Alt-BEAM Archive
Message #01181
To: Steven Bolt sbolt@xs4all.nl
From: Bob Shannon bshannon@tiac.net
Date: Sat, 27 Feb 1999 18:48:19 -0800
Subject: [alt-beam] Re: Li-ion cell charger
Steven Bolt wrote:
>
> 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.
Actually you need to check the specs for your cell. Hard carbon cells
have different demands than graphite based Li-Ion cells. Some may be
trickle charged.
> 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.
Right, this is the hard carbon vs. graphite issue.
> 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.
Li-Ion is far safer than the hydrogen cells needed to feed a fuel cell.
If you want to use a Li-ion battery, simply use the whole pack from a
small cell phone like the Motorolla Star-Tac. These often have all
the charger control circuits internal to the pack.
Its real hard to beat the energy density of Li-Ion.
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