Discharging Nickel Metal Hydride batteries

[Beemer]

I use Nickel Metal Hydride AA cells to power my photo flash equipment.  These are Energizer 2500mAh and I always charge them for 7 hours to ensure longer total life.  In use these are seldom discharged below 50% and I think I should be discharging them to say around 10% before giving them a full charge.   I read elsewhere that a resister in series with silicon diode would be appropriate.   Would a 10ohm resister be correct?

[Zero999]

You should use a proper charger which uses peak detection to terminate the charge, rather than a crude charger which terminates after a certain length of time has elapsed.

[alm]

Agreed about the proper charger, NiMH doesn't respond very well to slow charging, and overcharging can be harmful.

I believe that it might be a good idea to discharge NiMH batteries once in a while, but certainly not every cycle, this will just cause extra wear. I would make sure not to draw too much current (half the charge rate or so, something like .2C, i.e. 500mA for a 2500mAh cell), and make sure to terminate the discharge before the voltage gets too low.

There's plenty information available about how to treat NiMH batteries, you can probably find useful information at rcgroups.com, since they deal with lots of battery packs.

[Beemer]

You should use a proper charger which uses peak detection to terminate the charge, rather than a crude charger which terminates after a certain length of time has elapsed.

I already do

[Zero999]

If that's true, it shouldn't need 7 hours to recharge 2500mA cells which have only been half discharged.

[Beemer]

Agreed about the proper charger, NiMH doesn't respond very well to slow charging, and overcharging can be harmful.

I believe that it might be a good idea to discharge NiMH batteries once in a while, but certainly not every cycle, this will just cause extra wear. I would make sure not to draw too much current (half the charge rate or so, something like .2C, i.e. 500mA for a 2500mAh cell), and make sure to terminate the discharge before the voltage gets too low.

There's plenty information available about how to treat NiMH batteries, you can probably find useful information at rcgroups.com, since they deal with lots of battery packs.

For several reasons even intelligent multi-stage and temperature monitoring chargers can fail to charge all cells equally if the cells are not equalised before charging.   The result is reduced capacity of the battery and in the case of electronic flash results in excessively long capacitor charge time.

Partially discharged NiMH have a charge/discharge number (lifetime) significantly higher than the specification 500 full cycle charge/discharge.   I therefore do not believe that my equalisation process discharging to 1.2V per cell (the minimum) will result in cell deterioration.   Modern chargers used for emergency services e.g. police and fire have dischargers built-in to the charging station.

I will experiment with two diodes in series one with 0.7V the other with 0.5V (if I can find one) in series with each of the five cells in the battery pack.

thanks for replying

 

[Beemer]

If that's true, it shouldn't need 7 hours to recharge 2500mA cells which have only been half discharged.

You are correct which is another reason for me wanting to discharge to 1.2V/cell

[alm]

For several reasons even intelligent multi-stage and temperature monitoring chargers can fail to charge all cells equally if the cells are not equalised before charging.   The result is reduced capacity of the battery and in the case of electronic flash results in excessively long capacitor charge time.

Intelligent chargers are not prefect, they can sometimes miss the 'knee', especially at low charging currents. But I don't see how discharging them to the same voltage helps. Any good charger monitors each cell individually, so what does starting voltage matter?

Partially discharged NiMH have a charge/discharge number (lifetime) significantly higher than the specification 500 full cycle charge/discharge.   I therefore do not believe that my equalisation process discharging to 1.2V per cell (the minimum) will result in cell deterioration.

Agreed, partial cycles cause less wear than full cycles (an argument for not fully discharging them each time). Is 1.2V the 10% point? I don't have the voltage-capacity curve memorized, but it seems more like 50% to me. Is the voltage-capacity curve constant for cells of the same batch/age? Again, I'd take a look at what RC people do, since they have a fair amount of experience with caring for multi-cell packs (which is basically what 4 AA cells in parallel is, except that you charge individually).

Modern chargers used for emergency services e.g. police and fire have dischargers built-in to the charging station.

Don't they use lithium batteries by now? I assume they do something like discharging to 10% every so many cycles/hours?

I will experiment with two diodes in series one with 0.7V the other with 0.5V (if I can find one) in series with each of the five cells in the battery pack.

