[TriEmbed] lithium batteries

[Pete Soper]

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 /unsafe to recharge/). 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:

http://www.candlepowerforums.com/vb/showthread.php?235164-Using-Li-ion-cells-in-LED-flashlights-safely

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.

And some of info is counter-intuitive, such as the advice about charging 
current (low current charging results in shorter life).

The cheapo Chinese TP4056-based 
<http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdf> 
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.

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).

-Pete

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