NiMh charging formula

I have a "dumb" NiMh/NiCd charger.

For 9 volt batteries, it specifies 25 milliamps.

The charger manual gives this charge formula.

Charging time(h) = 1.2 x battery capacity (mAh) -------------------- charging current (mA)

13.44 hrs.

Is is formula useful or is there something better that factors in the starting Voltage ?

Thanks.

Reply to
Andy K
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** The "starting voltage" is almost meaningless - with NiMh and NiCd cells it is no indicator of a cells state of charge.

The formula applies to charging a *flat* battery, but long as the computed time is more than 12 hours some overcharging is not harmful.

... Phil

Reply to
Phil Allison

None that I know of... Two popular methods of charging NiMH batteries are Delta V and Delta T. Delta V - terminates the charge when the cell reaches full charge and the terminal voltage rise rate increases. the technique is prone to noise and may terminate earlier.

Delta T - terminates the charge at a 1degC rise rate. You can also terminate the charge at 35C. Dont exceed the max cell temp.

Overcharging a NiMH may cause the cells to gas Hydrogen, as the hydrogen no longer is absorbed into the cell. This happens at charge rates above 0.1C acording to the wiki.

Adding a thermistor to the charger and terminating the charge at 35C would not be a bad idea, since your above the 0.1C rate for the 9v battery ( assuming its a 200mah )

Cheers

Reply to
Martin Riddle

You have my sympathy.

This should explain how it works:

The problem is determining the EOC (end of charge) point. It can't be done using the battery voltage because that varies with the age of the battery. As you deduced, the starting condition is also important. For example a half depleted battery would only require about half the calculated charging time. Were you to insert a fully charged battery into this "dumb" charger, and then charge it for 13 hrs, you would probably be overcharging it. It might be possible to safely trickle charge the battery, but I've had no luck with that method.

NiMH and NiCd batteries also become hot only when overcharged. Some chargers determine the EOC point when they detect a rise in battery temperature. That doesn't work because by the time the battery gets warm, it's already overcharged and on its way to the dead battery bin.

So, you're left with two choices.

  1. Be very careful about knowing the state of charge of your battery and controlling the charge time. One accidental overcharge and your battery is dead.
  2. Invest in a proper "smart" charger.
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Reply to
Jeff Liebermann

Not true.

Early NiMH batteries were quite intolerant of overcharging, but modern batteries are much more robust. Not quite as robust as NiCd, though.

In fact, some amount of overcharge is necessary in order to keep the cells in a battery balanced.

--
RoRo
Reply to
Robert Roland

Check the manufacturer's data for much more detail.

To determine end of charge there are two methods- one is that the temperature of the cell is measured and the other is to detect a slight *decrease* in cell voltage with time (which I think is actually a side effect of cell heating).

For either of those to work at all you need to have enough current to get signficant heating in the cell, 25mA won't cut it.

There's usually some other conditions about not starting high current charge until the per-cell voltage is within range, and shutting down if the voltage gets too high or too low. Also a top-up charge is sometimes called for after the high-current charge is done.

The dumb way is to assume that the cells are discharged and use a timer, but overcharging kills the cells over time, so you should get it right.

It's also rather easy to make the cells vent hot hydrogen gas and electrolyte mist, but they don't tend to (themselves) spontaneously combust like lithium cells.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

On a sunny day (Sun, 26 Oct 2014 11:08:32 +0100) it happened Robert Roland wrote in :

I have a about 6 or so 'eneloops' that were all damaged by normal NiMh chargers. And one that was damaged by me trying to charge with a lab supply (at 1/4 C). All show increased internal resistance and varying internal resistance, so varying voltage in use, or in other words: noise, reduced power. Regarding the impossibility of charging eneloops in a safe way i have decided not to buy these anymore. The old Panasomic NiMhs I have have been abused and overloaded and what not a million times, and are ten years old and seem undestructible. Those do not retain charge as long as eneloops though. Panasonic bought eneloop recently... so whatinaname. Will try some cheap NiMh next time I need power, its not always needed to retain charge a long time. On the other side of the spectrum the Duracell AA NiMhs I have self discharge in a day or 2. So it there a general rule? I think not.

Anybody any RTGs for sale?

Reply to
Jan Panteltje

I beg to differ.

Let's see if I understand what you're saying. In a series string of NiMH cells, one must overcharge the good cells in order to bring the not-so-good cells up to the good cell voltage? Is that what you're saying? If so, this sounds like a variation on "battery conditioning" which works well at killing batteries.

