multi Li-Ion maths check needed

I`v just finished a simple Mod to an old 1960`s Pifco flashlight (it`s 100% easily reversible I make sure of that) and I need to check my maths on the Li-ion batts I`m using.

I`m using a pair of 18350`s held together with a Neodymium magnet, and that fits pefectly in the body of the torch. the batts are rated at 900mA and will be driving an LED drop-in running at 150ma.

Obvs I don`t want my cells to drop below 2.5v, so I divided 4v by 3 and get 1.3v, 1.3x2 =2.6v (perfect!) this says to me that I can use a third of the overall power in my batts quite safely before needing to recharge. that equates to 300mA of the total 900mA available.
since I`m drawing 150mA that means I can use it safely for 2 hours before I need a recharge.

Does that sound about right?


Both batts are sync charged and brand new from the same batch.

Lithium ion cells do not have a linear discharge curve so your math won't work like you intend it to. In addition, the discharge curve changes based on discharge rate.

Offhand, if you are pulling 150ma from 900mah cells... that's around 6 hours in a PERFECT world. I'd say use 3 hours to be safe and you will be fine.

https://siliconlightworks.com/li-ion-voltage

Wow, 3 hours! that`s surprised me, but after looking at that graph I can see why. and to think I was going to call it 1 hour to be on the Very safe side if my maths had worked out LOL

Katherine Alicia said:

.... I`m using a pair of 18350`s held together with a Neodymium magnet ....

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This can be very dangerous

If the magnet becomes dislodged, you could easily create an accidental short circuit

There are other potential issues, too. Unless the cells are near identically matched, they may discharge unevenly, resulting in the voltage of the weaker cell falling below the safe range much faster than "expected" (or calculated)

archimedes said:

This can be very dangerous

If the magnet becomes dislodged, you could easily create an accidental short circuit

Click to expand...

I`m kinda glad you said that, it was a concern I had (even though there is no damage to the plastic coating and protection ring around the anode). I was/am considering a plastic washer idea made of something a bit thinner than the magnet (1mm) with a 8mm bore that`s the same size as the magnets diameter, thus avoiding any movement of the magnet.
what I really need is some PVC pipe that`s a perfect fit for 18350s and 18650s, but alas I`ll have to wait until the shops are all open again so I can have a look around. Thanks for confiming that niggle I had

You should consider using protected cells. Then you won't have to calculate or guess how long you can run your light safely. Also with unprotected cells, if one cell is not as good as the other or dies early your math won't work anyway.

Katherine Alicia said:

.... I was/am considering a plastic washer idea ....

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Yes, you are correct that is indeed a safer approach. Some people who post about using magnets are not fully aware of the risks, which is why I mentioned it.

Icarus said:

You should consider using protected cells. Then you won't have to calculate or guess how long you can run your light safely. Also with unprotected cells, if one cell is not as good as the other or dies early your math won't work anyway.

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As many protection circuits cut off at ~ 3.0V, that would also provide a substantially safer minimum voltage than discussed in OP.

For myself, I don't like Li-Ion cells to spend any length of time below 3.5V (and certainly not < 3.0V)

Icarus said:

You should consider using protected cells. .

Click to expand...

haha! I was just about to post another thread asking if you can buy these protection circuits seperately! :-D
I like my unprotected cells as they are to be honest, but for certain applications (such as this) I`d be happy employ a small protection circuit as a standalone unit. I`v just never come across one before???
The smaller the better too, I imagine they`re about coin size and need access to both terminals?

EDIT: nevermind, I think i`v just found some DIY PCBs that you can retrofit to your batts :twothumbs

The longer size of the protected batteries might be enough so as not to need your magnet spacer as well.

True, I just hope that they`ll fit after doing it, there`s just enough room as it is. but my Real worry is soldering the strips to the battery! my iron is more than capable of doing it (100w gun type) but I`v heard of batteries exploding when being soldered to, the top cap isn`t so bad to solder to, it`s body that tends to cause the *BOOM*

Soldering lithium ion cells can damage them; factories use special spot welders to put tabs the on. Best to get some quality protected button top cells, particularly if you are using multiple cells in series, as leaving the light on is likely to reverse charge one of the cells, possibly setting off the fireworks.



If you really want to use unprotected cells, use only one cell so you can't reverse charge anything. Most LED drop-ins will run on a single Li-ion cell.