Fully charged new Ni-MH battery gets used up in seconds

[niemand]

I purchased a new FDK Ni-MH "A" cell to replace the battery in an electric toothbrush (Oral-B Braun Professional Care SmartSeries 5000). In removing the original battery, I left the original tabs in place by carefully ripping the tabs off the battery terminals. Then when I put in the new battery, I placed it in such a manner that the original tabs would rest against the new tabs on the new battery with some tension--no solder used at all.

I fully charged the battery and test ran the toothbrush a few times with the new battery fully charged, about 6 to 10 seconds each time, and it ran fine each time. When I was just about to put the insides back in the housing, the display showed the battery needed recharging. It took 12 hours to fully charge the Ni-MH cell but only 30 seconds of usage to drain it. I measured the battery voltage to be 1.40 v.
Could the battery be bad? I purchased it from Digi-Key a couple of weeks ago; the date code is "SEP-2020".
Could this problem be due to not using solder on the battery terminal connections (perhaps intermittent arcing during motor vibration causes battery drain)?
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[CaptDon]

What do you mean 'used up' As in the motor slows to a stop?? Isn't this the same unit with wires getting hot and a motor that seems to draw about 5 times to much current??

[niemand]

What do you mean 'used up' As in the motor slows to a stop?? Isn't this the same unit with wires getting hot and a motor that seems to draw about 5 times to much current??

"Used up" meaning "spent", "drained". The battery needs recharging to be used again.
This is not the same unit with wires getting hot on a struggling motor.

[james_s]

You need to either measure the current the motor is drawing, or measure the capacity of the battery. Voltage alone doesn't tell you much, many chargers have a capacity measurement mode that will cycle the cell and display the actual capacity.

[IanB]

I fully charged the battery and test ran the toothbrush a few times with the new battery fully charged, about 6 to 10 seconds each time, and it ran fine each time. When I was just about to put the insides back in the housing, the display showed the battery needed recharging. It took 12 hours to fully charge the Ni-MH cell but only 30 seconds of usage to drain it. I measured the battery voltage to be 1.40 v.

1.40 V is more or less fully charged. If the battery needed recharging it would measure around 1.2 V or so.

Most likely the poor electrical contact caused by lack of welding or soldering has confused the electronics into thinking the battery is drained when it really isn't.

[Mechatrommer]

make sure there is no contact shorted between:
1) motor's body and its tabs (battery terminals)
2) battery's +ve tab to its body (gnd)

maybe you can run off from pieces of wires to debug, you may start to buy measuring instrument such as DMM to see how much current draw etc. btw, maybe its also time to call it off, its been 8 years of service, if motor's carbon is used up, further attempt to repair maybe costly more than the toothbrush itself, i bought a pair of "Braun DB4010 Oral-B Advance Power Electric Micropulse Toothbrush" about a year ago, i see the cost each is about the same as your A size nimh battery... but i'm not sure its original or clone, but it works wiggling i guess like yours.. i'm surprised after a year, i havent changed its pair of AA batteries. ymmv.

[niemand]

[...] btw, maybe its also time to call it off, its been 8 years of service, if motor's carbon is used up, further attempt to repair maybe costly more than the toothbrush itself, i bought a pair of "Braun DB4010 Oral-B Advance Power Electric Micropulse Toothbrush" about a year ago, i see the cost each is about the same as your A size nimh battery... but i'm not sure its original or clone, but it works wiggling i guess like yours.. i'm surprised after a year, i havent changed its pair of AA batteries. ymmv.

This toothbrush had been sitting in its original box unused until I took it out last year to replace my dying Professional Care 9000 which was around 15-16 years old. So this SmartSeries 5000 has seen only about 1 year of use.

[bdunham7]

Your cell may have high capacity but not a high discharge rate.  This would cause the voltage to drop excessively under load and the toothbrush BMS may look at that, not OCV.  You do need to attach the tabs, but also measure how much current the toothbrush draws and at what voltage drop and compare that to your cell spec.

[Mechatrommer]

so have you checked the spring is not shorting the battery +ve tab and its metal body?

[Gyro]

Three threads for one knackered electric toothbrush? 


[Edit: Or is that two knackered toothbrushes in the space of a couple of days?]

[niemand]

Three threads for one knackered electric toothbrush? 


[Edit: Or is that two knackered toothbrushes in the space of a couple of days?]

Two toothbrushes, only one of which is knackered as of now because of its apparently bad motor. Three different questions because they are different concerns. Have I done anyone wrong?

[Fraser]

Having done this battery replacement myself many times, you must solder the battery to the contacts in the toothbrush. Any intermittency here will cause the battery monitor to declare the cell discharged. The current draw from the motor effectively causes a brown-out on the controller power rail.  The current involved in the charging of the cell is low so any resistance in the contact points is tolerated. The higher current draw of an electric motor reveals any resistance or intermittency in those contacts though.

Also, place insulation tape over the damage that you have caused to the cells heat shrink

Fraser

[magic]

Given that there are holes in those tabs, I would try to join them with a small bolt and nut. Seems less risky than sticking a hot iron in there.

