Appropriate Charge Rate for NiMH Batteries

[Scooby Doo]

I own the Maha MH-C9000 WizardOne battery charger and I would like to know the appropriate charge rates for the following batteries:

1. Duracell Rechargeable AA 2650 mAh
2. Energizer Rechargeable AA 2200 mAh
3. Rayovac IC3 AAA 800 mAh
4. Rayovac Rechargeable 1.2V AA
5. Rayovac Rechargeable IC3 1800 mAh


Thanks.

[Marduke]

0.5-1.0C is the proper charge current.

However I recommend not charging the IC3 cells, as they require a special charger and will quite possibly self destruct on any other charger.

[Scooby Doo]

For a charge rate of 1.0 C, how long would it take to charge the above batteries?

I read on other sites charging batteries for 1 to 2 hours is the best way for prolonging the batteries lives. Is this correct?

I also own the Energizer 15 minute charger. I used this charger, before owning the Maha charger, to charge some of the batteries listed above but the charger had damaged those batteries, would I be able to bring them back to life using the Maha charger? I assume the excessive heat had damaged the batteries.

Is Sanyo Eneloop batteries considered to be better than the above listed batteries?


Thanks.

[tino_ale]

All this is throughly explained in the charger manual IIRC

[SilverFox]

Hello Scooby Doo,

Welcome to CPF.

Charging at 1C takes just over 1 hour. Charging at 0.5C takes just over 2 hours. Charging at 2C is not recommended, but if you did it would take just over 0.5 hours.

When using a charger that uses -dV to terminate the charge, the strongest end of charge signal is generated when you charge in the range of 0.5 - 1.0C.

15 minute charging is quick and it has its place, but you pay for this convenience in reduced cycle life. Once a cell is damaged physically, there is nothing you can do to bring it back. If the performance of the cell drops due to uneven electrolyte distribution, you can revive the cell through using the Break-In function on the C9000.

15 minute charge rates tend to damage the internal components of the cell as opposed to storage which can also involve damage to the internal components, but at first it usually be reversed. At any rate, the best way to try to improve your cells performance is to periodically run a Break-In function on them.

The IC3 cells terminate the charge by disconnecting the internal circuit when the cell internal pressure reaches a set value. A non-IC3 charger will think that you removed the cell at that point, and when the switch resets it will think that you inserted a new cell and begin charging all over again. If you are planning on charging IC3 cells on the C9000, you must remove them as soon as they first shut off.

Tom

[travelinman]

I charge all my Nimh cells at 200 mah. But then, I'm retired and my time is worth nothing and I have all day/week/month to wait.

[Marduke]

I charge all my Nimh cells at 200 mah. But then, I'm retired and my time is worth nothing and I have all day/week/month to wait.

That is actually really bad for the cells. Chances of missed termination, and hence cooking the cell is greatly increased.

[TorchBoy]

I charge all my Nimh cells at 200 mah. ...

Quite apart from the practice being bad if it's not terminated, NiMH cells have a capacity in mAh, and are charged in mA for a certain number of h.



Yes Scooby Doo, Eneloops are great.

[travelinman]

That is actually really bad for the cells. Chances of missed termination, and hence cooking the cell is greatly increased.

I guess that all depends on which charger you are using. If your charger terminates at the appropriate time reliably then there's no problem. If you find that it terminates properly after "x" number of samples, then its OK to trust it.

You can even (and I have) checked the current flowing into the cell and compare that with the time it has been on the charger and calculate the amount milli amp hours sent to the cell on the cheaper chargers with no readout for that. I constantly check the readout on my 900 and it has never exceeded the cells safe capacity, therefore has always terminated properly.

I know some have found chargers that have done that at 200 ma, but I guess mine was not made on a monday or friday.

[brted]

It seems like you would want to charge them as slowly as you can, provided you don't risk missing the termination. Since there have been problems missing termination lower than 500 ma, then I charge my AA's at 600 ma. Charging at 0.5C to 1.0C seems to be fine, but the faster you charge a battery the more heat you get and heat is bad, right?

If a battery has less than 1200 mah (like AAA's) then I use 0.5C.

[Marduke]

If the cell is healthy, it will not get hot (just warm) when charged at the proper currents (0.5-1.0 C).

While you may think you are being kinder by charging them slower, you are actually putting your cells in an early grave.

[jayflash]

Yes, SilverFox and other sources have indicated a .5C - 1C is better for the NiMH chemistry and will result in better performance than a slow charge.

[45/70]

Most sources agree that NiMH's are best charged at a 0.5-1C rate.

I think the misconception that a slow rate is good for NiMH's is a hangover from the days of NiCds. The simplest, and cheapest chargers were ones that approximated a forming charge or 0.1C hence, the 14-16hr chargers they used. NiCd's are somewhat tolerant of slow charging, but for example, Battery University states on their "Do and don't" page, that a fast charge is preferred over a slow charge for NiCds as well.

