What does it say on the manufacturer's technical datasheet or charging application note?
If they were NiCd, you'd have no problem. The consensus appears to be that NiMH don't tolerate long-term overcharge. Opinions vary on short-term overcharge. I expect you're doing about the best you can do with simple schemes.
The difficulty is not in the equation. The difficulty is in determining full discharge. How do you determine that? Do you measure the voltage? under what conditions? OR do you just charge it when the performance of your device drops? OR at random? The range between 1.1V and 0.9V is huge depending on the load current you use to test. And the device probably quits long before that. I know because I have a computer hooked to a programmable power supply and load fixture and I've tested it on a many different cells with widely varying history.
One example application is the emergency flashlight. You can't just charge it when it goes dark. Even in the best of conditions, when it's starting to dim, you know you should charge it IMMEDIATELY... but you've only got one more thing to do...so you do it. More batteries are damaged from overdischarge than anything else. For it's intended use, the emergency flashlight needs to be at FULL charge all the time. So does your cell phone and your car battery and your electric drill and... Even low-drain applications are not immune. Do you want your alarm clock to go dead at 4AM on the morning of your big presentation.
Bottom line is that you expect your device to perform for the entire duration of the need...even when you have no idea how long that is. That requirement often leads to systematic overcharge.
But: I expect you're doing about the best you can do with simple schemes.
If you've done something like solder on the cells, you can expect some variations in self-discharge and capacity. Some amount of overcharge tends to keep them equalized.
In the dark ages, I've charged NiCd and NiMH batteries with a timer. It's a good way *IF* you know the capacity of the battery and the state of charge. Capacity is easy as it's in the spec sheet. However, a given battery can have radically different capacity values at different charge/discharge rates. To make the numbers look huge, manufacturers use a slow discharge rate. The battery also has a charging efficiency, where it takes more coulombs to charge the battery, than it can deliver. I use 75% for most batteries. Once you know all that, and the state of charge, you can calculate how long you need to charge the battery. If the battery gets warm at the end, you're overcharging and have probably killed the battery.
The catch is that if you forget to discharge to a known setpoint (i.e.
1V) you could easily overcharge the battery. Also, if you use a timer, and the power fails during the charge, you could also charge for longer than anticipated. Rather than go through this ordeal, technology exists to detect the end of charge. You can get the chips and make your own, or just buy a commercial NiMH charger.
More detail: Note the comments on difficulties with trickle charging NiMH batteries. (Note: Isador Buchmann is the founder of Cadex and author of the BatteryUniversity.com web site).
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
This is one of those rare cases where I have to disagree with Mr Lieberman. How warm is warm? "Getting warm" is an indication that charging is complete, or near-complete.
Many NiMH manufacturers claim their cells need to be "slammed" to get maximum charge. I'm reluctant to do this. However, I've charged NiMH cells in those "15-minute" chargers, and though the cells got hot, they were not destroyed. (I no longer do this. I have about 20 cells of 2500mAh and higher capacity, so I never need a quick charge.)
The best way to charge batteries is with a charger that lets you set the charge rate, and watches for the signs that indicate the cell is charged, such as the MAHA C9000. I generally charge at 0.3C or 0.4C, which is considered on the low side for NiMH cells. Contrary to what Battery University claims, I've never had problems with the cells overheating, or the charger failing to halt at around 1.42 to 1.45 volts. (Note that his gripes are principally directed at "consumer" chargers.)
The C9000 displays the battery voltage, so I can choose to stop charger at whatever voltage I feel comfortable with. I'm not stupid enough to let any charger operate without checking it occasionally. If you like, stick a timer on the charger to shut it off.
That's easy. This is Figure 1: from this article: The graph is for NiCd but NiMH is similar. I've also created my own temperature curves for my experiments in extremely rapid NiCd charging. Not that the temperature starts to rise at about 75% of full charge and has risen 45C (113F) above ambient at 100%. That's more than "getting warm". That's hot.
I believe I demonstrated this in the past. See: It seems that you have to charge cycle a NiMH battery about 3 times before it will deliver full capacity. However, the difference is small. For example: shows that the first charge cycle results in a 1180mA-hr capacity, while the third charge cycle increased it to 1360mA-hr or about 15% increase. Is that what you mean by "slammed"?
My favorite NiMH charger is a Radio Shack 23-1305 30 minute NiMH charger. The cells get warm at the end of charge. I haven't bothered to measure the temperature. Again, if you look at the previous article showing the temperature graph, the cell should be 113F at full charge. Hopefully, the charger slows down the rate of charge before reaching 100% or it probably will get that hot.
