Battery Types

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Question:
Is there any concern about using a different battery type? Do NiCad and NiMH charge the same?

    Mad Dog


Hiya Mad Dawgs,

No concern.

Short answer:

The characteristics of each are different, and you can see various data showing subtle arcs on graphs for different cell chemistries and constructions. The term "battery" we commonly use refers to the fact that a bank of cells go into making one of useable voltage or current. If you want to find out more, Use Sherlock to search using the industry term "secondary cells" (which refers to rechargeable types. Primary cells are the "disposable" kind). If anyone is interested, I can provide a list of battery sellers, many of whom also post brief overviews, with nice cutaway graphics showing the construction of various types of cell, and with dimensions of cells ( so you can determine whether you have 4/5A cells, or sub C ).

I'm trying to keep this simple and practical, just giving the Bottom Line for, you know, Practical Inventors and Innovators (a dying breed I fear). So I'll confine myself to...

Choosing a Rechargeable Battery

There are three commonly available types of rechargeable cell. Lead-acid, NiCD and NiMH. Lithim, mercury and zinc-air have their benefits, but are not rechargeable. Of the three common types, the one that's best suited for repeated charge/discharge cycles is lead-acid. It can be recharged hundreds of times without damage, if it's not cycled to zero too often. It's available in all kinds of sizes and voltages, and emergency-light types are probably the best value for the money. Unfortunately they are just too heavy to be popular for portable devices. Early camcorders had to use them, because NiCds didn't have the energy density to supply the heavy demand of the early machines. But if you don't mind the weight, they'll put out more power, and last longer, than any other type.

NiCds have been around for a while now, and manufacturers have squeezed all they probably can from that technology. As you know, they suffer from 'memory' problems, and while they can put out very good current when fresh and charged, they can only be cycled a limited number of times before the cells are exhausted, and will no longer charge. beyond a certain point, no amount of reconditioning will bring them back to their new condition. As well, they lose their charge all by themselves, and have a short 'shelf' life. If only one cell in a battery fails (usually due to deep cycling), the entire battery dies, and the only way to repair the pack is to find and replace the bad cell.

NiMH (nickel-metal hydride) is the latest technology, with a performance envelope a little better than NiCds, and without the 'memory' problem (actually, it's just much less acute). Because of improvements in technology, a cell of the same dimensions as an earlier NiCd cell, will often provide around 50% more current, and current my friends, is what makes your PB run longer! You can chrage these in the same chargers you invested in for your NiCds, and there are chargers on the market that claim to be "tweaked" for NiMH. That may be true. I build my own chargers, so I calibrate them specifically for the pack I intend them for, but any charger with an output matched to the cells you're charging will work, provided you remember to disconnect the cells when they are charged. Ideally, the charge current rate should vary over the charging cycle, and that requires sophisticated charger circuit design, which we won't get into today. You're welcome.

Incidentally, many of you won't know that you can recharge a regular carbon cell, usually two or three times without any problem. Because they are not specifically designed for this, and are not vented to allow charging heat and gas to escape, they won't put up with this for more than two or three charges, but those recharges will bring a carbon cell pretty much back to it's new state, and it's worth doing, considering the cost of batteries. There is a danger of creating hairline splits in the case, with subsequent leaking (I've been lucky so far, and I've been doing this for years) so you may want to be carefull where you install recharged carbon batteries. And remember, I'm talking about common carbon cells (sometimes called "heavy-duty") - - do NOT recharge alkaline cells, which are most people's favorites because of their long shelf life, because they WILL split open when overcharged. Yechh. They sometimes split open anyway, and it can be messy. Use bicarb, water and a small brass wire brush to clean contacts that have been soiled by a leaky alkaline. A small exacto knife is great for scraping hard-to-reach contacts clean. (long answer)


I really hope these long posts aren't annoying anyone. My intention is to share practical knowledge it took me years to dredge up, and countless hours in dusty libraries (remember those?) to uncover. It pleases me no end, to see that there are still people out there who are willing to break out of the box, and make something they need themselves, from scratch if need be. Hey, it keeps us off the streets, right? Hope this was useful.

Question:
Thanks for the great information. Can you tell me if Lithium Cells would work better than NiMH cells for a Powerbook battery?

    thanks,

    Mad Dog

Answer:

Lithium Ion cells are really ideal for low-current needs. They have the best discharge rate of all the commonly available cells, but aren't as strong when it comes to high-current applications. This makes them ideal for clocks, smoke detectors, PalmPilots, or anything else which presents a small but constant drain. For applications like these, they'll outlast any other common battery. And they can be recharged!

A Powerbook represents a heavy current drain, however. You COULD use them, if you have access to them cheaply, but you'd want to use enough of them in parallel to assure you of meeting the heavy current drain. That's do-able!

Let me know the power requirements of the model you plan to put them in. You may want to consider an external Lithium-Ion battery pack, because you may need more cells than you can jam into the original battery pack case.

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