Dave

[Mr Happy]

Battery University states ... that a fast charge is preferred over a slow charge for NiCds as well

In fact I believe NiCd's can take even faster charging than NiMH, 2C or beyond.

[alfreddajero]

[Turbo V6 Camaro]

glad i searched

I have been going off the c-9000 book at .3c to .1c i ahve been doing all my cells at 700mah charge and 350-500 discharge

should i step it up some?

[Mr Happy]

glad i searched

I have been going off the c-9000 book at .3c to .1c i ahve been doing all my cells at 700mah charge and 350-500 discharge

should i step it up some?

Not 0.3C to 0.1C, 0.3C to 1.0C. In general, the highest rate your charger can do up to 1.0C is best. If your charger can't go that high, just use the highest it can manage.

[J_C]

That is actually really bad for the cells. Chances of missed termination, and hence cooking the cell is greatly increased.

Bad perhaps but I wouldn't say really bad. That rate is at or below 0.1C for most modern NiMH AA, cells that size can sit at 200mA current for many hours past being fully charged and you'll only lose maybe a dozen % of the lifespan in the long term, which can be an acceptable trade-off. Not ideal, but in the end we're talking about batteries costing a couple bucks a piece. Much commotion is made about idealisms in charging NiMH, but they really aren't all that picky to recharge providing they don't get excessively hot. Many batteries will be hotter at the end of a 1C rapid charge than they would get sitting at 200mA charge current for several hours past being fully charged.

There is insufficient evidence to conclude a rapid charge rate will result in better performance, more of an urban myth since we know a slow charge is completely capable of fully charging a battery, it just has to terminate not too soon and not too late which is the trickier part. What it will do instead is waste less power but if you're on the grid the difference won't matter.

On the other hand, I've 3 different cheap chargers that never have a problem sensing Delta-V cutoff when charging at about ~0.3C (700mA) with all the batteries I've put in them. Maybe charging circuits are getting better than they used to be, and since a basic level of features comes with even a $10 charger these days, not much reason to do without cutoff circuits in them unless it's a series pack being charged and so the charger either doesn't exist or is much more costly.

[SilverFox]

Hello J C,

I beg to differ...

There is enough evidence to support charging in the 0.5 - 1.0C range that the battery manufacturers actually recommend it.

Tom

[J_C]

See Sanyo's FAQ page, they clearly recommend against it.

http://us.sanyo.com/Batteries/FAQs

Can I use a "Quick Charger" to charge an eneloop battery?


Though it is possible to charge an eneloop battery in a "Quick Charger", it is not recommended. We recommend charging eneloop batteries in a NiMh charger that is 2 hours or more. Charging eneloop batteries in a "Quick Charger" can reduce the overall life of the battery.


How long does it take to charge an eneloop battery in the eneloop 4-position charger?


From 4 to 7 hours

(more on linked page)
Same is true of most battery manufacturers, they make several chargers with under 0.5C rate.

[Marduke]

See Sanyo's FAQ page, they clearly recommend against it.

http://us.sanyo.com/Batteries/FAQs

(more on linked page)
Same is true of most battery manufacturers, they make several chargers with under 0.5C rate.

I suggest you read the technical specs and not the dumbed down FAQ written for illiterate consumers.

Sanyo recommends 1C charge...
http://us.sanyo.com/dynamic/product/...k-49579479.pdf
http://us.sanyo.com/dynamic/product/...k-35338904.pdf

Energizer recommends 1C charge (page 19)....
http://data.energizer.com/PDFs/nicke...ide_appman.pdf

Duracell recommends 1C charge...
http://www.duracell.com/oem/recharge...el/methods.asp


Believe it or not, there is little correlation to what a company's chargers do (designed purely for cost/profit reasons), and what the best practices say to do (chosen purely for technical reasons). Until the day comes that the bean counters making the decisions are actually knowledgable about the products they sell (fat chance), there will always be a discrepency. However, we here has CPF as knowledgable consumers (usually) have the advantaged of knowing the proper methods, and knowing what equipment will give the best performance for the cells you choose.

Edit:
Panisonic also recommends 0.5-1C charge...
http://www.panasonic.com/industrial/...rgeMethods.pdf

Rayovac also recommends 1C charge...
http://www.rayovac.com/technical/pdfs/NM715_2100MAH.pdf
http://www.rayovac.com/technical/pdfs/NM724.pdf

[J_C]

There's nothing profitable about their batteries not meeting their published specs. They do not stipulate you must use any one of the methods they recommend to achieve the specifications for their cells.

There are certainly things that shouldn't be done, but anything except 0.5C to 1.0C isn't one of them.

Again I will remind that there is a slight reduction in lifespan from some charge rates and overcharging, but they are $2 batteries not your long lost child or bars of gold. The more important factor is convenience which can lead to rapid charging in some scenarios or not in others. The majority of the planet does not rapid charge and proves the successful use of lower charge rates despite people who obsess about tiny differences and most of them with unscientific tests posed as "fact".