I've noticed that my NiMH battery collection (flashlights, cameras, GPS, radios, etc) spent much of their time in the charger simply recovering from self discharge. So, I invested in some Sanyo Eneloop batteries, which have a much lower self discharge rate. I took some photos yesterday with a camera that has been bouncing around my car for about 5 months. That battery indicator showed about 3/4 charge. Had this been the older batteries, they would have been almost totally discharged.
Nice charger/analyzer. I dunno the break-in and battery-forming features. Stressing the battery doesn't seem like a good way to increase battery life though it might produce rated capacity earlier. However, it's a much better charger than the common consumer junk chargers, and would probably be a good charger for the OP.
Have you tested your battery for rated capacity after one of your charge cycles? The MH-C9000 will run a discharge mode test and display battery capacity.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
shows that the first charge cycle results in a 1180mA-hr capacity, while the third charge cycle increased it to 1360mA-hr or about 15% increase. Is that what you mean by "slammed"?
No. It means hitting it hard with a high charge current.
I've never had that problem -- as far as I can tell. I've had cells that sat around for the better part of year deliver 20 or 30 flashes, with rapid recycling.
Most of my cells have been close to rated capacity. In one case, a cell was about 20% low, and MAHA replaced it.
Nice charger/analyzer. I dunno the break-in and battery-forming features. Stressing the battery doesn't seem like a good way to increase battery life though it might produce rated capacity earlier. However, it's a much better charger than the common consumer junk chargers, and would probably be a good charger for the OP.
Agreed. Thomas sometimes sold the C9000 for ~ $40, but those days seem over. I have two, so I don't have to wait if a lot of cells need charging.
Yuck. Why would I want to do that? If there were dendrites in NiMH batteries like in NiCd, then perhaps it would make sense. Besides if such a high current blast was necessary to produce a proper battery, or to produce bigger numbers on the data sheet, the manufacturers would already be doing it.
Where did you see this recommendation? I couldn't find anything with Google under "NiMH slamming" variations, except under National Institute of Mental Health. I did find this blurb that recommends against the practice:
Perhaps my cameras and GPS require a higher minimum state of charge than your flash? When I used to leave my conventional NiMH batteries in the camera for a few months, it would usually be good for one or two photos before complaining. I always kept a spare set of charged NiMH with the camera in case I run out. They were also only good for a few shots.
Scroll down to the two pink graphs near the bottom of the article. The conventional battery is down to half capacity in 75 days, and 1/4 capacity in 150 days.
Incidentally, this article: covers quite a bit of ground on dealing with NiMH batteries, including trickle charging, and a home made NiMH computah controlled charger: and a simplified USB powered charger: I'm tempted...
I didn't know that Maha made batteries: $4.25/ea for AA in a 4 pack is not cheap. Somewhat more expensive than Sanyo Eneloop. I dunno.
There are other low self discharge NiMH batteries available. Duracell Pre Charged Rayovac Hybrid or Platinum: Kodak Pre-Charged
I'm finding prices from $50 to $140 (including tax and shipping).
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Yuck. Why would I want to do that? If there were dendrites in NiMH batteries like in NiCd, then perhaps it would make sense. Besides if such a high current blast was necessary to produce a proper battery, or to produce bigger numbers on the data sheet, the manufacturers would already be doing it.
Where did you see this recommendation? I couldn't find anything with Google under "NiMH slamming" variations, except under National Institute of Mental Health. I did find this blurb that recommends against the practice:
"Slam" is my choice of words.
I can't give you any references, but I've repeatedly seen in the documentation for NiMH cells, that hitting them hard is necessary to get a "full" charge. You are repeatedly warned NOT to charge them below 0.3C, and higher values are preferable.
Most of my flashes are 500-series Canons, which are hardly wimpy li'l critters.
Scroll down to the two pink graphs near the bottom of the article. The conventional battery is down to half capacity in 75 days, and 1/4 capacity in 150 days.
That's not what I would consider "rapid" self-discharge -- other than, perhaps, compared to a lead-acid battery.
Incidentally, this article: covers quite a bit of ground on dealing with NiMH batteries, including trickle charging, and a home made NiMH computah controlled charger: and a simplified USB powered charger: I'm tempted...
I don't know if they make them, but they definitely sell them under the PowerEx brand. Thomas sells four for about $12. Considering that a charged
2500mAh cell gives about as much runtime as an alkaline cell, it doesn't take long to amortize the cost.
I'm finding prices from $50 to $140 (including tax and shipping).
The eBay prices are nuts.
