Fully charged new Ni-MH battery gets used up in seconds - Page 1
Forum section: electronics / beginners

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 01:23:51 am
content: 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)?" alt="" class="bbc_img" />" alt="" class="bbc_img" />" alt="" class="bbc_img" />" alt="" class="bbc_img" />

author: CaptDon
author_rank: Super Contributor
timestamp: September 08, 2022, 01:38:00 am
content: 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??

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 01:53:46 am
content: Quote from: CaptDon on September 08, 2022, 01:38:00 am"Used up" meaning "spent", "drained". The battery needs recharging to be used again.This isnot the same unitwith wires getting hot on a struggling motor.

author: james_s
author_rank: Super Contributor
timestamp: September 08, 2022, 01:59:13 am
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: September 08, 2022, 01:59:25 am
content: Quote from: niemand on September 08, 2022, 01:23:51 am1.40 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 08, 2022, 02:11:53 am
content: 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.

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 02:42:10 am
content: Quote from: Mechatrommer on September 08, 2022, 02:11:53 amThis 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.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2022, 03:20:37 am
content: 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 08, 2022, 03:28:11 am
content: so have you checked the spring is not shorting the battery +ve tab and its metal body?

author: Gyro
author_rank: Super Contributor
timestamp: September 08, 2022, 08:50:34 am
content: Threethreads for one knackered electric toothbrush?[Edit: Or is that two knackered toothbrushes in the space of a couple of days?]

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 10:35:47 am
content: Quote from: Gyro on September 08, 2022, 08:50:34 amTwo 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?

author: Fraser
author_rank: Super Contributor
timestamp: September 08, 2022, 10:41:37 am
content: 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 shrinkFraser

author: magic
author_rank: Super Contributor
timestamp: September 08, 2022, 10:51:18 am
content: 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.

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 10:55:17 am
content: Quote from: magic on September 08, 2022, 10:51:18 amHmm...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.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 08, 2022, 11:02:35 am
content: 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.

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 11:27:21 am
content: Quote from: BeBuLamar on September 08, 2022, 11:02:35 amYou are referring to the 16-year oldProfessional Care 9000with the struggling motor on the other thread.I answered the same question there.This thread concerns theSmartSeries 5000, which doesn't have the motor problem.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 08, 2022, 11:31:00 am
content: Quote from: Gyro on September 08, 2022, 08:50:34 amthe deal breaker is, no progress in prognosis. which forced me to chime in as humble as i can, hint... V = IR

author: jpanhalt
author_rank: Super Contributor
timestamp: September 08, 2022, 11:34:15 am
content: 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.jpgThe 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.

author: MrAl
author_rank: Super Contributor
timestamp: September 08, 2022, 01:32:28 pm
content: Quote from: niemand on September 08, 2022, 10:35:47 amIt 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.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 08, 2022, 02:57:14 pm
content: First, as others suppose, solder the tabs to the battery to ensure good conductivitySecond, 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).

author: tunk
author_rank: Super Contributor
timestamp: September 08, 2022, 03:57:23 pm
content: Quote from: bdunham7 on September 08, 2022, 03:20:37 amDo you have the specs for it? If the runtime is e.g. 40 minutes thenit should draw ~4A (2700mAh cell). According to the digikey pagethe discharge rate is 490mA, and there's a graph in the datasheetwith 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

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 05:58:03 pm
content: Quote from: Mechatrommer on September 08, 2022, 02:11:53 amI 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 amnotgetting 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.

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 05:59:13 pm
content: Quote from: Mechatrommer on September 08, 2022, 03:28:11 amI amnotgetting a short between the spring and the motor's body.

author: james_s
author_rank: Super Contributor
timestamp: September 08, 2022, 06:36:24 pm
content: 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.

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 06:42:42 pm
content: Quote from: MrAl on September 08, 2022, 01:32:28 pmThe 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:

author: james_s
author_rank: Super Contributor
timestamp: September 08, 2022, 06:52:28 pm
content: That's effectively just a power supply with one half of a transformer, the other half is the coil in the base of the toothbrush. The charger is part of the toothbrush itself.

author: mikerj
author_rank: Super Contributor
timestamp: September 08, 2022, 07:55:02 pm
content: Quote from: niemand on September 08, 2022, 05:59:13 pmThat's not what people are saying. That spring looks like it could very easily short the outside can of the cell (the -ve connection) to the +ve battery terminal. Is there something else you've left out from this assembly, e.g. some kind of insulating disk to ensure that spring cannot contact the outer body of the cell? Relying on the green shrinkwrap on the cell to do this will result in disappointment and maybe smoke.

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 08:15:31 pm
content: Quote from: mikerj on September 08, 2022, 07:55:02 pmNow I understand. The photo shows the full assembly, nothing left out, except for the housing. I will find a way to tape up that exposed spot. "Smoke" is right. I caused that rip when I was trying to insert the spring, which scraped the shrink wrap while another part of the spring came into contact with the solder on the motor (as can be seen in the photo), and the spring heated up fast, and there was smoke.

author: niemand
author_rank: Contributor
timestamp: September 08, 2022, 10:56:18 pm
content: Quote from: james_s on September 08, 2022, 06:36:24 pmI've made an attempt to read current draw, but nothing registered (DMM displayed "0.000"). The battery is 3/4 charged, reading 1.44v. I think the current is not showing in the meter is because of the same reason that the battery needs to be soldered in order for the toothbrush to function (as Fraser suggested above). I don't have wiring or test leads supplies that can simulate a more solid (soldered-like) connection.Interesting observation: Before I took out the battery from the handle, as soon as I lifted one of the tabs and put it back in place to once again make contact with the battery tab, the "3/4 charge" display immediately changed to "empty battery" icon. It never changes back to 3/4 charge. I would have to recharge the battery again (put everything back in place and put the handle back in the charger). Even after I've rigged it up to measure current draw, the display would only show the "empty battery" icon." alt="" class="bbc_img" />

author: niemand
author_rank: Contributor
timestamp: October 20, 2022, 04:41:03 am
content: Thanks to Fraser's sharing of his experience, the toothbrush is now working excellently after I soldered the connections to the battery (the new battery's tabs onto the tabs left behind from the previous battery). The toothbrush can now, with one full charge, deliver 22 sessions of brushing with an average brushing duration of 2 minutes and 45 seconds per session. Even after the 22nd session, the toothbrush didn't stop dead, but I decided to recharge it at that point because the motor became noticeably less vigorous. Prior to the new battery, the motor would just stop dead in the midst of the 5th session.