Possessed 18V battery pack - what is going on please? I must know  - Page 1
Forum section: electronics / beginners

author: GMan123
author_rank: Newbie
timestamp: February 11, 2021, 08:41:46 am
content: Hi all.Newly registered here but on and off long time lurker - I bought my first multimeter, an Amprobe 37XR, after watching one of Dave's shootout videos about 10 years ago.I'm no EE but I'm very comfortable measuring batteries, fuses, and doing simple repairs. I was hoping someone here could help me understand these very strange voltage measurements I'm seeing.It's a new 18V 2AH battery pack (Li-ion, generic power tool thing). I have two of them here, both new, but purchased separately so they could be very different ages.One reads normally at all times while the other seems to be possessed - itKNOWSwhen you are trying to measure it - and further, it knowsHOW MANY TIMESyou are trying to measure it - and it willSKEWthe readings accordingly! (lol a quantum battery???)I'm using a blue BM235 and a few cheap chinese meters (sadly my Amprobe accuracy is way off nowadays).The symptoms are so weird I don't really know how to summarise them but I'll try.Out of the box the battery measured 15.18V which I already considered low.I started it on the charger but noticed it had stopped after 10 minutes or so, I knew that couldn't be right (it's a 2.4A charger).Level 1 Weirdness; voltage droops (no load) during basic measurementBattery appears to be fully charged. There are no battery status LEDs on-board, but two separate chargers both stop charging it after approx 30 seconds.Multimeter DC volts very briefly displays 17-18V while rapidly decreasing to around 15.6V. It takes a few seconds settling the last few counts.It gives me the impression its true resting voltage is higher when I am NOT measuring it.Min.Max mode never manages to capture higher than about 18V at the start. I don't have access to any scopes (or know how to use them).Several different multimeters show the same odd sequence of readings.The other (good) battery measures a stable 20.5V as you'd expect.Level 2 Weirdness; parallel measurements cause a further drop in voltage readingAt some point while using a second meter to verify these unexpected readings, I briefly connected both meters in parallel and saw the reading decrease further to around 12 something volts.My multimeters are both in voltage mode, with leads in the correct sockets.I definitely didn't accidentally short the battery through the meter. And I didn't cross the meter probes to cause a short either.So I don't understand what caused this sudden and rapid decrease in voltage reading.Measuring the GOOD battery identically, both meters just show a normal 20.5V.Repeating the parallel reading on the bad battery, verifies it's about 12.5V that both multimeters drop down to.It's not instant, it takes a couple seconds showing decreasing updates.Level 3 Weirdness; teasing immediately after useI don't know why, but I put it on an 18V worklight from the set and expected to see dimmed LEDs. But to my surprise it lit up brightly.Immediately afterwards, you can measure 20.7V normally on this battery, either with single multimeter or with 2x meters in parallel. No difference.So this explains why the chargers refuse to charge it - it seems to already be fully charged.But after sitting idle for 4-5 minutes, the voltage reading abruptly changes to around 15.6V again.I've repeated this and verified the same results several times.- Note I haven't tried using it on the worklight or a tool for more than a few seconds - I don't understand what is going on and didn't feel comfortable potentially testing the safety circuitry by trying to discharge it longer, until I solicited some help here.- So it may be that put on a tool, it might suddenly stop working after 10 seconds or 1 minute - I just haven't tested that yet....I am pretty sure a multimeter in voltage reading mode can't cause a battery to explode.But I didn't understand what was happening and seeing a rapidly decreasing voltage like that reminded me of when I've purposefully shorted simple alkaline and Ni-MH cells, so that's why I got a bit skittish about it. Although I couldn't understand how, I didn't want to risk have a shorted Li-ION pack on my hands so I didn't leave the probes connected in that state for long. And I haven't tried using 3x meters in parallel.I did some reading this week to learn how voltage measurement works (charge a capacitor and measure time to drain?) - but I'm none the wiser.Clearly I don't understand enough to comprehend what I'm seeing during this measurement. Or why the battery seems to read normally for some minutes after being used in a tool.If someone can explain this I'll happily just exchange it and be done with it. I do actually want to use the battery.But if we need t open it up to measure individual cells, or look for dodgy solder shorts, or pinched wiring, then I'm prepared to do that to understand. I'm very curious.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 11, 2021, 09:07:12 am
content: Looks like you get fooled by the internal BMS (battery management system). It'd turn on the output only when the "correct" load is connected, otherwise you measure some leakage or residual charge of capacitors.All rechargeable Li battery packs have some kind of BMS built in, it's required for Li battery cell safety reasons. Sometimes it's "abused" for vendor lock-in of batteries. There's quite a variety of BMS around, so I can't tell what to expect with that particular battery.

