Gentlemen,
NiMh cells are 1.2V IIRC. Would I be right in thinking that these should be charged individually? IOW, if there's say 6 of them in a caddy providing 7.2V, you couldn't just hook them up to say 8V and recharged them all in series?
cheers,
CD
NiMh are OK done like this, but it's a shade more tricky with lithium, as cells may not all be exactly the same which leads to some cells overcharging and others undercharging, and overcharged cells are...
KABOOM!
In the model community, where maximum charge is a must chargers and batteries have balancing connectors to allow each cells voltage to be monitored and clamped to prevent overcharging
NiMH are ok with it because they burn off the overcharge as heat. That's fine assuming the current is low. If you want to fast charge NiMH at a high current then you do need per-cell connections, since there's only so much heat they can take, and one method of detecting charge termination is by spotting this temperature rise which needs probes on a per-cell basis.
You still get the same variation in charge states as lithium, but you are safe to overcharge some cells while waitng for the others come up to full charge.
(a decent charger will back off the charging current as the voltage increases so as not to overcharge other cells in the pack too much)
Theo
Other text in the article says the NDV detection is weak, corresponds to a 5mV drop in voltage, and is only really visible if the cells support fast charge.
Nickel Cadmium bears some similarities, such as the same difficulties detecting the endpoint.
You can do anything you want with batteries (the exception being the ones that catch fire). But depending on the extremes of behavior, not following best practice can be pretty expensive in the long run.
I could wear my wool suit in the shower. Nothing prevents it. But that would not be a "best practice" :-)
Paul
I didn't find this article very helpful, I'm afraid. They make frequent reference to "C" without defining what "C" is. They also didn't specify what the charging algorithm is. So two key omissions at least.
"C" is the batteries Ah capacity. A charge at "C" implies a charge in Amps at the Amp-hour capacity of the battery.
I thought the article was good.
Why do people use abreviations, when they spell it out anyway..?
They do have an article on that, wouldn't hurt them to link to it from the first mention of "C" in the NiMH article ...
Well, a lot of devices do do that, and the end result is often some dud cells where they get reverse charged by the unit when its running as they do not mach. Having said that, Recharging them one at a time seems hit and miss also. I have found about half have a very long life and the rest, not so much. I put this down to the way they are in series when used. If you get a number which match, then they seem to carry on that way, but have one that goes down before the others and its knackered in a few weeks. Now that has been my finding, and only mine, so if you are lucky with choosing the paris or however many are in series, then you get a long life. Brian
I would say that they do not like being in series in any way, see my previous post. Brian
Well the plan is to just keep them in series (there are 15 of them providing about 19V when fully charged) and hook them up to a 20V supply via a suitable current limiting resistor so they charge very slowly and with minimum stress. I'm still not sure if that's a good idea or not, though. Background. This is to power up a vintage Hacker Sovereign radio made in about 1970 which is a bit of a battery scoffer and eats through 2x PP9s within 3 months. It just seemed like I was for ever swapping out PP9s. So bought four 4x packs of JCB NIMh AA cells and am trying my luck with them.
C is capacity divided by charge time in hours
It then becomes the charge current. So a 5Ah battery charged at one C is charged at 5A. At 2C , 10A, and so on. It is a cell capacity-independent way to specify how fast a particular chemistry can be safely charged (or discharged: they are different).
So a 30C rated battery may safely be discharged in two minutes. Ideal for jump starters.
If its charge rate is 1C however, it needs an hour to be safely recharged.
It's a shorthand, and it does have its uses.
Some chemistries, you can trickle charge forever (abuse) a cell, at C/10 or C/20 ("capacity over ten" or "capacity over twenty") . That's NiCd. That's how Black and Decker used to charge my screwdriver. No precision charger there. Charge circuit cost them ten cents. Capacity over twenty means it would take twenty hours to charge up.
You can't do that for Lithium ion. It calls for a precision charger, with cutoff behavior, to stop charging the cell when it is full. You may charge at C/3 (fast charging?), but it still requires prompt termination (no trickle charging after it is full). The reason it has to be a precision charger, is it drives the device to the edge of stability. That's how you "pick" the capacity, by determining "when it starts to smoke".
Lithium Iron Phosphate is a bit more tolerant. Some of those have been powered by tiny solar cells (trickle the current in continuously), without them catching fire. But those also have lower energy density.
The unit of C is scalable. It doesn't matter how big the battery is. If C was 3000 maH, C/3 charge rate is 1000ma or "The key advantage of the lithium-titanium based technology is the high speed charge and discharge. Typical charge and discharge rates are defined up to 5C (in comparison with 0.5C for LFP). The peak discharge rates are up to 10C continuous"
And that is what the Proterra transit bus uses. A blade charger lowers onto the roof of the bus, and the bus charges at 500kW. Their charge cycle doesn't quite fill the bus battery full, but it does achieve a charging interval of ten minutes, before the bus drives off. It's a bit better than the car characteristic.
An ultracapacitor is not a battery (a battery has a non-linear curve, the ultracapacitor is linear and voltage drops at a constant rate when discharged at a constant current draw). But compared to a battery, it charges at 2000C. Those can do a million charge cycles, but aren't practical for vehicles (except for drag racers). The limitations on those would tend to be thermal. Even with wide tabs on each cell, the "peak" rating for the device implies you'd come close to burning the tabs off. Dropping a screwdriver across a suitcase full of those, would be "very bad for your screwdriver". They sell a six-pack of those (sort of looks like an automotive battery), to people who put a 1kW sub in the boot, and the device "buffers" the car battery,and improves the battery impedance characteristic. When the sub thumps, it thumps with authority.
The charge rate, in units related to capacity C, gives you some idea how whizzy an item you are dealing with. Or potentially, how dangerous it might be (ultracaps would not be fitted with protection devices if you really wanted to harvest their characteristic properties).
Automotive batteries don't have protection devices, because you "want those 600 cranking amps". A buddy in uni, was wearing a loose fitting watch with metal strap while working on his Mini, and the strap somehow shorted the battery. This is why they tell you not to wear metallic jewellery when working around such things. His wrist had the link pattern of the watch, burned into his wrist. You don't wear the watch while that heals :-) Once you see an injury like that, you'll have new respect for the battery in the car.
Paul
That amounts to single stage constant current, with a complex termination characteristics unrelated to absolute cell voltage.
Nickel metal hydride is similar.
Now, at one time, I used to read articles (not from that site), claiming one chemistry was NDV, the other was plateau, and the claim it was plateau was because nobody could observe the small amount of NDV :-)
That means, for precision charging (charging not based on time, as a very crude indicator it is done), is best done on individual cells. It's easier to spot the signal for the noise, when working with individual cells. But it still requires a "fast charge" NiMh, to ensure the NDV is the mentioned 5 millivolts or so. That's why the charging method has multiple terminations, including "time", if you blow past all the other termination events (max temperature).
All the observations are from the same root cause. The temperature rises when the cell is full. The pressure rises. The voltage behavior is affected by the pressure inside the cell. They're trying to pick a proxy for the physical effects, without measuring the physical effects.
If NiCd or NiMh had pressure sensors and two terminals on the outside of the device, that might allow endpoint determination. Or using thermistors (a bit more practical), you could measure the temperature inside the cell. NDV as a proxy, is fine, as long as the cell and charger, can work within the limitations.
There used to be some 8AH D-cell sized NiCd for sale. Why didn't I buy some ? Well, they weren't fast charge (it would not be reasonable to pour a lot of amps into them). No charger would ever be offered, on the same web page as the cells. How many charge cycles would you get out of a thing like that, without an intelligent charger ? I think development would have continued on those... if it was not the end of the NiCd era.
Paul
That was *very* well explained; many thanks, Paul.
Well there are NiMH PP3s (the little 9V batteries) which have to be recharged with the cells in series as they are made with (presumably 6) cells in series.
Nickel cells can be left on trickle charge indefinitely when placed in series. They all end up fully charged
But are there any fast chargers for PP3s?
Seven actually.
1.2 7 * . 8.4 ok
Of course. Any delta peak charger with adjustable charge current will work .
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