Keysight U1252B charging investigation and debug - hot battery! - Page 1
Forum section: electronics / testgear

author: ArcticGeek
author_rank: Regular Contributor
timestamp: October 18, 2016, 01:48:47 pm
content: This is my saga of a recently discovered charging issue on a recently acquired U1252B multimeter that was acquired as "parts only" on our favorite auction site.I actually have a total of 3 U1252x meters. One is a U1252B that is essentially brand new, an older U1252A, and a recently acquired U1252B that was used and sold as "parts only". The new U1252B and the older U1252A seem to charge okay, but the recently acquired U1252B does not charge properly. The symptoms of the charging problem sound identical to the issue discussed in this thread:https://www.eevblog.com/forum/testgear/keysight-u1252b-battery-charging-behavior-(hot-battery!)Other than the charging not working, the meter works perfect. The first time I charged the battery I didn't really pay that close attention; I left it to charge overnight. The next morning I noticed that the outside case of the meter was quite warm where the battery compartment was so I removed the battery and found it was quite hot. I immediately thought of the issue that bitseeker described in the other thread. So last night I started digging in to see if I could understand the problem a little more.The first thing I did is started swapping batteries around, this didn't help. The problem always followed the meter rather than the battery. Regardless of which battery I used, the faulty U1252B would always overcharge the battery and it would become hot. Adding yet another multimeter in series with the battery leads showed that the charging current was identical between all 3 meters; roughly 150mA during the self-test as well as during the 220minute charge cycle. But the bad meter doesn't shut the charger off. Even when the battery was 11.124V, the charger remained on. Not good. This explains why the battery is getting hot.The next thing I did was disassemble all 3 meters to see if I could see anything unusual on the broken unit compared to the 2 working ones. Nothing unusual found; no missing components, burned components, or broken components that I could see visually.To satisfy my curiosity I attached a current probe to the battery lead and looked at it the charge current on the scope, and I was somewhat surprised what I found. The charge current is actually sinusoidal with a frequency of approximately ~75Khz, as shown in the first attachment. The green trace is the current into the battery, measured on the red wire of the battery snap. The yellow trace is the voltage at the terminals of the charger, AC coupled at 20mV/div. The charger is plenty beefy, 24V at 2.5A, and its pretty clean right at the terminals of the meter. The good U1252B had a similar current profile, that is shown in my second attachment. The good meter has slightly higher frequency (81Khz), but current is approximately the same.This 75Khz stuff kinda bothers me. I don't know a lot about NiMH chargers, but I think most work by sensing a delta-V and shutting off the charger when it senses a change. But what bothers me is how the charger can accurately measure the voltage of the battery when it has ~10" of wire length on the battery leads. These 9V battery snaps on these meters extend about 5" beyond the PCB, and with both red and black wires that's a total of about 10". To boot, the wire is probably 22 or 24 ga. at 80Khz, the inductance of that wire becomes significant. Could it be that the wire length makes a difference on the battery voltage sensing? Hmmm....just a thought.More debug will continue...stay tuned

author: bitseeker
author_rank: Super Contributor
timestamp: October 18, 2016, 07:35:20 pm
content: AG,Thanks for starting this investigation thread. Your meter does indeed sound like mine did, and the problem was in the meter.I never looked at the charge current with a scope, though. That's very interesting and odd. I've never heard of a Nickel-based charger using a sinusoidal current. The frequency sure sounds like noise from an onboard buck converter.Since the voltage looks pretty steady during charging, it's probably OK for NDV detection, despite the funky current. The overcharging, though, sounds like a detection problem. It's weird, though, because on mine, the charge level meter worked fine, yet during charging it was not able to properly show the charge level and, hence, didn't detect when charging was completed.

author: TheSteve
author_rank: Unknown
timestamp: October 18, 2016, 07:53:30 pm
content: Hmm, is someone at Keysight selling the bad meters out the back door?Maybe you guys should compare serial #'s.I would have expected them to use a linear charging technique to ensure there was no noise generated - a 24 VDC adapter would have been pretty high for that.End of the day pulse charging the battery still isn't unreasonable. I would have expected a much squarer looking pulse though - what is the rise time of your current probe? Unless noise was an issue and they did some extreme rounding of the pulses.

