Nickel-Cadmium (NiCd). These have long been the most commonly used rechargeable batteries for electronic applications. They are inexpensive yet work very well and have long service life: up to 1500 charge cycles. But NiCds require proper handling: always discharge fully before charging, and exercise them regularly (discharge/charge monthly). But they suffer from a severe "memory effect" which can diminish the lifetime and restrict the capacity. If you only partially discharge the battery before recharging it, it will "remember" that level and can no longer be discharged any further. Repairs for this include a few cycles of deep discharging (to 1 volt per cell, not zero) and full charging, though the service life may be shortened. Prevention is the best course: don't recharge NiCd batteries until your equipment says they're empty, or use a smart charger that automatically discharges first, then charges. Failure to fully discharge can reduce the service life to one-third (500 cycles best case). Store NiCd batteries in any state of charge, between -20 and 45 degrees C, at low humidity, and restore for service by performing two or three discharge/charge cycles. Charge at least once per year to avoid loss of performance or leakage.

Nickel-Metal-Hydride (NiMH). These batteries have up to 40% more capacity, and 50% more energy density (energy per weight), than NiCds. Compared to NiCds, NiMH batteries cost more and have shorter service life (number of charge/discharge cycles): up to 500 full charges or even more partial charges. They exhibit only a slight memory effect, which can be avoided by fully discharging the batteries once every 30 cycles. Canon's NiMH charger for the EOS-1D performs this full discharge when you press the "Refresh" button (be prepared to wait a day for the discharge/charge cycle to complete). NiMH batteries should be stored partially charged, between -20 and 35 degrees C, and recharged at least once per year to avoid excessive self-discharge.

Lithium-Ion (LiIon or Li+). This newer technology offers double the capacity and three times the energy density (energy per weight) of NiCds. That means a pound of LiIon batteries could power your equipment three times longer than a pound of NiCd batteries. They exhibit no memory effect, have much lower self-discharge rate than NiCds, and don't need to be regularly exercised. This is great, but: Lithium-Ions are expensive, are unsuitable for high-current applications (like photo flash), require complex electronic "smart charge/discharge" controllers (built into the equipment), are easily destroyed by over-discharge (which is automatically prevented by most equipment), are damaged or destroyed by recharging under temperature conditions outside the range 0 to 40 degrees C, and age rapidly (lifetime may be two years whether used or stored).

	"I/V" Curves for Lithium Ion Discharge

On the right is a voltage and current plot for discharge of a LiIon quad-cell (14.4 volt) pack at constant power. Most applications of these cells do draw "constant" power -- that is, independent of the battery voltage. Therefore, the current has to increase as the battery voltage drops. With Lithium-Ion, the voltage drop-off is precipitous near the end of discharge. Beyond this point, the battery will quickly suffer damage or destruction. All equipment designed for LiIon batteries will carefully monitor the discharge state and shuts off before full discharge (about 3.0 volts per cell, or 12 volts in this quad-pack) can be exceeded.

If you use photo equipment in cold weather, be aware that the battery capacity (energy recovery) diminishes rapidly below freezing... much more so than with NiCd or NiMH, though the technology is improving with better electrolyte formulations for low temperatures. For applications like digital cameras, laptop computers, and cell phones, LiIon batteries are by far the best, yet the Canon top of the line digital cameras use NiMH batteries (go figure). Prolonged storage should be in the partially discharged state (40% capacity), below 15 degrees C. Partially charge once per year to avoid overdischarge, though the two-year lifespan could make this recommendation somewhat irrelevant. As with NiMH (but not NiCd), LiIon batteries will sustain a greater number of shallow, rather than deep, discharge/charge cycles. Note that Lithium-Polymer is a new and promising battery technology that, despite all the hype, does not as yet address the shortcomings of LiIon.

Rechargeable Alkaline. Less common than other types, these actually have substantial capacity and double the energy density of NiCd. They're best for low-current (low-power) applications because of high internal resistance.

Sealed Lead Acid batteries are sometimes used to power "portable" flash equipment. They are similar to automotive and marine lead acid batteries, except they are full sealed: there is no need to add fluid and no acid spills if they're tipped over. Lead acid has the advantage over other rechargeable chemistries when lots of capacity is needed at reasonable cost. The main disadvantage is the weight: 50 pounds or more for a typical flash setup (we're talking powerful studio-type strobes, not hot-shoe flash). There are nice bags and shoulder straps to better manage the load. But there really is no alternative if AC power is unavailable and studio-type flash will be used. The weight does restrict the distance over which the equipment will be hauled -- no backpacking with this gear! Lead acid batteries should be fully charged before storing.

Postscript: Future Batteries

Battery technology is being driven by huge market demand in portable electronic devices, of which cameras are only a tiny segment but major beneficiary. The chemistry of future batteries may be completely unlike the types discussed here, or batteries as we know them may be completely replaced by portable fuel cells or some other emerging technology.