Different use by device of NiMH and alkaline cell

[krw]

It'll allow the device to get the maximum life out of the battery without damaging the battery or the device. It's a good thing, IMO.

[The Natural Philosopher]

There is no such thing.

You can keep discharging a battery forever down to the last picovolt, but it still wont be fully discharged

It may be terminally f***ed though.

#

Well there you go. My point is proven. Not even up to the quality of insult I experienced as a 7 year old.

You cant even get that right...

[Johnny B Good]

Obviously, the device is a digital gadget *designed* to be powered from a single primary or secondary cell over the voltage range circa 1.6 down to 1.0 volt.

Like my now ancient but still working Nikon Coolpix 3Mpxl, P&S camera which was designed to cope with either a pair of AA NiMH or Alkaline or a single Lithium primary cell shaped to fit into the 2xAA battery compartment, it needs to be configured by the user as to which (in this case) of the three possible battery types it was being powered from in order to accurately assess how much remaining battery life was left when the voltage started reducing so as to safely cease operation whilst avoiding doing so prematurely.

The need to 'safely cease operation' arises out of the fact that there is obviously some form of File System in use for storing the recordings which would almost certainly become corrupted by a sudden loss of power.

Apart from the lower starting voltage of a NiMH cell (1.35v) compared to a fresh Alkaline cell (1.6v typical), there is also the huge difference in the discharge voltage plot.

The NiMH cell, like its predecessor, the NiCad cell, has a very flat curve dropping fairly quickly to 1.33v before levelling off to a very slow decline for a good 90% of its useful discharge cycle ending at the

1.2v mark whereby it may only have 5% or less of its stored energy available in rapidly dropping the final 200mV to the end point voltage typically chosen for the classic Alkaline/Zinc Carbon primary cell, ie one volt per cell.

The Alkaline cell has a discharge curve similar to its predecessor, the humble zinc carbon cell. This is one whereby the voltage starts off at around 1.6v then gradually declines at a pretty steady rate down to the

1v per cell end point. there's no "almost flat voltage plot" as with the rechargeable cell equivalents (NiMH or NiCad) so a decline down to 1.2v doesn't imply the imminent collapse of voltage as it would in the case of a rechargeable cell.

The power management circuitry incorporated into the recorder needs to be configured by the user since there's no algorithm for the power management to figure this out automatically with sufficient reliability.

A worn out NiMH may well look like a part used Alkaline cell or vice versa - it's best for the user to configure the cell type since the power management can then more accurately assess the condition not only of the state of 'charge' but also the serviceability of the cell in question and accurately reject flat Alkalines or worn out NiMH cells).

Be thankful that this option to select 'battery' type is available since it proves that 'battery power' was not added as an 'afterthought' to something originally designed to be powered from a wallwart like a DAB radio. :-)

[Johnny B Good]

Back in the late 70s/early 80s, consideration was being given to improving reliability of NiCad powered devices by replacing a battery of say 5 AA Nicads with a single D cell to provide the nominal, in this example, of 5volts by using a high efficiency low input voltage switching inverter (circa 95 to 97 % efficiency at 50 to 100mA tops) to power the battery operated portable gadget.

The idea was to eliminate the oh so common hazard of reverse charging the weakest cell in a string which would reduce the capacity yet further, swiftly aggravating this condition with each fresh attempt to recharge and squeeze the *stated* capacity out of the battery, resulting in the whole battery being rendered worn out despite the remaining cells more than likely having better than 80% of their original capacity left.

The problem wasn't too bad when *only* a pair of such cells needed to be employed since the task of detecting when the battery was in danger of reverse charging one its two cells was considerably eased, allowing the power management to shut off the equipment well before there was any danger of damaging the weakest cell.

A two cell NiCad battery only provides a usable voltage in the range of

2.7 down to 1.3 volts which for most portable kit of the last century was insufficient without some form of low input voltage high efficiency switching converter. Unfortunately for the end users, this was an idea that was regarded as a needless manufacturing expense by most makers of domestic battery powered kit (it was far cheaper to simply incorporate a 3 or 4 cell AA battery compartment and have their customers shell out that little bit extra on batteries).