author: bitseeker
author_rank: Super Contributor
timestamp: October 18, 2016, 09:09:34 pm
content: Quote from: TheSteve on October 18, 2016, 07:53:30 pmI dunno. The bad one I had was branded Keysight, not Agilent, and if I remember correctly, manufactured in March 2015 (serial started with 55).QuoteThe adapter they provide is really overkill for this product: 24V, 2.5A. It was probably intended for something else, originally, but since the voltage was right, why not use it. The banana plugs on the cord are actually an adapter with a barrel jack on the other end, shrink wrapped to the lead from the power supply.

author: ArcticGeek
author_rank: Regular Contributor
timestamp: October 18, 2016, 09:11:06 pm
content: TheSteve,The current probe used was a Tektronix TCP202 - it has very good bandwidth of 50Mhz. It's rise time is spec'd at <7ns. In addition, I have 2 of those current probes and I double-checked my measurement - both probes measured the same current waveform.I still have a hunch that the frequency content will make it difficult for the charger to sense the actual voltage. It's possible that they put a good LPF filter on the battery sense leads and it will measure the voltage properly even with this sinusoidal current. But it's also possible they have inadequate or no filtering at all on the battery sense voltage and they are getting erroneous readings. This would explain why bitseeker's meter seemed to measure the battery voltage properly when in normal use, but it didn't measure the voltage properly during charging. It could also be that they are the hairy edge of the charger working properly with its voltage sensing...and some meters just won't work with those long wire leads on the battery connector. Just a hunch for now though.Using a pulsed current is okay....but I don't understand why they designed the charger the way they did. With only 140mA of charge current, why not use a linear circuit for charging? And why they use a 24V 2.5A charger makes no sense either. They could have used a 12V 200mA charger and it would have been adequate. Perhaps they had a surplus of these 24V supplies.Tonight I'm going to try swapping the boards between a good meter and the faulty meter, and perhaps see where the battery leads are connected on the PCB. The fun continues!

author: bitseeker
author_rank: Super Contributor
timestamp: October 18, 2016, 09:16:21 pm
content: Quote from: ArcticGeek on October 18, 2016, 09:11:06 pmLooking forward to it!

author: ArcticGeek
author_rank: Regular Contributor
timestamp: October 19, 2016, 01:19:52 pm
content: Well, I didn't get as much debug done last night as I had hoped for....the wife made me help clean the house instead - ugh!I did, however, manage to get a little debug time on this issue. I started by checking the battery "fuel gauge" that the U1252x meters have; this is what comes up when you press the "Bat" key on the unit. I wanted to see if all of my meters measured the battery level the same if I connected them all to a lab supply and varied the voltage. The U1252B has a battery voltage setup in the setup screen to set whether you are using a 7.2V or 8.4V battery. The U1252A does not have this setting in the setup screen, it is fixed at 7.2V. The following is my results:Voltage Agilent U1252A Faulty U1252B (7.2V setting) Faulty U1252B (8.4V setting) Good U1252B (8.4V setting)-------- ----------------- --------------------------------- --------------------------------- -------------------------------6.60V 25% 25% 0% 0%7.20V 50% 50% 0% 0%7.70V 75% 75% 25% 25%8.25V 100% 100% 45% 45%8.90V 100% 100% 75% 75%9.40V 100% 100% 100% 100%As you can see, the faulty unit's battery measurement in normal runtime mode matched up well with my other 2 units. I didn't bother with checking the good U1252B in the 7.2V battery setting, as I am convinced it measures the battery voltage fine.The next thing I tried was to just observe what happens when I try to charge a fully charged battery in each of the units. I monitored the battery voltage with another multimeter while the charging was occuring. The results as follows:Good U1252BTrial #1 - upon plugging in the charger, the unit beeps 5 times and then the sinusoidal charging begins once the self test starts. The voltage on the battery goes up fairly quickly to 10.947 and then starts to drop (neg delta-V). At approximately 10.920V the charging stopped. I wasn't able to get a good voltage reading of the charge termination voltage because the multimeter I was using to monitor the voltage just happened to shut off from its auto-power-down at that exact instant - grrrr! The charging was complete when the multimeter timer reached 218 minutes (it starts at 220 min and counts down).Trial #2 - upon plugging in the charger, the unit beeps 5 times and the charging begins when self test starts. The battery voltage climbed to 10.940V, then started dropping. Charging stopped at 10.920V when the timer was at 217 min.Faulty U1252BTrial #1 - upon plugging in the charger, charging startsIMMEDIATELYbefore the 5 beeps occur. The battery voltage climbed to 10.877V then starts going down. Battery voltage dropped all the way to 10.774V and was still dropping when I unplugged the charged when the timer was at 203 minutes. The charger should have shut off by now.Trial #2 - upon plugging in the charger, charging startsIMMEDIATELYbefore the 5 beeps. The battery voltage climged to 10.758V then slowly started going down. Battery voltage dropped to 10.670V and was still dropping when I uplugged the charger. Charge timer was at 203 minutesU1252ATrial 1 and 2 - Upon plugging in the charger, the unit beeps 5 times and then charging starts during the self test. I didn't have time to monitor battery voltage and charging, it was getting late.The very interesting thing here is that the faulty meter does not hold off the charging before the 5 beeps - it starts the very instant the charger is plugged in. This behavior is definitely different from the good meters that I have. I thought this might possible be because of a firmware level since the faulty meter is a slightly older Agilent branded meter rather than my good Keysight branded, but the much older Agilent U1252A has the same behavior as the new (good) U1252B. This behavior might be related to the issue.More debug to come.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: October 19, 2016, 02:32:29 pm
content: Thanks for the detailed information. I recently received a U1252B plus a charger in a separate auction. I have a battery on order since mine was missing.I will check my meter for the same charging problem.

