Rechargeable battery

Types

NiCad

Low capacity, slow charge rate and toxic cadmium. A typical AA Nicad gives 0.45Ah from a 15hr charge. 1.2v per cell.

NimH

AA NiMH typically 0.8Ah - 2.8Ah, charge rates from 5hrs to 15 minutes. 1.2v. New NiMH are mostly low self discharge (LSD), older ones steadily discharged themselves. LSD NiMH are usually labelled as ready-charged, something that's not practical with earlier NiMH.

Rechargeable alkaline

A rechargeable variant of alkaline. Not in general use in UK, can be bought from abroad. They don't last as well as NiMh and NiCd, and have short lives if deep discharged. They provide 1.4v per cell when recharged, an advantage for appliances that don't play well with NiCd and NiMH. Wikipedia article

Lead acid

Large, heavy, lowest cost per capacity. Mainly used in cars. Acid spills out if tipped.

Sealed lead acid

SLA are spillproof variants of lead acids. Mainly used in alarms, golf & mobility buggies, UPS, etc.

Lithium ion

Li-ion cells store more energy than the same size NiMH, and charge faster, hence their main use in laptops. Correct charging is essential, or they catch fire.

NiFe

Historic forerunner of NiCd. Seldom seen, but one manufacturer of industrial sized cells does exist. Extremely long lived, some made in the 1920s still work. Most can be rejuvenated by replacing the electrolyte. Initial cost higher than lead acid, but lifetime far longer.

Are rechargeables worth it?

Good AA rechargeables ones now give similar capacity to alkaline, around 2.8Ah. At £1.50 a cell lasting 500 charges that's 0.3p per charge. Alkaline AAs at 17p each are thus 56x the cost.

Zinc carbons are much cheaper per battery, but give even less capacity per cost.

The cost of the recharging electricity is trivial. A 2.5Ah 1.2v cell holds 3 watthours, or 0.003 kWh. Allowing for inefficiencies, 0.005kWh costs 0.005x13p = 0.065p. You can recharge 15 for a penny.

• Alkaline cost: 17p each
• Rechargeable cost plus electricity: 0.365p per cycle

Where capacity isn't so important, cheap rechargeables at half the price typically give around 1/3 the capacity.

When they're not as good

Discussion of the apps where alkaline are better is here

Cordless tools

NiCd 1.3-2 Ah are most common. Energy capacity = voltage x Amphours. The quality of nicads varies widely, affecting how long they last in trade use.

Li-ion powered tools cost more, and are less likely to come with more than one battery pack. The advantage is faster recharging.

Supercapacitor powered tools are rarely seen. These have low capacity, but extremely fast recharge. The number of charge cycles is not limited as battery chemistries are. ohgizmo

Battery failure

Cordless tool NiCd battery packs can be recelled, but its too often not worthwhile. Recelling with budget cells may be cheaper, but how long they last in non-diy use is another matter.

NiCd and NiMH cells that won't hold charge can in theory be zapped with a moment of high charge current to burn out the shorting dendrites and make them work, but zapped cells quickly re-short.

Li-ion cells have limited life even when not used. A li-ion battery that's little used lasts longest if stored half charged in a freezer, sealed in a bag to keep it dry. Defrost before use, eat before sell by.

A seldom seen option is to run a 12v - 18v tool on a lead acid battery on a short lead. It does at least provide many times the battery capacity. Current draw is upto 20A or 30A, so the flex should be rated to at least 5A (30A wire would only be needed for continuous operation.)

Lead acids can sometimes be repaired by replacing the electrolyte, but handling toxic lead compounds and acid for the cost of a new battery isn't usually worthwhile.

See also

Wikipedia