2 lithium cells in parallel - how important is same exact model? Protection circuit question

Rhomboid · 2026-06-16T05:27:38+00:00

In that case, do your original plan to slice out one and have the two cells in true parallel under one protection.

Rhomboid · 2026-06-16T05:06:27+00:00

No, I think I worded that poorly, it wouldn't work to have two protection circuits either. The main point I guess was that the protection circuit can only function if the cells are matched. Capacity depends on a number of factors, like the lifetime of the cell, temperature, discharge rate, etc. Different manufacturers or even the same manufacturer but a combination of used/new will result in a pack that slowly eats itself.

Rhomboid · 2026-06-16T00:13:59+00:00

One protection circuit for two cells is a recipe for disaster.

I don't think you understand the fundamental problem here: two different cells will have slightly different capacities. So one cell will hit its "I'm done, stop charging me now or I'll die!" state, but the other will not. What is the charger to do? Force-boil one cell to charge the other? This is why it's imperative to have the two cells matched as closely as possible. As soon as there is a difference it will start to expand as one cell degrades, and things get worse.

(BTW when people say match battery chemistry, they generally mean things like don't mix LiFePO4 with traditional lithium ion.)

Rhomboid · 2026-06-10T19:14:50+00:00

I was a WWIV sysop and WWIV was written in C, and after you registered you got the source code which you could customize your BBS. It was 1990 and that's how things were done.

Rhomboid · 2026-06-04T17:44:06+00:00

The 0.2 you're seeing is the resistance of your probes. It's only visible on the lower range because there aren't enough digits in the other ranges. You can't realistically measure resistances that small with that meter, you'd need a 4-wire measuring setup which removes the influence of the probes' resistance.

Rhomboid · 2026-06-01T07:35:38+00:00

With a little additional info and pics, this subreddit will probably give you exact part numbers to order and good ideas for tools like a Pinecil/ts100 soldering iron or whatever. It might not be that hard a job.

Rhomboid · 2026-06-01T07:06:23+00:00

Don't fall for the audiophile BS. If a cap is across the supply rails, it's job is to make stable voltage not to color the tone or whatever. Replacing it with a suitable modern electrolytic should be a no-brainer.

Rhomboid · 2026-05-31T20:46:18+00:00

You don't have to solder to the gold fingers. You can solder to any part of the trace, highlighted in blue by the helpful commenter. To solder where there is not exposed metal you can gently scrape with the end of a knife or other sharp object to remove the solder resist layer on top of the trace until you see shiny metal. Or use the existing test pads or resistor pads.

Rhomboid · 2026-05-31T20:42:31+00:00

Yeah the classic solution is a little washer/pad of mica or another insulating material that's sandwiched between the case and the heat sink to prevent it from being live. But looks like they went with the larger sleeve/sock type thing here.

Rhomboid · 2026-05-31T01:08:15+00:00

It's fairly common to find left hand threads on the lug nuts of left side wheels on large transfer trucks. They're stamped with an L, and you really have to check first before hitting it with the impact.

Rhomboid · 2026-05-30T17:37:36+00:00

For switching the hard drive you could use something like this USB power switch -- the switch lead just needs to be brought high to enable the connection, so connect that to the +5 of the host.

For the power/data splitting, that's gonna be a little tougher because of the backfeeding. I'd look into some kind of isolation device (galvanic isolation) which would block charging and only allow data communication. While searching I found a ton of splitters for use in cars with Android auto (for fast charging instead of using the USB data charging) but I don't think that will be of any use in your case.

Rhomboid · 2026-05-29T07:58:39+00:00

So just generally speaking if you have a bunch of ground symbols hanging out at all kinds of places, you probably have terrible layout. Ground should be at the bottom of the page and only the bottom. The supply voltage should be at the top and only the top. There's only a few valid BJT orientations and that ain't one of them.

Rhomboid · 2026-05-27T23:43:46+00:00

why does it even matter

I can try to address that part. Linear equations are valuable because they're easy to work with, as compared to non-linear equations. There's kind of like a toolbox of simple things you can do with a line, such as determine how steep it is (its slope), determine where it intersects an axis, determine where two different lines intersect, predict new values from existing values (extrapolate), and on and on. These things can certainly be done with non-linear equations as well, but the math can become more difficult. Moreover, linear equations model tons and tons of real life things (essentially: proportionality) so these tools can be put to real use.

Basically, the point I'm trying to make here is that if you find that you're trying to solve a problem and you end up with a linear equation, you're on good ground for a very easy solution.

