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Starter battery, wee!

A project log for today's assorted project ramble "grab-bag"

Assorted project-ideas/brainstorms/achievements, etc. Likely to contain thoughts that'd be better-organized into other project-pages

eric-hertzEric Hertz 07/14/2020 at 00:360 Comments

forgot to turn off my headlights, and listening to the radio... guess what happened.

So, I figured it was time to run an experiment brewing in the ol' noggin' since I saw here of someone charging their car starter battery from an 18V cordless drill battery, and not long after seeing that, @Ted Yapo tried [nearly successfully] to get enough charge outta a coin-cell...

Well, I've got a *bunch* of USB power-packs... three in series gives 15V.

Nice thing about these over, say, a regular ol' battery [even a cordless drill battery] is they have boost-converters built-in... so as they discharge, they still put out 5V. It's a bit like a built-in joule-thief. Also, built-in current-limiting, which could be a mixed blessing depending on how it's implemented... oh yeah, and thermal-protection!

So here's a weird culmination of coincidences:

the way I wired 'em up was via bulky usb power cables I built a while-back... their housing hot-glued to the connector.

Then I inserted my ammeter between it and the cigarette lighter outlet...

2.4A --Perfect.

The batts are rated for 2.1, but I looked up the chips once, and they're set to 3A.

I thought maybe it was actually limiting, 'cause 15V -> 12 would mean for a lotta wire resistance to only be 2.5A...

So I figured the limiters doing their job and plugged it in straight. Nope. One of the batts quit outputting. Grabbed another, tried again with the meter, 2.4A... again trying direct, again kaput batt. 

OK, so, apparently, again in the last few completely unrelated projects, my ammeter's resistance had a big effect; this time quite handy. 

I estimated ~1ohm between the car batt and my usb batts, to give that 2.4A... so, yahknow, whatever tiny resistance is in my meter was *just* right.

There's a coinkidink!

These things got *hot*... enough to melt the glue, still, not so hot as to shut down... though i watched pretty closely.

I estimate about 8W lost in the hookup, and 24W, then, into the car batt... 24Wh, actually; it happened to take almost exactly an hour to drain the batts 'till one went kaput.

Weird thing, someone walked-up needing a jump of his own... and just when my charging completed he got his started. I tried mine, and... yeah! I guess it worked! Nice to've had the backup of his services-offered, anyhow.

Other interesting observations: the one holding the least charge [as indicated by its bargraph] seemed to drain fastest [makes some sense, lower voltage, higher amperage to boost]. Though, oddly, the one with the most charge, seemingly draining the slowest, [and, oddly, with the most airflow for cooling] went kaput a couple times. Can't quite wrap my head around it, but I guess it was prb for the better, as I bet the others probably didn't mind a break. ...and let that glue cool/harden.

Interesting metaphors of man-hours, differing-abilities [and stubbornness], John Henry, and managers insisting on coffee-breaks...

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Random-ish thought also revealing the coinkidinkness of it all... that if I'd tried four batteries at 20V? Well, if my setup was otherwise unchanged, we'da been at ~7A... and... it just wouldn'ta worked. So, I guess some amount of luck or coincidence or something that put pretty much all those variables pretty much randomly thrown together not only at a functional point, but also near-maximal.

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