Take for example the crazy supposed facts about battery temperature rise opposed to voltage rise for Delta -V termination. The supposed facts ignore the constants.

If you take one battery 5% away from it's Delta -V threshold and put it into a charger, and compare it to another battery which had been fully drained and recharged up to 5% away from Delta -V, the latter battery is hotter than the former. Delta -V is not temperature dependent, it just happens that a change, a variable one, facilitates it which is a far different situation than predicting end of charge from an absolute temperature or rise over ambient temp.

So it goes with most studies. Only when you take all the information do you find the glaring holes in the research.

What all these battery companies do is guarantee their product when you use their chargers which operate below 0.5C charge rate. I don't make that guarantee, they do. It is not important to try and eek out the last % capacity or life from a battery, we got along fine with less performance for many years and in the years coming will have even more.

Once again, the majority of the world has no problem carrying out their lifes, using myriad types of devices charging at a rate you don't like. Despite it being less than a hypothetically ideal rate, it is fine to use and the world stands constantly ready to prove that every moment.

There is no better proof than almost everything and almost everyone (manages to use their devices and batteries fine without a care in the world about some narrow charge rate).

[Marduke]

"That's how everyone is doing it" is not a valid reason for how something is supposed to be done. Lots of people do lots of things incorrectly, that doesn't make it right.

The point of the discussion here of what charge currents to use is not about what lots of people do (incorrectly, even by manufacturer standards), but what is BEST.

And as silverfox described, much of the world using slow charging DO have issues, they just don't know it. They think short life cycles and worsening performance are the norm and don't have the knowledge that the grass really is greener on the other side.

Lots of people drink and drive and get away with it, speed, swim less than 30 minutes after eating, etc. But that doesn't mean any of those are good ideas either.

There is no doubt, given the choice between the two, fast (1C) is better than slow, as proven by documentation above.

Where is your proof of the opposite?

[Marduke]

As for you temperature dependance question, feel free to view the Energizer document in great detail.

Also, try artificially cooling a cell during charging at 1C and monitoring the temp and voltage. Whoops, the -dV signal just disappeared. So much for that theory....

[sygyzy]

Great advice. I wrote it all down.

What's the appropriate DISCHARGE rate?

[travelinman]

You really do like to don't you

[clintb]

You really do like to don't you

I don't see Marduke as provoking in the least. On the contrary, he dug up plenty of factual information, and presented a distilled version of what the folks (mostly SilverFox) have discovered in actual tests. If you have some good test numbers to backup the one, consumer focused, document you've posted, then please, by all means, give more info.

[wapkil]

Hello J C,

I beg to differ...

There is enough evidence to support charging in the 0.5 - 1.0C range that the battery manufacturers actually recommend it.

Can you (or someone else) direct me to the places or documents describing why fast charging is better than standard charging? I have read it many times that fast charging is recommended but I couldn't find any clear explanation why.

It is obvious that if the charging current is too low for the charger to terminate correctly and it overcharges the cell it will be bad for the cell life. I think it may be the most important reason for fast charging current recommendations.

If we assume though that the charging will be terminated correctly, I don't know why 0.1C can be worse than, say, 0.75C. As I understand, every charging is shorting the life of the battery. If fast charging is better it would mean that these negative effects are lower with higher currents. It may be true but it's counterintuitive for me - I would think that it's usually better to perform similar chemical reactions slower, not faster.

[Mr Happy]

If we assume though that the charging will be terminated correctly, I don't know why 0.1C can be worse than, say, 0.75C. As I understand, every charging is shorting the life of the battery. If fast charging is better it would mean that these negative effects are lower with higher currents. It may be true but it's counterintuitive for me - I would think that it's usually better to perform similar chemical reactions slower, not faster.

It's a bit of speculation, but here's a hypothesis for you. It is possible that higher charge currents reach more of the chemicals in the battery.

It is commonly observed that flowing things take the path of least resistance. For instance, if you have a trickle of water flowing down a river bed the water will follow a path down the middle and leave much of the channel dry. The reeds at the edge will not get watered. On the other hand if you have a large flow of water the whole river bed well be flooded right up to the edges.

It could be the same with charging an electrochemical cell. A full charge on the battery requires all of the electrode material to be reached by the charging current, but there is no guarantee that all of the paths through the cell have the same resistance. Small currents may follow the path of least resistance and overcharge some parts of the electrodes while leaving other parts undercharged. While larger charging currents may do a better job of flooding the cell and reaching every part of the electrodes.

As I said this is speculation and may be wrong, but it is something to think about.

[TakeTheActive]

...It is obvious that if the charging current is too low for the charger to terminate correctly and it overcharges the cell it will be bad for the cell life. I think it may be the most important reason for fast charging current recommendations...

We also have the relationship between Charge Current and Crystal Size:

• Low Charge Current encourages Large Crystals
• High Charge Current encourages Small Crystals

Add to that, Self Discharge also encourages Large Crystals and we start heading towards Separator Damage.