Where's the bottom on NIMH batteries
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Albany Tom
Enlightened
Re: Where\'s the bottom on NIMH batteries
0 volts. Full discharge of a nimh or nicad won't hurt it. (Contrast to a lead-acid cell, where full discharge will cause some damage.)
The danger is in a pack, where taking the whole pack to 0 volts will end up taking one of the cells to negative voltage, that is trying to reverse charge it. That will hurt the battery. How far you can take a pack down depends on how well matched the batteries are, and how many cells in series.
0 volts. Full discharge of a nimh or nicad won't hurt it. (Contrast to a lead-acid cell, where full discharge will cause some damage.)
The danger is in a pack, where taking the whole pack to 0 volts will end up taking one of the cells to negative voltage, that is trying to reverse charge it. That will hurt the battery. How far you can take a pack down depends on how well matched the batteries are, and how many cells in series.
carbonsparky
Enlightened
Re: Where\'s the bottom on NIMH batteries
<font size="2" face="Verdana, Arial">As Tom mentioned you don't want to go all the way. Also if the batteries have not been charged in a while, one may have discharged farther than the other(self discharge). This could cause the one battery to reach zero and start reverse charging. You can avoid this by freshing up the charge on long setting batteries before using.
Re: Where\'s the bottom on NIMH batteries
You can d/l the datasheet from major Nimh battery manucfacture like Sanyo,Panasonic,GP etc and see the discharge curve. All of them are recomended the threshold at 1 Volt.
For Nimh or Nicd cell, at 1 Volt with load about 99% stored energy is already spent. So there is no reasonable power that you can squeeze out anymore.
Especially for Nimh, if the cell is pushed further into voltage reversal, the polarity of both electrodes is being reversed, resulting in an electrical short. Such condition cannot be corrected and the cell will suffered permanent damaged like lost of it's capacity or worst total dead e.g. in the batt pack.
So becareful, don't discharge below 1 Volt or sometimes 0.9 V is the last resort. If you look on various power IC like boost converter, many of them use strange minimum voltage cut off like 2.7 V or 1.8 V aren't they ? This are for the protection for avoiding excessive discharge and to ease the designer so they dont have to add another protection circuit for these Ni based power source.
You can d/l the datasheet from major Nimh battery manucfacture like Sanyo,Panasonic,GP etc and see the discharge curve. All of them are recomended the threshold at 1 Volt.
For Nimh or Nicd cell, at 1 Volt with load about 99% stored energy is already spent. So there is no reasonable power that you can squeeze out anymore.
Especially for Nimh, if the cell is pushed further into voltage reversal, the polarity of both electrodes is being reversed, resulting in an electrical short. Such condition cannot be corrected and the cell will suffered permanent damaged like lost of it's capacity or worst total dead e.g. in the batt pack.
So becareful, don't discharge below 1 Volt or sometimes 0.9 V is the last resort. If you look on various power IC like boost converter, many of them use strange minimum voltage cut off like 2.7 V or 1.8 V aren't they ? This are for the protection for avoiding excessive discharge and to ease the designer so they dont have to add another protection circuit for these Ni based power source.
Albany Tom
Enlightened
Re: Where\'s the bottom on NIMH batteries
The reverse charge thing is bad, but I think with 2 cells, it's not much of a problem. To test it, try this - if the LED still lights with only 1 fully charged NiMh, then you might have a problem. If it doesn't, you're set. Reason is that worst case is 1 fully charged cell, 1 dead cell driven into reverse charge. That leaves 1.25 volts for a 2.5 V light, that probably kicks out at 1.8 or so anyway. My bet would be that the dc-dc just shuts down, and ends up protecting the battery by either accident or design.
Cool how a regular, non-LED flashlight won't do that. It, rather, gives you a nice low impedence path to fully toast one of your batteries. (Score 1 LED)
The reverse charge thing is bad, but I think with 2 cells, it's not much of a problem. To test it, try this - if the LED still lights with only 1 fully charged NiMh, then you might have a problem. If it doesn't, you're set. Reason is that worst case is 1 fully charged cell, 1 dead cell driven into reverse charge. That leaves 1.25 volts for a 2.5 V light, that probably kicks out at 1.8 or so anyway. My bet would be that the dc-dc just shuts down, and ends up protecting the battery by either accident or design.
Cool how a regular, non-LED flashlight won't do that. It, rather, gives you a nice low impedence path to fully toast one of your batteries. (Score 1 LED)
Re: Where\'s the bottom on NIMH batteries
I'm trying to decide if I should make a low battery indicator. It's more junk to stuff in there, but I could set it at a little over two volts and make a red led light up.
I have a four led light with a dimmer. When alkaline batteries get low, around maybe 1.1 volts, it won't go to full brightness and I can feel it in the dimmer as there is no brightness increase above about 75 or 80% of range. So that tells me the batteries are low.
Sounds like more experiments required.
