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A brief summary of NiMH cells and their charging techniques, and why smart chargers are usually not so smart.
Nickel Metal Hydride cells are almost a legacy cell, in that they are designed to literally fill the gap of alkaline and zinc cells. In reality, lithium ion technologies are a much better option for future products. But where it's useful to be able to swap cells for instant power replenishment the NiMh cells have a safety advantage over lithium cells.
Things worthy of note in this video. Smart chargers may not recognise a fully discharged NiMH cell. If the cell is given a slight boost from another cell or slow (dumb) charger, the smart charger will recognise it again and charge it.
If a cell is old or has low capacity, a smart charger may end the charge cycle prematurely - sometimes within a minute. If that happens, check the cell has clean contacts and if that doesn't help then it can be charged to its absolute full capacity reliably in a traditional dumb charger that doesn't do any end of charge monitoring.
I almost forgot to mention the Rayovac IC3 charging technology. I think the product failed because it was sold as premium technology at a high price. It was the ultimate in precise and fast charging, allowing a 2000mAh cell to be charged in just 15 minutes with no risk of venting.
Other brands tried to compete using thermal charge end detection, but they were nowhere near the simplicity and reliability of the IC3 system.
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Let's talk about nico metal hydride cells, how to charge them and the problem with smart chargers. So i've got a selection of cells here, not all of which are nickel metal hydride, but i'm going to put some cells in i'm going to put in this cell. That is probably going to not charge up to a very high capacity. It's been recognized, i'm going to put in an old nickel cadmium cell, which has not been recognized and we'll see if it is recognized later on, and that has also not been recognized and i'll explain in a moment why that is so.

I'm going to take this one out because i want to actually demonstrate i want to see if that one does actually in any way take a charge, but that is now charging at 200 milliamp, milliamps and uh. If i click this button, it will show how many milliamp hours it's put in and, more importantly, it'll show the voltage across the cell right. So let's talk about nickel came in first and because nickel cadmium was the precursor to nickel metal hydride nickel. Cadmium goes back a long way, but when the european directive came in for getting rid of things like reduction hazards, substances and things like that, they didn't want the cadmium and landfill, so they chose to switch to nickel metal hydride cells and in a way that was Good because there were many advantages for a start, a nickel metal hydride cell, even a cheapy one like this - has relatively good capacity compared to the old nickel, cadmium ones.

The nickel cadmium ones were also very prone to self-discharge at a fairly high rate. Mda used the old nickel cadmium power tools like the ryobi battery pack, we'll know just how rapidly they'd self-discharge it was extremely annoying. You fully charge it up. You go back later on.

It's discharged that lithium has blown everything out of the water, but the advantages of nickel cadmium over in the nickel metal hydride were that it's it's a very robust battery technology. It's really rugged has a very long life um and it will operate at extremely low temperatures. Down to about minus 20 degrees celsius, so nickel, cadmium, still like this yellow cell here it still has its place and it's used in medical applications and also emergency lights, where it's just left on continuous trickle charge all the time and it's one of the most robust Technologies for that next comparison is to between nickel metal hydride, like this premium eneloop cell and uh lithium cells. It's very hard to compete with lithium, but if you were to put these in your pocket - and you were to short these out, the nickel metal hydride would uh potentially get very hot if it's shorted with keys, but not as hot as the lithium cells.

If you shortly themselves out, unfortunately, can go into thermal runaway, it's not the lithium! That's blowing up! It's just the fact that contains so much energy, and it all comes out at once that it causes problems, particularly that's why a lot of the manufacturers distanced themselves from the 18650s. They said, they're not suitable for personal use. They should only be built into equipment and in a way, they're right. If people stick these in their pockets, that's when you see people's pockets going on fire.

The whole point of that thing of the manufacturers putting that out was to basically say we're not liable because we told you so but moving on from that. If i had to choose a technology, it would be live, po4, lithium, iron phosphate. It's got lower energy density than traditional lithium ion cells. Well, it is lithium ion, but the ones we normally use the ones that charged a 4.2 volt.

They said charges to a lower voltage but uh. Although it's got lower energy density, it's much safer. Its electrolyte isn't flammable that's one of the problems with these. It's the flammable electrolyte vaporizing igniting.

They tend to feel the sizzle and let the fumes out, but not too badly um, but also they're super robust they'll. If you put these in storage uh this one, the lithium-ion phosphate would be more stable in storage. This would lose its energy density over time. This would keep its energy density and this one's going to charge for a lot more cycles.

