Battery Capacity/Time – C/20

For a 200 ampere-hour capacity battery, charging current would be Charging Current – 200/20 5 10 amps

In other words, you would limit this battery’s initial charging current to 10 amps. You can blast your battery with 200 amps and charge it in 1 hour, but you will prematurely age and kill it—it will not deliver its full, useful life to you. The graph at the lower left in Figure 9-3 shows the result of current-limited C/20 rate charging during the first 20 percent part of the charging cycle. The voltage rises gently and your battery is very happy.

Charging Between 20 to 90 Percent

In the middle of the charging cycle, you can charge at up to the C/10 rate. This is the fastest rate that efficiently charges a lead-acid battery. This rate is not as efficient as the C/20 rate—more energy is wasted in heat if you recall the I2R losses—but it gets the charging job done faster. At even less efficiency (and more risk to your batteries), you can bump it up to higher C/5 or C/3 rates during this period of recharging if time is essential to you, and if you closely monitor the battery’s temperature so its operating limits are not exceeded and you don’t wind up “cooking” it. Charging current would be 20 amps at the C/10 rate for the 200 ampere-hour battery. Figure 9-3 shows that voltage, after a step increase when current settings were changed, rises slowly to its 90 percent state-of-charge value of approximately 2.50 volts.

Charging Between 90 and 100 Percent

At this point, you want to drop back to the C/20 rate or, ideally, switch to a constantvoltage method. If you switch to constant voltage set at the deep-cycle battery’s full charging value of 2.58 volts, Figure 9-3 shows the result—current provided to the battery drops rapidly during this last 10 percent of charging and your battery is very happy while receiving its full charge.

Charging Above 100 Percent (Equalizing Charging)

You learned about equalizing charging in Chapter 8. It is needed to restore all cells to an equal state-of-charge (to “equalize” the characteristics of the cells); to keep the battery operating at peak efficiency; to restore some capacity of aging batteries; to restore floatcharged or shallow-charged batteries to regular service; and to eliminate effects of sulfation in idle or discharged batteries. Equalizing charging is controlled overcharging at a constant-current C/20 rate with the charging voltage limit raised to 2.75 volts. It is done after the battery is fully charged, and maintained at this level for 6 to 10 hours.

Equalizing charging should not be done at rates greater than C/20. Equalizing charging should be done every 5 to 10 cycles or monthly (whichever comes first); only in well-ventilated areas (with no sparks or smoking) as it produces substantial gassing; and only while close attention is being paid to electrolyte level, as water consumption is substantial during rapid gassing periods. Figure 9-3 shows the step increase in the voltage to 2.75 volts and the increase in current back to the C/20 level.

Now let’s look at the time involved in using the ideal approach to charge our hypothetical 200 ampere-hour capacity battery: