Ideal rate is between 0,5C and 1,0C.
Going above 1,0C is not recommended for heat reasons, unless you have good passive/active cooling.
Below 0,5C means that termination can not be achieved correctly, and therefore, it actually hurts cycle life a bit more than at 0,5C. Not noticeable though.
TLDR: Optimal rates are between 0,5C and 1,0C. Going below isn’t bad, just not optimal.
For an Eneloop Pro (2500) that would mean charge rate of at least 1250 mA.
I don’t know too many NiMH chargers that can push more than 1000 mA. Does that make them bad?
Use a charger designed to charge NIMH AA and AAA
In a Sky NC2500 I set the charging to 0.5A for AA and AAA NIMH.
I don't charge them in chargers made for Li-ion even if the charger specs say NIMH supported.
Plenty of good chargers reviewed by HJK seem to terminate properly at rate significantly lower than C/2, good chargers also have a temperature detection in combinaison, for example the opus bt-c700 seem to have no issue at 400mA (halves to 200 at the end) or 200 (100) on AA eneloop. I’m not sure this recomendation is still valid, maybe before with less sensitive chargers.
It also doesn’t matter if the charger use voltage termination like the maha C9000, lii-500 and others.
While it certainly is true that there are good chargers reviewed by HJK that terminate charging of NiMH cells at rates below 0.5C, there are also very good chargers that might not, in some cases. For example, I just received a MC3000 this week, which HJK clearly adjudged to be an excellent charger. However, it is noted in the HJK review that it is best to use a charging rate of around 0.5C with NiMH cells in the MC3000, as it was found that the charging of these cells might not terminate correctly at rates below 0.3C.
I think all of the basic points raised by previous posts in this thread are 100% correct, but I just wanted to add the above. I think 0.5C is a good recommendation and perfectly fine for the health of NiMH cells. The new Eneloop cells are quite robust and can easily handle that rate in a quality charger. Of course, if you are charging an older, or well-used, or lower-quality cell, you should always take special precaution to err on the side of moderation in charging current rate, and use a quality charger with appropriate safety features.
A rate of 0.5C or around 1A usually works fine with NiMH and -dv/dt termination. Voltage termination is very problematic, it changes with the temperature and with the chemistry and not all cells uses the same NiMH chemistry.
As some people knowns I have designed my own charger (Requires computer control and have no display or other user interface, article will be posted when I have finished it). It has never failed on a -dv/dt charger (With 0dv/dt and a timed backup), but voltage termination (At 1.52V, ut voltage did not reach 1.48V) has failed on one battery.
When combining -dv/dt and 0dv/dt termination it is a question of time, the -dv/dt will terminate significantly faster than 0dv/dt (At low charge rate neither will work).
I did look at NiMh charging some time ago: Battery Charging NiMH
I wouldn’t go above 1C, they can get hot.
OK, I will never go over 1C, mostly I chrage AAA at 400-500mA and AA at 700-1000mA, depending also on the charger current selection and the capacity of the cell.
But after some experiments I saw if I charge a AA with 1500mA it stays cool for a long time, only at the end it will get hot. But some cells in some chargers get at the end hotter, even if only charged with 500mA, than the cell at 1500mA if not charged with this current to near the end.
So it would make sense to charge with a big current for most of the charging time but reduce the current a short time before the charge is finished. Should be possible if the charger decrease the current at a specific cell voltage.
Is it correct or am I missing something?
But only my Opus-chargers do decrease the current at the end automatically, my other chargers don´t do it and with most chargers it´s not possible to decrease the current while charging. With some chargers you need to eject and re-insert the cell, with others you can press and hold the Mode-button to change it but useally the charged capacity will be ressetted. Nitecore and some Xtar allow to change the current while charging and don´t reset the charged capacity. But it would mean you have to observe it to change the current.
I just always charge NiMh at .5c, and no lower than .33c so as to avoid missed termination.
Charging NiMH perfectly is like an artform. Like every charger and every battery is a little different. I hate it.
