I was told to only trickle in a few mA when the cells are that low, I use ~5 mA which might even be a bit high.

They can be sitting there all happy and OK at low voltage, and when you start charging them you create problems. Any excess charging creates damage at low voltage. Patience is required.

good call! I kinda thought 100mA, well, I kinda heard from someone online, to use a "low current, say 100mA." They were discussing 18650 cells, and iirc the handwavy logic was <1/10th of the normal charge-rate which is ~= the amp-hour-rating/2... but, that's all /very/ handwavy, so I basically erred on the side of caution from that number [100mA], since my cells were three in parallel; ideally 33mA, then, to each cell..

But, of course, e.g. if one of the three was shorted internally, and the others weren't, then that /one/ would take the full 100mA...

Which is why I didn't start with 300mA.

But, also, being that I just used a 50ohm resistor in series with a 5V source, that means that it'd be 100mA [worst case] if shorted and less than 100mA as the battery voltage rose... Using a constant current-source seemed a bit risky to me, not knowing much about chemistry. I kinda figure it two with two extremes: the voltage measured on the battery, being 0.07V [which coulda just been my fingers touching the probes] was so close to 0V that A) a cell might be internally shorted [zero resistance between poles], or B) they might be essentially filled with water [infinite resistance between poles], in which case a constant-current source would increase the output voltage to as high as it can, trying to force in the set current... From that perspective a 5V source with a 50-ohm current-limiter, really, might've even been risky, 'cause, if it was an open-circuit [water], i'da been putting 5V at its terminals! I might've been wiser to start with, say, a 3.3V source, or even a AA battery and suitable resistor.

The guy I got the idea from, on youtube, actually does it by /directly/ connecting a good/charged cell of the same size to the "dead" cell! He does that for about 30sec. That could potentially pump AMPS into the dead cell! Could be like shorting the good cell. Flames are definitely possible, there; that's a different realm, I wasn't ready to take that leap ;)

Anyhow, you're prb right to start with even smaller currents... And probably even more important for smaller cells. I think it also wise to monitor the voltage on the cell, and the current going into it, as well as the temperature... Even with 5mA, I dunno if I'd be comfortable running it overnight unattended on a wooden workbench strewn with papers a foot away [until, at least, I saw the voltage increase a bit]. OTOH, if you believe the hype, Li-Ions that low can do weird things on their own, and some folk consider anything below 2V dangerous to even have in a drawer... hah! Everything has risks, I guess.

When you do this, how do you determine when it's OK to use a regular charging method? I mean, you don't wait for a full charge of a 1.5AH cell at 5mA [1500+/hours/] do you?

I only go to 2V that way, but I don't think it is linear at the bottom.

If I had a safe place to do it, and wanted to make sure I only kept good cells, I'd probably do it with higher current like the other guy. But I'm just trying to get what life I can out of the cell with the least chance of damage. When you do it at high current, if nothing blows up then you "heal" the shorts by burning them out. Have a safety container. I'd rather do high current or low current than medium current, though. I don't have a good reference for any of that, that's just my organic synthesis from reading application notes.

I also only go to 4.1V on old cells.

I've heard of "burning off" some of the internal crystalization/oxidation with high-current/voltage... I remember attempting it with NiCd, NiMh, and lead acid... I think I even hooked a 1.2V AA to a 20V power supply briefly. Not sure how much luck I had; really, I've a long history with flakey rechargeable batteries, unless I buy new, and even then, sometimes...

LiIon, though, seems to me a bit of a different beast. Spewing acid is gnarly, but white-hot flames...? Gotta have the right environment for that kinda experimentation...

4.1V on older batts is a smart idea; charging these with a charger made for newer, I found that /most/ of the time was spent trying to get from 4.1V to 4.2V. Also, I read that thing about 4.1 allowing for more cycles...

2V is about where I switched from 100mA to 1A... I guess "great minds think alike"

I think you'll find that cells of that age are NiCd.

it's clearly marked "Lithium Ion".

I was surprised, as well. Having encountered many machines in/of the era.

In fact, in doing some research, it seems the first Li-ions on the market were 'round the same year this was released. It also has TFT during the DSTN-era... imagine it prb cost a pretty penny.

Unless you're referring to the coin-cell stand-by battery...

I meant the standby. 6 cells to get 7.2V sounds right. Or was it 3 cells? LiIon is 3.something V. I got confused by your winding narrative which cells you were trying to charge.

