Add a backup battery on the MTK GPS module.

Hello Everyone, we all know our GPS board (the one that comes with the APM1) has no battery to backup data.

This has the disadvantage to make the GPS not very precise in the first minutes after power up and that every startup is a cold startup (at least one minute to get a 3D fix).

This is because the GPS needs find the satellites without a previous almanac (that is information on position of the satellites visible in that location at a specific time).

As you understand almanac is VERY important. Also considrer that the almanac is sent by every satellite and requires 12.5 minutes to transmit (or so).

From Wikipedia:

 "Finally, the almanac allows a single-frequency receiver to correct for ionospheric error by using a global ionospheric model. The corrections are not as accurate as augmentation systems like WAAS or dual-frequency receivers. However, it is often better than no correction, since ionospheric error is the largest error source for a single-frequency GPS receiver. Each satellite transmits not only its own ephemeris, but transmits an almanac for all satellites"

Of course this is not sufficient to have a perfect position: sky visibility, surrounding objects and number of satellites are the key factors. 

This is why after a FIX (fixed blue led) number of satellites is very low (5 to 8) and this get higher (10 or 12) after some time of flying.

So, because of this, I decided to bring up an old hack made by Dcaspi, an old friend who worked also on the former Ardurpirates code.

The hack requires some soldering skills, so I suggest you to do it only if you are confident with it.

Beware if you fry the board or GPS unit, I cannot (nor Dcaspi) be responsible for it...

This mod works and has been used for a while.

This is the original location:

http://code.google.com/p/ardupirates/wiki/MTK_GPS_backup_battery?ts...

Have fun!

Emile

Introduction

If you use the DIYDrones MediaTek? GPS, you're probably aware of the fact that, on each activation, it make take a very long time to get proper satellite lock. The reason is that it always starts from scratch; it is not aware of it approximate geographic location, not of the time.

The reason for this is lack of backup battery. Data is completely erased once you disconnect the main battery.

The GPS module itself has a backup battery input. Here we show how to connect an external battery to that input.

What You Need

You will need a a battery holder and a small, 3V, coin-shaped lithium battery. These can be purchased at any DIY electronics store.

I also used a piece of perforated prototyping board, but this is for convenience and not a must.

How It's Done

Below, you can see the GPS module board layout. Cut one trace as shown, and connect the two leads from the battery holder to the marked places.

Here is how the setup looks:

This is how it looks from below. Note the place where the trace is cut:

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Comment by Dean Wynton on May 31, 2012 at 3:09am

Emile the APM2.0 uses a 124ohm resistor from the 3.3v feed to the backup pin. Should this be done also?

Comment by ThomasB on May 31, 2012 at 3:24am

Thanks for this usefull informations/pics  :)

Comment by Nick Arsov on May 31, 2012 at 3:47am

Just an advice - keep the battery cables away from the antenna, otherwise they will disturb/lower the sensitivity. Lay them towards the comms cables.

Comment by Paul Mather on May 31, 2012 at 5:02am

Didn't the MTK board from 3DR used to have a battery on it? Why did they get rid of it?

Comment by Mark Murray on May 31, 2012 at 5:48am

Hi

This is good news! It looks like something very similar can be done on APMv2, except that there is a convenient resistor to remove (R3) giving a pad to connect the battery to.

I'll probably go a step further by using a 1F SuperCap in place of the battery, and a couple of signal diodes (1n4148 types) to drop the 5V down to about 3.4V and prevent discharge when the main power is off. The SuperCap hasn't the power-off life of a battery, but I think it may be adequate.

M

Comment by emile on May 31, 2012 at 7:34am

@Dean, I currently don't have an APM2 so I cannot help.

It would be nice though if anybody could give it a try and post their mod.

Since the GPS is the same, it would probably be a similar procedure.

@Paul the old board had a battery but was discarded because there were some problems which I don't remember/know.

@Mark, could you give use some info on that?

Comment by Mark Murray on May 31, 2012 at 7:41am

@emile: Er, do you want me to give more info or are you offering to give me more? :-)

M

Comment by Nick Arsov on May 31, 2012 at 7:43am

It shouldn't be any problem with adding a backup battery. I attach the documentation of 3329 .... see p.10.

http://inmotion.pt/documentation/diydrones/MediaTek_MT3329/mediatek...

Comment by emile on May 31, 2012 at 7:57am

@Mark: use = us ... typo.

@Nick this is a good start point the Doc says the Backup pin should always be connected in order to work, so probably to attach a battery one should remove the actual trace giving the 3.0V 

Comment by Mark Murray on May 31, 2012 at 8:45am

@emile: Sure! I'll see if it works first, then put the full story up here.

The intention is to do something like this (using lousy ASCII-art for the diagram)

5v(+)--->|--->|---\/\/\/\---(A)---|(-----(O) Gnd

This forms a string of components from 5V to GND, in order

1N4148 Signal diode

1N4148 Signal Diode

150Ohm 1/4W Resistor

1F "SuperCap" Battery backup capacitor 5V (any big 5V supercap should work, like 1.5F, 2F etc).

Point A is connected to the GPS unit's VBACKUP pin. Previous connections to VBACKUP are removed/cut; for APMv1, this is the same cut as above, for APMv2, this means removing R3.

Power retention won't be brilliant; several days from a quick back-of-envelope calculation. It would be very adequate for a day's flying. You could have multiple battery-pack changes, without having to go through the bother of a full cold boot on any startup other than the first, or if you fly often enough, very seldom have cold boots.

Hope this helps!

M

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