I was wrong. Thomas's holiday deal is the charger, a generic carrying case, four Immedion AA cells and a plastic case for the latter, for about $52. Given the cost of the "accessories", the charger nets at $40. This would be a very good time to grab a C9000 from Thomas Distributing.
I think I've decoded the effect. It's a cure for "voltage depression", where the NiMH battery decides to operate fairly normally, but at a somewhat lower voltage. The usual culprit is over charging. I've had it happen to me and almost tossed a set of good batteries. The recommended fix is to cycle charge the batteries a few times and it should recover. However, the not so graceful fix is to discharge the battery, and then to a radical quick charge, being careful not to overcharge it again. Allegedly, this avoids having to cycle charge the battery several times, probably at the expense of battery life.
See voltage depression section near the bottom of the page:
I'm not familiar with the flash, but I suspect is does not have the continuous high current load of a camera with it's focus and aperture motors grinding away for many minutes.
Well, it's rapid enough that I was recharging almost everything I owned every 3 months or so. If it were only one device, I wouldn't complain, but it was a mix of GPS, flashlights, cameras, phones, walkie-talkies, FRS radios, clocks, etc.
Well, the plan was to save money by using rechargeable batteries. I usually buy Costco Kirland AA batteries: at about $0.30 each. Sanyo Eneloop NiMH batteries are costing me about $3.00 each. So, if the NiMH battery lasts 10 charge cycles, I break even. So far, that's been the case, so for cost savings, rechargeable wins.
However, that has NOT been the case for convenience. I can leave an alkaline AA battery in my various devices for many months, and not have a self discharge problem. I find myself carrying spare alkaline batteries for these devices just in case the battery decides to discharge itself. Extra points for devices that suck power when turned off. Therefore, for convenience, conventional rechargeable NiMH is a problem. The low self discharge batteries are a big improvement and may just solve the problem. I'm not keeping score, but I'm definitely not recharging anywhere near as often as when I was using conventional NiMH batteries.
Good price, but the web pile is back to $52 (including shipping) and without the AA cells and plastic case:
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
I've had good results with the older Enloop charger...the one with the single led that can charge an odd number of cells.
There are two categories of dendrites. Dendrites that have already shorted need to be vaporized with short-duration high-current. That clears the short and blasts a bigger hole in the separator. Causes higher self-discharge and faster dendrite regrowth. Back in the day when NiCds were expensive and shorted ones were free, it made a lot of sense for hobby projects.
The second class is those that have not yet pierced the separator. For those, I'm a fan of burp charging. Same process you'd use for plating when you want a smooth surface. I did some experiments, but I had nowhere enough data to draw a conclusion. And I concluded that the cost of powering the computer and the programmable power supply and load probably exceeded the benefit.
The biggest problem with charging batteries is determining when to stop.
I had a friend who did radio repair for the city. Back in the day, police radios were terminated by a thermal switch. That worked surprisingly well, but only because the battery was predictably depleted during the shift. If you stuck a charged radio into the charger, you did some damage. He had a lot of failed batteries from people who left the radio off all day and stuck it back in the charger.
There's so much variability in temperature, thermal time constant, voltage gradient, etc. that it's best to measure change than absolute value. I'm a fan of 0deltaV termination with timer and temperature fail-safe for NiMH cells.
Most DSLRs use lithium-ion batteries. I've never measured the current drain, but I doubt it approaches what's required to charge an electronic flash from a dead start. And unless you're using the focus-track feature, auto-focus operation is intermittent.
--> discussion of rechargeable versus disposable batteries snipped
How about getting back to the original subject, "Best Method to Slow Charge..."
Here is additional information that seems to support that formula:
I just bought a pair of 2500 MHA NiMH "C" cells that state on the package, "Recharge in just 2.5 hours with 1250 mAh rapid charger or
15 hours with a 250 mAh standard charger". Note: There was no charging information on the 1500 MHA NiMH "C" cells I used in my battery pack project. The 1500mAh is a different brand than the
2500mAh. John
Perhaps it might be useful if you would disclose the brand and model number of these "C" cells? The typical "C" NiMH battery has a capacity of 3000 to 5000 ma-hrs, not 1500, which sounds more like a "AA" battery or possibly a sub-C size battery.
The 1.4 factor is: 1 / 1.4 = 0.7 = 70% charging efficiency. That's a reasonable number.