author: bitseeker
author_rank: Super Contributor
timestamp: October 21, 2016, 08:01:17 pm
content: AG,Good stuff you're collecting here. Regarding the "fuel meter", do you get erroneous levels on the bad meter during charging? Mine would show 75%-85% during charging regardless what the actual voltage of the battery was.As for firmware, I don't have a version number but my meter was from 2015, so newer ones can be affected as well.

author: ArcticGeek
author_rank: Regular Contributor
timestamp: October 24, 2016, 01:13:54 pm
content: QuoteNo, the fuel meter appears to work properly during both charging and normal operation. I had my suspicions when I first read about the troubles you were having with your meter, so that's why the fuel gauge was the first thing I checked. The fuel gauge shows 100% long before charging terminates, as the 100% threshold is approximately 9.40V....and charging termination is usually around 10.2-10.5V. From what I have observed the threshold values for the bar graph do not change between normal mode and charging mode.As for debug progress, I didn't get as much done the last several days as I would have liked because I was out of town, but I did manage to get a few things done.The good U1252A and U1252B charge behavior is identical. The charger is off for the first ~5seconds when you first plug the charger in, this is when the unit beeps 5 times. After the 5th beep the charger is enabled and the sinusoidal charging begins. If you watch the battery voltage as it charges, you can see the battery voltage reach a plateau and then start dropping steadily. Both units will stop the charger when the battery voltage drops 25-30mV below the peak voltage. From the little research I have done on NiMH charging, this is good and expected behavior.The faulty unit enables the charger immediately when you plug the charger in, it enables the charging before the first 5 beeps occur. In addition, it never shuts the charger off. At one point I thought the problem might be in how the unit sensed the voltage since the charge current was switched at 75Khz, but I have since ruled that out. On the good units I can extend the battery leads using cheap 18" clip leads and those unit's behavior is not affected and they still work well. If the impedance of the wires was causing errant/erratic behavior I would expect the additional clip lead length to have an affect, but on the good units the extra length of wire didn't seem to bother. Furthermore, I can put another multimeter in series with the battery to measure the battery current and that doesn't have an effect either.My thinking is the uP inside the meter is responsible for enabling/disabling the charger using some sort of GPIO pin. To that end, I thought it might be interesting to probe each of the pins that are used to connect the main board to the daughterboard of the meter. I probed each signal one by one using a scope while plugging in the charger. I was looking for a high->low or low->high transition on any of the pins that would coincide with the charger being enabled. None of the pins showed any signs of signals that toggled at that time - bummer. However, there were a couple of pins that appeared to be clock/data....its possible that the use that as a way to perform the GPIO.My next step is to swap the daughterboards between a good and faulty unit and at least isolate which board is causing the issue.More to come

author: bitseeker
author_rank: Super Contributor
timestamp: October 24, 2016, 09:26:31 pm
content: Sounds to me like you're still making great progress.It's interesting that your fuel gauge is fine. On my new meter, I have the same 100% level for a while before C-End. So, confirmed that's still the intended behavior on a good one.The U1253B had aservice noteabout terminating the charging cycle too early due to incorrect data in the EEPROM. I wonder if something similar is causing some of the U125xB not to stop when it should.