I wish there were more lithium. I want my phone to have a lithium iron phosphate battery, but i think it's life, it's high life is probably why they don't put them in phones uh because they want you to buy a new phone every year. Oh, that was a bit conspiracy theorist, but anyway, what's happening here. This one is uh, take your charge.

This is good, actually it's doing better than it did before when it put a grand total of one milliamp hour in and then decided it was fully charged. I shall come back to that moment, i'm going to bring the notepad in and i'm going to talk about uh how to charge these cells. So the simplest possible circuit for charging a nickel metal hydride cell is say, for instance, plus 5 volts, a nice handy voltage from your usb power supply a diode optional, just to stop it back feeding to the supply when you unplug it uh a resistor to limit The current and the cell, and that is it, zero volts. That's all you need to charge the cells and you think what happens if you over charge it? Well, you can't, if you keep charging a nickel metal hydride cell like this as long as you don't do it to a ridiculous level as long as you don't smash amps through it um, it can do something very clever if i say draw the outline of a Cell, like this so there's the cell there's the pip in the end, here's electro materials inside when it gets to the end of charge bubbles form on the electrodes.

It liberates gas most cells do that and when that happens, with a nickel metal hydride or nicad, it does what's called self-catalysis as long as the current's not too high. It can convert this gas back into liquid electrolyte and in doing so, the temperature rises a little bit in the case of the old nickel, cadmium ryobi batteries. What they used to do is they had a thermal switch in them. They had a group of cells and in that group of cells they had a little thermal switch sandwiched down the middle and at the end of charge, the way it detected the pack was charged was the thermal switch would cut out and it would either break the Current or just break a control signal to the actual controller charge controller, and it would turn it off very straightforward, and this compares very favorably in a way, but pros and cons to lithium cells, where you cannot trickle charge a lithium cell.

If you keep trickle charging that the voltage will go above 4.2 volts and the standard lithium ion cell and it will potentially go unstable, it will damage it at the very least. It will damage it at worst case it may initiate uh failure of the cell um. Let's talk about the discharge curve of uh nickel metal hydride, which also makes it fairly useful, makes a bit annoying when you're trying to replace alkalines with it. If this is say, 1.5 volt 1.5 - and this is 1.2 - and this is zero volt.

It starts off at full charge round about 1.5 and rapidly goes down to one point two, but then hogs one point: two all the way to the end then suddenly dies off at the end and again, if, in the days of the early in the especially like Nickel, cadmium power tools, you'd be drilling a hole and suddenly you'd hear the motor go and it would just suddenly go down to a lower speed because one of the cells had just reached the end and it just fell off a cliff and it just basically the Voltage just dropped that constant voltage is another a nuisance. This is where lithium wins again, because the lithium, if you measure the voltage across a lithium cell, it will give you an indication of the charge state. That's in it. If it's around about 3 volts, it's pretty flat, if it's 4.2 volts it's fully charged, if it's 3.6 or 7, it's got about half the charge in it.

It's a really handy way of doing that. Also, when you put this into a charger, it will charge it from any level whether it was almost full or almost empty. It will charge up to exactly 4.2 volts and then it will stop and you'll know exactly that this cell is fully charged. That does not happen with nicometro hydride cells.

That is one of the problems of these smart chargers. So that is the trickle charger. But if you want to charge at higher currents, because uh, typically speaking, to allow self-catalysis to take place, it has to be below 0.1 c. What that means is if the charge capacity of a cell say, for instance, this one is about - let's say it's: 8 800 milliamp hour.

You have to keep the charge current to about 80 milliamp hour. It has to be a 10th of the capacity of the cell. Typically, to actually allow that process to continue, if you over charge them continually in a trickle charging circuit, the pressure will build up and then it will vent gas out the end. The pressure build up is an interesting thing, because uh there was a battery technology.

Was it's not available anymore? It's a shame. It was the ultimate nickel metal hydride cell. It's a shame. They discontinued them.

That was real vac. But let's talk about the way, this type of charger will uh charge the cells. Now this at this point in time, this cell is taking a charge it's currently at about 29 milliamp hour. This one has not taken a charge, and this one, when i put it in also didn't want to take a charge.

Let me show you why and how to actually recover these to actually make them charge quite safely. It's normal. If you have three cells in series in a battery pack, it's worth mentioning. One of these is a cell.

Two of them in series is a battery. That's where the whole cell battery thing comes from, but you have these uh cells wired in series, and you have them powering a load like a lamp and i'll show you this with a conventional current flow. I should zoom down there shouldn't i. I should so using conventional current flow from positive to negative the current's flowing around and the kind of equal through all the cells.

The middle cell goes flat. First, they never have identical capacities, so one cell's always going to go down. That's the point in your power tool that used to be the sudden. The speed would drop as one of the cells went flat and what happens then is the current still keeps flowing it's pushed around by the other two cells, but now it's pushing making this side of the cell positive and this side negative and over it can actually Damage the cells, if you actually reverse charge them like that, that's what's happened here.

These cells are either very flat or they've been reverse charged. When you put them into the charger, it can't see them because it's looking for a voltage to actually get an indication. The cell is in there if that happens. This applies to nickel, cadmium or nickel.

Metal hydride take about a power supply or bridge bridge a good battery to a bad one. It's a very rough way of doing it or use a dumb charger. This is a trickle charger that charges 100 milliamps continually and if you put them in something like this, it will not only recover them. It will actually charge cells like this fully uh.

Just take a lot longer to do it, but you can put a charge onto this to bring the voltage back up to the correct level and once it's got a voltage on it, the charger, the smart charger will detect that voltage and it'll start charging. Let's recover this in the same way, this makes me wonder how many people put their cells into the smart charger. It doesn't charge them, so they just say: oh that sells dead and they put it in the bin. That is not true.

It's detected this cell as well and will now have a go at charging it. Let's talk about the fast chargers and charging much higher currents above the point at which self-catalysis can occur and it can recombine the gas. So let's take a look at the options here. You have negative delta v.

Negative delta v sounds very stylish. What happens at the end of charge with negative delta v? Is the charger puts the charge voltage across it and then takes that away and just measures the voltage and it's looking for the point at which, at the end of charge, little bubbles occur on those electrodes and for a slight load across the voltage will suddenly drop At the end of charge, and that's how it detects it now, it used to be quite a vivid drop of nickel cadmium. It's a very small drop with nickel metal hydride and, as such, many of these chargers can miss that and if you put a battery into charge, this has just reported these two as being full when they're not i'll, show you why that is. It's put.

It's trickle charging, but it put almost nothing into them and i'll show you why that is so uh the negative delta v um. It looks for that change in voltage and it's very easy to miss it. So the other system it uses is no change of voltage. No change of voltage means it measures the voltage across the cell and if it detects the voltage, just isn't rising anymore if static, it ends the charge.

At that point, the uh other thing it can use is delta t which is uh. My preferred way of i mean ultimately the change in pressure. One is better, as i'll tell you in a moment the uh. What's that called ic3, but the delta t is temperature change.

This unit does actually monitor temperature. I think some of them have a little thermistor. Actually sorted onto the end of each of the electrodes and they can actually detect a sudden rise in temperature because when it reaches the end of charge and these the self-catalysis is occurring, it can't take anymore. The pressure builds up, but also because that chemical process has taken place.

It warms the battery very slightly in a low current trickle charger. It's not going to be much, it's not going to be even detectable, but when you're charging at 500 milliamps or an amber whatever the rise in temperature is vivid. So it goes from being a fairly ambient temperature. It will suddenly rise up and it will detect that as the end of charge, some also have a last resort, they will just say if it's not charged in a certain turn time, it will just turn off, but if you feel batteries and they're getting very hot In the charger that means they're charged, usually um and uh, there should be.

Basically, you should stop charging that point. It's the curse that smart chargers sometimes do weird stuff, one of those things, the negative delta v, and also the fact that they are looking for a upper voltage threshold. It's designed to stop people putting things that alkaline cells in these. If you put an alkaline cell in the voltage will go from this of say: 1.2, it will very rapidly rise up to this 1.5 and that's the point.

It rejects that cell in the case of these cells, because it's got a high impedance and because it charges at the high current spikes, it will often reject low capacity cells. You know things like the classic: kodak dollar store pound shop cell or this daewoo one will often not take a proper charge in these, even at the low current setting. If that's the case, then put them in a charger like this, and it will absolutely guarantees to charge the cells up to their full capacity that one originally only took a charge about one milliamp hour. It's up to 49.

There, let's see where the voltage is 1.4 volt, that's doing all right! It's recovering well, but when i put it in here and gave it a charge for a couple of hours, it actually took about 300 milliamp hour uh charge. So it must be in a good few hours. I had it in this then so it the cell, was good. It's just that this unit was not really recognizing it that its internal resistance was too high for this to put a charge into it.

There is another curse with these smart chargers. One i got from maplin with high capacity cells and i put them into charge, went out the room forgot about it came back later. There was a strange smell. The maplin charger was a very smart computerized charger and it crashed it crashed putting full current out to those cells, and when i was looking around the room, looking for what, where the smell was coming from checked, all electronic items saw the cells, all the heat shrink On them was basically just ripping and peeling back and shriveling it actually glued itself.

It stuck it melted onto the back of the charger, which is a bit scary and i immediately turned off and the cells tried to take the cells out, but they were very hot. So i just left them in place and i put it into a metal container just in case anything went really wrong with the cells because they were so hot uh, but afterwards, when i turned it off and on after i get the cells out. It's because it's computerized, it said ah everything's fine ready to charge cells it, but as soon as that's happened, you kind of lose confidence, that's something you have to keep in mind with smart charging systems, sometimes they're too smart. So these things these ones have not taken a charge, these ones, if i put them into this cheapy, i think this came from poundland again.

If i put them into this cheap charger, it would be guaranteed to put a charge into them and likewise the trickle charger. Might take a long time to put a charge in, but it will charge whatever cells you put in up to their fuel capacity. Another thing i've come across of these even putting good cells in sometimes randomly it will just terminate the charge prematurely. You get three identical cells that have been used in the identical applications and you they've got the same.

Basically, the same capacity, two of them charge up to say they take another one thousand milliamp hour because they weren't fully discharged the other one will only take 100 milliamp hour and the reason for that is that it's done a false recognition of an end-of-charge state. It could be a bad battery contact, it could just be the chemistry creating noise inside, but sometimes it just messes up i'd hope this one's going to actually fail the battery. It's it's, not it's going to put a charge into this time, but that uh. I think that sums most of it up nicometro hydride still has a place.

It has a place for devices that have the standard battery holders, although that helps if the equipment can actually recognize 1.2 volts. But i ultimately have a horrible feeling that uh lithium is going to win overall, but until then uh the good quality eneloops and things especially with their super low self discharge, are definitely still a viable option. But if you've got a smart charger, just remember that sometimes they're, not so smart, and just because it's not charging a cell might just mean it's just needing that cell needs boosted in a dumb charger with something else just put a voltage onto it. Don't just think the cells failed because it may just not recognize the cells in place, but that is it.

Nickel, metal hydride cells quite interesting technology. Oh darn bonus extra footage because i forgot one of the most important breakthroughs that just never ever carried on and it was railvac's ic3 in cell charge control. Let me draw a rough doodle. What i think was going on here.

The in-cell charge control was the ultimate way of charging a nickel metal hydride cell it guaranteed. It was going to be charged right up to the end to fuel capacity. It was never going to be overcharged and it could be charged in 15 minutes. Even a 2 000 milliamp cell could be charged literally with amps without over charging it here's how it worked.

There was an extra metal ring around the side, the positive pip, and when you put into the charger a metal cup made connection with that outer ring, plus a little stud in the middle meant, it could charge ordinary cells as well. The middle stud just basically put in a trickle charge as such, so it would just gradually fill the cells up slowly. The outer one put the high current in the high current uh bridged onto the middle's middle stud, but when the end of charge was reached and these little bubbles started forming on the electrodes and the pressure built up as soon as that happened - and this is i'm Guessing this is how they did it. A pressure switch inside basically disconnected the outer ring from the center stud, and that ended the charge process, which meant that you know if the charge of the uh, if you put the battery in it, was already fully charged.

It would start charging super high current, but the bubbles would form instantly cut off if the cell was completely empty 15 minutes later, it would have charged it fully till those bubbles formed and it cut off, but you couldn't over charge it because the pressure inside was What actually determined the end of charge - and it meant that the it was the perfect charger, so that was the rail railvac ic3 uh railvac uh. Unfortunately, i don't know why it didn't continue. Maybe it's because they were more expensive to make too specialist, but it didn't continue and they discontinued. It was such a shame because, technically speaking, it was the perfect nickel metal hydride cell.


