Life of eneloop batteries (before they stop holding as much charge)

[NY]

We have eneloop 750 mAhr BK-4MCCE NIMH batteries (2x AAA) in our DECT phone handsets. This is the same capacity and technology as the OEM batteries which were supplied with handsets (BT 8500).

Until recently everything was fine, but now we've noticed that all the handsets suddenly cut off after about 20-30 minutes (phone calls to parents can last a while!) without any warning bleeps for "battery low" (*). Afterwards the phone doesn't even have enough charge to turn on. However often the battery-charge indicator will show full as soon as the handset is placed on the charger, and will then have enough charge to run for at least a few minutes.

It's as if the discharge characteristics of the battery are no longer what the handset is expected, so the voltage is going straight from what the phone regards as "charged" to what it regards as "dead" without gradually decreasing until it reaches the warning level, with enough remaining charge to keep going until we've had chance to get another handset to continue the call.

Is that likely to be a handset problem or a battery problem. The batteries are a year old (bought 5 Jan 2022) so they are not old. The packaging (I kept it!) says "After 1 year, retains 90% capacity and after 10 years, retains 70% capacity" and "Most suitable [for] DECT phone". Obviously DECT phones spend most of their time on the charger so the batteries (and the charging circuit in the handset) has to be tolerant of being permanently on charge, rather than being disconnected from the charger when they are full, as you'd do with most appliances.

It's interesting that all the handsets seem to have started doing this at roughly the same time; maybe the one that is most often used for calls is a bit more prone to it than the ones that are used less often so remain charged rather than undergoing discharge-charge cycles.I would imagine that even the frequently-used phone has undergone far fewer that the "recharge up to 2100 times" limit.

OK, so the packaging may contain a certain amount of marketing bullshit, but even allowing for that, these batteries seem to have failed very, very soon.

Any comments?

(*) That's for calls with speakerphone turned OFF: I know that speakerphone eats battery charge very quickly.

[Theo]

This seems to be a common problem, and one I've had with various different brands of batteries - this points to the phone being the problem.

One thought was that the batteries which are designed for capacity are not ideal for the never-running-flat mode of operation of a DECT phone, which is more about frequent cycling. I tend to buy Eneloop Lite, which have a better cycle life and a lower capacity, than regular Eneloop (the ones you have). I would rate 750mAh as 'high capacity' and I'd expect a regular DECT cell to be 500-550mAh.

Another thought is that certain brands of phone overcharge their batteries - I have Gigaset as prime suspect here, but yours isn't one of those (unless it's a rebrand?). The handset charging doesn't seem very smart.

At the end of the day, they would be much better using lithium cells and a proper BMS. But they don't do that. So we're left with batteries being a regular consumable.

Theo

[Rod Speed]

NY [e-mail] wrote

I had Eneloops, 2x AA, in my Panasonic KX-TCD735ALM cordless phones.

Mine were quite different in capacity to those supplied.

I got a similar result eventually, not even being able to answer an incoming call without the handset turning off when answering the call.

I found that replacing the batterys with Active Energy rechargeables from Aldi fixed the problem.

My Eneloops lasted much long than that.

Not clear that they are actually permanently on charge. The Panasonic KX-TCD735ALM does have a very smart charger and for a while I did use it to charge rechargables that the not very good charger would fail to charge.

Yeah, I got that result too, tho I only have two handsets.

Eneloops are notorious for not lasting as long as they should.

I'd just replace the Eneloops and see how that goes. You don't need the very slow self discharge in a cordless phone handset if you always put the handset back on the base when the call is over.

I almost never use the phone in other than speakerphone mode unless the other end says that the call breaks up, normally with the other end in a noisy environment.

[Michael Chare]

I have the Gigaset DECT handsets and have had similar problems even using eneloop batteries so an recommendations forn the best batteries to use would be appreciated.

I recently wanted to call HMRC regarding a tax return. There was a warning about wait time so I thought the battery in the DECT phone might go flat. I used a wired phone which fortunately has a loud speaker instead. After about 30 minutes I was able to speak to a human who told me how I could make a formal complaint about a problem with their website.

[Martin Brown]

The handset chargers are complete crap and destroy batteries by continuing to trickle charge them at far too high a rate. The result is that the battery loses its ability to supply current when asked to.

You describe the symptoms perfectly. Terminal voltage falls from nominally fully charged to too low to work shortly after the battery is asked to do any work. The very act of ringing can sometimes be enough.

It is a cheap and nasty charger problem. There is always a tension between keeping a battery ready to do the maximum amount of work and its longevity. Most DECT mobile phone chargers make a questionable choice.

They typically destroy a set of batteries completely every 3-4 years - although some are worse than others. One of my "identical" set of charges is less damaging to batteries than the others.

It is all the time that they spend on the charging stands and a very dumb charger that does the damage. You can't blame the batteries for this they are being wilfully abused by the equipment.

[NY]

Same here: I got out a wired phone for use when I was stuck in a long queue waiting to talk to the self-assessment tax officer to sort out why the amount of tax that I owed was *less* than expected.

When I'd filled in the tax return, I'd ticked the box saying that although my self-employment earnings were below a threshold, I wanted to pay voluntary National Insurance (so I maximise the amount of state pension I will get). And it turned out that there was a bug in their system: I know I ticked the box and it showed up on the PDF tax return that the online system sends you: I could see it in front of me as I was on the phone to HMRC. But the tax officer said it was not entered on their system, and she couldn't do anything about it, even after checking with her supervisor. She referred me to the NI people, which would have meant another long queue and then probably back to the self-employment people after that, because HMRC departments haven't got the hang of talking to each other, and everything must go through the punter. HMRC's level of customer service really is pathetic.