author: ArcticGeek
author_rank: Regular Contributor
timestamp: October 25, 2016, 02:02:49 pm
content: Yea, I saw that service note from Keysight, but it only applies to the U1253, so I didn't feel it would apply (but I did keep it in the back of my mind).I did make some good progress though. My hunch about the charger being enabled/disabled by a GPIO pin was correct. After removing the boards from a good U1252A and the faulty U1252B, I started probing around to see what happens at turn on when the charger is enabled. I started by measuring continuity from the red lead of the battery snap and going inward. With a bit of reverse engineering, I found that the charging switch circuit to the battery is what I show in the first attachment.It is interesting to note that there are some slight difference between a U1252A and a U1252B. U1252A has a PNP transistor in a TO-126 package for Q5, the U1252B has a SOT23 P channel FET. The PCB is actually etched to accommodate either type of transistor. In fact, the pads for the FET are labeled G,D, and S and the pads for the transistor are labeled B,C, and E. I have pictures showing both boards.Low and behold, when I scope the collector of Q7 I see that signal going low right after the 5 beeps - this is what enables the charger on the good unit. That trace is shown in attachment #4. The green trace is the charging current and and yellow trace is the collector of Q7. You can see that the charge current is turned on when the collector of Q7 goes low.Probing the faulty meter, I do not see that charging current being enabled/disabled by Q7 and Q5. Instead, I see the charger running prior to the falling edge of the Q7 collector voltage. This trace is attachment #5.So is Q5 defective? Using an ohmmeter I don't observe anything unusual between the drain and source, it measures open circuit. For a quick "fix" I replaced Q5 with a 2n3906 PNP transistor...and viola! It works! The charging current is now held off during the 5 beeps until it is turned on by the Q5 and Q7. Looks good! This trace is shown in the last attachment.So now the charger can be turned on, but I still don't know if it shuts off properly when the battery is charged. I need to get a replacement FET or transistor and then try it, but I suspect it will work. Stay tuned for the next installment.

author: bitseeker
author_rank: Super Contributor
timestamp: October 25, 2016, 08:54:38 pm
content: Excellent sleuthing, AG. A bad transistor sure fits the behavior. It can't turn off the charging, so it also can't keep it from starting prematurely. I concur on your assumption that changing that transistor will now fix the C-End. The thing that's a bit odd is that the meter doesn't show C-End (at least on mine it didn't). Could it rely on sensing that charging has stopped before changing the status on the screen?

author: ArcticGeek
author_rank: Regular Contributor
timestamp: October 28, 2016, 07:46:39 pm
content: Well, I confirmed that replacing the transistor solved the charging problem. Charging now completes as it should and the charger is shut off, I verified this operation several times using two different rechargeable batteries. In each case, the charger shut off after the battery voltage plateaued and dropped 30-35mV from the peak voltage.Bitseeker, I think I will take back by earlier statement that I thought the "fuel gauge" works well during charging - I think it actually lies. I happen to notice that right at the time the charging was almost complete, the fuel gauge only showed 85% full. I don't think the firmware of the meter is quite in sync with what the charging algorithm is doing; as in this case we were only seconds away from the charger shutting down and the bargraph did not reflect that. I've only tried this a couple of times on just one battery, but I'm guessing this might be why your meter showed that charging wasn't complete. Just a thought anyway.I'm pleased that my newly acquired meter is now 100% working - although even if I hadn't got it working it still would have been a useful meter and just use a non-rechargeable battery

author: bitseeker
author_rank: Super Contributor
timestamp: October 29, 2016, 09:13:51 pm
content: Thanks for the wrap-up, AG. Glad to hear that replacing that transistor did take care of it. It's also a relief that you experienced the bogus fuel gauge during charging. The odd thing is that it sounds like you still experience it even after replacing the transistor. Perhaps there are values in the EEPROM that are set per meter, not globally.If you hadn't found the cause, you could still use a rechargeable battery, but charge it outside the meter.