From LED flashlights to GPS units to home blood pressure monitors, the humble AA and AAA batteries still rule. Making full use of these versatile batteries (and going green) means using rechargeable NiMH batteries, and using the right chargers for them.
Timer chargers are the cheap chargers often bundled with rechargeable batteries in supermarkets.
They do work, but can overcharge batteries and shorten their lifespan. They are reasonably safe if the batteries are drained before charging, but will overcharge already-charged batteries.
The Energizer Rechargeable Battery FAQ states:
“Using the batteries to end of life prior to charging is recommended when a timer based charger is used. A smart charger monitors the cell condition during charge and prevents overcharging and subsequent negative impact on battery cycle life.”
The longer the charge time, the safer. The “fast” 15 minute or 1 hour chargers should be avoided as these can overheat the batteries, further reducing their lifespan.
How to identify timer chargers
- Fixed number of hours to charge batteries (can be 15 minutes, 1 hour, 4 hours, 12 hours).
- Cannot charge one battery alone, must charge 2 or 4 at a time.
- Need to be switched off or unplugged from the wall socket (to reset the timer) before the next set of batteries can be charged.
Topping-up batteries before they are fully discharged is useful
- Fully discharging a battery (to less than 1 volt) can reduce its lifespan.
- Unused NiMH batteries can lose some of their charge after a few months (self discharge).
- Charged and discharged batteries are often mixed up in the same drawer.
“Smart” or “intelligent” chargers allow topping-up and avoid overcharging by accurately monitoring the voltage of the batteries.
They also often have advanced features such as
- Safe full-discharge of batteries (discharge until 1 volt).
- Reconditioning overcharged batteries (multiple charge/discharge cycles, this can take a few days).
- Measuring the actual charge capacity of a battery (this can take a few hours or days).
How to identify smart chargers
- Will be prominently labeled as such by the manufacturer.
- Can charge any combination of batteries, including just one (independent charging circuit).
- Often displays battery’s charge state (empty, 1/3, 2/3, charged) on the front panel.
For international travel, auto-switch chargers that can run off both 110 volts AC and 230 volts AC are essential.
For use with solar panels, the battery charger should have a 12 volt cigarette lighter plug, or a 5 volt USB plug (depends on the solar panel).
Battery Charger Brands
Maha (PowerEx) and La Crosse Technology are known for their smart chargers.
More well-known consumer brands such as Sony and Eveready also sell smart chargers. Care should be taken as their smart chargers can look similar to their timer chargers.
Investing in a Good Charger
Considering that a NiMH AA battery costs a few dollars, and that it could last 500 to 1000 charges with proper use; a smart charger could easily pay for itself in a few years.
Timer chargers should be avoided. Shortening the lifespan of batteries doesn’t just waste money, it also isn’t environmentally friendly.