On Mon, 20 May 2019 15:18:29 -0700 (PDT),
wrote:

I purchased a 3 pack of the model 350 at Costco. Everyone one of
them has this issue. 3 months is about right. Put new batteries
in all of them and a few months later they all are virtually
drained of battery life.
Matt C

That would be a Duracell Durabeam Ultra 350. The current version sold
by Costco is the Durabeam Ultra 380, which is the same flashlight
package except all 3 flashlights are black. I would guess(tm) that
the 350 was last sold in early 2017, so you probably bought "old
stock" that was sitting on the shelf for about 2 years.
https://costco97.com/duracell-led-flashlights-3-pack/

A unique feature of these Duracell line of flashlights (350, 380, 500)
is that they use FOUR alkaline cells instead of the usual three. This
is an attempt to squeeze more lumens out of the flashlight at the
expense of battery life. The driver (current regulator) chip used may
have been originally designed to run at 3.0 to 4.0v for one LiIon, and
maybe 3.9 to 4.8v for three alkaline cells, but might have problems
with 5.2 to 6.4v for four alkaline cells. My guess(tm) is that these
lights used the usual AMC7135 current regulator IC which is rated at
350ma from 2.7 to 6.0v.
https://www.electroschematics.com/wp-content/uploads/2016/04/acm7135-datasheet.pdf
In my limited experience tinkering with some of these chips, I found
that 6.0v really is the absolute maximum voltage that can be used and
that some (not all) chips will blow up at around 7v. Running these on
4 alkaline cells seems a bit close for comfort. Unfortunately, I did
not check for leakage current through the driver. It's easy enough to
do. Install 4 brand new alkaline cells and measure the current drain
when off. If you measure any leakage current, you can guesstimate the
standby life of the battery by dividing the capacity of a single cell
by this current. For example, if you measured 0.1ma of leakage, all
four AAA cells will be depleted in:
750ma-hr / 0.1ma = 7,500 hrs
which equals:
7,500hrs / 24 hrs/day = 313 days
or about 1 year. Based upon your 3 month experience, and that the
package was probably sitting on the shelf for a while, I would
guess(tm) you should see about 0.35ma of leakage.

However, all is not lost. There are LiIon inserts made to fit in
place of alkaline battery holder. This is commonly done with
flashlights that take 3 alkaline cells:
https://www.youtube.com/watch?v=yNo_k_LOWMM
It can also be done with an 18500 LiIon cell, if you can find one.
I've done much the same thing except I used a hard rubber chemistry
cork instead of a wine bottle cork. However, I haven't seen anything
for doing it with 4 alkaline cells. That's probably a waste of time,
so I suggest you recycle the flashlight and buy something that runs on
a LiIon cell and not on alkaline cells.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558