September 16, 2016

Lithium Ion batteries

I have become charmed by rechargeable lithium batteries, in particular 18650 cells. I have found that old laptop packs are an excellent source of these if you know what you are doing. Lithium ion cells charge to 4.2 volts, but are often specified at a "nominal 3.7 volts". Certain variants charge to 4.35 volts. There are other chemistries as well, but 4.2 volts is far and away the most common thing you encounter.

Harvesting used batteries

Most people should not do this. Maybe even I should not do this.

Safe practices

These things are dangerous. You absolutely should get a high quality charger designed explicitly for lithiums and learn about the hazards. Lithium batteries can catch fire, explode, and cause fires, particularly if abused. Abuse can be either overcharging or discharging below a safe level. Most people should use only protected cells.

A solid rule is that no lithium cell should ever be discharged below 3.0 volts. 3.5 volts is a recommendable point to take a battery out of service and recharge it. Batteries should not be stored fully charged. It is recommended to charge a battery to 3.8 volts and store them in that state.

People say that at 3.3 volts, from 92 to 98 percent of the battery capacity has been used. It is also said that at 3.6 volts (at rest) is pretty much empty. Under load, a battery may drop as low as 2.5 volts when discharged.

Keep this table in mind:

4.2 volts 100%
4.1 about 90%
4.0 about 80%
3.9 about 60%
3.8 about 40%
3.7 about 20%
3.6 empty for practical purposes
<3.5 = over-discharged
Storing batteries not in use in a refrigerator at about 40 percent charge level is probably a great idea. High temperatures are definitely not good.

Chargers

I use a XTAR VP2 and adore it. There are other good chargers. You can go nuts and buy a "hobby charger" like the RC model people use and become a total battery nerd. I am tempted, but they are pricey.

Protected versus unprotected cells

For safety, most people should be using protected cells. These have a built in circuit to prevent both overcharging and excessive discharge. Most of these circuits cut off the batter when it drops to 3.2 volts. If you do what I do and use cells pulled out of some device, these are unprotected cells. The device itself (the laptop pack) contained a circuit board that performed the protection function. If you use unprotected cells, you are taking a risk. If you use a good charger, you are protected on that end (but should still charge in some fire-proof location). Monitoring discharge is entirely up to you, and good advice is to recharge often and not take chances.

Lithium ion versus lithium polymer

This is a topic with a huge amount of misinformation circulating, and I won't claim to lay it to rest here. For most people, there is no difference, so just forget about it.

I think of these two types of batteries as the same thing, except that lithium polymer batteries have a soft package. The is almost but not quite true. There is some advertising hype that lithium polymers offer higher energy density. This is true, but not enough to really care about. On top of that, the lithium polymers cost signficantly more. So unless you need an unusual thin battery shape, don't worry about it. Also certain lithium polymers can produce extremely large currents, which may be of interest to the RC model community, but not to me. My interest is flashlights and low power electronics.

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