LiIon battery chargers have the same problem with series connected cells. The difference is that while NiMH cells might survive some overcharge, LiIon cells short and die rather easily. To solve the problem of maintaining identical and balanced cell voltages, the RC (radio control) industry uses a "balance charger": These have two connectors to the battery. One pair of terminals for the high current charge/discharge current, and another connector that goes directly across each cell to monitor and control the charge current for each cell individually. Something like this might also work with NiMH, but I haven't found any balance chargers that will do it. I suspect that's because LiIon and mutations are the future, while the use of NiMH is decreasing rapidly.

One of my experiments was to see how quickly I could fast charge NiCd and NiMH cells. Since these chemistries only get hot when overcharged, I suspected that I could safely charge them at ridiculous rates, as long as I didn't overcharge. Trying to detect the 5mv drop in terminal voltage at EOC was futile as warm contacts would ruin that measurement. Most cells were 2000 ma-hr AA NiMH cells that managed to deliver about 1200 ma-hr at 1C discharge loads[1]. I would then quick charge them at up to about 5C (10 Amps) for about

12-15 minutes. Knowing the initial SoC (state of charge) and cell capacity was critical.

Of course, I intentionally overcharged some cells, just to see what happened. I determined that things change at about 75% of capacity. Below 75%, the charging efficiency is 100%. If I charged at any rate to 75% of full capacity, I could discharge the cell (at C/10) and get all of it back. Above 75%, everything would change. If I counted coulombs and charged to 100%, the charging efficiency dropped from

100% to about 85% with the loss going up in heat. The heat was a waste of charging energy. It also caused the cell to deteriorate in both capacity and self discharge rate.

Notice that the cell does not magically stay cold at 99% of charge, and explode at 101% of charge. It's an exponential curve, that starts at about 75% and climbs rapidly. The 100% point seems to have been selected to produce a reasonable number of charge cycles, safe charging temperature, and approximate correlation with the -5mv NDV (negative delta voltage) EoC point. If you want your NiMH batteries to live forever, just set the EoC to about 75% and don't go anywhere near 100% charge. I attempted to reproduce the NiMH graph at: which shows loss of cell capacity after 300 charge cycles. At 100% charge (1C charge, 1C discharge, 4 hr cycle for about 100 days), using a commercial smart charger, my results were fairly similar to the graph. However, at 75% of charge, there was little decrease in capacity up to 700 charge cycles, when I gave up. Unfortunately, I discovered too late that my battery holder had a problem and that the contact resistance had ruined my internal resistance measurements.

Anyways, I think you can see what might happen as you approach 100% charge with an NiMH battery. I don't think going over 100% is a very good idea.

"Charging Nickel-metal-hydride"

[1] NiMH cells are rated for capacity at C/5 discharge rate to 1.0v. For a 2000 ma-hr cell, it takes about 10 hrs to run the discharge test. Typical data sheet:
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Reply to
Jeff Liebermann

Lol, have you considered using the charger built to charge them?

Duh, that is the point of the Enloops. I believe Enloops are some 50 to

100 times slower to self discharge.

Uh, are they dropping the Enloop name, I doubt it. Does Panasonic have a line of slow self discharge NiMH? I expect not.

--

Rick
Reply to
rickman

Series cells (and capacitors) naturally unbalance due to slight variations in self-discharge. There's eventually a risk of some cells being reverse-charged while others still have enough voltage to make the pack seem usable. That's immediately the end of the battery pack's life, maybe in a violent way.

Balancing overcharge is very mild; just enough to compensate for varying self-discharge. NiMh, NiCd, and lead-acid have no problem at all with this. It ages lithium-ion batteries more rapidly so some packs have a shunt voltage regulator on each cell.

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Reply to
Kevin McMurtrie

Thanks for all the responses.

Andy

Reply to
Andy K

The official Sanyo NC-MQN06 charger is a rather slow charger. Looking at the docs, it estimates 7 hrs charge for 2 AA cells. Working backwards from the stated capacity of 2000 ma-hr, that would be: 2000 ma-hr / 7.0 hr = 286 ma or about 0.15C, which is rather slow. The quality of this charger is debatable:

I haven't tried using the current models: Note the differences in charge control: BQ-CC16 Smart Charge 1/-delta-V /Timer BQ-CC17 delta-T /Timer BQ-CC18 Timer cut Notice the timer to prevent killing the battery by leaving it in the charger forever. Yes, you can kill a NiMH by continuous trickle charging. If you must buy one of these, I suggest the BQ-CC16 as the other are probably battery killers.