"So, then, I broke apart the battery pack and found 6 18650 li-ion cells [li-ion existed 25 years ago?!]... three parallelled, in series with another three parallelled. And, each parallel-combination measuring 0.07V. Yikes."

Yeah, I should have realised the NiCd part was irrelevant.

well, maybe it's relevant in that nicd can be depleted to 0 and recharged... if not for that, I might not've even been able to power this on at all. Heh.

Also, standby batts are amazing in general... swap out the big one while it's running? On coin cells?!

interesting aside... I've long wondered how such systems can both "charge" the battery *and* run off it at the same time: e.g. a phone crunching numbers might be using more than 2.5W, while connected to a computer via USB [5V,500mA=2.5W]...

I think, now, I can /vaguely/ picture how that's sometimes implemented; and the system necessary to enable that /particular/ functionality, that way, may very well be responsible for the fact many such devices /can't/ run without a battery, even when the attached "charger" [power supply] unquestionably can handle the device's highest power-demands, even when the device /knows/ it can [e.g. negotiated 2A from a "charger"]. 

I think it's a matter of necessary circuitry, and needing physically different circuits for the two cases [or more]... huh. And, then, needing to account for things like cheap/thin wire-losses, in determining which circuit to switch-in, when.

They just charge slower if the phone is being used, to stay under the limit, there are power managements ICs for this sort of thing. Also recent charging specs are not limited to 500mA or even 5V. As for cheapo charger cables, just toss them out or use somewhere requiring little current, they are a waste of money, however little.

*sigh*... this is the real world, wherein sometimes we must make do with what we've got, got access to, or have thrust upon us.

Charger circuitry has to deal with cheap cables. PMICs come in various technologies/qualities. And nothing I've ever owned, short of a $4 SD-card reader with a micro-B-adapter has USB-C.

The whole point of this post is about reviving and repurposing /old tech/.

When I was a kid I found an old motorcycle battery. I spend an inordinate amount of time trying to revive it by connecting it to a car battery charger. It never held much charge after all that effort. Since then I've realised if a rechargeable battery is on its last legs, nothing you do will add much life to it, and it's better to just toss it.

Of course if you have more time than resources, then by all means pass the time that way. After all, many of the retro projects here, including my own, could be replaced with modern equivalents working better, and tne projects are just for the heck of it. But I think one has to be realistic with what can do with old tech.

Indeed... There are cases where the "go-to" mentality is "garbage" or even "dangerous" when in fact, if you know what your goals are, then that garbage is /exactly/ what's necessary to get the job done.

Your motorcycle battery experience is a prime example. Yeah, you're probably not going to make it like-new, but I bet you discovered that at least briefly it would take a charge... so, then, now you know that for when your car battery is on its last legs and you took too many short trips...

do you walk 20mi back to the nearest auto-parts store and carry it 20mi back

do you pay a towtruck $300 where a refurb $40 part is all you need

or do you get a jump-start and drive there in first-gear to keep the RPMs up on that alternator?

...

BTW: Properly Refurbished lead-acid car batts work surprisingly well, cost far less, are almost certainly better for the environment than even recycled, and helps put food on a skilled person's table, rather than supporting the shooting of yet another endangered species by a billionaire.

I probably used up more electricity trying to revive that motorcycle battery than all the use I got out of it later. So it was fine as a kid's pastime but for serious recycling it's better to do it systematically. I'd probably take it to the recycling centre if I came across it today. It was acting as a door stop IIRC.

It all comes down to the situation. If you can revive it for little effort and it's useful to you after that, by all means pass the time that way.

There is a reason there is an aftermarket for phone and laptop batteries. I've bought those and recycled the worn ones responsibly. No point busting a gut trying to do the impossible. I reckon I've been responsible in continuing to use the electronics when other people would toss it. The laptop in question is actually a netbook from 2008 that was loaded with XP, but runs AntiX Linux adequately. The phone I gave away while there was still decent life in the replacement battery.

I haven't scoured the local e-waste bin since the pandemic hit but I have found useful things in there which I tried to find a new home for. A PVR which powered up fine was given to somebody who restores DVD players and similar for charity recipients. Laptop power packs have been rehomed. Lots of non-USB phone chargers which I may or may not be able to repurpose for project PSUs. Many Ethernet patch cables, fortunately 5E or 6. Can't do much with ADSL modems, ADSL is on the way out here. Some of the stuff may go back into the bin. That's how it is.