Nitecore MT40GT battery question

Just picked up a Nitecore MT40GT and wonder if there are any precautions one should take when running two cells in series? I'm using Nitecore 3400.

Standard precautions; as can be reviewed in the sticky topics in various sections of this forum.

Such as: The two cells must be the same brand, the same capacity, the same charge state.

Get two new batteries that are identical, and keep them together, just for this flashlight.

Don't overcharge the cells, don't over discharge, don't short circuit the cells, don't use/store the cells outside of approved temperature range. Keep the cells away from moisture; do not damage the cells, do not use damaged cells.

Only use charger that is approved for that type of cell. Charge the cells when you can attend; not the best to charge lithium ion cells unattended. Remove the cells from the charger when charge is complete.

Keep the cells/flashlight out of reach of children; only let competent personnel have access to the cells/flashlight.

Be aware of, and heed any battery indicator features that come with the flashlight.

Power down the light when it feels too hot.

A few basics off the top of my head:

-Use protected cells (which it sounds like you will be)
-Don't mix brands, capacities, or cell types (i.e., use two identical cells)
-Make sure cells are closely matched. For example, I use 4 identical Orbtronic protected 18650s in my Fenix TK61. All show 4.20v off the charger, and when left overnight 3 show 4.19 and one shows 4.18. I've also tested them a couple times after about a month of occasional use, and the cells still have the same relation to one another, i.e., 3 might be at 3.98v and the odd one will be at 3.97v. I'm not sure what the maximum safe variation is considered to be, but I wouldn't use two or more cells that vary in voltage by any more than .01v, since it seems to be fairly easy to put together a set that meet this criteria. I also have 3 of the same cells in my Solarforce S1100 and 2 always show the same votage, while 1 is .01v less.
-Once you find a good set (or pair in your case) mark them and keep and use them together in that specific light throughout their lifetime. The cells for my TK61 have "TK" marked on their labels with a Sharpie, and my Solarforce S1100 cells have "SF" on them.

edit: Derp! Beaten to the punch yet again! :twothumbs

Thank you for the info. I don't have enough sells for all my lights but have two brand new cells I use between my two single cell lights. I wanted to also use those two cells for the MT40GT light. If I charge them both up before using them in the MT is this ok?

erict18650 said:

Thank you for the info. I don't have enough sells for all my lights but have two brand new cells I use between my two single cell lights. I wanted to also use those two cells for the MT40GT light. If I charge them both up before using them in the MT is this ok?

Click to expand...

Just to add to all the good info above-

Ideally you would start with two new cells but in this case it sounds like both your [not very old] protected nitecore 3400 cells are the same age so , the most important thing as already stated above is you measure [ multimeter required] the starting voltage and the finishing voltage, ie say they both start of at 4.18 volts, you run the light down to say 3.6v and check that they are even or with in .03 volts say 3.61v and 3.64v you will be fine. Keep measuring and check for start and finish voltage deviation for the first couple of charges [ i measure every time ]
Because your cells are protected they will trip to zero volts if they are using to much current. For more info read up on cells in series and dangers of reverse charging,
Although not ideal, you can use them.

swan said:

Just to add to all the good info above-

Ideally you would start with two new cells but in this case it sounds like both your [not very old] protected nitecore 3400 cells are the same age so , the most important thing as already stated above is you measure [ multimeter required] the starting voltage and the finishing voltage, ie say they both start of at 4.18 volts, you run the light down to say 3.6v and check that they are even or with in .03 volts say 3.61v and 3.64v you will be fine. Keep measuring and check for start and finish voltage deviation for the first couple of charges [ i measure every time ]
Because your cells are protected they will trip to zero volts if they are using to much current. For more info read up on cells in series and dangers of reverse charging,
Although not ideal, you can use them.

Click to expand...

Do you have a link for info on cells in series and reverse charging?

I use two unprotected NCR18650GA cells in my MT40GT. But if you aren't very confident in lithium ion, then I'd go with protected cells. Definitely as mentioned above use two of the same brand, and capacity..........and at the same state of charge. The easiest way to do that is to charge both up at the same time or one after another if you only have a one bay charger.

It's also handy to have a voltmeter to test the batteries and get a feel for the amount of discharge (voltage level) vs the amount of use time.