The recommended current for the older BK3-LCC AA cells is 1000 ma for

1.1 hrs. Recommended charge current for the newer AA cells is 2000 ma for 1.1 hrs. They are currently on the 4th generation cells. See the various data sheets: What this means is that all Eneloop cells are not the same and that the various model chargers are not interchangeable.

I've had very good luck using Eneloop AA batteries in my various cameras (Canon S5-IS, Canon A40, etc). I've been using my cell phone and tablets as cameras more often and have put the digital cameras aside. I need to use them recently. 6 months of sitting on the shelf and they were about 80% of full charge. If I had done that with a conventional NiMH battery, they would all have been totally discharged. Highly recommended, but be careful what you use for a charger.

It's much more. The Eneloop cells drop to about 85% in about 1 month,

80% at 2 months, and stay at that level forever. See the 2nd graph. The Panasonic press releases claim 70% of charge after 5 years of storage:

It's still being sold as Eneloop. Dunno about other LSD (low self discharge) calls from Panasonic.

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Reply to
Jeff Liebermann

...

Hi Robert, Jeff and others

The worst thing for any rechargable (except NiFe) are reverse-charging of e.g. one cell. That happens because cells have different: (1) self-discharge characteristics (2) capacity (3) maybe different internal impedance as a function of charge-level.

Reverse-charging even one cell will shorten its live hugely. And when one cells is dead the whole battery fails alot faster than maybe 200 full cycles? - and not the advertised 1000-2000 full (100%) cycles.

The best you can do for your serially connected cells are to survey your cells when discharging.

But this will typically cost a microcontroller and some wires to each cell end, and a power switch that is controlled by microcontroller in question.

The system is called a Battery management system (BMS):

formatting link

If a cell is overcharged, but is will shorten the cell life some, especially if the cell vent - which means that it looses water (=higher internal impedance).

-

Li-ion (excluding LiFePO4!) cell must not be overcharged or else it also get a lot shortened life - or even destabilize.

If a Li-ion (excluding LiFePO4!) cell is over-discharging (0V and not even reverse-charged), it will destabilize. And when later charged it will be prone to burning and explosion.

It a Li-ion (excluding LiFePO4!) get to high a charging current or discharging current, it will destabilize.

That is why (nearly) all Li-ion (e.g. computer) Li-ion batteries have a BMS.

-

LiFePO4 can handle overcharging, but this will shorten the cell life some, especially if the cell vent - like any other chemistry - which means that it looses water (=higher internal impedance).

-

It is possible to buy battery BMS-controllers e.g.:

  • TI: bq20z60-R1, bq29330, bq294xy...
  • Atmel: ATA6870N *n +(ATmega32HVE2 or ATmega64HVE2)

Atmel ATA6870N Li-Ion, NiMH Battery Measuring, Charge Balancing and Power-supply Circuit datasheet:

formatting link

Look at the sources here:

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-

There is even an interface standard (Smart Battery System, SBS):

formatting link

A battery with a BMS is called a smart battery:

formatting link

A charger that can "talk" with a battery with a BMS, is called a smart battery charger:

formatting link

Glenn

Reply to
Glenn

On 26/10/14 17.31, Jeff Liebermann wrote: > On Sun, 26 Oct 2014 11:08:32 +0100, Robert Roland > wrote: > >> On Sat, 25 Oct 2014 20:45:38 -0700, Jeff Liebermann >> wrote: >> >>> One accidental overcharge and your >>> battery is dead. >> >> Not true. > > I beg to differ. > >> Early NiMH batteries were quite intolerant of overcharging, but modern >> batteries are much more robust. Not quite as robust as NiCd, though. >> >> In fact, some amount of overcharge is necessary in order to keep the >> cells in a battery balanced. > > Let's see if I understand what you're saying. In a series string of > NiMH cells, one must overcharge the good cells in order to bring the > not-so-good cells up to the good cell voltage? Is that what you're > saying? If so, this sounds like a variation on "battery conditioning" > which works well at killing batteries. ...

Hi Robert, Jeff and others

The worst thing for any rechargable (except NiFe) is reverse-charging of e.g. one cell. That happens because cells have different: (1) self-discharge characteristics (2) capacity (3) maybe different internal impedance as a function of charge-level.

Reverse-charging even one cell will shorten its life hugely. And when one cells is dead, the whole battery fails a lot faster than maybe 200 full cycles? - and not the advertised 1000-2000 full (100%) cycles.

