Talk:Nickel–metal hydride battery

Power density numbers are wrong

The tagbox says 300-1000 W·h/kg while the lead says it's 75. I believe the former is correct. Maury Markowitz (talk) 01:46, 8 April 2010 (UTC)

You are confusing power density and energy density. Article seems ok.Greglocock (talk) 04:34, 8 April 2010 (UTC)

Removed memory effect section

The sole source (web page) for the section saying that NiMH's suffer from a memory effect no longer exists. I tried to find its new location on the same site, and actually discovered that the site suggests that NiMH's *don't* suffer from memory issues:

"A properly designed application with Ni-MH batteries will result in neither permanent performance loss nor perceivable temporary capacity decreases from this characteristic." Duracell

All other sources I found also said that NiMH's don't suffer from memory effects. MichaelBluejay (talk) 16:29, 12 May 2010 (UTC)

Linden and Reddy's Handbook of Batteries, 3e (2002), discusses the "Voltage Depression (Memory Effect)" in section 29.4.9.

"A reversible drop in voltage and loss of capacity may occur when a sealed nickel-metal hydride battery is partially discharged and recharged repetitively without the benefit of a full discharge."

The explanation given is that only a portion of the active materials are cycled, and the uncycled portion "change in physical characteristics and increase in resistance." This is consistent with the explanation by Sato, Takeuchi and Kobayakawa, Journal of Power Sources, 93 (2001) 20-24, where the authors report that a small portion of the positive electrode active material is converted to gamma-NiOOH in a reversible manner. Data from Duracell is also provided in Linden to show the effect and that it is largely or fully reversible (figure 29.13). Note that the Duracell site is talking about "properly designed application;" not "properly designed battery" or "technology." In my experience, the NiMH industry has attempted to distance itself from association with the better-known and more damaging memory effect found in NiCd. Tom Hopper (talk) 10:11, 3 March 2011 (UTC)

High voltage

What happens if I charge NiMH batteries with higher voltage. Or the voltage for AAA/AA batteries is a standart? Only the mA charging rate is different? —Preceding unsigned comment added by Leonardo Da Vinci (talk • contribs) 11:21, 21 May 2010 (UTC)

In general, nicad and NiMH batteries should be charged constant-current -- or something approximating it -- not constant-voltage. WilliamSommerwerck (talk) 23:46, 10 November 2011 (UTC)

writing and content problems

I started fixing the section on delta-T charge termination, then discovered the article has problems with writing (it's badly overwritten in places) and possibly misplaced content. I would rather discuss these with the original writer, before jumping in. WilliamSommerwerck (talk) 17:50, 19 September 2010 (UTC)

There are more than 500 contributors to this page, I suggest you go ahead and fix the style and rely on the Wiki feedback mechanism to tell you if you've gone too far. --Wtshymanski (talk) 13:22, 20 September 2010 (UTC)

Editing

As an example of how this article got into the mess stated above, please notice some anon IP desparately wants to add the following gibble-gabble to the article in the touching belief that it expresses the idea of cell reversal "more clearly":

This is because the current -shared and imposed by all the cells in series- creates a reverse voltage across each cell's internal resistance. This resistance increases as each cell is discharged, to the point that the reverse voltage at a discharged "weak" cell can become greater than its forward voltage

As a general rule, if you have a large clause set off by hyphens, you're not writing very clearly. All the gibble about "internal resistance" is unecessary to explain that if one cell gets reversed, the battery gets hosed, which is the point. We don't need hundreds of extra null words to express the simple idea. --Wtshymanski (talk)

and the "citation" that is supporting this gibble is talking about ni-cad batteries, not ni-mh. The citation is a Web page that is a summary of a paper, and doesn't talk about "internal resistance", "current sharing" and the rest of the gabble. --Wtshymanski (talk) 17:06, 22 October 2010 (UTC)

Well, well, well. This [1] and this [2] say that one of the particular *advantages* of the nickel-metal hydride system is its ability to be reversed (at moderate currents) without damage. And this [3] does talk about the dangers of cell reversal but without respect to current level; it also mentions cell resistance is substantially constant during normal discharge, contradicting the gibble gabble. And this [4] says prolonged reversal will result in hydrogen evolution and perhaps venting. --Wtshymanski (talk) 19:06, 22 October 2010 (UTC)

