Rechargeable Batteries. The complete household handbook.

A disposable battery converts chemical energy into electricity in one direction. When the chemistry is spent, the cell is done. A rechargeable battery uses a reversible chemistry: push electricity back into the cell and the reaction runs backward, restoring the original chemicals.

The two rechargeable chemistries that matter for household use are nickel-metal hydride (NiMH) and lithium-ion. They work differently, come in different sizes, and serve different jobs. This handbook covers both, because a household that uses flashlights, radios, and portable power may encounter both.

NiMH is the chemistry inside every rechargeable AA, AAA, C, D, and 9V battery sold at a retail store. It delivers 1.2 volts per cell instead of the 1.5 volts a disposable alkaline delivers. Most devices tolerate that difference. NiMH cells are safe, forgiving, and inexpensive per charge cycle.

Lithium-ion is the chemistry inside 18650 and 21700 cells, which are the cylindrical cells that power high-performance flashlights, portable power stations, and cordless tools. They deliver 3.6 to 3.7 volts per cell, pack significantly more energy per gram, and require specific chargers and careful handling. They are not drop-in replacements for AA or AAA batteries.

Every rechargeable battery carries a milliamp-hour (mAh) rating. This number tells you how much charge the cell can hold, measured in milliamps delivered over one hour. A 2,000 mAh cell can deliver 2,000 milliamps for one hour, or 200 milliamps for ten hours, at least in theory.

In practice, the number on the package is the best-case lab measurement at room temperature with an ideal load. Real-world capacity is lower, because temperature, discharge rate, and cell age all reduce it. A cell rated at 2,500 mAh may deliver 2,200 mAh in a flashlight and 1,800 mAh on a cold night.

Higher mAh means longer runtime between charges, all else being equal. For AA NiMH cells, capacity ranges from about 1,900 mAh (low-self-discharge economy cells) to about 2,800 mAh (high-capacity standard cells). For AAA, the range is roughly 750 to 1,100 mAh.

The trade-off: higher-capacity cells tend to have higher self-discharge and shorter cycle life. The most popular low-self-discharge cells sit in the 1,900 to 2,000 mAh range for AA, which is a deliberate compromise: slightly less runtime per charge, but the cell holds that charge for months and lasts for over a thousand cycles.

Compare mAh within the same size and chemistry. A 2,000 mAh AA cell does not compare meaningfully to a 3,500 mAh 18650 cell, because the voltage is different and the devices are different.

A good charger extends battery life. A bad charger shortens it. The difference is how they detect a full charge.

Basic trickle chargers push a low current continuously and rely on a timer to stop. If the timer is generous (and most are), the charger continues pushing current after the cell is full, generating heat and degrading the chemistry. Cells charged this way lose capacity faster and deliver fewer total cycles.

Smart chargers monitor each cell individually. They detect the slight voltage drop (called negative delta-V) that signals a NiMH cell is fully charged, and they stop charging that cell while continuing to charge others. Each bay operates independently, so cells of different capacities and charge states can sit in the same charger without the weaker cell being overcharged or the stronger cell being undercharged.

Features to look for in a smart NiMH charger: individual bay charging with independent cutoff, a negative delta-V detection circuit (not just a timer), a discharge or refresh function for reconditioning old cells, compatibility with both AA and AAA sizes, and a display that shows charge status per bay.

For lithium-ion cells (18650, 21700), a dedicated lithium charger with CC/CV (constant-current, constant-voltage) charging is required. Never charge lithium-ion cells in a NiMH charger, and never charge NiMH cells in a lithium charger. The charge profiles are different and mismatching causes damage or safety hazards.

Rechargeable batteries do not last forever. Every charge cycle causes a small, irreversible change in the cell chemistry. Over hundreds of cycles, capacity gradually declines until the cell can no longer hold enough charge to be useful.

Capacity fade. Every NiMH cell gradually loses maximum capacity. A cell rated at 2,000 mAh might deliver 1,700 mAh after 500 cycles and 1,400 mAh after 1,000. When a cell no longer holds enough charge for your use, it is done. This is normal aging, not a defect.

Overcharging. Continuing to push current into a full cell generates heat, which degrades the cell chemistry and shortens total cycle life. This is the most common cause of premature battery death, and it is almost always caused by a basic trickle charger that relies on a timer instead of detecting full charge. A smart charger with negative delta-V detection prevents it.

Over-discharge and polarity reversal. Running a NiMH cell completely flat, or leaving a depleted cell installed in a device that continues to draw current, can drive the cell below 0 volts (polarity reversal). A polarity-reversed cell may never charge correctly again. Remove batteries from devices you are not using, and do not mix old and new cells in multi-cell devices, because the weakest cell runs flat first while the others are still pushing current.

The "memory effect" myth. Older nickel-cadmium (NiCd) batteries suffered a real memory effect where partial discharges reduced usable capacity. Modern NiMH cells do not have this problem in any meaningful way. You do not need to fully discharge a NiMH cell before recharging it. Topping off is fine.

Heat. Storing or charging NiMH cells at high temperatures accelerates capacity loss. Store batteries at room temperature, not in a hot garage or a car in summer. Charge them in a ventilated space, not inside a closed cabinet.

A pack of four NiMH AA cells costs $8 to $16. A smart charger costs $20 to $40. Total starting investment: about $30 to $55 for four cells and a charger.

A pack of four disposable alkaline AA cells costs $2 to $6, depending on the brand. If you replace batteries in a device every two weeks, that is roughly 26 packs per year per device, or $50 to $150 per year on batteries you throw away.

With weekly or biweekly cycling, rechargeables break even within two to four months. With monthly cycling, break-even takes about six to twelve months. After that, every charge cycle saves money, because the electricity cost to recharge a NiMH cell is negligible, roughly a tenth of a cent per charge.

For devices that sit idle for months (a stored emergency radio, a rarely used flashlight), the math changes. If you only replace batteries once or twice a year, disposable alkaline or lithium cells cost less over the device's life, especially when you account for self-discharge. Low-self-discharge NiMH cells narrow the gap, but the break-even may take several years.

The clear winners for rechargeables are high-drain devices used frequently: game controllers, wireless mice, camera flashes, children's toys, and lanterns during camping trips. For these, the savings are real and fast.