Unevenly drained batteries

[dervishMe]

Hi all,

I like to check the voltage of the drained batteries from my flashlights before replacing them. What happen when the two batteries from a same light don't discharge evenly? Example today one of the AAA from my little used Inova Radiant measured at 1.05 v and only .49 v for the other. The light out was quite dim.

This is not the first time by the way.

Henry

 

[FlashCrazy]

Nothing to do with the light... it's the batteries. Shouldn't happen too often, unless you're using off-brand and/or really old batteries. Even if they have the same voltage when you put them in the light, they could be mis-matched in terms of capacity or internal resistance.

 

[DM51]

I'm moving this to the 'Batteries Included' section.

 

[SilverFox]

Hello Henry,

Welcome to CPF.

Usually, the discharge is stopped at around 2 volts for a 2 cell light. This prevents over discharge.

We spend a lot of time analyzing cells and I pick cells with similar capacities to use in multi cell lights. That way they stay better matched.

It is better to recharge often than to run your cells all the way down until the light is very dim.

Tom

 

[zipplet]

I recently experienced this with my L2D-CE and some GP 2700mah nimh cells. I ran the light until it dropped out of regulation - and then for a further 5 minutes. The rubber tailcap boot swelled (gas!) and I measured one battery as flat and one still had power (I used a simple tester, couldn't be bothered getting out the DMM).

As silverfox says, it's better to recharge often...

 

[Mr Happy]

The real problem with using rechargeable NiMH cells in flashlights is the eye is terrible at recognizing how bright something is without a reference point. You can detect sudden changes in brightness, and you can compare two lights to see which is brighter, but given one light alone you can't tell whether it is full brightness or half brightness. So by the time you think your light is going dim, it is already way past the time where the batteries are being damaged.

If you discharge a cell on the C9000 it stops when the voltage reaches 0.9 V, and the voltage invariably rises back to 1.1 V or more after it stops. So if you find a cell that reads less than 1.0 V when you test it without load on a voltmeter, it almost certainly was being reverse driven in a multi-cell flashlight and therefore was being damaged before you stopped using it.

For maximum life of your batteries, I think you should stop using the light and recharge them before the open circuit voltage is below 1.1 V (1.2 V for Eneloops). That basically means you have to stop and recharge before you see the light dimming. For preference use a light with a low voltage cut-off so that you know for certain it is time to recharge.

 

[zipplet]

Excellent points, I wonder at what voltage the fenix driver decides to suddenly drop to a low light output. I hope it's in the safe zone.

 

[Mr Happy]

I think many lights are designed with alkalines in mind, since 99% of the general public doesn't use rechargeable batteries. To get the most out of alkalines they need to be sucked right down to the lowest voltage possible. Quite different from NiMH cells.

 

[dervishMe]

Thanks for the info, guys!

But I am using regular alkaline batteries.

Henry Dervishme

 

[Mr Happy]

Due to variations in manufacturing tolerances, no two batteries are exactly alike. So if you run two alkaline cells in a light, one of the cells will most likely run out before the other one, and then you will see a difference in voltage when you check them. For both cells to show the same voltage they would need to have been identical and this is quite unlikely for the most part. So to see a difference in voltages is probably more likely than to see the same voltage.

 

[dervishMe]

Hi Mr. Happy,

Thanks for the reply. Just wondering why the difference is so great. Both batteries are same brand, from the same package.

 

[Mr Happy]

I'm not sure why the difference is so great. Other people have observed the same thing and it has been thought of as something of a mystery.

One thing it could be is that when a battery is nearly dead, there is a big change in voltage for a small increase in drain. So at the end of a battery's life, any difference in voltage is magnified compared to when the battery is fresh.

