Electronics > Repair

Reversed battery on Fluke 85 III and now dead

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So I had four 30C 250mAh LiPo batteries laying around that I bought for my Blade mCP-X helicopter that did not fit well.

It seems like every 6 months, I get the low-Batt indicator on my Fluke 85-iii because is one of the earlier models that drains the battery quickly when in standby (and I tend to forget my meter on all the time).

So when the low-Batt indicator lit up yesterday, I decided to replace the 9v with two of these LiPo batteries in series.  I'm an active RC hobbiest so I always have a charger available, but never have 9v batteries.  This LiPo arrangement would allow me to quick-charge the battery in 30min every 6 mos for my Fluke, and I'd stop having to buy these $4 9V alkalines (and have to stop my project to go to the store to get the battery).

UNFORTUNATELY: When I soldered the 9v battery connector lead (Radio Shack #270-0324) onto the 2S1P 250mAh pack, I made the mistake of connecting the red lead to the postive and black to negative.  AND in my infinite wisdom, failed to check that the polarity on the connector metered out correctly (my meter had a dead battery and I was too lazy to go find my cheap meter).

Well, when you use the 9v battery connector lead as a source for a 9v battery eliminator, the polarity is reversed so the leads are the reversed colors.   Connecting my LiPo pack in reverse polarity apparently took out my Fluke because it's dead  :-\.  I opened it up and checked the fuses and they were fine.  I can't imagine the Fluke doesn't have reverse battery protection given that it's very easy to connect a 9v battery reversed while trying to snap it on, so I'm not sure why it died.  I can only speculate that the reverse protection circutry in the Fluke is designed for a large internal resistance battery like a 9v alkaline and when I connected the hi-C-rated LiPo battery reversed, it took the protection circutry out ?

I opened it up and didn't smell any magic smoke.  I downloaded the service manual but it didn't really have anything for testing the 9v source power circuitry and didn't contain a schematic on that area, so I'm not sure where to start.  I did the standard things... checking voltage was making to the board from the leads, testing the fuses, etc.

I'd appreciate any ideas some of you might have.

Thanks for reading,


Sorry to read. 

Just first thoughts before getting complicated: do you have a working 9V alkaline battery to reconnect to the 85-III and insure its really dead?  This removes any issues with the DIY battery pack cells or connectors.

If dead still, its possible the power protection circuitry for DMMs in the 80s series will be minimally changed from series 1.  I'd download Series 1 manual and there is a schematic to get a general idea of its location on the mobo; it also has a detailed hardware troubleshooting guide particularly for the power supply.

Keep us posted, good luck!

+1 on the connecting the original 9V supply for testing.

Not 100% sure, but I think the series III power supply is the same as the series I.  There is indeed a reverse-polarity protection diode.  If your modified battery popped it, the next check is the CMOS hex-inverter package (CD4069), which is connected across the battery all the time, regardless of switch position.  Five of the gates are used for the beeper, but one is used for power logic.

Be sure to check Vdd and Vss with respect to the COM jack, as I recall it's +3.0V and -3.2V, respectively.

Yes, I have tried a good working 9v.  I guess a lot of inexperienced people come through these forums, so I can understand the fundemental suggestions on the obvious  :D.

Thanks a bunch for the schematic on the series I - I hadn't thought of checking on the earlier series for it.  ...as well as the further troubleshooting suggestions.  I'll update here if I find out what blew.


I have no experience with the 85 specifically, but I can offer a tip. A simple reverse-polarity protection that is commonly used is a reverse-biased diode across the supply leads. The intent is the short out the supply (battery) if connected backwards, to prevent the reverse voltage from reaching the device circuitry. Often enough, this causes that diode to fail shorted, which will prevent the device from working even with normal polarity. Check for a reversed diode across the battery inputs, and check if it is shorted.


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