A Schottky diode might satisfy that 0.5V. Keep in mind that the forward voltage depends on the current, so even with just 0.4V over a regular silicon diode, some small amount of current might keep flowing. Not sure if this is significant for battery use, especially if you terminate it reasonably early (don't let it sit for days). I would use a constant current sink in series with something like diodes for voltage cut-off. A resistor might be good enough since the voltage doesn't change very much (between 1.5V and 1.2V).

[Beemer]

For several reasons even intelligent multi-stage and temperature monitoring chargers can fail to charge all cells equally if the cells are not equalised before charging.   The result is reduced capacity of the battery and in the case of electronic flash results in excessively long capacitor charge time.

Intelligent chargers are not prefect, they can sometimes miss the 'knee', especially at low charging currents. But I don't see how discharging them to the same voltage helps. Any good charger monitors each cell individually, so what does starting voltage matter?

I do not use use a quick charger so reduce the risk of hydrogen gassing which can occur with 1 hour chargers.   Photographers are frequently in a rush to get the cells into their flash and then the cells have been charged to different levels.   The battery capacity and number of flashes is then seriously compromised. 

Partially discharged NiMH have a charge/discharge number (lifetime) significantly higher than the specification 500 full cycle charge/discharge.   I therefore do not believe that my equalisation process discharging to 1.2V per cell (the minimum) will result in cell deterioration.

Agreed, partial cycles cause less wear than full cycles (an argument for not fully discharging them each time). Is 1.2V the 10% point? I don't have the voltage-capacity curve memorized, but it seems more like 50% to me.

Yes 1.2V is the 10" point

Is the voltage-capacity curve constant for cells of the same batch/age? Again, I'd take a look at what RC people do, since they have a fair amount of experience with caring for multi-cell packs (which is basically what 4 AA cells in parallel is, except that you charge individually).

The discharge curve of NIMH is very linear but I do not know what happens to the curve when the internal resistance (initially 0.05ohm) start to change with age.

Modern chargers used for emergency services e.g. police and fire have dischargers built-in to the charging station.

Don't they use lithium batteries by now? I assume they do something like discharging to 10% every so many cycles/hours?

Germany is the largest user in Europe and I am friends with the guy who sells the specialist battery chargers to all the national essential services.   He tells me that these chargers use pulsed charging voltage to optimise performance.   That said there is no question that Lithium is several times better even over ENLOOP NIMH but I am a stingy Scot

I will experiment with two diodes in series one with 0.7V the other with 0.5V (if I can find one) in series with each of the five cells in the battery pack.

A Schottky diode might satisfy that 0.5V. Keep in mind that the forward voltage depends on the current, so even with just 0.4V over a regular silicon diode, some small amount of current might keep flowing. Not sure if this is significant for battery use, especially if you terminate it reasonably early (don't let it sit for days). I would use a constant current sink in series with something like diodes for voltage cut-off. A resistor might be good enough since the voltage doesn't change very much (between 1.5V and 1.2V).

thanks

[Zero999]

Leave it connected to a Joule Thief?

The LED should turn off when the battery voltage drops below about 0.6V.

[alm]

By the way, I am by no means a battery expert, but you should be able to find other links (like the Battery University one) for more authoritative information.

I do not use use a quick charger so reduce the risk of hydrogen gassing which can occur with 1 hour chargers.   Photographers are frequently in a rush to get the cells into their flash and then the cells have been charged to different levels.   The battery capacity and number of flashes is then seriously compromised. 

From memory, these are the types of charging.

• Ridiculously fast chargers, those 15 minute chargers that cook the cells.
• One-hour chargers that charge at around 1C.
• 'Quick chargers' that take a few hours, and charge at something like 0.3-0.5C.
• Slow chargers: 0.1C for 16 hours or so. The old dumb chargers without any timer/shutoff used to do this.
• Trickle charging at 0.05C. This just to top them off and compensate for self-discharge, not to charge a discharged battery.

I wouldn't recommend those 15 minute chargers if you want your cells to live for more than a few cycles. Slow chargers don't work very well with NiMH, because it's hard to detect the voltage knee at low levels. Some sources actually recommend the one hour charging to prevent crystalline formation (often referred to as memory effect), but 0.3-0.5C also works fine. It is important that the charger correctly detects the end, since NiMH doesn't like being overcharged. The knee is easier to detect at high current, but missing it is more harmful.

