Learned something interesting about Madmax...

[PlayboyJoeShmoe]

I bought 4 new Eveready 1850mah NimH batteries last night. Just to 'see', I placed two of them straight from the package into my mini.

It was as bright as ever for a while, and I now know what to look for when the batts are dying... a gradual decrease in output that when it got low enough for me to shut it down had right at 1.00v left in the batts.

So it doesn't "fall off a cliff" as I thought it might.

I'm more pleased then ever that I got a Madmax!!!

 

[Masala]

hmmm sound good!

 

[JollyRoger]

Especially since you're using nimh batteries, which have a nice and flat discharge curve, you're not going to see a steep drop in light ouput over the battery's life. Very nice indeed...but you should charge the nimh at the first sign of dimming....to prevent them from being damaged.

 

[PlayboyJoeShmoe]

Update!

With 1.31V measured in the 1600mAh Panasonics last night, I ran it today for a bit over 2 hours before it dropped so I could see it.

I figured I would go ahead and kill 'em, so I can have fresh batts for the Dallas trip.

Also noted. I ran my new Eveready 1850mAh's in my Rayovac Prodigy with a 4.8v .75a bulb for a bit over 4 hours yesterday while conditioning them.

It was plenty bright for those 4 hours.

 

[Klaus]

Joe - using 1850ma NiMhs to run a 750ma bulb for 4 hours is pretty impressive - did you invent a self-charging or never-depleting type of setup or is there a typo somewhere ?

Klaus

 

[Deleted member 684]

Nah, he merely forgot to calculate in day light savings... that means -1 hour and that means 3 hours of light. He didn't specify how bright does he think bright is so let's take it as 2 hours. :wink:

 

[logicnerd411]

Originally posted by JollyRoger:
but you should charge the nimh at the first sign of dimming....to prevent them from being damaged.

<font size="2" face="Verdana, Arial">Really? Why?

 

[Klaus]

NiMh don´t like being discharged below 0.9V

Klaus

 

[Jonathan]

Originally posted by Klaus:
NiMh don´t like being discharged below 0.9V

Klaus

<font size="2" face="Verdana, Arial">Actually NiMH _cells_ are fine with discharge below 0.9V. In general, there is very little reason to discharge below 0.9V per cell, however, because so very little energy is available at this point.

The problems occure with NiMH _batteries_, meaning a series connected string of cells.

NiMH cells are damaged by being charged in reverse. When you have an NiMH _battery_, all of the cells will have slightly different capacity. In general, unless the cells are _perfectly_ matched, one will discharge first. Since the other cells sill have charge, they can keep the current flowing. But since the cells are connected in series, this means that the _discharge_ current is still flowing through the fully discharged cell. In other words, it is being discharged 'past' zero, or _reverse_ charged.

The whole '0.9V per cell' thing is just a darn good rule of thumb for preventing this reverse charge failure mode. However it is only a rule of thumb, and there are situations in which it doesn't work. One is with very long series chains of cells, eg a 20 cell battery. The nominal voltage is 24V. If one cell happens to discharge first, and start reverse charging, then the battery voltage will drop to about 21V (you lose the 1.2V normal voltage, and then you lose >1.2V in the reverse charging process); but the 21V is > 0.9V per cell, so your system might not notice the reverse charging.

Another situation is under extremely high rate discharge, where the fully charged voltage is below 0.9V per cell because of the internal resistance of the cell. Clearly in this situation you need to discharge to a lower voltae (0.5V per cell) in order to use the battery.

Still another situation is with short battery packs, eg 2 cells in series, where a reverse charge situation would drop the battery voltage to zero...here you can safely discharge to <0.9V per cell because reverse charging is so unlikely.

-Jon

 

[Roy]

As I understand it, that is the difference between the Madmax and the Badboy. Madmax has no low voltage shutoff and will run the batteries down to zero or close to it. Baadboys have a low voltage cutoff that is designed to protect rechargable batteries.

 

[Klaus]

Jon,

agree with you that 0,9V is a number which does have something simplifying to it - and yes the cell reversal is the biggest enemy - but based on what I know and read I can´t agree with your point that single or dual cell setups can be discharged beyond that at no risk - seems no matter where you look its indicated that deep discharges tend to shorten the cells life - while as you stated there is no point in doing so anyway as almost nul enery is left down there.

I´m not really an rechargeable expert beyond a (heavy) user type of profile but everywhere I looked those points were seen the same way I do and where my own experience leads to as well.

