EEVBlog / BM235 LCD & backlight not working

[ErnstN]

Hi folks,

I'm looking to fix a broken LCD display and backlight of my EEVBlog / BM235 multimeter.

When I last replaced the batteries I accidentally inserted Lithium batteries. That made the device behave funky and might have killed the screen / backlight function. Looking at the display under an extreme angle, I can see an oil-like flair-up originating from the corner, I've attached 2 pictures. Could that be the result of the backlight burning out and taking the display with it?

Without the display I can't obviously know for sure the devices still functions, but when I switch the device in "connection-test mode" or "EF mode" the audio signals seem to come at the right moment ... so something is still working.

Any suggestions on how to go about, where to find a spare part and did somebody else encounter this?

Your help is much appreciated.

[AVGresponding]

That optical effect is just diffraction in the front protective lens, nothing to do with any problems with the LCD.

It's caused by light reflecting back through micro-stresses in the moulding (caused as it cools), and makes it behave like a diffraction grating from certain angles.



EDIT: What type of lithium cells did you put in?

[ErnstN]

Good to learn the optical effect isn't a damaged display. I've tried a number of different "off-the-shelf" batteries; Philips long life, Energizer Ultimate and now Duracell alkaline.

[AVGresponding]

AA cells within the usual voltage limits shouldn't damage the meter. Though I don't have an EEVBlog meter (don't lynch me!), I have a wide selection from Brymen and other manufacturers. I have had no issues with Ni-MH (1.2V nominal) or Ni-Zn (1.6V nominal) in any that use AA, though I use Li-ion PP3's in the one's that use that size. Meters that use AAA so far I only used Ni-MH LSD cells.

[J-R]

AA/AAA lithium primary batteries have a very high voltage when new, over 1.8V.

[Fungus]

That's not usually enough to kill things, but...

...you're right. Lithiums are very bad for BM235s. Apparently the main chip is designed for 3.6V and there's no voltage regulator.

https://www.eevblog.com/forum/testgear/the-bm235-does-not-accept-lithium-batteries/

https://www.eevblog.com/forum/testgear/bm235-lithium-battery-mod/

Seems like you might have damaged it.

[AVGresponding]

Ni-Zn have a cell voltage of over 1.8V when freshly charged too, not had problems but as I already said, I don't have a BM235.

No regulator? That sucks balls, and is a bit of a disappointing design choice by Brymen; Lithium primary and Ni-Zn cells would both have been available at the time of design. Does the user manual mention not to use these chemistries?

[Fungus]

Does the user manual mention not to use these chemistries?[/color][/size][/b]

I checked and it only says "Use 1.5V AAA".

No specific warnings against Lithium or Ni-Zn.

[Brumby]

I checked and it only says "Use 1.5V AAA".

That's a pretty clear message against batteries around the 3.6V range ... such as lithiums.

[AVGresponding]

I checked and it only says "Use 1.5V AAA".

That's a pretty clear message against batteries around the 3.6V range ... such as lithiums.

Not really. The nominal voltage of alkaline primary cells may be 1.5V, but a fresh cell will have an OCV of around 1.65V, so a 2-cell battery will have 3.3V, above the nominal 2-cell of Ni-Zn of 3.2V. Also bear in mind the nominal voltage for a AA Lithium is 1.5V (we are not talking about lithium thionyl chloride here, but cells such as Energizer L91 Ultimate Lithium).

Also bear in mind the nominal voltage is what a fresh cell outputs under a nominal load, and depending on application, actual load voltage on a fresh cell can more more or less than this.

https://data.energizer.com/pdfs/l91.pdf

[ErnstN]

> Apparently the main chip is designed for 3.6V and there's no voltage regulator.

thanks for weighing in all, seems like I've killed the device. Expensive lesson in chemical makeup and too bad that battery form factor's aren't a reliable affordance.

[Brumby]

Not really. ... ...
... ... (we are not talking about lithium thionyl chloride here, but cells such as Energizer L91 Ultimate Lithium).

OK ... I should have clarified further or, better still, stopped here:

That's a pretty clear message against batteries around the 3.6V range.

[Brumby]

... seems like I've killed the device.

We're sorry for your loss.   

[AVGresponding]

Not really. ... ...
... ... (we are not talking about lithium thionyl chloride here, but cells such as Energizer L91 Ultimate Lithium).

OK ... I should have clarified further or, better still, stopped here:

That's a pretty clear message against batteries around the 3.6V range.

Yes, but unless you actually read the datasheet or test them before using them, you might be forgiven for thinking these cells are 1.5V, since that is what is printed large on the retail packaging.

It might be worth approaching Energizer with this in mind, with a view to having them pay for a replacement meter.