Comment by ssl-3 | Hacker News Reader

Comment by ssl-3 | Hacker News Reader

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UPSs still typically use lead acid batteries. These don't particularly mind being at ~100% SoC for years at a time (they like staying charged), which is very good for this application.

And it's easy to keep them at around 100%. For instance: For a nominal 12v battery, one can very nearly just feed it 13.8v (plus or minus some temperature compensation) and have results that are actually pretty good and close-enough to 100%.

And when things go very wrong, lead acid batteries tend not to catch on fire.

It's a natural fit for UPS duty.

The lithium batteries in cell phones are a different thing. They hate being at 100% SoC and degrade fairly quickly in that state.

They can be pretty happy at around 50% SoC, but it's hard to actually keep them there because of the way the voltage ramps [don't] work with lithium chemistries: It's almost flat through the middle of the curve.

And cell phones don't let us do that anyway: It'd be great if there was a "Compute mode: Just do your best to keep this battery near 50%" setting to click on, but there is not. (And we could hack that in, except: We don't own our phones.)

And when things go wrong, they can become exothermic in ways that are not exactly ideal for an unattended device sitting on a shelf.

I use a chargie device which is a hardware level charge limiter

Cute. I didn't know those existed, and they do make sense.

It's only solving part of the problem though: Sure, it can keep the charge at around a self-reported 50%. But the device itself doesn't necessarily have a good idea of what 50% even means, especially as the months and years tick by and cells age, unless it is allowed go closer to the extremes (closer to 0 and closer to 100, sometimes).

The reasons for this get weird. Near the edges of the curves (0 and 100), the BMS can use voltages to figure out where things are. This works well.

In the middle of the curve, voltages are damn near flat and cease to be an indicator of state-of-charge, so the BMS counts coulombs instead. This works too (for awhile).

Except they're not ideal batteries. They age even if they're not being used at all, and shit happens just as it does for any other non-ideal entity. So on a long-enough timeline, even a perfect coulomb counter doesn't necessarily have any way to accurately report state of charge.

The way that this problem is dealt with on a phone that actually gets used like phones get used is that the BMS can reset the coulomb counter during the peaks and dips of a normal charge cycle, based on voltage.

But charge cycling doesn't happen even with a Chargie-equipped phone sitting on a shelf and running Docker containers, does it?

Put a wall plug timer on it, charge it 50% of the time.

No.

You should definitely add a /s lest anybody take this seriously. It's quite funny though. Especially recommended if you're hoping to burn your house down.

For people that didn't know, this is probably the fastest and riskiest way to destroy your battery quickly

If you're going to do this be sure you swap between charging and discharging as often as possible, go as low as your timer will allow. 30 minutes charging, 30 minutes discharging should allow you to reach house fire in less than a month.

Like your comments. Question. How does a BMS count coulombs? I mean what is the technology used?

Q = ∫idt, so you just need something that regularly samples the current (typically measured via a shunt resistor) and accumulates it over time. This is typically done by a dedicated low power chip with an integrated differential amplifier for reading out the sense resistor, ADC, and nonvolatile memory - these are often referred to as "gas gauge" chips. This functionality can also be found on higher end all-in-one battery management chips.

Great. Thanks.

You can add custom charge cycling on a schedule if using the Chargie app. One could for example have it normally stay at 50% but do a full charge/discharge cycle weekly.

Overall though, it would probably make the most sense to physically remove the battery from any phone used as a dedicated webserver, and then have it connected to an external UPS instead.

Also cute. That's pretty excellent. I think I'd like to bring such a rig down to 10% and up to 90% once a month or so, based on what I think I know.

And yes, agreed absolutely: It safer to pull the battery, and not particularly difficult or detrimental to do so for the kinds of applications I'm envisioning here that involve a disused phone doing server-ish stuff.

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