As long as you let the cells sit in the charger long enough for all to terminate, they should have about the same charge (assuming identical internal resistance and capacity, which is why it's recommended to use and charge them together in packs of four). What difference would discharging make (apart from occasionally to break up crystals)? If you take them out of the charger before the charging was terminated, they may be at different levels, not much you can do about that except charging faster so they're finished sooner.

Germany is the largest user in Europe and I am friends with the guy who sells the specialist battery chargers to all the national essential services.   He tells me that these chargers use pulsed charging voltage to optimise performance.   That said there is no question that Lithium is several times better even over ENLOOP NIMH but I am a stingy Scot

Pulsed charging is definitely superior, but is rarely used for off-the-shelf AA batteries I believe. I think the pulses of discharge and charge are much shorter than completely discharge and charge, so I doubt that you get the same effect by discharging them to 1.2V before charging them. I don't think there's an easy why to use rechargeable lithium cells in standard electronic flashes. Some people use primary lithium cells, but these are obviously not rechargeable, so they are a significant expense.

[saturation]

I recently wrote this review of a fairly comprehensive NiMH charger.  Its fairy detailed regarding the needs of NiMH.

http://www.epinions.com/review/Maha_Powerex_MH_C9000_WizardOne_Charger_Analyzer_for_4_AA_AAA_epi/content_512329485956

The Maha is, IMHO, one of the best AA/AAA charger out there that does what its supposed to do, keep cells charged, performs IEC test of capacity and allows user based protocols, for those who have figured out an improved charging scheme beyond those supplied by Maha.

Its a PWM charger.

As for discharging them down to improve their health, which means like new mAH capacity, its called 'reconditioning' or 'restoring', but the cutoff is 0.9V for NiMH.  As the cells age NiMH could be unacceptable in a mix of 3 characteristics: discharge voltage, mAH capacity, and self discharge rate.  I was amazed I could restore dead NiMH cells back to life, stagnant up to 10 years old, after reconditioning.  

Compared to NiMH, Rechargeable Lithium are better batteries for power, W=VI , but they also possess the risk of fire and explosion.  Li cost more to make, but the industry has made made no attempt to make it a drop in replacment for AA or AAA, and thus affect its price due to economies of scale from higher consumption.

Li lack packaging standards.  Its made in a variety of dimensions and voltages to fit various forms, so again not produced in scale to rival AA or AAA, pack prices remain higher per mAH compared to NiMH.  One is force to buy a different pack for different devices, and likewise different chargers for different voltage ratings.

Li is considered 'hazmat' when shipped for commercial use, and given the power consumption of many modern digital devices, NiMH is more cost effective as a power source, limited mostly by its form factor, AA or AAA.

[Beemer]

By the way, I am by no means a battery expert, but you should be able to find other links (like the Battery University one) for more authoritative information.

I do not use use a quick charger so reduce the risk of hydrogen gassing which can occur with 1 hour chargers.   Photographers are frequently in a rush to get the cells into their flash and then the cells have been charged to different levels.   The battery capacity and number of flashes is then seriously compromised.  

From memory, these are the types of charging.

• Ridiculously fast chargers, those 15 minute chargers that cook the cells.
• One-hour chargers that charge at around 1C.
• 'Quick chargers' that take a few hours, and charge at something like 0.3-0.5C.
• Slow chargers: 0.1C for 16 hours or so. The old dumb chargers without any timer/shutoff used to do this.
• Trickle charging at 0.05C. This just to top them off and compensate for self-discharge, not to charge a discharged battery.

I wouldn't recommend those 15 minute chargers if you want your cells to live for more than a few cycles. Slow chargers don't work very well with NiMH, because it's hard to detect the voltage knee at low levels. Some sources actually recommend the one hour charging to prevent crystalline formation (often referred to as memory effect), but 0.3-0.5C also works fine. It is important that the charger correctly detects the end, since NiMH doesn't like being overcharged. The knee is easier to detect at high current, but missing it is more harmful.

As long as you let the cells sit in the charger long enough for all to terminate, they should have about the same charge (assuming identical internal resistance and capacity, which is why it's recommended to use and charge them together in packs of four). What difference would discharging make (apart from occasionally to break up crystals)? If you take them out of the charger before the charging was terminated, they may be at different levels, not much you can do about that except charging faster so they're finished sooner.

