Of automatic battery chargers and safety timers that are too short

I was looking through the manual for Radio Shack’s battery charger, part no. 23-425. (I was bored! Really!)

The manual lists automatic shut off detection as “negative delta v and zero slope detection.” The chart says that it’d take about ten hours to charge a 7000mAh niMH D cell. It also says that it also has a safety timer set to ten hours.

I got to wondering: What if I wanted to charge 9000mAh cells on that thing? Would it be as simple as running it through two cycles? It should shut off on the second cycle when the cells are actually finished charging, right?

Another question: What exactly is the timer for? Most cells take a lot less time than ten hours to charge in that charger, anyway. If the primary detection failed, those cells would be fried before the timer went off. Is the timer just set to try to prevent a fire or something?

In a word, yes.

You’re right, the charger would shut off on the second cycle, so you’re okay (BTW, you’re better off with a slow charge because battery life will be extended. It’s just inconvenient. This is true for car batteries as well, so if you ever have to charge one up because you left the lights on, buy the smallest, cheapest Sears battery charger you can find and let if charge overnight. Avoid jump starting if you can; once the car starts, the alternator charges it so fast the battery’s plates may become sulfated, decreasing life) The timer is so that if the batteries are shot, a condition that usually means they draw a continuous charge but internally “leak” so that they never “fill up”, the charger doesn’t run forever.

For charging car batteries, you charge them the same way they discharged them. If they went down slow from sitting for a couple of months then they have to be charged slow otherwise damage will occur. If they discharged fast from leaving the lights on, then they can be charged at a higher rate. No harm can come from charging a battery too slow, harm can come from trying to charge one too fast. In the example you gave a higher rate can safely be used.

No. First off the alternator while capable of producing a huge amount of amps only trickle charges a battery. The alternator only will produce what the car needs to run plus a very small current to charge the battery. The problem with jump-starting a dead battery is that you usually don’t drive far enough to charge the battery after the jump-start. An alternator is designed to keep a fully charged battery fully charged, or to charge an almost fully charged battery.
Also for the record sulfated is a term to describe
A) A battery that has been discharged over time and the sulfate on the plate has become hard rather than spongy and needs a long slow lo amperage charge to restore the battery. Charging a battery does not cause sulfation. Discharging a battery does.
B) Sometimes used to describe any dead battery

-Who wound up helping to write a repair manual on the subject.

Thanks. Looks like I’m good to go, then.

I would use a charger with a longer charge time. However, the device I’m planning on using them in (kaboom box) probably goes through those 10 D cells in a couple of hours. I wouldn’t want to wait a week between uses.

Can’t see it. For this to be true, the alternator would need to have a regulator that “knows” the current necessary to run the car and all accessories. Since this current varies drastically (largely depending on what accessories are on), the alternator has no way of knowing how much current to put out to run everything and charge the battery slowly. To put it another way, the alternator doesn’t monitor battery charge current separately from the vehicle operating current and thus can’t differentiate between the two. Also, consider what happens when you jump start a car after a deep discharge: the alternator belt often squeals because the alternator is grunting so hard to put out the large current necessary to charge the battery.

Alternators have voltage regulators that continuously monitor its output current so as to maintain a constant battery voltage. If the battery voltage is way low, alternator output current will be high (as will the drag on the engine). If the battery is close to fully charged, alternator output is much lower. Modern voltage regulators also have current limiters to limit charge current to avoid alternator damage.


Art the system does not monitor the battery seperate from the loads, since it is all the same circuit. Also the system does not monitor current flow. The regulator monitors voltage.
When a load is added to the system (turn on your headlights) the voltage in the system drops, the regulator detects this and increases the field voltage in the alternator and current flow (and voltage) increases to make up for the increased load. Turn the rear window defroster, and the the same thing happens all over again. Turn off the lights, and defroster, the voltage increases and the field voltage decreases and the charging output decreases. (both voltage and current)
Regardless of what the dynamic load is there is a programmed steady charge rate which may be very high for a few minutes, but will taper down to a very low rate rather quickly.
If the rate were to remain high, the battery would be destroyed from overcharging.