author: GMan123
author_rank: Newbie
timestamp: February 11, 2021, 09:34:15 am
content: Thank you. Yes if I only had one battery and saw this, that would be my assumption too - BMS.But I have two of the same model battery and they behave differently. I am able to measure the second one normally at all times whether it's freshly charged, or been sitting for a week.These are very cheap own-brand batteries/tools from a cheap department store - I only bought them because it was on clearance and I want to learn how to use these tools. But my point is I doubt they've spent much money on intelligence BMS in these batteries. Not even my Dewalt batteries cut off voltage as you suggest.These only have 2 connections between battery and tool, and 2 between battery and charger. Not much room for intelligent communication between them all.I guess it's possible they're different internals, despite being the same model number on the outside.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 11, 2021, 10:31:46 am
content: Quote from: capt bullshot on February 11, 2021, 09:07:12 amJust for the record, such blanket statement is not true at all, far from it. Packs with no internal electronics are typical, especially in the past. I don't know about the recent few years, but just some 5 years ago many power tool packs were just cells, passive protection, and often the center taps brought out for the external BMS (inside charger, and inside the power tool, not in the pack as you describe).BOSCH power tools some 10 years ago did not even bring out the center taps! Packs were Samsung 13Q cells in series, treated as a "single" higher voltage cell, and worked just fine, I measured completely in-balance end-of-life packs. Don't know what they do today, probably have added the electronics in the packs as this is more common today.Strange behavior is often due to BMS, but do not assume there always is one. Even if there is one, do not assume anything about it's suitability for any protection function, especially if the packs are used outside the intended device. Such assumptions are unsafe. I have seen a dangerous trend of false sense of security regarding poorly understood or badly designed BMS systems. Interrupting large currents reliably is already challenging, and many BMS systems are copy-paste designs from BMS IC appnotes written by an intern. How do I know? I did that myself too, copypasting BMS IC appnote and ending up with a shorted MOSFET and overcharged battery pack. I should have known better.

author: The Bootloader
author_rank: Contributor
timestamp: February 11, 2021, 04:32:30 pm
content: Quote from: Siwastaja on February 11, 2021, 10:31:46 am+1. One of the reason cell balancing is now more common/required is because cell balancing issues increase as the batteries are charged with a higher current.A few years ago people did not mind having to charge the tools for hours. Now everyone wants fast charging.Also, on the darker side, BMS can also allow for battery "copy protection"... which also explains why more and more manufacturers use them

author: amyk
author_rank: Super Contributor
timestamp: February 12, 2021, 02:33:28 pm
content: As others have mentioned, it could be a "smart" battery.There's only one way to find out...

author: GMan123
author_rank: Newbie
timestamp: February 14, 2021, 02:14:03 pm
content: Yes I'm already aware of BMS / balancing / safety circuitry inside battery packs. But I have 2x batteries here with identical model numbers. This doesn't explain why only one of them would be a 'smart' battery.These really are very likely the cheapest 18V battery packs available in NZ. NZ$19 (US$14) will get you the 1.5AH version. That's the full regular retail price!I highly doubt there's anything but the bare minimum of protection circuitry and maybe cell balancing in there.As you say I must open them up to know for sure. I can recognise a capacitor from a diode, but my knowledge doesn't really extend to analysing circuits. I'll recognise if they have balance wires but that's probably all.As curious as I am I think I'm just going to return everything. One of the packs or the other, is not behaving right and I will forever be looking over my shoulder after charging and storing them.I was hoping someone might be able to point me in the right direction for understanding why measuring it with two meters in parallel would show a different result from measuring with just a single meter. That seems a generic question regardless of the presence of BMS. But maybe my issue description was too long to solicit many views. I appreciate you taking the time anyway.

author: epongenoir
author_rank: Contributor
timestamp: February 15, 2021, 03:27:50 pm
content: A voltmeter has an internal resistance (Say 10MΩ), if you place two meters in parallel you load the circuit with half the resistance (Assuming two identical meters).If the source impedance (in this case we can call it internal resistance, composed of battery+BMS switch open) is high, even 5MΩ will drop the voltage significantly.If the BMS switch is closed, we can assume the battery has a very low internal resistance, say 0.01Ω. This will give you the same voltage even using two meters.If it's not clear I can provide you with schematics