And now the offered citation "Solid state batteries" doesn't look any firmer, at least the bits I can see with Google Books. Pages 242-247 are part of a section called "Definition of full cell/battery parameters", and the accessible bits don't talk about cell reversal in general nor nickel-metal hydride batteries in particular. And the other reference is still talking about nickel-cadmium battery cell reversal, again, not relevant to nickel-metal hydride. The phrase "current sharing" in connection with a series string is also just wrong, there's no choice about current "sharing" since it's a series circuit. The proposed passage matches inaccuracy and obscurity with prolixity. --Wtshymanski (talk) 21:15, 1 November 2010 (UTC)

Gibble back in again, and none of the offered references support the "internal resistance" increasing. Wouldn't it be more sensible to quote some actual literature related to NiMh batteries instead of this highly original interpretation of two items that happen to mention "battery" ? --Wtshymanski (talk) 18:03, 2 November 2010 (UTC)

Claimed voltage of charged cell

Article has an unsourced claim that the open circuit voltage can be 1.4 volts and even as high as 1.5 volts. Real world batteries are unaware of this. Indeed the electronegativity of the materials used to construct NiMH batteries is such the highest possible E.M.F. is 1.37 volts. 86.176.69.42 (talk) 16:40, 17 January 2011 (UTC)

Verbrugge and Tate provide graphs showing OCV of just below 1.35 V one hour after end of discharge @ 90% SOC and just below 1.45 V one hour after end of charge @ 90% SOC. OCV values are shown ranging between about 1.22 V and 1.45 V depending on SOC and whether the previous step in testing was discharge or charge. See Figure 3 of Verbrugge and Tate, Adaptive state of charge algorithm for nickel metal hydride batteries including hysteresis phenomenon, Journal of Power Sources 126 (2004) 236 - 249.

Linden and Redy's Handbook of Batteries, 3e (2002), provides data from GP Batteries showing discharge voltage at the start of discharge ranging between 1.4 and 1.45 V (figure 29.4 and 29.5). Personally, I believe that this is a residual effect from charge (i.e. no pause between end of charge and start of discharge), but in any case it also fails to contradict the claim for high OCV. Tom Hopper (talk) 10:35, 3 March 2011 (UTC)

Patent encumbrance in electric vehicles

The article is worded in a way that implies that a single patent, granted in 1982, is still encumbering deployment of affordable NiMH batteries in electric vehicles today. That cannot be the case, because any patent granted in 1982 would have expired in the USA by 2002. If, today, there are still patents in effect which would encumber the deployment of affordable NiMH batteries in electric vehicles, then those patents must necessarily be newer than the 1982 date mentioned in this article. And if that is the case, then what are those additional patents? 24.222.2.222 (talk) 12:16, 18 February 2011 (UTC)

Amperage limited charging method using solar cells

I do not have the manuals from which I learned the amperage limited charging method. In this method, the charging rate is amperage limited, rather than limiting the voltage. A voltage at around 150% of the voltage of the battery (or series of batteries) is applied in reverse. The exact voltage is of small importance as long as it is not too high. The amperage must be limited to one tenth the capacity (denoted C/10) or less. NiMH batteries are designed to accept up to a C/10 level of amperage for charging. More than C/10 will cause hydrolysis of the electrolyte solution (which is KOH in water). This method is ideal for charging directly from solar cell panels, as solar cells are amperage limited in their output. Ordinary solar panels designed for 12 volt systems (output ~15 to ~20 volts) can be used to charge 12 volts series NiMH batteries as long as the current produced by the panel does not exceed C/10 of the battery . A 6 volt solar panel (output ~7 to ~10 volts) can be used to charge series connected NiMH meant for 6 volt electronics. As an example a AA NiMH battery rated at 2500 milliamp hours can be charged at an amperage of up to ~240 milliamps. A series of 2500 milliamp hour batteries uses 250 milliamps as well. With charging in parallel, such as four of these 2500 milliamp hour batteries in parallel, a charge of up to <10 amps may be applied. I used this method to charge batteries to run a short wave radio and a night lamp. It works well. My Flatley (talk) 00:42, 25 September 2011 (UTC)

Storage

What are optimum long-term static storage conditions for NiMH batteries? Not to maintain immediately available charge, but to minimize permanent capacity loss? Some sources say to store at about half charged, other sources say that for Ni chemistries charge state is not important. Cool temperatures seem desireable, but better not to freeze? If storing for years, would it be better to trickle charge at a very low level, to avoid full discharge? (batteryuniversity.com/learn/article/how_to_store_batteries) Different manufacturers seem to say rather different things: (www.powerstream.com/Storage.htm) There is general agreement that charging behavior after storage may be unusual the first cycle, and capacity may be regained over a few charge-discharge cycles. -96.237.13.111 (talk) 04:44, 27 November 2011 (UTC)