Li-Ion safety cap DIY

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divxdude

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Apr 21, 2006
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i'm a big noob so if theres flaws in this plan please tell me.

i'm planning a safety cap for my unprotected 18650
Li-Ions.
Attaching a so called protection board to the end add as much danger as it prevents. the board is asking to get broken if used in a flashlight its bound to happen..
needs a crush proof shell.

they must never be shorted or recieve a reverse current
so i'm planning a fuse and a high capacity diode soldered to
+ and tucked into a pvc plumbing cap to be epoxy'd onto the cell to stay.

the diode will prevent reverse current and act as a CUTOFF to prevent depletion past the cell and safety limits so it needs to appropriately rated.

the fuse will prevent dead short and excessive drain rate
but will kill the unit if the cap cannot be removed so big fuse will be protected by smaller fuse outside the cap. now you may repair YOUR DEAD SHORT BOOBOO.

also inside the cap will be a wire for charging tied to the diode before comes out of the cap. it will be a like a cordless phone wire with a tiny connector that is keyed or shaped to prevent reverse connection or improper insertion. a waterproff connector mounted under the cap would be ideal but not enough room for that i fear..

the cap cannot block vents under the + button so will near equal the cell diameter butted to the outer rim and attached with something like fuel tank repair epoxy "clay" .

the cap will have holes equal or exceeding the vent holes
under +. the holes will be wax sealed

these are only partial Li-Ion precautions but very important ones that everyone should consider before tinkering with these. the rest can be handled by the charger and/or a protection pcb outside the light or device.

am i missing any important dangers in this plan?
 
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coldsolderjoint

coldsolderjoint

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diodes have a voltage drop.. i would think you need to up your charging voltage a litlle?
 
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HarryN

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Not an expert by any means, but Li Ions are rechargeable, so it is not really necessary to prevent reverse current. What is important is to prevent the voltage from going below approx. 3 volts during discharge. Most diodes have a Vf of approx. 0.3 volts (at the rated current), so I don't think this is the answer.

A fully protected cell also contains such items as over voltage protection, over temp protection, etc.

Frankly, protected 18650s are so common and inexpensive now that perhaps it would be easier to just use one of those and add shock protection to the light. Pilas from JSB are a good example. That is the concept I am following in my custom light.
 
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divxdude

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yeah k , dont need that.
 
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divxdude

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also what custom light are using for 18650 cells?
any lights fit that ?
 
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divxdude

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so there isnt a good diode for cutting off current at 3v

nevermind..that doesnt add up..not enough volts.

a luxeon flashlight is the only thing that makes sense here.
the lux will stop current at 3v? no?
 
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divxdude

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k
 
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divxdude

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what sort of amps will a dead short produce?
i bought 3A fuse.
 
coldsolderjoint

coldsolderjoint

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a "dead short" is 0 (thertical) ohms.

So when you divide voltage by 0, you get infinity for amps and then the universe explodes.
 
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divxdude

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so...seriously..

will the luxeon stop current at below 2.8v?
is there a simple regulator that will shutdown at 2.8v?

i can install protection at the charger if it doesnt have it.
with a epoxy'd cap on + nobody gonna be mis-charging it unless its me anyway.
 
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divxdude

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i still want to add a heat alarm too.
can be very simple...no?

or just a thermal fuse.

edit:
okay i guess epoxy may be drastic..
this super gooey double sided foam tape
they glued this stuff in with might be a good
idea tho.
 
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