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Old February 7th 18, 04:48 AM posted to
Jeff Liebermann
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Posts: 2,699
Default Battery Replacement on Lights with Internal Li-Ion Batteries

On Tue, 6 Feb 2018 10:52:50 -0800, sms

My wife's Lezyne Deca 1500XXL stopped taking a charge, at all. Taking it
apart, I saw that the batteries were made in July 2015. Not too good for
it to stop working that soon. These lights don't have user-replaceable
batteries, but by removing two screws I was able to open it, and the
battery pack does have a connector on it so at least they didn't solder
it directly to the printed circuit board.

I don't see a problem. If your wife used the light every day for
about 2 years, that would be 730 charge cycles. That's about the
correct lifetime for a 60% DoD (depth of discharge). See table 2:

Charging the batteries in parallel with a 2A USB charger is also about
right. For two alleged 2800mA-hr cells in parallel, that would be
about 0.35C charge which is quite safe.

I've found that the cells that die quickly are usually helped along by
a charge circuit that overcharges the battery. 4.2v should be the
absolute maximum. LiIon loses about 10% of it's capacity during the
initial rapid discharge from 4.2 to 4.0V. While I don't like losing
the 10%, the battery will last much longer if only charged to 4.0v.
Charging to 3.92v yields the best compromise between two failure
mechanisms (electrolyte oxidation and growing crud on the anode). See
"summary" section:

It's a 2 cell 18650 pack with the batteries in parallel, and a
protection circuit board shared between the two cells. The cells are
allegedly 2800mAH, for a total of 5600mAH.

Seems rather high. Note that batteries are tested at a 0.2C
discharge, which yields larger numbers than the usual headlight
discharge rate. The Lezyne Deca 1500XXL claims 1500 lumens, which
also seems a bit high. Assuming 120 lumens/watt at a nominal
3.7V/battery, that's:
1500 / 120 / 3.7 = 3.4A
to run the headlight at full brightness, or 1.7A per cell.
Meanwhile, the cell capacity is tested at:
0.2 * 2800 = 0.56A

The closest I could find
on-line was a 2x2600mAH parallel pack

Notice that his version:
is series connected, but claims only 2200ma-hr. Kinda looks like the
same cells, but wired differently. Looks like the one you purchased
might be lying about the battery capacity. You'll find out soon
enough if the battery is discharged faster than expected.

Ready to buy a battery discharge tester
and a lux meter?

I moved the connector from the old pack to the new pack, plugged it in,
and closed it up. Seems to work fine now.

Yeah, for how long will it work? Dig out your digital volts-guesser
and measure the EoC (end of charge) voltage. If it's over 4.2v, that
may be why it died early. Also, see how long it will run. You don't
want it going out prematurely on the initial test run. Maybe carry a
2nd headlight for the initial testing.

Jeff Liebermann
150 Felker St #D
Santa Cruz CA 95060
Skype: JeffLiebermann AE6KS 831-336-2558

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