After almost four decades, I expect the advances in switching voltage converter technology have finally reached the stage whereby it is now possible to supply a hundred milliamps or more at 5 or 6 volts using a single 15AH D cell instead of resorting to a battery pack of four AA or C cells to do the job much more expensively.

Even if you can only purchase your D sized NiMH cells in pairs, it would be reassuring to know that you will be able to get the full service life out of each one without any risk of reverse charging damage that would otherwise see you chucking away a whole set of four cells for the sake of just one bad cell.

The biggest "Fly in The Ointment" with such a scheme, lies with the problem of high resistance contacts inherent with the classic "Torch Battery" design of AA, C and D cells which is aggravated in the case of rechargeables being expected to survive some 500 to 1500 charge/discharge cycles ignoring the inevitable deterioration of the contacting surfaces which one shot primary cells neatly avoid by the simple fact that each fresh set of primary "Torch Battery" cells comes complete with a brand new set of untarnished contacts. :-(

[Johnny B Good]

Nicads and NiMH can be discharged completely, unlike Lithium secondary cells. The problem with NiCad and NiMH is the risk of *reverse* charging the weakest cell(s) in a battery string due to trying to squeeze the battery pack 'dry' when used in kit that makes no provision to prevent over-discharging the battery (typically something designed primarily to be powered from primary cells such as a torch or a portable radio designed near the end of the last millenium (either side of the transition into the third millenium).

Flattening a single Lithium cell below somewhere around the 2.4v mark is a no no. Completely discharging NiCad and NiMH cells individually is perfectly fine as long as you're not trying to flatten a whole battery's worth of them via the end terminals of a string of cells. If you can seperate the cells to discharge them individually, that's fine but for something like a NiMH 9v PP3 where you can't separate the individual cells, you can't 'flatten it' without damaging a cell or three from the reverse charging effect on the lowest capacity cells.

Even if all the cells used to make up such a sealed battery pack start off perfectly matched in capacity, it doesn't take too many charge/ discharge cycles for variations to occur and the risk of reverse charging to become ever more real. Trickle charging at the C/30 rate can help mitigate imbalances in the state of charge of the cells but even this is not enough to keep the effective capacities of the cells balanced.

Eventually, even when avoiding the hazard of reverse charging, the whole battery will become compromised by the weakest cell in the string which will determine its effective service life. There's a lot to be said in the favour of a "One Cell 'Battery'". Indeed, this is exactly the solution for a lot of modern day Lithium Cell powered gadgets such as Smartphones, Tablets and so on which use just a single prismatic Lithium secondary cell.

Here, of course, you have the advantage of the rather higher cell voltage (typically regarded as being nominally 3.6 or 3.7 volts - the equivalent of a 3 cell NiCad or NiMH pack) which is high enough, even after high efficiency switching conversion to a lower voltage level, say

3.3 volts, to drive modern electronics, including the 2 watt TX stage of a mobile phone. Also, another factor driving the 'Single Prismatic Cell Solution' is the fact that you can pack a little more energy storage into a given volume of 'battery space' compared to a two or more cell battery pack. This improved 'energy density' is further enhanced by elimination of the individual cell voltage monitoring chip(s) normally built into multi-cell lithium battery packs since the device itself can monitor every single cell in a mono-celled 'battery'. :-)

[Johnny B Good]

If you think about it (a digital recording device that obviously has some form of file system built into it, just like the case of the digital camera), it's more than just "A Good Thing", it's an *essential thing* to avoid unnecessarily frequent file corruption events due to sudden loss of supply voltage.

The device, just like that 12 year old Nikon Coolpix of mine, needs to be able to get a useful life out of the possible battery types it has been designed to operate with without being 'caught short'. I doubt you'd need to reprogram the battery type each time you replace the 'battery' since it's almost certain to have a tiny primary lithium cell or a super cap to maintain the real time clock and associated 70 odd spare registers available to store the various settings, including the battery type last programmed.