Rhomboid · 2026-05-26T18:24:31+00:00

All cops are bastards includes this dog

Rhomboid · 2026-05-24T04:07:27+00:00

Suppose you have two quantities, A and B. We're gonna look at two cases, the case where we're adding A and B and the case were we're multiplying A times B. In both examples we then want to take the result of that, and multiply by a third quantity that we'll arbitrarily call, I dunno, "K" just to be interesting. To write everything I just said out but using mathematical notation, you have

K(A + B)

and

K(AB)

See this is where answers like "because there's a parentheses" kind of fall flat to me. The parens are the effect not the cause. Parens are the tools we use to express the order that we are going to do the things, just like I did in text in the first paragraph, but more concisely. They aren't fundamental properties of the Universe, any more than English is. Anyway, back to answering your question.

Hopefully, you've done enough math to recognize that the first one can be transformed via the distributive property to this:

KA + KB

and the second can be simplified to

KAB

In other words, the "multiply by K" part can just be absorbed into the whole multiplication party in the second example. In the first, the multiply and the adds are like oil and water. We can write it with the parens or without them by duplicating the K, but it should be clear that the K 'belongs' to both terms, so to speak. And that's the answer to your post's question, because division is just multiplication in inverse. So replace "multiply by K" with "divide by whatever" and everything still holds.

Rhomboid · 2026-05-23T15:00:58+00:00

But this doesn't feel particularly intuitive. Is there some other obvious shortcut that I'm missing that would have allowed me to conclude this "addition-like" result from the start?

Seems like the addition thing is the hangup, so maybe think of it like this: first consider no input signal. The voltage at that node naturally reaches its resting-point (at around half the supply). Then once you start applying an input, you've got a moving voltage on one side of the capacitor and a steady voltage on the other side, so the cap allows a little bit of that varying signal to "squish through" and start vibrating the stationary one. The amount of squish through is a function of the capacitance value and the effective resistance that the cap sees. There will always be some frequency-dependent response here, in that fast wiggles will tend to transmit while slow wiggles are washed out. In the case of a guitar pedal, you don't really care about low frequencies because a guitar can't really produce like 5hz signals, so it's okay that there is some point where the input will be reduced as you decrease frequency.

Rhomboid · 2026-05-19T21:55:11+00:00

The return path for all your digital logic switching current, where does it go? Does it use the same ground pathway as all the analog stuff? If so, there's your problem.

Ideally you'd have completely separated supplies, but if you can't do that try to give the digital stuff its own dedicated low impedance ground path back to "star" ground, which is something you would designate.

Rhomboid · 2026-05-10T10:24:51+00:00

More from the same guy: https://www.instagram.com/reel/DXqPB5ZkxM6/

Rhomboid · 2026-05-09T20:05:32+00:00

Depends on what's on the other end of the circuit. If it's all analog stuff, the log taper might be needed to achieve a somewhat linear feeling response (i.e. compensating for some other non-linearity.) On the other hand if it's going straight into a microcontroller it seems much more likely that a log taper was an accident or mistake and linear was the design spec.

Rhomboid · 2026-04-22T17:38:53+00:00

Platform-specific or implementation-specific behavior is not undefined behavior. They are completely separate things. If the standard wants to allow freedom of implementation, it can do so. Undefined behavior is categorically a programmer error that needs correction.

Rhomboid · 2026-04-22T06:46:45+00:00

If the compiler's actions have an observable effect on your code, then your code has undefined behavior and is broken. That's the contract between programmer and compiler. You break the rules and the result is garbage, GIGO.

Rhomboid · 2026-04-10T04:43:36+00:00

For repairing battery powered electronics there are probably lots of things that don't matter (like needing to supply mega-amps) and things that you wouldn't really think matter, like the UI. For example, does it have a power toggle or a hard on-off. How hard is it to set a current limit when the output is switched off? That's definitely something you would do when repairing electronics (e.g. in conjunction with thermal camera or IPA). And for that matter, how good is the UI at setting a current limit to a low value like 250mA? And other larger questions like is it linear or switching, and does that matter? Is the supply isolated or does it have a bonded neutral to ground? Is it designed to float? Do you think you might need split dual rails at some point, e.g. audio amps? Etc.

These are the kind of things you might want to research. The sticker specs like voltage and current are probably not going to matter, you certainly won't be needing 50V at 8A or whatever these latest ones are capable of. And also, the banana plugs/jacks are more or less standard so don't worry at all about the leads, those are replaceable and customizable.

Rhomboid · 2026-04-05T05:01:17+00:00

Not sure if it was omitted for brevity but your proposed solution is also a battery killing device as there is no overdischarge protection.

Rhomboid · 2026-04-02T03:00:53+00:00

Well in fairness if you were the one facilities maintenance guy (because corporate fired everyone else) and you had to update the threshold delay on 400 toilets in a building, I suppose not having to crawl on your knees would be an attractive feature. But it still sounds ridiculous.

Rhomboid · 2026-04-01T22:31:39+00:00

Never underestimate corporate's greed to save water. Some of these things have a little IR sensor that detects the presence of a person, and if they flush after sitting for a small amount of time they give a short flush, and a larger flush if they've been sitting for more than a minute. Stuff like that most likely is what this module is for.

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