I've written a formal complaint and asked them to sort it (I enclosed the printout of the page in the tax return). But the Royal Mail tracking hasn't recorded that the letter has been delivered yet, after a week.

The UK really is going to hell in a handbasket at the moment.

[NY]

And you can't take the handset off the charger when you notice the battery is charged, because then it will gradually get used up as the phone listens to the base station for a call, so it will be low/flat by the time that happens.

The chargers really need to handle trickle charging and voltage monitoring properly, so they stop charging the batteries when their terminal voltage says "full" and then periodically re-check the voltage in case the batteries have started to discharge through normal usage (listening to base station). But keeping permanent charging current flowing through full batteries does not sound good for them :-(

[Clive Arthur]

<snip>

Gigaset here too. Mine were originally NiCd cells, I tried NiMH for a while but have now gone back to NiCd from eBay. They last a couple of years, and certainly longer that the NiMH.

[Brian Gaff]

What is happening is that the battery in effect has a smaller area of its electrodes working. Draw some current. the voltage drops away since its impedance is high. They are usually like this due to a poorly designed charger that cooks the cells once they are fully charged and this gradually means that the working parts get poisoned by overcharging. I've watched good chargers that simply test the voltage under load and pulse the charger. These seem not to cook the batteries. It is a bit of a bodge though, as to my mind there needs to be some kind of sensor system or a third connection that monitors things.

Brian

[Dave W]

My Gigaset A120 handset has two Duracell NiMH AAA rechargeables in it. I never leave it on the charger because it's bad karma, as you describe. I put it on the charger when the bars go down to one.

[NY]

How do you cope with the fact that sometimes when the phone rings or you come to make a phone call, the battery has discharged (due to the normal "listening to the base station" circuitry), so you need to charge it before you can make/receive a call, depending on when you need to use the phone in relation to your charge/discharge cycle throughout the day?

Why is it that DECT phones only last about an hour (less if "squawkerphone" is turned on) on a battery charge, but a mobile phone will last a lot longer on a call? Is it partly that mobile phone batteries have a third terminal for monitoring the terminal voltage, so can determine better when the battery is full so as to stop charging, whereas normal two-terminal AA batteries don't have that? Or is it that mobiles normally use Li-Ion batteries which perhaps are more tolerant of over-charging?

[Rod Speed]

Never seen that with my Panasonic KX-TCD735ALMs and I almost always use them in speakerphone mode.

Clearly not.

Nope.

[Paul]

Slowly filling a NiMh, makes it impossible to detect the endpoint.

When chemistries like that, the manufacturer says the cells can be charged at the 1.0 C rate, that makes it a lot easier to design a precision charger. The negative slope endpoint can be detected (negative dV/dT), compared to other noise sources in the circuit.

That article says, if your NiMh device is unwilling to charge the cells properly, you should switch to NiCd which is more tolerant of abuse. They are not infinitely tolerant, but more tolerant than NiMh. If you choose to use NiCd, check the battery compartment frequently for signs of electrolyte venting and corrosion.

Paul

[NY]

Can a device which is designed for NiMH use NiCad instead? If there is any end-of-charge logic, won't it be designed for the charge-amount versus voltage characteristics of one battery technology, so there is a risk of making things worse if the end-of-charge state never gets reached or else the charging stops too soon when the batteries are only partly charged.

[Martin Brown]

I think you can get away with leaving them off the base station for a few hours every now and then and it will help a bit but it would be nice if the charging regime of DECT bases was more battery friendly.

It is one of those situations where if you use your phone a lot and move them around so that each gets equal wear you can probably extend the battery service life. OTOH if they spend almost all their time on charge and are seldom used then the batteries will gradually be destroyed by overcharging. It is hard to damage a rechargeable battery at C/100 charge rate but eventually it will result in failure.

Next time I have chance I will measure what my DECT base stations do.

It is a bit like with UPS's - maximum runtime is a selling point but it comes at a cost (to the user) of frying the batteries more quickly. Then they make a profit selling over priced extra batteries every few years.

Mobile phone chipsets are very much more efficient than DECT and do only a minimal ping to stay in touch with their base station. In an area with very low or no mobile signal the battery life of mobiles is compromised as in drops like a stone. Spending all its time pinging at maximum power "ET phone home".

[Martin Brown]

If it is a smart charger then it won't be right. You are gambling that either it isn't or that the NiCad is better able to handle the abuse.

I recall an amusing historic MFU when NiFe cells were withdrawn from service use and the new improved NiCads were charged using the same SOP. The result was most of them used as emergency standby power were ruined within a year by the wrong charging procedure (big expensive batteries).

NiFe could stand any amount of abuse and still be OK, NiCad were a bit more tetchy.

[Andy Burns]

I have previously had early battery death from Philips DECT handsets, my current Gigaset is pushing 7 years old and still has original GP Battery NiMH 750mAh in both handsets and they live in their charging cradles 24x7

[Chris Green]

Yes, it does seem to be a bit random the way batteries last or fail in DECT handsets. We have a mix of different Gigaset ones and there seems little rhyme or reason to battery life. I do think that lower capacity (i.e. around 700mAh) NiMh seem to last longer. Our oldest ones are now 13 years old and I'm pretty sure I've only changed the batteries once in any of them.

[The Natural Philosopher]

yes.

If there is

Other way around. Both technologies look for a delta peak, but with NiMh it is rather small. Nicads work in NiMh chargers, not always the other way around.