Rocketman

Protection circuits for Li-ion DIY e-cigs

There’s been a little controversy about Chinese batteries and in particular the little protection circuit board attached to a Li-ion cell to call it “protected”. It the first photo is a protection board removed from a cheap 18650 cell. The 6 pin chip on the board is the brain. It contains a voltage reference and several comparator circuits to monitor maximum charge voltage, minimum discharge voltage, maximum continuous current and short circuit detection. The chip turns the back-to-back mosfets in the 8 pin chip on or off. Referring to the next photo, the mosfets are labeled M1 and M2. They are shown with shunt diodes that conduct in reverse when the mosfet is turned off, a limitation of the manufacturing process as the die structure will conduct in reverse. The mosfet on the left is the discharge switch and the one on the right is the charge switch. Under most conditions both are on and the controller turns them off when necessary to protect the Li-ion cell. The on resistance is about 20 to 25 milliohms (0.025 ohms) each. So the path resistance is 40 to 50 milliohms through both switches which drops about 45 millivolts per amp . 3 amps, the continuous max current limit, drops about 135 millivolts.
The third photo shows a higher current version of this board that has an extra set of mosfet switches connected in parallel. Same functionality, but a higher current capacity and about half the voltage drop. The path resistance of the parallel mosfets (all 4 turned on) is 20 to 25 milliohms which drops about 25 millivolts per amp . 6 amps, the continuous max current limit, drops about 135 millivolts.
The one you select depends on how you are going to use it. If you plan on using Low Resistance cartos or attys, the low current version may cause you problems. With a freshly charged cell, a cold LR carto, the first puff will draw the most current. If close to the max current trip point of the controller chip, it will shut down, turn on, turn off, turn on, etc. If you are using a boost circuit to provide higher voltage the current drawn from the cell will be even higher and is more likely to cause problems with the low current board. If you only plan on using standard resistance cartos and attys 2.2 to 3.4 ohms, and vape at cell voltage (about 3.7) then the low current board would be adequate. The cheaper Chinese 18650 cells have this version of protection circuit.
If your current requirements get close to or exceed 3 amps (from the cell) then it would probably be best to go with the higher current version of the board.
For a dollar or two this protection board gives you overcharge, overdischarge protection, and short circuit protection and if you pick the correct version for your application, relatively little in-circuit loss.

Hope this helps,

Rocky

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tek

ahuh... gives me a idea.
be back in a moment

berger

Does that still require a strip from one end of the battery to the other correct? I mean if Im looking at the wrapper of the cell I should see a raised part of that strip in the there...

Steampunk

If you hear a <BOOM> and/or <CRACKLE> from the R.I. area...

berger

Now Im worried about the eastern seaboard power grid....

Steampunk

As long as my AC is still working, it's cool.

Er, no pun intended...

We had a bit of a 'blackout' Thurs right in the middle of the hot part...Whew!

berger

at 7:00 PM Thursday evening it was 105...no ac would suck...just a tad

Rocketman

The protection circuit board still needs to connect to the positive end in order to sense cell voltage. The "raised" portion may be difficult to "see".
If the overlap in the wrapper is in just the right place it could mask the bump for the strip. If you doubt your cell is protected it may be necessary to remove a little of the wrapper at the negative end (the safer end) to see. Then recover with a little black electrical tape. I wrap both ends with tape when I put a cell into an aluminum box anyway, or put a thin layer of insulation inside any tube mod.

Steampunk

Not only that, my Mom was sitting in her recliner...reclined! (It's a POWER chair and has no aux crank!

Teri

The highest we've had this week was about 78. We're at 77 now, but expecting about 85-90 today. I hope that heat wave breaks before we have to travel.

RY4

Dang I'm jealous, I need to come where you are. The heat and humidity are awful here.

Teri

C'mon... We'll make room for ya'.

RY4

I am soooooooo tempted right now. But you know that.. After my surgery next week, I'm really going to be bugging Monte.

Rocketman

Ever feel like your ecig could be just a larger bigger? Like what is a realistic limit for size in an 'at-home' e-cig? Sure, no one wants to be seen in public with some big ash mod, how embarrassing !
and a Pass Through? Not quite as convenient if you have to get up and move about, is it? So what's the answer to keeping your charge cool?

How about this one?

20 foot USB cord, charger, and a parallel 18650 box mod. Since the cord is only passing charge current, not vaping current a pair of these cords for 40 foot could probably allow you to move all around the house without having to stop to recharge, like never

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Steampunk

I hand my K00K Crown to The Master...

Rocketman

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Steampunk

even if you search Too big to flush...

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Rocketman

Bikes is back,



and yes that is grammatically correct.

RY4

LMAO Rocky! Make sure you take a plunger with you.....

Rocketman

Plunger shaped mod, great idea.

RY4

Just don't get it stuck to your mouth..... ROTFLMAO!

Steampunk

There's an image I could have avoided....

tek

eww