I have an 123 on FW 1.08 (earlier model, late nineties, 20 yrs old!) and rebuilt the battery pack with Sanyo NiCd batteries (KR-1800SCE) once, but that did not last very long and I have since been running the scope on power adapter only. I'm digging this up because I've always been interested in fixing this, and now I need to be able to use the scope on batteries. This will either be a DIY NiMH pack or maybe even Lithium. I'll try to recap and add more info in the hope it may help others.
There used to be a forum/discussion on
http://www.flukecommunity.com/forums/showpost.php?p=6844 "123 battery pack, difference BP120 <-> BP120MH?" with some information on the battery pack by someone looking for the NiMH ID resistor. They seem to mix up their links regularly, the new forums are now called "Fluke Nation" and seem to be outsourced to
http://flukecorp-cx.custhelp.com/app/utils/login_form - however, this now seems to be inaccessible without registration/login. The thread now is at
http://flukenation.com/posts/9d37b58388. Another, more recent one, "BP130 battery" at
http://flukenation.com/posts/32b879babc.Back to the charging:
The charging circuit and procedure are very well described in the ScopeMeter Service Manual. There are a few different versions of the manual, but the power/charging section overview seems to be mostly identical. The 2003 version has added information about the battery packs and ID resistors:
Two different battery packs are possible: as a standard Fluke 123 is equipped with a Ni-
Cd battery, Fluke 124 has a Ni-MH battery that allows a longer operation time. Both
instruments will also function on a battery pack different from the standard type. The
installed battery type is read by the D-ASIC via BATIDENT/BATIDGAR (pin B5): for
Ni-Cd there is 0 ? between BATIDENT and – of battery, for Ni-MH this is 825 ?.
Battery pack models:BP120 - original NiCd -obsolete-
BP130 - Battery Pack NiMH for ScopeMeter 124 -obsolete-
BP120MH - NiMH Battery Park for Fluke 123, Fluke 43 and 43B - The BP120MH replaces the earlier models BP120 and BP130 alike. SKU 2544081: $129.99 (
https://www.flukeonlinestore.com/products/batteries-and-chargers/battery/bp120mh-rechargeable-battery-pack-for-43-43b-123)
No mention of the 123 V1 with last FW 1.08 supporting/using the NiMh ID-resistor so far.
Firmware Upgradeshttp://www.fluke.com/fluke/uyes/support/software/ScopeMeter_Firmware_Upgrades.htmhttp://en-us.fluke.com/support/software-downloads/fluke-scopeMeter-123-series-version-1-firmware-upgrade.htmlV1.08 changes do not mention battery support, I can't find changelogs for earlier firmwares.
How the charging works (see service manual):Nominal battery voltage: 4*1.2 V = 4.8 V ("If no battery pack is installed, VBAT is approximately +15V.")
The power/charge supply is a 100 kHz switch mode regulator in the P(ower)-ASIC, which in turn is controlled by D(igital)-ASIC.
Charge conditions/cycle:
- Battery Voltage (VBAT, via BATVOLT) <5.2V (is this a typo and it should be 4.2 V?), Battery temperature 10..45 °C
- 500 mA until dV/dt, up to 6 h (fast charge)
- 200 mA 2 h (top off)
- 60 mA trickle up to 48 h
The lower limit seems to be 4.0 V, the instrument then enters OFF mode (see "3.2.5 Start-up Sequence, Operating Modes").
Safety measures:- both ASICs monitor temperature, bimetal switch shuts off at 70°C.
- "The P-ASIC limits the voltage VBAT to 7.4V via its internal Control circuit. This situation arises in case no battery or a defective battery (open) is present." This voltage is probably set by R513 / VBTHIGH.
Thoughts on the Lithium battery hackRegarding user rsivan's mod starting at
https://www.eevblog.com/forum/testgear/fluke-123-scopemeter-and-battery-replacement/msg279033/#msg279033They modified VBTHIGH to 8.2 V, doubled the charging current by halving IBAT sense shunt resistor, and modifying the voltage feedback BATVOLT.
Lithium batteries are charged constant current followed by constant voltage, determined by absolute cell voltage:
https://www.digikey.com/en/articles/techzone/2016/sep/a-designer-guide-fast-lithium-ion-battery-chargingWithout modifications, a single lithium battery cell (3 ... 4.2 V) would be overcharged, but 2 in series might not have that problem, as the charger hardware supposedly limits the voltage to 7.4 V (just about the nominal 2*3.7 V, so quite a lot of headroom, but not using the full battery capacity [by far]).
However, they would be discharged down to 4 V (2V)!
Can the lower level be modified, too?
User rsivan has determined threshold levels for the battery voltage with regard to battery indicator and shutdown, though the theory does not seem to match their value:
0.95 V for battery X icon ((0.95/0.67)+3.27 = 4.687)
0.8 V for shutdown threshold ((0.80/0.67)+3.27 = 4.464)
rsivan gives 5.9V for their battery warning threshold, so (5.9-3.27)*0.67 = 1,7621.
The calculation is derived from service manual section "7.5.3 Slow ADC"
"BATVOLT (N501 pin 78), must be {0.67 x (VBAT-3.27)} Volt.", (Measure VBAT on TP504 = N501 pin 3), so:
(7.4-3.27)*0.67 = 2.7671
(4.8-3.27)*0.67 = 1.0251
(4.0-3.27)*0.67 = 0.4891
Something is off!
Regardless of the 4V shutoff treshold, the +3V3GAR linear regulator is always on, draining the battery (slowly). Assuming a 0.65V V drop by the transistor, we're also at just below 4 Volts minimum (there still might be current drawn by the regulator and/or other parts that are constantly connected).
So for lithium batteries it seems one would need to rely on the charger's overvoltage cutoff (instead of dV/dt), but also on the batteries protection circuit's shut-down function to disconnect the battery at undervoltage, though I don't know how the charger circuit reacts to that and how the protection circuit would "recover":
Overvoltage -> Battery protection disconnects -> Vbat rises to 15 Volts (?) as power control assumes there is no battery
Undervoltage -> same if battery appears to be disconnected.
Any input on this appreciated.
The battery types on Wikipedia :
https://en.wikipedia.org/wiki/Nickel%E2%80%93cadmium_batteryhttps://en.wikipedia.org/wiki/Nickel%E2%80%93metal_hydride_batteryhttps://en.wikipedia.org/wiki/Lithium-ion_battery