 

[Isthereanybodyoutthere]

I am no expert in batteries ,but some one told me that the batterie closest to the bulp/led is used more/drained faster
Thats why he rotate the batteries after some use (non rechargeable ) to get most out of them

Could some make a test
Would be simple
Mark two batteries ( a and b )
Fully recharge them
a in front
light on for 20 minutes
Measure the voltage on the cells
Do it 3 times to be sure
Do it again ,,but this time b in front

If in both cases the front one is most drained ,,he was right

Anny one done this

 

[Burgess]

I've experienced this same thing with Alkalines.


And, of course, when a flashlight uses 4-cells,

the probability seems to increase.


I've simply chalked it up to Manufacturing Tolerances.

:sigh:
_

 

[zipplet]

The battery closest to the head does not drain more quickly. Both are drained at the same rate - you can prove this if you would like to. However a given battery may drain more quickly because of manufacturing differences, age/use (NiMH), etc. I know where this myth surfaced from. If a light is dimming and you open it and swap the batts around, during that time the batteries recover a little so it appears you are getting more energy from them, when you infact are not.

Burgess: I think it's manufacturing tolerances (with alkalines) which is why I like the way a light such as the L2D-CE gives you a suddden drop in brightness as it drops out of regulation. Time to switch out ASAP.

 

[Lynx_Arc]

it is quite possible the hottest battery is drained the fastest as in most cases heat can affect chemical reactions usually increasing them. The battery near a bulb could get hotter but usually the difference is negligable

 

[mighty82]

But have anyone actually had a nimh battery destroyed due to reverse charging? I have done a couple of tests with my L2D, where I have put in 2 mismatched batteries. One had a capacity of 2420 and the other one 2310. I set it at high and waited till it dropped out of regualtion and a few minutes more.

When I measured the voltage one battery was 0.9 volts and the other one was 1. I have tried this several times and they never seem to be that far apart. I would think it's a non problem as long as you dont run it on the lower levels all the way down. If you use it in the low level just turn the head once every hour or so to see if it will run in turbo, if it doesen't, change the batteries!

 

[Mr Happy]

But have anyone actually had a nimh battery destroyed due to reverse charging?

I don't know for certain. But I have a set of XG 2400's that tested for runtime once or twice by putting them in a 4AA flashlight and timing until the bulb went dim. This was before I knew it might be a bad idea to do that. After I got my C9000 I tested them and they don't hold more than 1600 mAh any more.

So maybe I damaged them. Or maybe they were bad from the start. I don't know, but they are the only batteries I did that to, and they are the only batteries I have that test so poorly compared to their label.

Given what I know now, I won't take the risk of doing it again with good batteries.

 

[UnknownVT]

which is why I like the way a light such as the L2D-CE gives you a suddden drop in brightness as it drops out of regulation. Time to switch out ASAP.

Yes this was a possible "problem" I pointed out in my review of the Fenix L2(D or T) - because the head is basically the same as the L1 (D or T) the voltage goes as low as 0.9 Volts before the light drops out of regulation.

0.9V is OK for a single AA NiMH as long as one stops as soon as the light drops out of regulation.

BUT what happens when there are TWO AA's as in the L2(D or T) the head (being the same as the L1x) will only drop out of regulation below 0.9Volts -

I felt 0.9V was too low for 2x AA NiMH.......

However most people seem to be running the Fenix L2 (D or T) fine on 2x NiMH AAs.

Perhaps it's not the "problem" that I perceived?

 

[zipplet]

Hmm, I must admit I didn't think of the fact that the L1D/L2D share the same head. That's definately a problem (it being 0.9V, which is too low for 2x AA nimh).

It is a problem due to the number of reports of gassing including my own, but I guess most people recharge often or stop using the light the moment it drops out of regulation. I know it's still not good to keep it going that long but it doesn't seem to harm the batteries too much. Going further does (so far, every time I've run the light for more than a minute past that point it gasses).

Fenix should redesign the L2D head, it wont be compatible anymore with the L1D but it could be designed to drop out of regulation when you have only a few minues left before the critical voltage (0.9v per cell).