26650 battery mixing - protected and unprotecteds in the same light!!!

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lionheart_2281
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26650 battery mixing - protected and unprotecteds in the same light!!!

G’day all,
I recently bought this modded monster from 18sixfifty and have been looking at 26650’s for it. I was initially going to buy some KK INR’s for it, but then decided not to…read on…

So I needed like 7× 18650’s to top up the TK75vn KT and K40vn (hate swapping batteries between torches, I’m lazy) so I went to my local battery supplier Supersports600 . After initially getting the 18650’s I needed, I asked him what he would do about 26650’s for a torch that supposedly pulled around 13A (it actually pulls under 10A, he tested this). This is where it gets interesting…keep reading on…

He initially recommended running 3x protected 26650’s that he swore could handle the amp loading, however 3 of those didn’t fit, they were too long.

SO GET THIS: he said “well, 3 of those don’t fit, so WHY NOT RUN 2 UNPROTECTED 26650’S WITH 1 PROTECTED 26650. “. I said I was hesitant in doing this, as I have always been told to never mix batteries. He laughed and said “I designed the protection circuit in the Blazar NCRB’s that is also used in several other high end 18650’s, I can guarantee it is safe to do so.”

So here I am, now running in series 2 unprotected 26650’s with a protected 26650. Basically now I have the piece of mind knowing that now it is impossible to over discharge the 3 batteries as the low voltage protection of the protected 26650 will trip when it gets too low.

Might also be worth noting all 3 batteries have the same 4500 mAh.

I would also like to note that I do not recommend doing this, but the way this electrical engineer explained it to me, I’m willing to give it a try. I also intend to keep these batteries as a set.

Edited by: lionheart_2281 on 06/25/2014 - 03:16
MRsDNF
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I actually rung the guy the other day inquiring about those very protected 26650 batteries and he was not sure what cells they were inside the wrapping. So my question would be are you running three cells the same but two have protection and one doesn't or two different branded cells? Either way without doubting what he told you personally I stick to the same cells in all my multi cell lights and all protected batteries.

Edit. What was he using to check the current and what was he using to power the light when he was checking the current.

 

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Jerommel
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Not much can go wrong in this scenario.
It’s series, so when 1 batt quits all is shut down = series link is broken by the protection circuit.
Much more important is that you charge them equally and that they’re all identical cells.

mattheww
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Jerommel wrote:
Not much can go wrong in this scenario. It’s series, so when 1 batt quits all is shut down = series link is broken by the protection circuit. Much more important is that you charge them equally and that they’re all identical cells.

I am not so sure this is a great idea. If the unprotected cell goes flat first, things are likely to get interesting. So you want to make sure that unprotected cell has the highest capacity of the lot, that way it will ALWAYS be the protection on the other 2 protected cells that kicks in.

Jerommel
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I wrote:
Much more important is that you charge them equally and that they’re all identical cells.
lionheart_2281
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mattheww wrote:
Jerommel wrote:
Not much can go wrong in this scenario. It’s series, so when 1 batt quits all is shut down = series link is broken by the protection circuit. Much more important is that you charge them equally and that they’re all identical cells.

I am not so sure this is a great idea. If the unprotected cell goes flat first, things are likely to get interesting. So you want to make sure that unprotected cell has the highest capacity of the lot, that way it will ALWAYS be the protection on the other 2 protected cells that kicks in.

Being in series, won’t they always have the exact same amount of charge?

mattheww
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The cells will only end up exactly the same amount of charge if they each started with the same charge energy. That is easier said then done.

There are often difference between cells. Almost by definition protected and unprotected cells will be from different manufacturing lots and possible even different manufacturers, so the odds that you end up 3 perfectly matched cells are slim. Consequently if you make sure that the unprotected cell has the highest capacity, you can be certain that it will be one of the protected cells that will run down first and opens the circuit.

gadabout
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As the last two posters have said, as long as the three cells are evenly matched and disharge at the same rate then having one protected should work fine because we could expect the other two cells to be at a similar voltage.

In practice I guess they would have to be very mis-matched to put one cell into reversal.

Even though I use protected cells (Blazars mostly) in my Shocker and TN31, I try to keep them in matched sets and never pull one out for "other use".  That way they should all have the same number of charge cycles, storage temperatures, etc.  I often charge them in parallel in a holder from a Spark SG5 headlamp.

If you have a hobby charger, perhaps an occasional capacity test (every 6 months or so?) might be a good idea.  Or perhaps just get into the habit of measuring the voltage on all three when you pull them for charging and start to worry if they begin to read as significantly different.