Alt-BEAM Archive

Message #01056



To: Wilf Rigter Wilf.Rigter@powertech.bc.ca
From: Steven Bolt sbolt@xs4all.nl
Date: Sat, 27 Feb 1999 10:45:53 +0100 (CET)
Subject: [alt-beam] Re: Li-ion cell charger


On Fri, 26 Feb 1999, Wilf Rigter wrote:

> Speaking of 4V lithium-ion cells, they are ideal for our robot
> applications: very light and high energy density. What is the
> recommended charging circuit for these cells?.

You basically use a current-limited (about 0.3C) charge until a
voltage limit is reached and the current has dropped. The charger
must completely switch off when the cells are full - no trickle
charging allowed, because li-ion can't absorb overcharge. It's
likely to make the cell unstable. We're talking explosion and fire
here.

Before you even think about experimenting, there are a few things
to consider. First, there are afaik two different kinds of li-ion
cells. One has a charging limit of 4.1V, the other of 4.2V. In both
cases, the tolerance is +/- 0.05V per cell. The discharge limit
also differs: 3V in one case, 2.5V in the other. The difference is
caused by the electrode materials used, and the makers are
continuously experimenting with new stuff, probably requiring
different limits.

Now comes the bad news. The type of cell and the associated limits
are not mentioned on the li-ion packages I've seen. So if you take
one apart to make a nice `bot battery, you won't know the charge
and discharge limits. Charging to the wrong limit doesn't help
performance. It's not actually dangerous if you deal with the two
types I've in mind (at normal temperatures), but the requirements
of newer cells may differ enough to make it very dangerous indeed.

Afaik the makers sell li-ion only in packages complete with
protection circuitry. With reason. My best advice is not to use
these cells in experimental hardware, despite their attractive
energy density. Nicad and NiMH are quite good and cheap as well as
easy to charge. By using a temperature or delta-V measuring
charger, they can be charged faster than li-ion and NiMH is
environmentally ok. Something better will happen soon, I think.
Conrad is already offering fuel cells. Let's skip li-ion.

Best,

Steve

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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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