At least that's the strategy used by Nikon with their 3MPx Coolpix P&S camera until said lithium cell expired after about 6 or 7 years which I eventually got round to replacing about 6 months ago by wedging a slightly larger replacement cell into the camera body to save having to go through the date/time setting procedure on every power cycle. Indeed, that repair remains good despite the soldering iron abuse to the replacement lithium cell.

I'd had to locate it remote from the ridiculously tiny space allocated to the original cell on the circuit board using fly-leads soldered to the smallest lithium cell I could readily get my hands on - hopefully, assuming no lasting damage due to soldering the fly-leads, this larger cell should last a decade or longer due to its significantly greater volume. :-)

[The Natural Philosopher]

On 15/04/17 03:53, Johnny B Good lied:

No, they cant.

Nicds sort of recover. Nimh are dead dead dead.

Li-ion catch swll up and may catchfire.

The problem with NiCad and NiMH is the risk of *reverse* charging

More an issue with Li-ion

[therustyone]

There is a commercial version of the joule thief

Formerly known as the "batteriser" (which sounds a bit like a Scottish health food processor). Whether it's successful seems to be debatable.

[Max Demian]

Back in the 70s I had a pocket calculator with a vacuum fluorescent display rather than LEDs, powered by two AA batteries (alkaline or NiCad or mains PSU/charger). Since VFDs require an 18V HT, there must have been some kind of step up converter.

[charles]

NiCad in the 70s?

[krw]

Sure. NiCds were invented 100 years ago and in commercial production since at least the '50s. I had GE NiCds in the mid-'60s[*]. By '70 NiCds and plug-in charger modules were generally available in drug stores.

Sorta like these:

[krw]

No, he's right. As long as the aren't reverse charged (which happens in multi-cell batteries during discharge) these can be completely discharged. Actually, NiMH are unique in that they prefer to be stored discharged.

[The Natural Philosopher]

I was flying with them in model planes in the 60's

NiCd is WWII technology.

[krw]

Not essential (as in life supporting) but a good thing. Consumer intervention required doesn't make it "essential", IMO.

That's the way it should be (or flash/eeprom) but you're not even certain of your expensive Nikon.

Soldering Li cells? Uhhhh...

[The Natural Philosopher]

No, he is wrong

"NiCd chemistry prefers to be run completely flat, under load, to a loaded voltage of 1.0 volts per cell. If you plan to store the cell, you can short out the + and - terminals and store a NiCd cell that way. After storage, a cycle or two of charging and discharging will bring the cell back to its normal condition.

NiMh cells are run down to 0.9 volts per cell under load. Once the load is removed, the voltage will spring back up to 1.1 - 1.2 volts.

If you end up with a NiMh cell that is showing 0 volts resting, the cell is damaged. In an over discharged state, the electrodes inside the cell corrode. This results in impurities contaminating the electrolyte, and a loss of active electrode material. The cell is damaged. The longer the cell sits in this over discharged condition, the more damage is done."

This accords exactly with my experience of NiMh batteries in RC transmitters.

If youu leave em swtiched on, or leave em for more than a year switched off, you will never get the cells back.

With NiCd sometimes yoou did.

As long as the aren't reverse charged (which happens

Nope.

[tony sayer]

In article , Neon John scribeth thus

Thats not a bad old meter I've got a 8050A and its fine c/w batteries. The only problem with them these days is the LCD screen can be a bit faded but several, see U tube, have done replacement screen adaptions which do look very good.

Still use a model 45 Fluke, excellent dual display unit....

[whisky-dave]

I think I still have one, did it make a crackling sound when switched on mine was a pale blue dispaly IIRC. mine ran from 4 AA or a PSU.

[Max Demian]

No. why should it?

Mine had a blue display behind a green filter.

Can't have been very pocket sized with four AAs.

[Martin Brown]

You needed big pockets! HP35 and TI51's came with a belt clip case which would eventually fail dropping the thing on the floor. ISTR the battery pack on them was 3xAA equivalent in rechargables.

I found a "pocket" Casio calculator of that era or slightly later clearing my parents house. I have yet to test it. The previous ones having gone. I'm pretty sure it was the blue fluorescent type.

I had at one time a 10xnixie and thyrotron desk calculator and a purely mechanical one before that. On a good day I could do sqrt on it!