Using a timer is fine, if you're very careful. However, I guarantee that it will shorten the life of your battery. The object of the exercise is to charge the battery without going into overcharge. That's easy with a new battery, where the aformentioned timer formula works just fine. However, as the battery ages, it's capacity also decreases. Charging at the same 0.1C rate for the same amount of time is a guaranteed overcharge. I have some experience with that, having successfully trashed some rather expensive radio battery packs with timer controlled charging. In order to do it correctly, you would need to ocassional measure or estimate the battery pack capacity, and adjust the timer according to the reduced capacity. You would also need to know the state of charge, or discharge the battery to a known charge point.
"Harmful overcharge can occur if a partially or fully charged battery is charged with a fixed timer. The same occurs if the battery has aged and can only hold 50 instead of 100 percent charge. Overcharge could occur even though the NiMH battery feels cool to the touch."
In other words, to make the timer method work, you would need to know the present capacity and state of charge of each battery at the time of charge. If you keep detailed records and make accurate measurements, that's possible. I heard a talk by someone from NBC who did exactly that for maintaining the battery packs for portable cameras and recorders. Serial numbers on every battery pack, regular testing, and detailed records were required.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Sigh. I've got an itchy mouse finger hovering over the "buy" button.
I've done my share of NiCd cells directly across a 12v car battery for a few fractions of a second. No explosions, and a rather minimal success rate. The ones that worked didn't last very long.
Also known as "negative pulse charging" or "reflex charging". There are commercial chargers that do this. Pulse-Power: "Negative Pulse Charging Myths and Facts" I'm not convinced, but then I haven't tried one of these chargers.
Yep. Kinda like filling the gas tank with no gas gauge or automatic pump shutoff.
I know it well. The Motorola HT600 and similar "brick" radios. As an added bonus, Motorola may have designed the charger to destroy the battery packs. They also have to be modified to work with NiMH packs:
However, my favorite was a Kenwood something HT, that had small a red plastic window in the bottom of the battery pack. The charger had a corresponding window with a IR photo transistor in the charger base. This was suppose to measure the cell temperature and stop charging when it became warm. Unfortunately, a little dirt on either window turned the charger into a battery overcharger.
For NiMH, that's a problem. Trickle charging an NiMH battery makes the drop in terminal voltage very difficult to see. It's in the low millivolt region. I think (not sure) that Maha recommends something like 0.5C charge on their Eneloop just so that deltaV can be detected. Here's a hint, without detail: "manufacturers recommend 0.5C to 1C to (1A to 2A on eneloop) to make negative delta voltage detection more "detectable"."
I did some testing of temperature detections on individual NiCd cells when I was doing my ultra-rapid charging experiments. I stuck 8 thermistors on various parts of the cell, and proceeded to intentionally overcharge it. The mass of the cell was sufficiently high that the thermal lag was substantial. That won't make much difference for a slow charge, but can easily be catastrophic with ultra-rapid charge, where seconds count. The best sensor location appeared to be in the middle of the cell. Unfortunately, the plastic or paper wrap around the battery is a good thermal insulator. I had to scrape off the coating and connect directly to the metal case.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
MAHA has told me it will never provide a C or D adapter for the C9000. Not wanting to figure out how to reliably attach wires to the charger's terminals, I charge my C NiMH cells * in an Eveready charger with a NiCd / NiMH switch on it. I monitor the charge with my hand. When the cell gets a bit warm, I pull it.
I use C cells in a potato-masher flash and a classic Sony FM radio.
This principle isn't new. The basic system dates back to the late 60s. Motorola made a "pulse-discharge" charger for Honeywell flashes. The claim was not only that it could quickly recharge the power pack, but that it would often "heal" a badly performing or even "dead" pack.
"Modern Photography" gave the system a rave review, confirming that it worked as claimed. Several years later I asked Burt Keppler what happened to the system. He said Honeywell pulled it because there were too many instances of the packs exploding. When I asked him why he never reported this in "Modern", he had no answer.
You need better control than that. I've had better luck discharging a big cap into the cell.
Agreed. Trickle charging is for sissies. ;-) And only makes sense when you have no reasonable termination system...and then, it still makes no sense.
That's also my complaint about the C9000. If your cells have slightly high internal resistance, it refuses to charge them at rated current. If you crank the current down till they charge, you risk failure of charge termination.
These I bought at Harbor Freight. 1500mAh are Chicago Electric Power Systems (in my battery pack) item # 90149, and the 2500mAh (I just bought) are Thunderbolt Magnum item # 97864.
I am aware of partially charged batteries and not to charge them at the same length of time as charging fully discharged batteries.
You made very good points regarding aging battery's capacity going down and I will certainly keep that in mind.
I may consider rechargeable lithium batteries in the future, but I will do some research first to learn about their issues.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.