12 thoughts on “Charging nimh cells – smart vs dumb chargers”
  1. Avataaar/Circle Created with python_avatars Bod9001 says:

    The pressure thing might have issues with going to high altitudes or when it's low or high pressure due to the weather, though it would be pretty amusing being told not to charge your batteries while it's stormy

  2. Avataaar/Circle Created with python_avatars E Van says:

    I remember my RC cars in the 90s all used blue NiCd battery packs. I had an RC plane too that was pretty cool.

    Haha. I remember thinking that RC plane looked extremely similar to the small hand launched recon drones the military used in the 2000s.

  3. Avataaar/Circle Created with python_avatars Hola! Namenloser says:

    Its not as bad anymore with li-ion phone batterys my LG V30 is now 3.5 years old and after it lost like 20% capacity in the first year nothing changed since then
    My dad has an huawei p10 after 6 years it still lasts for a day

  4. Avataaar/Circle Created with python_avatars Gadgetboy says:

    Bonus comment! ๐Ÿ˜›

    I had one of those, and it had this really loud fan that blew through the batteries and charger circuitry. Now I know why the fan would vary in speed as the battery filled.

  5. Avataaar/Circle Created with python_avatars ROB Worrall says:

    Not Rambly at all Clive, there's lots of good info on this vid and I don't think there is any easy clear path or way to explain this subject, especailly considering all the different battery chemistries and their properties, I wouldn't know where to start explaining it to a general audience!

  6. Avataaar/Circle Created with python_avatars Microdoser says:

    I've built an 18650 into one of my products, but I have set the charging circuit to stop charging at 4V because I will get 4 times the number of discharge cycles when compared to charging it to 4.2V yet still get 75% of the rated capacity. I don't need the full capacity because it is just to provide a UPS for the device to enable it to shut down properly, and the extra life for the battery is well worth it.

    The equation is roughly Ef@Vch= 2^(10*(4.2-Vch)) where Ef is the enhanced life cycle factor (Ef = 2 would mean that the battery will survive twice as many charge-discharge cycles as Ef = 1), and Vch is the charge voltage.

    I liked the video, but then I like more in depth videos. I know they have less mass appeal, though.

  7. Avataaar/Circle Created with python_avatars Robert Blethen says:

    I saved 3 cells that I had tossed in the dead battery pile waiting to be disposed of thanks to this. Thank you Clive! Very good video! Can not count how many I have tossed over the years because of this.

  8. Avataaar/Circle Created with python_avatars Simon David says:

    I found the trick with dumb chargers revitalising "dead" cells years ago with car batteries. Same thing happens with overly smart 12V lead acid chargers. I would need to hook up my Halfords dumb charger for a couple of days before a smart charger would see it.

  9. Avataaar/Circle Created with python_avatars Michel Honig says:

    A real problem with these so-called 'smart chargers': the failure to detect drained (but working) cells. The better 'smart charger' has an option to 'force start charge' or 'start charge for drained cell with chemistry X' with the latter being a gentle starting charge. I usually check 'lygte-info' for proper (and less proper) chargers. My experience with e.g. SkyRC MC3000 and GyrFalcon is that they do detect drained cells. The 7Dayshop charger looks suspiciously identical to my Voltcraft (Conrad) charger ๐Ÿ˜‰

  10. Avataaar/Circle Created with python_avatars Paul Amos says:

    Very informative and not in the slightest bit rambling. When you have things to impart about products then limiting what you say is worse as you are not covering the subject to its full capacity ๐Ÿ˜Š๐Ÿ‘๐Ÿป

  11. Avataaar/Circle Created with python_avatars Chris Bruce says:

    the rayovac probably failed because of production costs. the cheaper a chinese factory can make the product the more the factory could sell making the profit higher. it just simple chinese business tactics as taught Yoda you have. that was spot on.

  12. Avataaar/Circle Created with python_avatars Gregory Thomas says:

    I have some Radio Shack NiMH cells that were manufactured in 2001…they are still going… though at a reduced capacity…so I use them for my coffee stirrer.
    Nite Core is my normal charger but every now & then it doesn't see them…I put them in a dumb lithium charger for about 3-4 seconds…it shocks them up and the NC can see them.
    The Rayovac IC3 was so good that the other battery companies told them to stop making them because it was making the other companies took really bad <mini-conspiracy theory rant>

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