21 February 2009

Battery Addendum

My earlier post on the newer (low self-discharge) battery cell technology could have benefited from a bit more homework. But hey, I don't do this for a living. It's just a lunchtime diversion. So, some corrections and expansions.

First, the Wiki article on the low self-discharge NiMH battery is useful. The article lists about 30 brands, all apparently produced by just five makers. So consumers have lots of choices.

I contrasted alkaline single-use batteries with NiMH rechargeables in terms of energy density, and said they were much lower. That is not true. Now, not all (non-lithium) single-use batteries are alkalines. There are several technologies used, some better than others. Most dollar-store batteries are probably non-alkalines and therefore have very little juice, much less than rechargeables. But the name-brand alkalines are in fact more energy dense than rechargeables, approaching 3000 mAh for the very best.

That being so, why do rechargeables last longer and/or perform better in some devices like digital cameras? I had thought it was related to energy density (mAh), but that is incorrect. The reasons are more complex, but one forum poster explained it nicely:

    It's true that alkaline cells have an impressive energy capacity, usually greater then 2500mAh. But this is only one of several factors that affects the performance of a cell. At certain points in their operation digicams have extremely high power demands. The power a cell or battery can deliver depends on both the voltage it can provide, and the electrical current it can deliver (the power supplied is actually the voltage multiplied by the current). The voltage is limited, it is fixed by the chemistry of the cell, so the only way to deliver that power is with a large current. The essential problem with alkaline batteries is that they have great difficulty providing these large electric currents. Alkaline cells are brilliant, however, in the right application. For low power devices, or devices that are used infrequently e.g. doorbells, radios, remote controls, etc., they are superb. They have a huge energy capacity, and a shelf-life of several years. But if you need bursts of high power, as required in a digicam, they are unsuitable - except in emergencies. (Many pro photographers keep some high quality alkalines in their bag so that they can always take a few shots if their rechargeables run out, etc.)
So, for low-power, long-term use, alkalines work great. But for electronics with higher power demands (cameras, music players, flashlights, portable gaming devices), lithiums and NiMH rechargeables perform much better. But the new low self-discharge NiMH batteries work about equally as well as any of them in most applications. As well as being cheaper and greener. For now, they retain their superbattery crown.

Addendum: See further update here.

No comments: