Mostly Eneloop in Maha Discharge, Capacity & Health Questions

[Bolster]

If you're inclined to help a new Maha C-9000 owner:

(1) At the end of a discharge, is the mAh reading (a) what was in the cell -- ie, how many mAh were actually drawn out of the cell and are now gone, or (b) what the cell's potential capacity is? I'm confused...seems "capacity" gets used to mean both "what's in the cell" and "what the cell is capable of holding," two different things.

(2) Depending on the above, what number actually tells me the mAh "potential" of a cell -- ie, the health of a cell? It's the mAh displayed after a discharge, right? I've read "The capacity of a cell after charging can only be measured by discharging. The discharged capacity is what counts." But does that imply a full discharge from a full cell? Or would the end-of-discharge measurement also be accurate for a half empty cell?

I see people reporting the 'capacity' of their cells after a full charge, and in fact the Maha charger reports this number as if it's meaningful. Wouldn't that be incorrect, if you can only determine capacity at the end of a discharge? If true, what is the meaning of the mAh number at the end of a charge?

(3) I probably have more Eneloops than I can keep working on a steady basis. And I don't use them for high drain applications. My use is more for: intermittent use flashlights and headlamps, TV and DVD remote controls, flash for camera, etc. Lots of these are sit-and-wait applications. Would it be reasonable to put all my Eneloops on a schedule of a Break-In-Mode once a year? More? Less? Would BIM be overmuch, should I be using Refresh & Analyze instead?

I wish the Maha instruction booklet gave guidance to keeping up the health of low-use cells. They say "R&A once every 10 cycles" and "BIM every 30 cycles," but what if a 'cycle' of usage is an entire year? How often to R&A or BIM under those conditions?

Thanks

 

[SilverFox]

Hello Bolster,

It goes like this...

You charge a cell up. The capacity of the cell is determined upon discharge. When you use a cell it runs for a length of time determined upon the cells capacity and the load. The capacity of the cell is determined upon discharging the cell.

The amount of charge that goes into a cell allows you to determine how well the charge termination worked. If you have a 2000 mAh cell and see that you put 3000 mAh into it during charging, your cell has been overcharged because the termination signal was missed. On the other hand if you have a discharged 2000 mAh cell and see that you only put 500 mAh into it during the charge, the charger terminated prematurely. You will have to charge again to end up with a full cell.

The "health" of a cell is initially determined by its discharge capacity and its ability to hold voltage under load. I view a cell as CRAP when it falls below 80% of its initial discharge capacity.

If you do a Break In once a year, your Eneloop cells will remain vibrant for a long time.

Storing cells can be difficult. The low self discharge cells store best at about 60% of full charge. Normal cells store best when they are discharged. However, to keep normal cells vibrant you need to do a charge/discharge cycle every 30 days. In undemanding use you can stretch that out to doing a charge/discharge cycle every 6 months.

While there is no such thing as "normal" use, I view normal use as going through a cells capacity in about 1 - 7 days. At the two extremes of this you would run a Break In once a month, or once a year if you are following the Maha recommendation.

It all comes down to performance. If you are happy with the way your cells perform, keep doing what you are doing. However, if the performance drops off, then a Break In may help. Also, if you run a Refresh and Analyze every so often it doesn't hurt, and may help.

Tom

 

[Bolster]

Thanks Tom! I've noticed you're kept pretty busy answering Eneloop/Maha questions on this forum.

The advice on how to handle lite-use cells (neither "normal-7-days" nor "storage") is particularly helpful, thanks. This lite-use is seldom discussed, is it. But I think a lot of cells probably experience lite use.

 

[Smells_Familiar]

Storing cells can be difficult. The low self discharge cells store best at about 60% of full charge. Normal cells store best when they are discharged. However, to keep normal cells vibrant you need to do a charge/discharge cycle every 30 days. In undemanding use you can stretch that out to doing a charge/discharge cycle every 6 months.

Hey Tom,

I've got in my notes that it's best to store LSD batteries at ~80% and in a cool place. To be honest, I forget what the rational was behind this (the 80%). Not that it's a big deal but could you tell me why it's theoretically better to store them at 60% vs. 80% or vice versa?

thanks!

 

[SilverFox]

Hello Smells familiar,

I believe low self discharge cells are delivered after being charged to about 80%. This is an excellent compromise between having a cell that is "ready to use" and minimizing any oxidative damage that may occur with extended storage at full charge. After packaging the cells may not be put into use for an extended period of time, so the slightly higher state of charge will render them still somewhat "ready to use" even after extended storage on the shelf.

I think the electro-chemistry favors storing at a lower state of charge.

Tom

 

[Bolster]

So...for stored Eneloops...after a 6-mo or yearly maintenance R&A or Break In, it would be wise to discharge back down to maybe 1100 or 1200 mAh?

Would this also be the case for lite-use or slow-use cells? (Remote controls, etc.) Or for lite/slow use, do we just charge up to full capacity and use?

 

[SilverFox]

Hello Bolster,

The best solution for stored Eneloop cells is to buy more flashlights and use them...

They are actually pretty tough cells. I would say just charge them up and store them and use them when needed. I am not sure the effort needed for optimum storage is extremely beneficial.

However, if you want to run a long term test for something like 5 years, you can charge them to 1200 mAh and store them in a zip lok bag in the refrigerator. Also you will need some cells charged to 100% and stored at room temperature. 5 years from now we can do a comparison between the stored cells and the other cells and then we will have a data point.

Tom

 

[Bolster]

Sorry, still confused about term "capacity." Does it refer to the amount that is/was in a cell? Or does it refer to the amount that the cell can/could contain? Actual, or potential? I've seen it used both ways.

When a cell is discharged on the Maha, does the final mAh number tell you the amount of energy that was in the cell? Or the amount the cell COULD hold in its present condition?

If it's energy that was in the cell (which I suspect), then the only decent measurement would be draining a completely full cell. Discharging a partially full cell would tell you virtually nothing, and would also be useless as a baseline measurement. Yes? No?

I'm asking, because I'm writing down final mAh numbers from initial discharge, and using as a reference point for improvement. But if I'm emptying partially full cells, and it measures amount of energy drawn out of the cell, then what good is it as a reference point?

 

[Wrend]

Sorry, still confused about term "capacity." Does it refer to the amount that is/was in a cell? Or does it refer to the amount that the cell can/could contain? Actual, or potential? I've seen it used both ways.

Yes, both ways, depending what you mean. Basically, for our use, capacity is an amount or measurement of mAh relative to a cell or a series of cells. This could be charged capacity, discharged capacity, capacity potential measured by a standardized capacity test, cumulative lifetime capacity potential, proportional capacity self discharge rate, and so on.

In general, when referring to a cell's capacity, this would mean its stored capacity potential.

Sometimes capacity (as in "C" rate) is used to mean the charge or discharge rate relative to the cell's capacity potential. So a "1C" discharge rate would (theoretically) fully discharge a cell that was fully charged after 1 hour. I however do not use the term capacity in this way and instead would use "1C/h" to show that it is a current rate.

When a cell is discharged on the Maha, does the final mAh number tell you the amount of energy that was in the cell? Or the amount the cell COULD hold in its present condition?

It tells you the amount of capacity it measured discharging the cell.

Also, note that "energy" is generally used to mean watt-hours, while "capacity" is amp-hours.

If you have three identical cells, one cell used by itself has the same effective capacity as two cells used in series, but the two cells in series have twice the energy (double the voltage). Two cells used in parallel have double the effective capacity and energy of one cell used by itself, but they have the same voltage.

If it's energy that was in the cell (which I suspect), then the only decent measurement would be draining a completely full cell. Discharging a partially full cell would tell you virtually nothing, and would also be useless as a baseline measurement. Yes? No?

I'm asking, because I'm writing down final mAh numbers from initial discharge, and using as a reference point for improvement. But if I'm emptying partially full cells, and it measures amount of energy drawn out of the cell, then what good is it as a reference point?

The break-in function does a standardized capacity test. I discharge my cells first so they're not needlessly overcharged during the fist portion of the test. Using either 1900mAh or 2000mAh as the capacity for the AA Eneloops is fine for the break-in test; either will fully charge them. I personally use 1900mAh for the AAs and 800mAh for the AAAs.

 

[SilverFox]

Hello Bolster,

Let's look at this like the gas tank in your car.

The capacity is the amount of gas that is in the tank.

With cells, the capacity is the amount of energy in the cell.

However, my analogy breaks down when you try to determine the capacity.

With the car you run it until it runs out of gas, then the capacity is determined by filling the tank.

With a cell you fully charge it and then the capacity is determined by discharging it.

Discharging a partially filled cell can provide useful information about an application. For example, let's say you charge your cell up and put it into a light. You use the light for a few days, then pull the cell and discharge it to see how much capacity is left. This can give you an idea of about how long your cell will last in that light. If you are going camping and plan to use that light, this information can help you decide how many spare cells you need to bring.

The best reference point to start from is the value given at the end of the Break In cycle.

Tom

 

[bbb74]

In my battery tracking spreadsheet, all the thresholds are customisable but I usually "default" to thresholds that highlight cells that need attention for:

- Fully discharging a cell every 15 cycles ... and at least once every 3 months
- Running a breakin every 30 cycles ... and at least once every year
(amongst other things)

So it covers low use and high use cells by using time and number of cycles to determine any maintenance required.

For eneloops I usually bump it up to 20 cycles/4 months for a discharge and 40 cycles/1 year for breakins.

Also, any batteries not in use I have in the fridge in the "shelfqueen" state, which means it automatically adds another month to the above (by default, its customisable). In the fridge, reactions are slowed down so any oxidation or self discharge happens at a much lower rate so less maintenance should be required. I do store mine at full charge. I haven't found much evidence to say that any other charge rate is better or worse. Some manufacturers and sources recommend storing at full charge. Not saying its right but I haven't found anything concrete to say otherwise. Anyway they're in the fridge so whatever is happening will be happening pretty slowly.

I don't use the freezer because the temp in there is pretty close to the lowest permissible on the spec sheets, and water is a component of NiMH batteries, and I'm concerned that if some of the ingredients separate a little bit and there are areas with a high enough water concentration they could freeze and cause damage (eg. to the separator).

 

[Bolster]

Thanks for the education, folks. Taking me a while to digest, will need to read multiple times.

When's the test?

 

[Wrend]

I'll go a little more fundamental and maybe easier to understand. Capacity is basically an amount of electrons. (1 amp is a rate of 6.241×1018​ electrons moving across the same point in a circuit per second. 1 amp-hour is the amount of electrons moved after 1 hour at a 1 amp rate, 2.24676×1022​ electrons.)* A battery cell can only effectively hold and store so many usable electrons worth of charge, which is its fully charged capacity potential.

In a battery pack, the capacity of one cell pushes and is pulled into the next cell in series when the battery is charged or discharged. The capacity of the last cell in the series is then pushed/pulled through the circuit back to the first cell. This is why you don't add the capacity of cells in series to figure out the effective capacity of a battery pack. Provided all the cells are the same, the effective capacity of the pack is the same as any one of its cells. However, since there are more cells in a pack pushing/pulling, the pack's "energy" potential does increase relative to the number of cells. The pushing/pulling of the electrons between the cells is the pack's voltage and is relative to the cell chemistry type and how fully charged they are.

*Don't worry, this won't be on the test.

 

[awyeah]

Out of curiosity, what would happen if you overcharge a NiMH cell? If the charger misses deltaV or for whatever reason doesn't terminate on time, and it does put 3,000mah into a 2,000mah cell, what would happen?

 

[desirider]

I would like to add to awyeah's question. Again, just out of curiosity, what would happen if we let regular, non-LSD NiMH cells to sit around for a long time like 2 years? Will the self discharge go into a deep discharge to the point of no recovery?

This situation happens often to me. I go through some old stuff and discover a gadget in storage that has rechargeable batteries and have been sitting around for too long (a baby monitor, for example).

 

[Wrend]

Out of curiosity, what would happen if you overcharge a NiMH cell? If the charger misses deltaV or for whatever reason doesn't terminate on time, and it does put 3,000mah into a 2,000mah cell, what would happen?

It depends on the rate you're charging them at. Overcharging at rates lower than 0.1C/h won't do significant damage, but should be avoided when possible. Higher rates will do more significant damage, and at very high rates it is possible the cell could "vent."

I would like to add to awyeah's question. Again, just out of curiosity, what would happen if we let regular, non-LSD NiMH cells to sit around for a long time like 2 years? Will the self discharge go into a deep discharge to the point of no recovery?

This situation happens often to me. I go through some old stuff and discover a gadget in storage that has rechargeable batteries and have been sitting around for too long (a baby monitor, for example).

Yes, it is likely that you will not be able to recover enough of the cell's usable capacity potential for it to work well. Still, you could try by doing a slow forming charge at 0.1C/h for 16 hours or so, followed by a slow discharge, then another slow charge.

NiCds on the other hand might still be fine.

 

[awyeah]

It depends on the rate you're charging them at. Overcharging at rates lower than 0.1C/h won't do significant damage, but should be avoided when possible. Higher rates will do more significant damage, and at very high rates it is possible the cell could "vent."

I see. What would you consider a "very high" rate? For example, I charge my Eneloops betwen .5C and very occasionally 1C (1000mah/2000mah) depending on how fast I need them. I would imagine that's probably not considered "very high"? Also, I would imagine that my C9000 would stop the charging process if they started to get too warm...


EDIT: I had the wrong numbers in there, corrected them.

 

[Wrend]

I've never had a missed termination with my Eneloops on the C9000, so I couldn't tell you for sure what would happen. The cells would likely get pretty hot, and could trigger other termination safety features of the charger.

Just as a rough estimate, I'd say "very high" to be at 2 amps (1C/h) or more. This largely depends on the type and condition of the cells and how long they're overcharged for. The cells getting too hot is the main concern. If they're too hot to hold, then likely some level of more significant damage has been done.

 

[awyeah]

Ah. I guess I'll stick to .5C then just to be safe

The FAQ for my charger says not to go below .33C except in break-in mode.

Also, see my edits in the previous post, I had my numbers wrong.

 

[Wrend]

The C9000 terminates charges pretty conservatively, so you likely don't have much to worry about either way.

I personally do prefer to charge the Eneloop AAs at between 700mA to 1000mA and the AAAs at either 300mA or 400mA.