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Li-Ion Batteries
There
are 2 problems with lithium ion batteries; they can explode if abused,
and they will become useless fairly quickly if treated badly. Happily,
it's pretty hard to explode a li-ion battery (the most common cause is
penetration; so keep your spare batteries in a tough, rigid
container!), but there are some basic things you should do to
minimise the risk. Only buy protected batteries; that is, batteries
with a protection PCB. This limits the max. discharge current (about
7A, so you won't notice this limit until you short the battery), and
prevents over-charge and over-discharge. Take the batteries off the
charger as soon as you can after they've finished charging;
overcharging is the most common way to damage a cell, or even cause it
to explode. Don't leave batteries badly discharged for long periods of
time; if your light shuts off because of the protection circuit, then
recharge the battery within a maximum of twenty four hours; leaving it
longer
rapidly ages the cell, lowering its capacity and making it more likely
to vent with flame. And (I hope this one is obvious!), don't short
circuit the battery.
A tough waterproof
container protects your spares
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Don't be Dim!
For
day to day use, they shouldn't be discharged beyond 80%, which
correlates to an open circuit voltage of about 3.7V, which is reached
just after the point of noticeable dimming on full power with the
average P7/single 18650 light (In my sample, 3.74V was the point of
dimming). If you want decent cycle life, don't discharge beyond this
level and swap batteries as soon as you notice dimming; bouncing off
the protection circuit as your limit for discharge is definitely a bad
idea. When you've just got in from a cold ride, let the cells sit in a
room to warm up a bit before you charge them; charging a cell with a
core temperature below 00C causes damage, though
they can be discharged
safely at -200C with
only slightly less capacity than a discharge at
+200C.
Keeping cells in a hot car isn't too bad for short periods of
time, but long term won't help them at all.
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Keep your cool
When
storing cells over the summer, discharge to about 40% (reached after
~40 minutes on full blast with a P7 light), or 3.9V (open circuit
voltage), and try to keep them somewhere cool, such as a basement, or
wine cellar if you're posh. Keeping the cells at 100% charge in warm
conditions is very bad for them, and they'll be near useless next year.
You
can tell if a cell is getting worn out by checking how well it holds
charge over about a week; if you fully charge it to 4.2V, then a week
later it reads 4.1V, then it's on its way out. If it reads 4.0V within
a week, then you've damaged the cell, and the risk of explosion is far
greater; I'd strongly advise you replace the cell(s) in question if
they reach this level. At £3 each, the risk of having it explode while
helmet mounted really isn't worth it.
Dedicated
charger: the safe way to charge cells
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Super
safe cells:
If you're really worried about exploding cells, you can consider IMR18650's
They have less capacity, so expect 30 to 40 minutes runtime. They have
no protection circuit, so it's especially important that you don't
overdischarge or short circuit them. Their advantages are a much higher
max charge/discharge rate and higher cycle life, and they won't vent
with flame no matter what you do. Apart from that, they can be treated
much as "normal" li-ion batteries.
With thanks to the following excellent sources of
battery information:
Battery University
Candlepower Forums
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