Here's the follow-up to the ongoing debate on Ni-Cads, and how to treat them correctly. The Internet community generally took this as the definitive word. ---------------------------------------------------------------------- From: myers@fc.hp.com (Bob Myers) Newsgroups: sci.electronics.basics,sci.electronics.design Subject: Re: NiCds: 0V and - X Volts ok. Was: NiCad tester needed. Followup-To: sci.electronics.basics,sci.electronics.design Date: 31 May 1996 19:41:26 GMT Organization: Hewlett-Packard Fort Collins Site Richard Steven Walz (rstevew@armory.com) wrote: > The fully discharged state IS 1.0 Volts! That's what the manufacturer > meant! If they SAID 0 Volts they wanted to sell more NiCads!!!! > -Steve Um, no. Most manufacturers recommend a discharge cutoff for batteries of about 0.5-0.6 volts/cell, since by that point you've left the relatively flat portion of the discharge curve and are now seeing a rapid voltage drop with continued discharge. They're trying to tell you to cut off the discharge before you see much possibility of reverse-charging one or more cells. But as other posters have already pointed out, there is nothing inherently bad about taking a CELL to 0V. To quote the Varta NiCd handbook: "NiCd cells may be stored IN ANY CHARGE STATE" (emphasis mine). From the way this and similar threads have been going, I'm wondering if all concerned understand the difference between "battery" and "cell". (snip and weld) You like the Varta technical data, SO... Form the Varta "Sealed NiCd Batteries Product Range and Technical Handbook" (my copy's from 1987, but I don't think that chemistry has changed all that much in 9 years) - "Sealed NiCd batteries may be stored for years regardless of the charge state they are in." 1.0 V/cell is set as the end-of-discharge voltage in capacity curves based on a nominal discharge rate of 0.2C, but this does NOT mean that it is *necessarily* dangerous to the cell to allow its voltage to go below this level. (In a constant-current discharge at 0.2C, 1.0V might be a good point to stop, just because AT THAT RATE, you're going to pass through the voltages between 1.0 and 0.0 pretty quickly.) In fact, many of the Varta specs actually list permissable discharge rates to an end voltage of 0.7 or 0.8V. There's nothing especially magic about ANY of the points between 1.0 and 0.0 V, other than the fact that you ARE on a very steep point on the curve and better be watching the discharge carefully. Switching sources for the moment, to a G.E. technical bulletin on the supposed "memory" phenomenon: "Realistically, however, 'memory' cannot exist if any one of the following conditions holds: A. Batteries achieve full overcharge*. B. Discharge is not the same each cycle (+/- 2-3%) C. Discharge is to LESS THAN 1.0 VOLT PER CELL. [emphasis mine]" * - I wanted to point out that "full overcharge" in the context of G.E.'s not is NOT in conflict with the notion that overcharging is responsible for voltage depression. You would need to read the full note to understand what they're meaning by "full overcharge" here. So here's at least one source which not only isn't concerned about 1.0V/cell being a minimum, but actually is recommending discharge BELOW that point! Again, the 1.0V/cell point isn't magic. The few times that I HAVE seen recommendations for an absolute, thou-shalt-not-drop-below-this-point sort of end-of-discharge voltage spec, it's been more in the 0.5-0.6V range. 1.0 V/cell is simply a convenient point at which to stop drawing the capacity curves, but this should NOT be taken as implying that something bad happens as soon as you get below it. > to this!! It means you haven't read doodley, maybe for decades!! I'll > wait for Bob Myers to officiate this one, and do it my way. I have > many of mine at an extreme age now performing wonderfully! And they > don't and have never needed to be "deep discharged". That's a MYTH! 100% agreed. Intentional deep discharge is just a waste of time and energy, even if we could ignore the risk of reverse charging in the case of NiCd BATTERIES. Bob Myers KC0EW Hewlett-Packard Co. |Opinions expressed here are not Workstations Systems Div.|those of my employer or any other myers@fc.hp.com Fort Collins, Colorado |sentient life-form on this planet. From: rstevew@armory.com (Richard Steven Walz) In article <4p7hc9$ot4@fcnews.fc.hp.com>, Bob Myers wrote: > >You like the Varta technical data, SO... > >Form the Varta "Sealed NiCd Batteries Product Range and Technical Handbook" >(my copy's from 1987, but I don't think that chemistry has changed all that >much in 9 years) - (snipped) Thanks a lot, Bob, and I shall regard this as the definitive answer for this question. As you seem to say, past steep negative rates of change of voltage, just after the cell passes through 1.0 Volts and heads for 0.6V is an important place to stop so that it doesn't carry it down to or past 0.0V, because it is going so fast at that slope of voltage decline. But you say that there is no certain positive state of charge that is particularly dangerous to the cell, provided we don't hit 0.0 or below into "negative"-land or overcharge it to a dangerous voltage or current level. And you seem to indicate that discharging it to 0.0 volts with a resistor is unnecessary at very least, and even potentially problematic around 0.0 V's? At least if anyone reads all of Bob's commentary, I think we get this out of it, that: discharging to 0.0 is "a waste of time", has the "risk of reverse charging", has been seen by him to be below most recommended specs for discharge as follows from his post above, namely that: ">Again, the 1.0V/cell point isn't magic. The few times that I HAVE seen >recommendations for an absolute, thou-shalt-not-drop-below-this-point sort >of end-of-discharge voltage spec, it's been more in the 0.5-0.6V range." He said: "thou-shalt-not-drop-below-this-point... ...in the 0.5-0.6V range." I take that to mean for this discussion that: I stand corrected that the discharge region around 1.0V is NOT actually "sacrosanct", but also that those who recommend complete discharge to 0.0 V per cell are potentially exceeding good recommendations for their cell life! So, in short, I needed to loosen my spec a tad, and the short circuiters need to listen up and realize that they: 1) Need not care in what state a NiCad is stored disconnected, and 2) don't need and likely should NOT be discharging them to 0.0 Volts or leaving a resistor across them for any storage reason or pre-recharge ritual!! ----------------------------- I am partly vindicated, and so are they, then, but NOT to the extent of shorting these cells to try to deep-discharge them waaaay past a half a volt, which should likely be THE farthest down you MAKE them go, though they can stand safe storage at ANY voltage level they themselves achieve through internal discharge. I discovered that 1.0 Volts is no holy-grail for NiCad cells as well, and that they can be allowed to discharge some ways down past 1.0, to as little as 0.6 Volts or so. Can you guys live with this understanding now? Or are you going to discount a VERY seriously experienced senior engineer for HP? I can live with it, can you? Is that about right, Bob? (I Cc:'d him). -Steve Walz -- -Steve Walz rstevew@armory.com ftp://ftp.armory.com:/pub/user/rstevew mirrored: ftp://ieee.cas.uc.edu:/pub/electronics/faqs/ftp.armory.com and Europe: ftp://ftp.cised.unina.it:/pub/electronics/ftp.armory.com and Oz: ftp://ftp.peninsula.apana.org.au:/pub/electronics/ftp.armory.com [END]