[niemand]

Given that there are holes in those tabs, I would try to join them with a small bolt and nut. Seems less risky than sticking a hot iron in there.

Hmm...The holes exist only on the tabs of the new battery. I would have to somehow drill a new hole on each of the tabs left from the original factory battery.

[BeBuLamar]

Did you ever try to spin the motor by hand and see how hard it is to spin it? It should be very easy. If not you have mechanical binding.

[niemand]

Did you ever try to spin the motor by hand and see how hard it is to spin it? It should be very easy. If not you have mechanical binding.

You are referring to the 16-year old Professional Care 9000 with the struggling motor on the other thread. I answered the same question there.

This thread concerns the SmartSeries 5000, which doesn't have the motor problem.

[Mechatrommer]

Three threads for one knackered electric toothbrush? 

the deal breaker is, no progress in prognosis. which forced me to chime in as humble as i can, hint... V = IR

[jpanhalt]

Check out the construction of NiMH batteries.  Good picture here: https://en.wikipedia.org/wiki/Nickel%E2%80%93metal_hydride_battery#/media/File:Nimh_disassembled.jpg

The case is the negative electrode (as with many batteries).  Hence a connection between the case and positive electrode will be a short.  Now look at your second to last picture of the positive electrode connection.  It seems possible that spring is making contact through the tear in the case and positive electrode. 

I agree on the need to ensure a good electrical connection either by soldering or spot welding the new battery.  Soldering a pre-tabbed battery to stubs of tabs left from an old battery is quite easy to do.  Just be sure nothing from the positive terminal has any chance of making electrical contact with the battery case.

[MrAl]

Three threads for one knackered electric toothbrush? 


[Edit: Or is that two knackered toothbrushes in the space of a couple of days?]

Two toothbrushes, only one of which is knackered as of now because of its apparently bad motor. Three different questions because they are different concerns. Have I done anyone wrong?

It looks like it could be the thread count police at work here ha ha.  Some people think they have to meter people's comments for some reason.  You cant ask more than one question or you go to jail :-)

But anyway, it sounds like it could be a problem with the contacts as you think.  If it worked before and doesnt work after the mod then it could be that.  You could test the battery yourself if you have some test equipment.  You would connect a resistor to the battery and measure the voltage and see how long it takes to run down.
1.40v may not be a problem though so it may just be that the contacts do not make good enough contact.
A battery that takes 12 hours to charge could take that long even if it was not discharged it depends on the charger model itself too.  Some will detect that it is full and not charge as long while others will go the whole time.

So to find out what this really is you might have to do some better testing.

[capt bullshot]

First, as others suppose, solder the tabs to the battery to ensure good conductivity

Second, NiMH batteries exist in a variety of specifications - e.g. ones that are good at low current usage and maybe high overcharge tolerance, and e.g. others that excel at high peak load currents, have low impedance but must be treated with extreme care to not degrade, and maybe ones that are mediocre to good at most disciplines.
You need a battery that has a very low impedance and can deliver high load currents for rather short time intervals (2 ... 5 Ampere for 2 minutes or so).

[tunk]

Your cell may have high capacity but not a high discharge rate.  This would cause the voltage to drop excessively under load and the toothbrush BMS may look at that, not OCV.  You do need to attach the tabs, but also measure how much current the toothbrush draws and at what voltage drop and compare that to your cell spec.

Do you have the specs for it? If the runtime is e.g. 40 minutes then
it should draw ~4A (2700mAh cell). According to the digikey page
the discharge rate is 490mA, and there's a graph in the datasheet
with 1000mA. So it may not be able to provide enough current.
https://www.digikey.si/en/products/detail/fdk-america-inc-a-member-of-fujitsu-group/HR-AUT/1203032

[niemand]

make sure there is no contact shorted between:
1) motor's body and its tabs (battery terminals)
2) battery's +ve tab to its body (gnd)

maybe you can run off from pieces of wires to debug, you may start to buy measuring instrument such as DMM to see how much current draw etc.
[...]

I am getting a short between the motor's body and each one of the motor's own terminals at the solder points on the circuit board. I think this is correct behavior, no?
I am also getting a short between the motor's body and the battery's +ve terminal.
I am not getting a short between the spring and the motor's body.

I need to learn how to measure current draw. If I have to run the toothbrush to measure current draw, I'll need to charge up the battery first; the toothbrush display is currently flashing "empty battery" symbol.
My Kaiweets HT118A meter has µA, mA, and A settings on the dial.

[niemand]

so have you checked the spring is not shorting the battery +ve tab and its metal body?

I am not getting a short between the spring and the motor's body.

[james_s]

You'll need to use the A range to measure Amps, and then you open the circuit and wire the meter in series with the battery. DO NOT attempt to measure current across the battery, that is a classic rookie mistake that blows meter fuses.

[niemand]

[...] it depends on the charger model itself too.  Some will detect that it is full and not charge as long while others will go the whole time.

[...]

The only charger I have is the one that came with the toothbrush, just a little stand that looks like a loop where I set the toothbrush after use: