Issue 143 June 2002
Invisible Components

Down the Drain

The 1.7-A bulb current determines the discharge rate, with the peak equal to the average. Assuming a 0.2-C drain for an SLA battery means 8.5 Ah, NiMh at 0.5 C requires 3.4 Ah, and a NiCd battery at 1 C would be only 1.7 Ah.

In this situation, the bulb’s high and constant current sets the minimum battery capacity. Normally, you can reduce the battery size by the circuit’s duty cycle: half an hour of use at, say, 100 mA would require only 50 mAh from the battery.

Cold temperature operation adds 25% and limiting discharge to half the rated capacity doubles the result. That SLA battery is beginning to look like a real monster at 20 Ah, the NiCd pack hits 4 Ah, and the NiMH is twice that.

Charger complexity affects the decision though. SLA batteries have dead-simple (albeit slow) chargers, NiCd and NiMH batteries are even worse, and lithium ion batteries are downright finicky. When you design a product, you must factor in the charger’s complexity. I’d rather not design an exotic, one-off charger for a device that will be used perhaps for two months each year.

Like the spotlight bulb, SLA batteries are cheap and readily available, which means replacements won’t be a problem. They use a simple charger and can withstand half a year of sitting around with no attention at all. They’re an example of a mature technology, which means they’re well understood and fully characterized.

Not to mention, of course, that I have several SLA batteries in my parts heap. They work well for powering amateur radio gear after those teeny NiCd batteries wear down!

I popped a charged 5-Ah SLA battery in the refrigerator for a few hours to verify its cold-weather performance. Figure 1 compares discharge cycles at 16°C (my rather cool shop) and 3°C, both ending at 11.4 V, equivalent to 90% depth of discharge.

Assuming a constant 7.5-W bulb resistance and eyeball-fitting the curves, the total battery capacities work out to 50 Wh and 37 Wh. The nominal capacity of 60 Wh shows you the effect of both high discharge rate and low temperature. The battery reaches half-discharge at 12.2 V after 1 h when warm and only half an hour at March temperatures.

I suspect drawing this much power under such adverse conditions will drastically shorten the battery’s life, at which point I’ll deploy another SLA battery from my collection. If there’s a real problem, I may have to dip into my NiCd stash.