<font size="2" face="Verdana, Arial">I'm making a light with an LS/O, two nimh batteries, and if the mailman ever brings it, a max1797 chip. And a dimmer, it's got to have a dimmer.
I'm trying to decide if I should make a low battery indicator. It's more junk to stuff in there, but I could set it at a little over two volts and make a red led light up.
I have a four led light with a dimmer. When alkaline batteries get low, around maybe 1.1 volts, it won't go to full brightness and I can feel it in the dimmer as there is no brightness increase above about 75 or 80% of range. So that tells me the batteries are low.
Sounds like more experiments required.
Re: Where\'s the bottom on NIMH batteries
--------------------------------------------------------------------------------
Originally posted by vicbin:
"For Nimh or Nicd cell, at 1 Volt with load about 99% stored energy is already spent. So there is no reasonable power that you can squeeze out anymore."
The fallacy of this statement is that it ignores the voltage drop you get on AA NiMHs under high current drain. For example, I drive a Carley 3.5 V 2.30 Amp Halogen bulb potted into a ceramic PR base in a PT40 with 4 @ AA NiMHs. Under this current, freshly charged AA's start off at 1.0 V per cell and drop from there.
At 3.35 Amps, 1800 mAh NiMHs start off at 0.85 V per cell.
--------------------------------------------------------------------------------
Originally posted by vicbin:
"For Nimh or Nicd cell, at 1 Volt with load about 99% stored energy is already spent. So there is no reasonable power that you can squeeze out anymore."
The fallacy of this statement is that it ignores the voltage drop you get on AA NiMHs under high current drain. For example, I drive a Carley 3.5 V 2.30 Amp Halogen bulb potted into a ceramic PR base in a PT40 with 4 @ AA NiMHs. Under this current, freshly charged AA's start off at 1.0 V per cell and drop from there.
At 3.35 Amps, 1800 mAh NiMHs start off at 0.85 V per cell.
Doug S
Flashlight Enthusiast
Re: Where\'s the bottom on NIMH batteries
<font size="2" face="Verdana, Arial">I bet that is one Bright light! No doubt that it would make a 5W Luxeon look wimpy. Cheaper too. I wouldn't expect the stock PT40 reflector to be able to handle the heat. Did you find a suitable aluminum replacement?
Jonathan
Enlightened
Re: Where\'s the bottom on NIMH batteries
However I would have expected somewhat higher voltages; for example, a panasonic AA NiMH datasheet suggests that at 3.1A their NiMH cells drop to 1.1V/cell for most of the discharge. These are 1500mAh cells; perhaps different brands of high mAh capacity cells also have higher internal resistance and thus more voltage drop under load.
-Jon
<font size="2" face="Verdana, Arial">lemlux, I entirely agree that the common '1.0'V/cell rule of thumb ignores the internal resistance of the cells and the voltage drop caused by this internal resistance. A better rule of thumb suggests that you use some fraction (say 80%) of the 'midpoint discharge voltage' (essentally the voltage _under load_ when the cell is half discharged, determined by looking at a graph of voltage versus time) as the ideal cutoff. This works out to about 1.0V/cell at low discharge rates.
However I would have expected somewhat higher voltages; for example, a panasonic AA NiMH datasheet suggests that at 3.1A their NiMH cells drop to 1.1V/cell for most of the discharge. These are 1500mAh cells; perhaps different brands of high mAh capacity cells also have higher internal resistance and thus more voltage drop under load.
-Jon
MrAl
Flashlight Enthusiast
Re: Where\'s the bottom on NIMH batteries
Hello there,
Albany Tom:
I like the point you brought up about LED's not killing batteries.
It's interesting that an LED+R connected to (a) battery(s) wont drain
down the battery all the way for years probably, or at least at the
same rate as the self discharge. The LED keeps drawing less and
less current as the voltage goes down. Also interesting is that
the LED stays lit, even if just a little. I've gotten usable
light out of a single Nichia at as little as 100ua of dc current.
Jeff:
If you are interested in a simple low battery indicator, take a
look at Zetex's undervoltage chips. In a tiny SM 5 pin package
they can light an LED with series R when the battery runs down
without any additional circuitry.
Good luck with your LED circuits,
Al
Hello there,
Albany Tom:
I like the point you brought up about LED's not killing batteries.
It's interesting that an LED+R connected to (a) battery(s) wont drain
down the battery all the way for years probably, or at least at the
same rate as the self discharge. The LED keeps drawing less and
less current as the voltage goes down. Also interesting is that
the LED stays lit, even if just a little. I've gotten usable
light out of a single Nichia at as little as 100ua of dc current.
Jeff:
If you are interested in a simple low battery indicator, take a
look at Zetex's undervoltage chips. In a tiny SM 5 pin package
they can light an LED with series R when the battery runs down
without any additional circuitry.
Good luck with your LED circuits,
Al
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