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There was discussion Monday night about unprotected lithium
batteries and how they need to be protected from over discharge to
avoid damaging them (and in some cases making them <i>unsafe to
recharge</i>). There was also vague murmuring and general anxiety
about the safety of this type of battery. I came across this
(classic UK-style humorous, but expert-sounding) treatment of the
subject and think it might be useful:<br>
<br>
<a
href="http://www.candlepowerforums.com/vb/showthread.php?235164-Using-Li-ion-cells-in-LED-flashlights-safely">http://www.candlepowerforums.com/vb/showthread.php?235164-Using-Li-ion-cells-in-LED-flashlights-safely</a><br>
<br>
In my situation I'm using unprotected ex-cellphone batteries for
very low power applications (involving a small fraction of the
current draw of an operating phone). But I realize I've got to use
Paul's "disconnect your own power" trick in conjunction with voltage
monitoring to automatically avoid over discharge or else find and
use a protection circuit specifically designed for lithium batteries
if I'm every going to leave the power switch on for very long. But
in the meantime if the thing is putting out 3.4-4.2 volts I simply
refuse to break a sweat. But some of the info above made me sit up
and take notice, for sure.<br>
<br>
And some of info is counter-intuitive, such as the advice about
charging current (low current charging results in shorter life).<br>
<br>
The cheapo Chinese <a
href="http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdf">TP4056-based</a>
chargers do leave a battery at under 4.2 volts (all at 4.18 for the
five freshly charged batteries I just tested) and the TP4056 data
sheet explicitly states that the charging stops when the float
voltage is reached and the sunk current falls below 1/10 of the
programmed charging current. Since the TP4056 limits the charging
voltage to 4.2 volts and monitors current exactly I'm cautiously
optimistic (but by no means certain) our UK friend writing the
advice above might be OK with this type of charger. And to repeat an
important factoid from the meeting: by default these little chargers
will deliver 1.2 amps to the battery if that's available. (PC USB
ports are supposed to be limited to 500ma, but I sure wouldn't
depend on that in this situation). A key, practical detail from the
UK advice is that the battery should get warm during charging, but
never hot. He colorfully describes the difference. The point being
that your mileage will vary. But I follow the charger vendor advice
to set the current limit resistor to .37C for the battery with the
smallest C I'm going to use. That translates to a 4k resistor for a
300ma limit for the 850-1100ma batteries I'm using. <br>
<br>
But based on this little forum article I'm switching my charging
over to a fireproof setting (inside a metal can sitting on a brick
fireplace).<br>
<br>
-Pete<br>
<br>
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