Huh?
Opened up an original Fluke battery pack. Crusty as you might expect but... what's that?? Some yellow modling came off, looks suspiciously like a PTC resetable fuse. Might be part of the puzzle.
Hey... a Polyfuse buried in the cell pack. Who'd have thunk it...?
I have no idea! Who? WHO I ASK YOU??!?
*snerk* Seriously though, might be part of the puzzle, but there's more to it. I blew one with an original pack as well, even though you could ask yourself the question how good that PTC still is after being soaked in battery puke. Still need to check the replacement packs, don't know if they have it or not. But still, must be more to it. There's a charging regulator there, with a max charging current of 1A. You couldn't blow that sense resistor with 1A, not with 2A either.
The two units I blew the sense resistor on were revived.
Going to set up some working 199Cs to monitor the charging cycles of the others. Or themselves. Oooh, that would be cool.
Another possibility: the charging circuit uses the NTC in the pack to monitor charging. It's possible leaving the back cover up disturbes the thermal balance. Or maybe it's positioned differently in the pack. So many possibilities.
Hmmm... Now I've had a chance to RTFM before posting, (
at ME) it's pretty obvious the charging function on this unit is quite a bit more complicated than a simple analog Ni-
xx charger.
Pages 3-23 to 3-26 of the SM show a number of things going on;
if I'm understanding this correctly, even when the unit is off, all power functions are managed by a dedicated Power ASIC. The following things are monitored, and everything below has to return some valid result or the mask software running in the main D-ASIC will not even wake up from IDLE (INACTIVE) mode:
BattVolt: Battery Voltage at R4112 powers part of the P-ASIC. It then can monitor the following:BattIdent set by a ID resistor in the pack: Evidently more than one battery type was planned; I'd guess Ni-Cd vs high-capacity Ni-MH. This will affect max charge current and cutoff/trickle mode voltage set by the P-ASIC.
MAINVAL: Voltage from AC adapter/Charger at R4104.
Batt Temp: The NTC in the circuit snippet above.
Charge Current: Measured through R4101, the one which keeps blowing up.
If all monitored points return a valid voltage, then the unit's main D-ASIC starts running and unit enters
Charge or
Operational & Charge mode (if ON button is pressed) and battery charging begins.
Charge current is then managed by the main D-ASIC based on VBatt vs TEMP over time.According to this material,
max charge rate is supposed to be no more than 1A. Obviously, something in the scopemeter's interaction with these aftermarket packs is making it possible for that limit to be grossly exceeded.I find myself wondering if perhaps the aftermarket packs IDENT resistor is wrong for the cells involved, or if possibly there is some other issue. Maybe something that needs a firmware update? Maybe old noisy power bricks with dried out caps? Maybe nonexistent/wrong/just plain crap Polyfuse? Maybe the TEMP NTC is the wrong type/value/slope?
Maybe the TEMP NTC in the OEM packs is corrupted by electrolyte similarly to the Polyfuse,
and the failure mode is actually different there? Same is also possible re the Polyfuse in them as well, now I think of it.
I've attached those pages below as PDF & ODT for easy reference. And here's the full SM again for convenience if someone else wants to kibbutz:
https://elektrotanya.com/fluke_192b_196b-c_199b-c_scopemeter.pdf/download.htmlHope that helps!
mnem