digital compass and magnetized batteries

[therustyone]

I've got a digital compass that runs off one AAA cell (the sort you can get on eBay), which recently went haywire on compass directions. After taking it to bits out of curiosity and running the PCB seperately without any problem, it was obvious the replacement battery itself (Duracell Plus) was magnetic and causing magnetic interference. I then found some discussion of this on a forum from around 5 years ago.

where can you get totally non-magnetic AAA cells in the UK, preferably leak-proof.

rusty

[Graham.]

> So where can you get totally non-magnetic AAA cells in the UK,

I hadn't thought of that, but I do know that sailors need to be wary of such things as spectacle and sunglasses frames that can introduce a deviation in the binnacle if they are magnetic.

[Reentrant]

> So where can you get totally non-magnetic AAA cells in the UK,

My Garmin GPS (76CSX) has a built-in magnetic compass sensor so it knows north even when you're not moving. This needs recalibrating after you change its two AA batteries. You start calibration from the menu and slowly turn round clockwise twice. Now I appreciate why.

[Tim Watts]

Hi,

A lot of batteries IME are in a steel outer case.

Unlikely - a compass would generally use a magnetic sensor (Hall or fluxgate) as you'd want a reading without having to walk in a straight line for some 10's of yards as is necessary with a GPS.

Cheers

Tim

[Man at B&Q]

You need to define "GPS compasses". They are not all created equally.

JFGI

MBQ

[dennis]

My magnetic compass needs recalibrating frequently, not just when you change batts. Its probably because even stuff in your pockets can have an effect. Fortunately its a toy and I don't need it.

[dennis]

AFAIK there is nothing in the gps system to allow a stationary receiver to know where North is. So all gps compasses are equal

[Terry Casey]

>

I had an electronic compass - I think it came from Halfords - in my pre SatNav days.

After it was fixed in the position, you pressed a setup switch, then drove round a car park in circles until it was happy. Used a 3V Lithium cell IIRC.

Of course, it was adjusting to the magnetic signature of the car - not the battery!

I went round a minesweeper once - fibre glass hull, stainless steel engines, etc.

It's magnetic signature was measured and electronically neutralised at the beginning of each trip, after which anything that could make a difference had to stay in the same place.

Thus, it was explained to me that if you start of with a box of Baked Beans cans in the galley, each can has to be put back into the box in e4xactly the same position after it is emptied. This carries on until all the cans are empty, then the box of empty cans is swapped for a another box of full ones, and so on ...

[Skipweasel]

More likely they're in a non-magnetic shell. The casing of most batteries is steel because it's cheap.

And his device is a flux-gate compass, I suspect. Nothing to do with GPS.

[Skipweasel]

MagneticRays!

[therustyone]

I've got the same calibration instructions (its made by Bresser) but that's not enough to remove the magnetic battery effect. I think it just calibrates the X-Y sensors. I do have another digital compass with a button cell instead of AAA so I hope that battery is non magnetic when it gets replaced.

I tested some Duracell AAA's near a traditional needle compass and they swing it all over the place, just the same as the electronic one.

rusty

[hugh]

Similarly with my Satmap

[geoff]

In message , hugh writes

Same with the one in my new car

not that I bothered

[therustyone]

[e-mail]...

that reminds me about Canvey Island's surviving WWII degaussing station for ships:

problem with GPS is the signal disappearing in dense woodland and you're soon totally lost. A magnetic backup is handy.

[JoeJoe]

>>> So where can you get totally non-magnetic AAA cells in the UK,

The compass on my HTC Desire mobile is calibrated by waving it in a figure of 8 movement (looks stupid, but it works).

[chris French]

not so much of a problem with lots of units nowadays.

My Garmin vista Hcx has a high sensitivity reciever and I can't remember ever having much of a problem, even under heavy tree cover. It can get a bit less accurate, but so much that it is a big deal.

[chris French]

In message , "dennis@home" writes

True, but many some GPS recievers (like mine), have a built in electronic compass, as well as the GPS signal derived one.

which is maybe what MBQ was referring to.

[dennis]

As in a magnetic one and hence suffer from the same problem as the OP has.

Possibly.

[Peter Parry]

There is no such thing as a "GPS compass". GPS can give a track when moving and a position when static. When stationary it does not know north from south east or west. This is an inherent characteristic of the GPS system and nothing can alter it.

To compensate for this "feature" some manufacturers build in a conventional flux gate compass which has no idea of position but does know where north is. This compass relies upon the earths magnetic field.and is as affected by nearby ferrous objects as is any other compass.

The nearest ferrous object is usually the steel cases of the batteries so most such compasses include some signal processing to compensate for nearby fixed magnetic materials. With a conventional compass the process of correction is done by adjusting compensation magnets around the compass b9ody, with flux gate compasses it is done by recording the error table as the compass is moved (see "swinging a compass").

[Man at B&Q]

[e-mail]...

Some only work when you actually move, so the gps signal is used to work out the direction.The sort "harry" was referring to when he said "I know GPS compasses do not rely on the earth's magnetic field to establish North".

Some have a built in electronic compass and so DO know where North is. The sort that "harry" seems unaware of.

MBQ