Klaus

Some quotes:

Number of cycles: The NiMH is rated for only 500 charge/discharge cycles. Shallow rather than deep discharge cycles are preferred. The battery’s longevity is directly related to the depth of discharge.

o Discharging NiMH batteries below ~1.0V can cause irreversible damage. NiMH batteries are great for electronic devices that stop drawing power below a certain voltage, but not so great for analog devices like flashlights that can run them flat.

So-called "conditioning" chargers are a hangover from NiCd, and are generally *not* recommended for NiMH -- they put the cells through deep discharge that can shorten life or even damage NiMH cells.

 

[PlayboyJoeShmoe]

It really was 4hrs guys. Really.

And 2hrs on the Madmax.

I guess I now have useless batteries, because a couple times I have dropped 2 or 4 of them to no light at all.

If it REALLY won't hurt the NimHs to charge them while they still have plenty of life, then I guess I'll do it nightly???

Whatever battery my Stinger HP uses stays on a 12hr chrager all the time. I think they are Nicd... what does that mean?

 

[cave dave]

I noticed a distinctive dropoff over the course of 15 min with the madmax. When it was as dim as an Arc AAA I took them out and measured them at ~1.0V each.

I think as long as you switch out when you are no longer getting that fine Luxeon light you should be just fine. I wouldn't let it drop below Infinity level though if I were you.

 

[BentHeadTX]

Playboy Joe,
Nicd means Nickle-Cadnium. As far as using them for flashlights, they are really a great battery! You can kill them to nothing at it is actually good for them to do that. They provide a tremendous amount of current to keep the regulator rocking. (NiCads are used in defibrillators for that reason) NiCads also last for over 10 years if you take care of them (run them down all the way, dead helps a lot)
The problem with NiCads is if you only use them for a little bit and then recharge them...they get a "memory" (best way to explain it) If you use your rechargables down to almost dead, use NiCads instead of NiMH... they will take that abuse much better.

 

[radellaf]

Umm...please visit http://www.cadex.com/ and read their online "Batteries in a Portable World" book. It's a definitive work. Some of the info here is getting to be mis-info.

 

[Doug S]

Originally posted by radellaf:
Umm...please visit http://www.cadex.com/ and read their online "Batteries in a Portable World" book. It's a definitive work. Some of the info here is getting to be mis-info.

<font size="2" face="Verdana, Arial">Nice link RAD. I hope that everyone follows your recommendation.

 

[Klaus]

Definetely a good read - some of my quotes above are from there

Klaus

 

[SilverFox]

All right, I'm confused...

Klaus is quoting that shallow discharge cycles will make NiMh batteries last longer,

But when I go to the cadex.com site I find this:

"Nickel-metal hydride requires regular full discharge to prevent crystalline formation. Nickel-cadmium should be exercised once a month, Nickel-metal hydride once in every 3 months."

Does this mean that Playboy sould only run his batteries down once every 3 months? It seems to imply that shallow discharge cycles will promote crystal growth that will limit the capability of the batteries.

Tom

 

[Klaus]

Tom,

both is accurate - a discharge once a month or so will prevent the cells from forming "crystals" which degrades the capacity. A charge or discharge at higher current is also good for the non-forming of those - you will have these capacity degrading crystals mostly when routinely shallow discharging at low currents.

Now on to the on first sight discrepancy: Shallow discharge vs deep discharge - I once read a nice explanation - think of a cell as bread - now you can slice the bread in small or think slices - thick slices being the deep discharge while think being the shallow discharge - slicing thick slices you get less slices than sliccing small slices while in the end you get the same amount of bread - just in different number of slices.

Back to NiMh cells - if you always recharge your cells after you used them without fully discharging them first before charging you get more cycles out of them than when you would have fully discharged them every time befor charging - get the idea ? Now this doesn´t mean you shouldn´t once in a while (like every month or so depending on usage pattern) fully discharge them to prevent voltage depression - from which NiMhs are affected too - but very much not as severe as NiCds - actually this voltage depression is what commonly is referred to as the "Memory-effect" with NiCds which it typically isn´t - but thats another story.

Klaus

 

[PlayboyJoeShmoe]

Okay. I'm more confused than ever.

At least two of the 1850 Evereadys are okay. I went to shine Mr. Madmax today and nothing happened. I figured I really did ruin the batts.

Later I tweaked the module a bit with the head screwed off and voila! LIGHT!

At my usage pattern I think I will charge on a weekly basis. Perhaps Sunday night.