Li-ion question.

P

piero

Guest
Hi everybody,

Wich suggestion do you have for a device equipped with a single
rechargeable Li-ion cell (900-1400mAh) continuosly powered from the
mains to keep the cell charged and safe?

I've read trickle charge is not allowed with such chemistry.

The charge rate should be kept very low if the cell was deply
discharged.

A simple solution could be a voltage source adjusted to 4.05V and
limited
in current. This could not reach the full capacity but would be safe. Or

not?


piero
 
piero wrote:
Hi everybody,

Wich suggestion do you have for a device equipped with a single
rechargeable Li-ion cell (900-1400mAh) continuosly powered from the
mains to keep the cell charged and safe?

I've read trickle charge is not allowed with such chemistry.

The charge rate should be kept very low if the cell was deply
discharged.

A simple solution could be a voltage source adjusted to 4.05V and
limited
in current. This could not reach the full capacity but would be safe. Or

not?
Click to expand...
I think it is a bit more complicated than that, for example you need to
protect the cell from over-discharge as well as overcharge.

There are ICs which do this available from many manufacturers, have a
look at:

http://www.maxim-ic.com/PowerSupplies.cfm

Gareth.

--
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To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris
 
On Sat, 08 May 2004 08:42:27 GMT, piero <pizanotto@tin.it> posted this:

Hi everybody,

Wich suggestion do you have for a device equipped with a single
rechargeable Li-ion cell (900-1400mAh) continuosly powered from the
mains to keep the cell charged and safe?

I've read trickle charge is not allowed with such chemistry.

The charge rate should be kept very low if the cell was deply
discharged.

A simple solution could be a voltage source adjusted to 4.05V and
limited
in current. This could not reach the full capacity but would be safe. Or

not?


piero
Click to expand...
Those cells are typically charged with a constant current until the
cell's voltage reaches 4.1 volts (or a voltage specified by the cell
manufacturer) and then the charger switches over to a constant voltage mode at
the specified voltage. The constant voltage mode is continued until the
current, which will continue to decrease, reaches 5 to 10 percent of the first
stage constant current. At that point, the charger considers the cell fully
charged and quits charging altogether.

Charging from a constant voltage source for an extended period is
something I've never seen recommended. I suspect that the cell may be degraded
if you attempt that.

I recommend that you build up a "normal" LiIon charger from chips
available from a number of sources and then build a timer that turns the charger
on at regular intervals of perhaps once per week. The on period should be long
enough to bring a fully discharged cell back to full charge. A normal charger
will only charge the cell for as long as the cell requires. If a fully charged
cell is connected to a normal charger, the charger will go through its cycles
very quickly with no chance of damaging the cell.

Jim
 
Personally I would be very wary home-brew of LI-ion battery chargers. See
the link below for what happens when you overcharge one. Admittedly this
was done deliberately, but you would want to be very sure your system was
fail-safe.

http://www.rcgroups.com/forums/showthread.php?s=&threadid=151687&perpage=15&pagenumber=1




--
Regards,

Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control
"piero" <pizanotto@tin.it> wrote in message news:409C9D7B.FDFFFAD5@tin.it...
Hi everybody,

Wich suggestion do you have for a device equipped with a single
rechargeable Li-ion cell (900-1400mAh) continuosly powered from the
mains to keep the cell charged and safe?

I've read trickle charge is not allowed with such chemistry.

The charge rate should be kept very low if the cell was deply
discharged.

A simple solution could be a voltage source adjusted to 4.05V and
limited
in current. This could not reach the full capacity but would be safe. Or

not?


piero
Click to expand...
 
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