NiMh minimum storage voltage

[AnthonyL]

I've received an appliance (actually a warranty replacement) which appears to contain 2 x 1.2v NiMh batteries - see

as the batteries are not intended to be user replaceable

Two previous units have just died, or rather, have lost their oomph.

Am I correct that I should be able to put a multi-meter across the charging terminals and expect to get 2.4v when fully charged?

The voltage on delivery read 0.91v and I wonder whether this is too low, representing poor storage. I'm told it is new stock.

I'm now 20hr into the instructed 24hr initial charging prior to use and just getting 1.4v.

[Max Demian]

Is that while charging, while operating, or what? Does it actually function after 20hr charging?

[The Natural Philosopher]

You should get a fair bit more tan that. I think nearly 3V

My 8 cell units peak out over 11V, but never quite make 12 :-(

It is low. But you should get away with it. If that is in fact the voltage for a single cell., If its for the pair I would send them back.

For te pair? return them

[Harry Bloomfield Esq]

AnthonyL submitted this idea :

Likely cheap Chinese cells, send it back again. Yes, fully charged two cells in series should show close to 2.4v. In parallel, or singles -

1.2v

[AnthonyL]

As stated the 1.4v was a check after 20hrs of charging. It's now just over the 24hrs and reading 1.54v.

I'm just putting the multimeter across the input terminals where the charging plug would go.

It functions not satisfactorily, insufficient pressure as per the one it replaced.

[AnthonyL]

The guide linked to above shows a 2.4v battery which is just two 1.2v soldered and wrapped together.

If putting the meter across the charging terminals is valid then the now fully charged 1.54v reading is consistent with the lowish pressure rather than jet that the new unit is providing.

[Paul]

You know the charging curve, cells come up over the knee fairly quickly. The meat of the charging curve should happen with a significant part of the voltage present.

This is the charging curve.

I would say your symptoms indicate one good cell plus one dead-short cell. When you contact them, make sure the next unit sent is "factory fresh". Or, the same thing will happen on the next one. This is not an accident. All the ones in the stockroom could be this way, if they're five years old.

Paul

[Robert]

Measuring the voltage at the charger input may be low due to circuitry tween plug and batteries. Need to measure across the cells themselves

[Harry Bloomfield Esq]

Paul has brought this to us :

Have we established whether or not the charger is actually charging?

[Brian]

NiMH cells are, in my experience, pretty long lived - I?ve some I bought nearly 20 years ago and they weren?t expensive ones. I top them up every month or so.

That said, they do fail and yours sound duff. I don?t recall one even taking that long to recover to 1.5 when initially fully charged.

Unlike NiCads, which sometimes respond to a BRIEF ? burst? of high voltage, followed by a normal charge cycle, I?ve never recovered a NiMH cell successfully.

[Tim Streater]

That's the trouble. They don't hold a charge unless they are Eneloops or similar.

[Paul]

I'm not sitting there with product in hand, but I would say the "charger" consists of transformer, one diode, one resistor and no "off" switch. It just keeps charging for as long as it is plugged in. It's the "C/20" method of battery abuse :-)

Circuits like that are pretty robust, and you'd have to drop it on the floor... and hit it with a sledge... to stop it.

If the unit charged in 2 hours (fast charge), then there would be an IC and there would be more options for functional failures. And the fast charger, complete with its "auto-off", would result in the batteries lasting for six or seven years.

When appliances have no automation inside (dumb as a post), you know that was done to make the product wear out and be disposed of faster. It's not like the manufacturer doesn't know how to make a quality design.

I had a Black and Decker battery screwdriver, that was designed this way, and it was just a "ruiner of batteries". The electrolyte would come out, and so on, from overcharging. I have another screwdriver that needs to be re-celled, but... if I put cells in it, how long will it last ? That "resistor trick" is just death to DIY repair. You would be re-celling every six months.

The advent of Lithium cells, with their safety issues, you might notice that now the manufacturers magically know how to design a charger. It's a miracle. If they use the "resistor trick", sooner or later there would be a fire and a law suit.

Paul

[AnthonyL]

I did and was assured the unit I was receiving was from new stock.

I've sent them all the readings and a video of the unit taking nearly

3 times as long to empty as their spec. All they've done is offer me another replacement.

My plan now is to break into the unit and check the batteries directly and probably try putting in Eneloops. But I've got to mess around de-soldering and rebuilding little connectors from what the internet videos show. Really quite unecessary to construct the unit with this level of inbuilt complexity.

As an aside the last electric toothbrush I purchased has replaceable batteries and I've been using 2 x AA Eneloops for about 2 years, recharging them monthly. Brilliant and sustainable.

[AnthonyL]

All I can say is there is 5.3v at the charger, and I've tried two different chargers.

How else do you suggest I check?

[AnthonyL]

The charger is one of those standard type of "wall wart".

AC/DC adapter Input 230-240v 40ma Output 3V = 100mA 0.3VA (as written)

Multimeter on the connectors gives 5.3V.

The first Waterpik I had went 10 months before it "ran out of oomph".

Its first replacement lasted 2 days before having poor pressure.

The replacement for that has never had pressure.

I'm not a household that usually has problems with battery devices.

[AnthonyL]

Once in a while I have an Eneloop that looks dead. I have a technoline computerised charger and that won't touch it, but then if I put a pair of batteries in one of the fast chargers I have, for usually no longer than 10 mins, it kicks back into life and I can fully charge it on the technoline.

[AnthonyL]

Thanks. I can't get to the cells, well, not without serious dismantling.

Any other way of getting around that problem? My multi-meter has a few options not that I'm that familiar with using them.

[AnthonyL]

ok, I've taken the unit apart and it's not the battery as far as I understand (I'm not an electrical/electronics engineer).

It appears there is a diode in line with the positive input to the batteries. Measuring across the batteries shows 2.78v which I assume is quite satisfactory for 2 x AA

Just to be sure I've got this bit right I've uploaded photos to:

showing

1) Battery and in-line diode 2) Voltage across battery 3) & 4) Readings with multi-meter set to diode mode

It will be helpful if someone could confirm my conclusion on the electrics before I turn my attention to the water path.

[The Natural Philosopher]

any NiMh that has been more than 18 months in stock is dead no matter how new it is.

cheap.