LiFePo4 Voltage Spikes from chassis to Cell Terminals - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: February 23, 2025, 09:54:37 am
content: Hi,I am building a 48V DIY LiFePo4 battery box and I observed the following.When you try to measure the voltage between the case chassis and eachof the cells, you get a voltage spike on the multimeter up to the full voltagesometimes.Cells are connected in series of 8, but the two series are not connectedtogether. All cells are covered in plastic wrap, and the box inside, aswell as the spaces between the cells, are insulated with plastic sheets allaround.What is causing that spike?There is no electrical connection between the cell terminals and the chassis!You can see the spikes on the following small clip:The oscilloscope ground is connected to the case chassis.Thank you.

author: SteveThackery
author_rank: Super Contributor
timestamp: February 23, 2025, 12:00:29 pm
content: Hmmm... it looks like the case is acting as one plate of a capacitor.

author: hgg
author_rank: Frequent Contributor
timestamp: February 23, 2025, 12:17:23 pm
content: The multimeter then is discharging it, but what is charging it again?Very strange.

author: benj38
author_rank: Frequent Contributor
timestamp: February 23, 2025, 02:49:30 pm
content: Very strange. Maybe some funny electrostatic charges?

author: hgg
author_rank: Frequent Contributor
timestamp: February 23, 2025, 03:00:36 pm
content: Just connected the chassis to the wall socket ground.Same thing. There must be an explanation.

author: benj38
author_rank: Frequent Contributor
timestamp: February 23, 2025, 03:11:46 pm
content: Did you verify it is not some artifact of the DMM? I.e., does it do this when you try to measure other voltage sources?

author: hgg
author_rank: Frequent Contributor
timestamp: February 23, 2025, 03:15:02 pm
content: Yes, that's the first thing I did. I picked up a different multimeter.It did the same and then I captured it with the portable oscilloscope you see in the video.

author: golden_labels
author_rank: Super Contributor
timestamp: February 23, 2025, 07:03:52 pm
content: IMO you are just picking up transients. Made worse by a long lead and perhaps even your own body providing decent coupling.Nothing to be worried about.

author: hgg
author_rank: Frequent Contributor
timestamp: February 23, 2025, 07:24:07 pm
content: I am not worried, I am just curious to find the actual cause.Transients from where? The oscilloscope probe when connectedis presenting a resistance between chassis and cell. I have alsotried grounding the chassis to the wall socket earth.You can see in the video that all the cells are at a positive potential,yet the oscilloscope picks up positive and negative voltage spikes.Always less or equal to the full battery string voltage.

author: rlaggren
author_rank: Newbie
timestamp: February 23, 2025, 10:49:36 pm
content: I'm not an experienced tech in this area. But here is a laundry list that comes to mind. It mostly builds from an effort not to assume anything.Is there any slightly conductive film on the batteries themselves?Are the batteries sitting in a _completely_ dry case? Are you _sure_?Try grounding the case to a good ground. _Is_ that good ground really good?Try reading voltage from different parts of the plastic battery shells and insulation. If you find something, try grounding that piece of plastic and see... Materials sometimes don't have exactly the properties you may assume they do.Is this _immediately_ repeatable or do you need to wait a minute or two to repeat the behavior? (from your vid it looks immediately repeatable, just checking - is it repeatable to the _same_ exact batt terminal and location on the metal case?)How is the metal case assembled? Absolute electrical continuity all around?What happens if you measure between the metal case and some other metal object (eg. computer or other instrument chassis)? How well is your house (and other instrument chassis) really grounded. What do you get measuring between house ground and the metal case?What happens if you remove the batteries from the case and then measure different points? Eg. around the bench and also to the (empty) metal case.If you physically rub the battery shells? Ditto the insulation around the batteries?Reading between battery posts (looking for spikes when probe connects)? Reading what shows when you probe the _same_ conductor (eg. battery terminal)?I doubt most of the above will elicit any additional info, BUT one of the tests _might_. Or maybe give you some ideas.Cheers.Rufus

author: SteveThackery
author_rank: Super Contributor
timestamp: February 24, 2025, 01:15:56 am
content: Quote from: hgg on February 23, 2025, 12:17:23 pmI'm thinking the current flowing through the multimeter is charging the outer case to whatever is the voltage you are probing in the battery bank. If you then move the probe to a lower voltage point you get a negative-going spike, if to a higher voltage point, a positive going spike.Or something like that.

author: Simmed
author_rank: Frequent Contributor
timestamp: February 24, 2025, 02:22:22 am
content: wait a mintuethe ground lead is held in your fingers?DUT > probe > 1000k 10x ?? > DMM > GND > hand ?

author: SteveThackery
author_rank: Super Contributor
timestamp: February 24, 2025, 11:39:09 am
content: Quote from: Simmed on February 24, 2025, 02:22:22 amIn the first post he says the oscilloscope ground is connected to the case chassis. That's why I think all we are seeing is a current spike through the scope as the case gets charged or discharged until it is at the same potential as the probe.There is no mystery here. If the OP has a device that will measure small capacitances he could measure the capacitance between the case and the battery pack. I bet it's a significant number of pF - enough to create a detectable current spike in the scope.

author: hgg
author_rank: Frequent Contributor
timestamp: February 24, 2025, 12:21:12 pm
content: Quote from: SteveThackery on February 24, 2025, 11:39:09 amIndeed, the DE-5000 is measuring 21.87nF from chassis to battery negative at 120Hz.All other multimeters measure 3.8nFSo the case with the batteries are acting like a capacitor, correct?What is charging that capacitor? Me?How?Can I do the measurements without me charging it?