Germany is the largest user in Europe and I am friends with the guy who sells the specialist battery chargers to all the national essential services.   He tells me that these chargers use pulsed charging voltage to optimise performance.   That said there is no question that Lithium is several times better even over ENLOOP NIMH but I am a stingy Scot

Pulsed charging is definitely superior, but is rarely used for off-the-shelf AA batteries I believe. I think the pulses of discharge and charge are much shorter than completely discharge and charge, so I doubt that you get the same effect by discharging them to 1.2V before charging them. I don't think there's an easy why to use rechargeable lithium cells in standard electronic flashes. Some people use primary lithium cells, but these are obviously not rechargeable, so they are a significant expense.

To answer your question which is basically why bother discharging..... Firstly I am using a 7 hour charger that does have a timer but not a cell temperature monitor.  Many times I do insert batteries late at night that are only 50% discharged.  These are needed for a morning shoot.   When I go at six am to remove them they are much hotter than I prefer.   This is effectively overcharging and not good.   I do have a 1 hour charger but that leaves the cells even warmer if they are around 50% discharged

By the way as a newbie around here....how do I make a quote from just a line or so of someone's post without having to include their whole post?  I tried selecting then replying using "quote" but that does not work

[Beemer]

Leave it connected to a Joule Thief?

The LED should turn off when the battery voltage drops below about 0.6V.

Yes, that would be a nice little kit to build and I could also use it for some low voltage toys I intend to build for my grandchild.

thanks

[Kiriakos-GR]

Personally , I belong to the Ansmann fun club,  after lots of research ..

Guessing about the battery status , as charge level , its an mistake .

The first tool , its an serious battery tester ,
and I do promote this one , as its the best ever made , with load .
https://www.eevblog.com/forum/index.php?topic=448.msg6650#msg6650

Technically  the NiNH are empty , if it drop lower than 1Volt .
The NiMH does not have memory effect ,  old NICD  had that problem.
Deep discharge has nothing to offer.

Now about chargers ,  I trust only the latest models from Ansmann , made at 2004 and after.
As battery station  ( multiple types of batteries to charge )  = Ansmann Energy 8  
As fast NiMH exclusively charger, with selectable speeds = Ansmann Energy 4 Speed .

Very happy with both .  
 
 

[alm]

To answer your question which is basically why bother discharging..... Firstly I am using a 7 hour charger that does have a timer but not a cell temperature monitor.  Many times I do insert batteries late at night that are only 50% discharged.  These are needed for a morning shoot.   When I go at six am to remove them they are much hotter than I prefer.   This is effectively overcharging and not good.   I do have a 1 hour charger but that leaves the cells even warmer if they are around 50% discharged

Then the 7 hour charger is not a proper charger that terminates based on -dV/dt or dT/dt, and the 1 hour charger sounds like it either uses too much current or terminates too late. Saturation's review gives a good overview. I would think that getting a proper charger would be the better solution, timed charging just doesn't work very well with NiMH, even when discharged to the same level, old/lower capacity batteries will always be charged sooner than new/high capacity batteries. I'm sure it's possible to tune the process by tweaking initial voltage and time, but it just doesn't seem worth the hassle to me. Ansmann, Maha and La Crosse all make good chargers. I like the Maha C-9000 that Saturation reviewed, but you do have to press a few buttons for every cell (unless you accept the defaults, which aren't bad for current AA cells), so for bulk charging, a simpler charger like the Ansmann Energy 8/16 or that 8 cell Maha charger might be a better choice.

By the way as a newbie around here....how do I make a quote from just a line or so of someone's post without having to include their whole post?  I tried selecting then replying using "quote" but that does not work

I copy the first quote line to the top of each quoted part, and make sure there's a /quote at the end of each part. Use the preview button to check.

[NiHaoMike]

What about stop the charge early? The Prius batteries last a very long time since they are rarely completely charged or discharged.

[TheWelly888]

By the way as a newbie around here....how do I make a quote from just a line or so of someone's post without having to include their whole post?  I tried selecting then replying using "quote" but that does not work

Beemer - Just highlight the unwanted text between the quote and /quote in square brackets and delete that. The Preview button will help you.

Nested quotes gets awkward!