EEVblog 1449 - What Causes Excess Battery Drain? (BM235)

[EEVblog]

A BM235 customer has a BM235 that drains the batteries in OFF switch mode, lets look at the PCB to find out the possible culprits. Potentially applicable to any battery powered product.

[Old Fart Analog Engineer]

Most likely that "giant plastic water pipe" overflowed with some liquid (probably sugary) and deposited and dried on those capacitors causing the excess leakage current.  Even more likely the dried conductive residue is on the function switch.  Take it Apart, Clean it off, Dry it and Robert is one of your parents sibling!  😉

[ormaaj]

Any way the diodes could become forward biased? You've got diodes, inductors, caps, and a switch - the ingredients for a momentary impulse current.

I don't actually know whether a diode can quickly degrade from a momentary surge through a mechanism other than junction overheating. Unlikely for a diode that size. The MLCCs are certainly very suspect.

[rob77]

actually i had a similar issue with high current draw of a different brymen meter here...

https://www.eevblog.com/forum/testgear/brymen-bm869s-rejecting-fresh-9v-batteries/msg959057/#msg959057

[G7PSK]

I had a digital tyre inflator /pressure gauge that used a coin cell, right out of the box it drained the battery over night and did several new batteries the same so I contacted PCL who made or sold them and they just sent me a new one. Having been told I could bin or do whatever I wanted with the first one I took it apart and had a look at the board , first thing that caught my eye was a big fat ceramic capacitor right across the battery so I removed it and not having a replacement I put the board back in and the gauge worked perfectly and the coin cell is still working the unit two years later. Not sure why the designers thought the cell needed a bypass cap. when the microchip had several of its own. Excessive battery drain is often down to leaky by pass capacitors and always worth a look if you have that type of trouble. 

[Kleinstein]

My suspicion would also be with the capacitors. The place shown on the PCB is an area of high stress, that should be avoided by MLCCs. Not as bad as the position of the inductors, but still a poor placement.

The caps are likely for EMI reasons. Hard to tell why on that side of the switch. With a large capacitor the surge current on switch on may be a reason to have the capacitor before the switch, but not really with those small ones.

Star grounding is good practive for low frequency precision. However repeated branching off from the ground is a bit odd and not good practive, more like a compromise needed. There should be 1 true star ground and than all critical traces going from there and one branch may be shared for the non critical parts. There may still be a few more star points for other signals, though usually not that many lines leaving there. 

[refd]

The addition of the 1N4007 was probably to shunt batteries that were inserted incorrectly.  The design of the case may not prevent contact for batteries installed backwards.  I don't have one of the meters so I can't say if there is an external power jack.

The capacitors on the battery are usually used to help shunt ESD discharge when the meter is off.  Without these caps to ground a high static charge when replacing batteries might bridge the narrow clearances in the power switch and make it to sensitive circuits.  Subsequent static discharges can make the diode and the caps leaky.  Since the meter with the leakage seems to be older it probably has seen it's share of battery replacements and thus could have been exposed to leakage inducing ESD.

[JohnWW]

Hi Folks

Went to use my BM235 for the first time in a month to find that the batteries had leaked.  Cleaned the effected areas of the battery contacts with Isopro. Now if i have a none powering unit.

Have check and I have voltage at the spring contacts. have made sure the springs are making contact with the copper pads but still no go.

Qustions for the brains trust:

is there something elso that i should look at to get the meter back up and running?

Has anyone done the remove of the diode and caps to stop the leaking?

Thanks

JWW
 

[Kleinstein]

The diode and caps are there for a reason. Normally if nothing is damaged the leakage from the 2 should not be so bad.
With the diode they may have got the wrong type of diode:  an UF4007 is way more leaky than an regular 1N4007.

For cleaning after a leaking battery IPA is not helping that much, as it is not good in desolving salts. It is usually more lots of water and maybe some dilute white vinegar or similar weak accid to remove copper carbonate that may form from corrosion and CO2 from the air. IPA or similar may than be the very last step to dry things up.