author: CaptDon
author_rank: Super Contributor
timestamp: February 24, 2025, 01:39:49 pm
content: Replies concerning house wiring mean nothing. Connecting the battery box chassis to house ground means nothing. They are batteries in a steel box and a hand held DMM and the measurements the O.P. is making are completely floating and not referenced to earth ground. Indeed the batteries in the box have a capacitive component in relation to the box. It is impossible to say at exactly what potential that capacitor charge will be. Chances are if you connect meter negative to the chassis and then probe the highest positive battery terminal you will get a positive voltage reading until the 'capacitor' is equalized. If you then probe the lowest negative end of the battery you will obviously see a negative voltage until the 'capacitor' charges or in these cases 'equalizes'. If that very simple and direct explaination doesn't satisfy HGG then no explaining ever will.

author: hgg
author_rank: Frequent Contributor
timestamp: February 24, 2025, 01:47:16 pm
content: What is charging the capacitor?When you connect the multimeter probes to chassis and most positive,the multimeter internal resistance will start discharging it, isn't that so?

author: CaptDon
author_rank: Super Contributor
timestamp: February 24, 2025, 09:33:42 pm
content: It only equalizes the charge to the positive side of the battery when probing the positive most terminal and the apparent voltage will slowly become zero with your meter connected, BUT, and try to get this through your head, THE negative battery terminal NOW APPEARS to be THE FULL NEGATIVE VOLTAGE below the chassis, but only for some amount of time while metering. IF THE LEAKAGE CAPACITANCE AND LEAKAGE RESISTANCE of every battery is roughly the same to the chassis you should be able to probe the center point of the battery and see very little difference to the chassis but immediately after doing so if you probe the most positive end of the battery you should see about 1/2 of the battery voltage but only for a short while until in normalizes that end of the battery stack to nearly zero volts. Come back a day later and probe the center point and again you may see only a tiny voltage difference. You don't understand and can't comprehend this voltage shift and if you don't understand basic electrical theory then no answer will be satisfactory to you. It's like Einstein trying to explain relativity to me, in a hundred years I still won't understand it but I'll have a nice headache. Your meter has very high input resistance and takes very very tiny amounts of current to register voltage. All you are seeing is the results of very very tiny leakage currents and even the containers of each individual battery in the box has some sort of leakage resistance even if it is in the Gigaohms.

author: benj38
author_rank: Frequent Contributor
timestamp: February 24, 2025, 10:39:17 pm
content: Quote from: hgg on February 24, 2025, 01:47:16 pmThe batteries.Quote from: hgg on February 24, 2025, 01:47:16 pmYes, it will discharge the capacitance between that terminal and the chassis (ignore leakage resistance, it is not necessary for getting the idea). Note that it is not a single capacitor you are dealing with.@hgg Imagine that every plate inside every battery has some small parasitic capacitance to the chassis, now draw this as a circuit diagram, look at it, and you will get it.Start with the simplest case of a single battery with the same capacitance to chassis from the anode and from the cathode. Your simplified equivalent circuit will look like two identical capacitors in series between the battery terminals, with the chassis being the midpoint between the two capacitors.

author: SteveThackery
author_rank: Super Contributor
timestamp: February 24, 2025, 11:01:38 pm
content: I wonder if this will help. In the first diagram I'm showing the battery pack and the case as two plates of a capacitor. The green bits represent where you can connect your probes.In the second diagram the 1M resistor represents your oscilloscope. The ground lead goes to the case, and the signal lead can be connected to any of the green connection points.Let's use the case as our reference point for any voltages we mention, and for convenience we'll call that zero volts (0V).When you apply the probe to any of the green points on the battery pack, you are connecting the case to that particular point via the 1M resistor.We will connect the probe to the left-hand point of the battery pack, which I have marked as 0V. This means that, with respect to our case, the left-hand end of the battery is at 0V, and the right-hand end is at +10V. All the intermediate points in the battery pack are positive with respect to the case. We can justifiably say that the average voltage of the pack is +5V with respect to the case.Now move the probe to the right-most point of the battery pack (the 10V point). The 10V end of the battery is now connected to the case, so all the intermediate points in the battery pack are negative with respect to the case. The average voltage is -5V with respect to the case.So, by moving the probe between those two points we are changing the voltage of the top plate (the battery pack) with respect to the bottom plate (the case).With respect to the bottom plate, the top plate is going from -5V to +5V and back again.Remember: the top plate (battery pack) and the bottom plate (case) form the plates of a capacitor, so when the top plate varies between -5V and +5V with respect to the bottom plate, current must flow in the circuit to charge the capacitor, first in one polarity, then the other polarity.The current flows through your oscilloscope and registers as a brief positive-going spike or a negative-going spike, depending on whether you are raising the overall voltage of the top plate or lowering it.Hopefully you can see that every time you move the probe to a different point in the battery pack, a brief positive or negative current will flow, registering as a spike on your scope.

author: hgg
author_rank: Frequent Contributor
timestamp: February 25, 2025, 05:03:52 am
content: Thank you for your explanations.

author: CaptDon
author_rank: Super Contributor
timestamp: February 25, 2025, 06:22:40 pm
content: Steve and Ben, good analogy!!! I was just sitting down to lunch and it hit me, 'multiple capacitors to the chassis' as opposed to a single capacitor solution!! Don't know why I didn't think of that sooner?

author: rlaggren
author_rank: Newbie
timestamp: February 27, 2025, 12:01:47 am
content: Great thread, great details. Thanks hgg for throwing it out there to start.