I’ve found with AAAs you kinda have to charge at 1C sometimes above if it’s low capacity AAA unless you can watch it like a hawk or the charger will just never stop unless it has some internal timer or heat sensor or something. And you can’t even do like a <0.1C 16 hour trickle charge because your charger doesn’t have a 75ma setting.
For AA’s 1amp seems to be fine. Because ya, how many chargers will do NiMH at 1250mah? I’ve never had a problem with a charger stopping a 2450mah AA in time at 1a but that same charger won’t stop a 750mah AAA at 0.5a. I think there’s more to it than just the C rate. But it’s different with every charger. That’s why I hate it.
0.5 C - 1.0 C
Rarely lower / Never higher.
Works with good cells but if you have something old/cheap cells it simply don´t work because it stays lower than 1,5V
Don´t know a charger which have 1250mA charge current but the Nitecore UMS-series, some Skyrcs, Powerex MH-C9000 and ISDT C4 EVO will let you choose 1200mA and 1300mA
And yeah, some chargers will terminate better at low currents than other at high current
Yeah i think it’s better to charge them in lower Ambs so they don’t get much pressure… i general… !?
Im charging my 18650 Li-ion batterys always at 0.5 Ambs on my XTAR VP2 so that they can charge more smoothly, plus i always have multiple spares batteries so i use them in a rotation… !
Probably it’s the same thing with all the battery types … !?
I also have a few AA Li-ion 1.5V 1600mAh with usb charging port, so i use a simple phone wall charger !
[quote=Johnny_Bravo]
Works with good cells but if you have something old/cheap cells it simply don´t work because it stays lower than 1,5V
[quote=Jeffgoldblum]
C
Could you explain “won’t work bec it stays below 1.5v?”
I always thought ideally it’s best (for the battery) to charge batteries of any type as slow as possible.
I have some older cells which reach less than1,5V, maybe 1,45V at the end, it´s the same in all chargers I use. Most of the charger don´t stop charging, the cells get very hot, I measured over 50°. The Nitecores have a good reputation also with NiMH but won´t stop charging in this case.
In theory the most chargers use -dV/-dT but it seems the voltage drop which is needed for this kind of termination will not happen. Maye a 0dV-termination would work with it but I don´t know which charger can do tgis.
I used to set at about 0.5 C on my old Lacrosse. It would miss termination occasionally and wrecked cells. Just let my MiBoxer do whatever it thinks it should do. It varies quite a bit in terms of the rate it will put into a cell but has never missed termination. Cells don’t get hot. Charge numbers appear to show very low charging loss. Seems way better than a fixed rate and in my experience has been safer for the cells.
i’ve tried so-called name-brand chargers (kodak, everready) and no-name chargers, and it seems that all of them will cook the cells and never auto-stop, even though it states that they will on the packaging.
The -dV/dT is nice in theory, but i have dismantled chargers and not found anything that looks like the sensors necessary to implement this approach, so i’m calling BS on this being used as auto-stop. Maybe the high-end chargers can do this.
It seems to me that overheating would be harmful to cell life, so i don’t let that happen anymore. But it means i have to babysit the charging session, and when they start getting warm i halt the charge. PITA method
What kind of sensor do you mean? Many chargers have a built in Voltmeter, some also a temperature sensor on the low side of the slot or in the cell contacts
Did it show the voltage of the cells?
I have a lot of NiMH, most of them works well with chargers but some cells not. These are the cells which stay on the voltage around 1,45V. Maybe I would never know this if I don´t had chargers with voltage-display.
I have also some chargers with temperature sensors but can´t remember a reaction at hot cells.
It does show the voltage but I honestly don’t really look at that much for the NiMH cells. IIRC, it seems to terminate right around 1.5. Cells fairly quickly drop down into the 1.4 something range. That will happen before I even get around to taking them off the charger. Haven’t really noticed a variation in charged voltage — 1.43 - 1.47 IIRC? The big thing is the IR. The higher the IR the slower the charger will charge. Cells with low IR will get a much higher charge rate. High IR cells will get a much lower charge rate. As I said, the cells don’t get hot in either case. The high IR cells just take a lot longer to charge and of course will not preform as well in a light.