When a ThruNight 2C goes BAD!!

Here's an original post, where I replied to Selfbuilt's recent review of the ThruNight Neutron series.

I've had the 2C for about a month now.

First, I don't notice the PWM at all! Second, on mine, the hotspot has a light-brown donut when white-wall shining. Third, the tail-standing is quite wobbly.

But wait!!! It CAN take RCR's can't it? The problem I've been having (for which I posted in a thread all its own) is that when using 2 TrustFire RCR's, (and after the light goes abruptly dark), from using it for several days, 1 of the cells is COMPLETELY DEAD (ZERO VOLTS). Meanwhile the second cell is STILL right around 3.7!!!!!! oo: I've tried this on 2 different sets of cells and it does the same thing. Right now, I'm using it with AW's, to see if it behaves the same way.

I'm wondering what the heck is going on with that!! :thinking:

Fast forward to today.........

I'm now thinking it's GOT to be the light which has an issue. Since having used TWO different sets of TrustFire 16340 cells and having the same problem duplicated, twice.. I then used 2 AW cells. AND THE SAME THING HAPPENED AGAIN :scowl:. So last night, with the AW's in there, I tested them. BOTH were over 4.0V. Didn't use it again. So this morning, I run it, at MEDIUM. The light is on for maybe 15 minutes... when it SHUTS OFF. Cells are tested again, with one being completely dead, and the other at about 3.7v. This CAN'T be a coincidence!! Right now, I'm trying 2 primary 123's to see how they react. The light won't accept 18650's, so if RCR's are an issue too... I ain't keepin' it.

This has got me spooked oo:.

Something's making the protection circuit trip and it makes sense that one battery is going to trip before the other. Once the first battery trips, the second won't trip because current is cut off. If you put the 0V battery in a charger for just a few seconds, you should be able to measure full voltage. You're not using magnets between the cells are you? Sounds like maybe something is pulling too much current from the batteries, either the light itself or a dead short. Have you taken some current readings at the tail?

Check for anything in the light that might be causing a short, like the springs being off center or deformed so that they end up touching the positive and the negative parts of the driver board. Make sure there aren't any burs or metal shavings in the tube. Also since the RCR123's are longer, make sure they aren't putting excessive pressure on the head or tail that could cause a short.

I think you are observing completely normal and expected behavior from a two cell light. It is nothing wrong with the batteries or the light as such, it is just what is expected to happen.

Basically, when a lithium ion battery reaches the end of discharge at about 3.7 V, the voltage "falls off a cliff". Look at any discharge graph to see what I mean.

Imagine two people running towards a cliff edge in the dark, where one is a few feet ahead of the other. When the first person runs over the edge of the cliff and screams the second person will stop dead in their tracks and be safe. Therefore you will have one person at the bottom of the cliff (at 0 V, with circuit tripped), and and the other still at the top off the cliff (at 3.7 V). Only if both runners are neck and neck (both batteries have exactly the same capacity and state of charge) will both runners end up at the bottom of the cliff together.

"BRTED" You're not using magnets between the cells are you? Sounds like maybe something is pulling too much current from the batteries, either the light itself or a dead short. Have you taken some current readings at the tail?

No. No magnets. And no, I've not taken any tail readings either.

Mr Happy said:

I think you are observing completely normal and expected behavior from a two cell light. It is nothing wrong with the batteries or the light as such, it is just what is expected to happen.

Basically, when a lithium ion battery reaches the end of discharge at about 3.7 V, the voltage "falls off a cliff". Look at any discharge graph to see what I mean.

Click to expand...

Hmmm... well I can't dispute the discharge graphs, but I've not read anything before about this being typical behavior. Doesn't standard practice indicate that Li-ion cells shouldn't be permitted to drop below 2.7 or thereabouts? Because these incidents certainly blow that theory, or make it impossible to monitor the cells health... prior to it jumping off a cliff!!! (BEFORE it jumps it should let me know it's reaching the edge!!!) I'm sorry, I just can believe this is "what is expected to happen". Admittedly, I've not been using RCR's all that long. And this initial "leap" into 16340's has begun with usage in series.

I can try the cells in another light and see if similar behavior repeats itself. Maybe that'll be a better indication.

RBWNY said:

"BRTED" You're not using magnets between the cells are you? Sounds like maybe something is pulling too much current from the batteries, either the light itself or a dead short. Have you taken some current readings at the tail?

No. No magnets. And no, I've not taken any tail readings either.



Hmmm... well I can't dispute the discharge graphs, but I've not read anything before about this being typical behavior. Doesn't standard practice indicate that Li-ion cells shouldn't be permitted to drop below 2.7 or thereabouts? Because these incidents certainly blow that theory, or make it impossible to monitor the cells health... prior to it jumping off a cliff!!! (BEFORE it jumps it should let me know it's reaching the edge!!!) I'm sorry, I just can believe this is "what is expected to happen". Admittedly, I've not been using RCR's all that long. And this initial "leap" into 16340's has begun with usage in series.

I can try the cells in another light and see if similar behavior repeats itself. Maybe that'll be a better indication.

Click to expand...

The protection circuit is what the "drop" is.

srfreddy said:

The protection circuit is what the "drop" is.

Click to expand...

Alright.... but is it meant to be this dramatic? Plus, there doesn't appear to be much in the way of runtime. I'll only have to use these cells for maybe a 1/2 - 3/4 a hr per day (utilizing low-med-high) for maybe 3 days before the "drop-off" occurs. Again though, maybe I'm more accustomed to the longer runtimes of primaries. :shrug:

RBWNY said:

Alright.... but is it meant to be this dramatic? Plus, there doesn't appear to be much in the way of runtime. I'll only have to use these cells for maybe a 1/2 - 3/4 a hr per day (utilizing low-med-high) for maybe 3 days before the "drop-off" occurs. Again though, maybe I'm more accustomed to the longer runtimes of primaries. :shrug:

Click to expand...

Rechargables have half the capacity of primaries, so I would expect so.

RBWNY said:

Alright.... but is it meant to be this dramatic?

Click to expand...

Yes. I don't know if "meant" is a good word, but a sudden trip to blackness is certainly what happens with protection circuits. It's a reason some people don't like that kind of configuration. They prefer their lights to fade gradually with a long moon mode at the tail end. You can have that if you choose a single cell light that runs from an AA cell or perhaps an unprotected 18650 cell in lithium ion.

Plus, there doesn't appear to be much in the way of runtime. I'll only have to use these cells for maybe a 1/2 - 3/4 a hr per day (utilizing low-med-high) for maybe 3 days before the "drop-off" occurs. Again though, maybe I'm more accustomed to the longer runtimes of primaries. :shrug:

Click to expand...

There is a recognized limitation of RCR cells compared to primary CR123A cells, in that RCR cells have a much smaller capacity (shorter run time). I guess 1 1/2 to 2 hours of running doesn't seem unlikely to me with RCR cells.

(I'll make no secret that for my own convenience I will take a 1AA light and put an Eneloop in it. It is plenty bright enough for most purposes unless I want an outdoor search light with a long reach. And with a light that can support it I can put a 3.2 V LiFe cell in it and make it as bright as any 2AA/CR123A light.)