The best you can do for your serially connected cells are to monitoring your cells when discharging.

But this will typically cost a microcontroller and some wires to each cell end, and a power switch that is controlled by microcontroller in question.

The system is called a Battery management system (BMS):

formatting link

If a cell is overcharged, it will shorten the cell life some, especially if the cell vent - which means that it looses water (=higher internal impedance).

-

Li-ion (excluding LiFePO4!) cell must not be overcharged or else it also get a lot shortened life - or will even destabilize.

If a Li-ion (excluding LiFePO4!) cell is over-discharging (0V and not even reverse-charged), it will destabilize. And when later charged it will be prone to burning and explosion.

It a Li-ion (excluding LiFePO4!) get to high a charging current or discharging current, it will destabilize.

That is why (nearly) all Li-ion (e.g. computer) Li-ion batteries have a BMS.

-

LiFePO4 can handle overcharging, but this will shorten the cell life some, especially if the cell vent - like any other chemistry - which means that it looses water (=higher internal impedance).

-

It is possible to buy battery BMS-controllers e.g.:

  • TI: bq20z60-R1, bq29330, bq294xy...
  • Atmel: ATA6870N *n +(ATmega32HVE2 or ATmega64HVE2)

Atmel ATA6870N Li-Ion, NiMH Battery Measuring, Charge Balancing and Power-supply Circuit datasheet:

formatting link

Look at the sources here:

formatting link

-

There is even an interface standard (Smart Battery System, SBS):

formatting link

A battery with a BMS is called a smart battery:

formatting link

A charger that can "talk" with a battery with a BMS, is called a smart battery charger:

formatting link

Glenn

Reply to
Glenn

The formula is useful, as far as it goes. Factoring in the starting voltage can be done simply by adding a shut off or taper circuit. (Of course best would be a good smart charger.)

The add on circuit is adjusted to detect the fully charged voltage (aka "terminal voltage"), then taper the charge to something like C/50, or shut the charge off completely. The initial state of charge affects how long the "dumb charge" circuit will take to reach terminal voltage. Once terminal voltage is reached, the add on circuit prevents excessive charging.

A TL431, a trimpot and 2 resistors is all you need for an add on circuit to taper the charge - the shut off circuit would need a few more components.

Ed

Reply to
ehsjr

Just curious - Do you ever have anything to write about that takes only one (not run-on) paragraph?

Reply to
John S

On 26/10/14 21.24, Glenn wrote: ...

Hi!

Forgot some smart battery charger chip examples:

AN667, microchip.com: Smart Battery Charger with SMBus Interface

formatting link

-

Do not want to program your own Smart Battery Charger? - e.g. SBS compatible chips can be bought ready-made:

linear.com: LTC4100 - Smart Battery Charger Controller:

formatting link

linear.com: LTC4101 - Smart Battery Charger Controller:

formatting link

-

Please note: The "smarter" the more likelihood of incompabilities between the smart charger and smart battery:

batteryuniversity.com: : Inner Workings of a Smart Battery:

formatting link
Quote: "... A speaker at a battery conference said, ?The battery is a wild animal and artificial intelligence domesticates it.? ... Ironically, the more features that are added to the SMBus battery and charger, the higher the likelihood of incompatibilities. ..."

Glenn

Reply to
Glenn

On 28/10/14 11.24, Glenn wrote: ...

Now at:

formatting link

Reply to
Glenn

Hey, Jeff -

I apologize for that comment. It was rude and uncalled-for. The only excuse I have is that I had just had a bad experience and let my anger take control.

Sorry.

Reply to
John S

Nope. I was a bit busy and didn't have time to make it brief. (Apologies to Mark Twain).

I like to read detailed answers and try to provide the same. I learn more from these than from unsupported one line answers and short quips, which are usually a waste of time. My rule-of-thumb is that anything I write should be something that people would want to read. I don't think anyone wants to read one line retorts, which are the fashion on Twitter.

No problem. I've done worse for much the same reason. Had you found one of my numerous mistakes and chronic math errors, I would have responded with either apocalyptic fury or thanked you before disappearing and going into a deep depression.

A few years ago, I was dealing with a kidney stone and was using writing on Usenet to distract myself from the pain. When not drugged, I wrote with a general lack of coherence and in a tactless manner. When drugged, much of what I wrote made no sense an hour later. Oddly, my math and spelling improved when drugged. My solution was to always delay posting a reply, wait an hour, read it again, and then either clean up the mess or just delete it.

I hope things work out for you.

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Reply to
Jeff Liebermann

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