GPS Battery

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Raining here, so I thought I'd look over my GPS module. Putting a meter to the battery I'm reading 1.65VDC. I believe it should be Nom. 3VDC. Can't tell as the solder straps cover the part number, anyone know the part number?

I can make out Mxx21FE, possible a MS621FE Seiko II brand

Dimensionally it matches...

MS621FE
Nominal Voltage(V) 3
Charge Voltage (Standard Charge Voltage)(V) 2.8 to 3.3(3.1)
Nominal Capacity(mAh) 5.5
Internal Impedance(Ω) 80
Standard Charge Discharge Current(mA) 0.015
Maximum Discharge Current (Continuous)(mA) 0.25
Cycle Life (Time)100% D.O.D(Depth of Discharge) 100
Cycle Life (Time)20% D.O.D(Depth of Discharge) 1000
Size Diameter(mm) 6.8
Size Height(mm) 2.1
Weight(g) 0.23
 
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I concur and I have done a slightly different GPS mod to my Solo. (I actually did this last November)

The datasheets for the u-blox 7N module refers to the backup battery that maintains the Almanac, Ephemeris
and time during power off, discharge current is typically 15uA. Settings can be saved in internal Flash EEPROM (or so all the product web
sites say), but not Almanac & Ephemeris.

https://www.u-blox.com/sites/default/files/products/documents/MAX7-NEO7_HIM_(UBX-13003704).pdf
https://www.u-blox.com/sites/default/files/products/documents/NEO-7_DataSheet_(UBX-13003830).pdf

That battery on the Solo is a MS621FE, which is the cell recommended by u-blox.

http://www.sii.co.jp/en/me/files/2014/11/BAC3011EJ-01C1409_web__.pdf

On page 6 of the battery datasheet we see:

voltage 3
capacity 5.5mAh
charge/discharge current 15uA (although later on the graphs indicate you can charge it at 100uA)
cycle life at 100% DOD 100 (Depth Of Discharge)
cycle life at 20% DOD 1000

When I disassembled my Solo after a week of intermittent use and measured the MS621FE voltage, it was 1.4volts,
indicating that it was very close to fully discharged, see page 7, 8.
Page 7 indicates a charge current of .1ma to 3.3v for 96 hours will result in the capacity being maximized, and results
in about 1% charge gain per hour. The u-blox datasheets indicate 1.6V minium for data retention.
But, to charge for 96 hours one would have to have the Solo powered on (flying or not) for that whole time.
If you have only one battery and at 2 hours per charge and don't fly at night, you MIGHT get 1-2 hours per day of charging time.
With 4 batteries, you might be able to get 12 hours of charging time, and at about 1% charge gain per hour (and .3% per hour charge
loss) you might actually gain some charge over the course of a day! That's if you fly (or power on) continuously for 12 hours straight.
Of course you don't need really need a fully charged backup cell.

At 15ua discharge current a fully charged battery MIGHT last 366 hours, or about 2 weeks, that's about .3% charge loss per hour.
(The Almanac and Ephemermis decay in validity as well during this time so the GPS would still need to
re-download some of each after extended power off time). Time, once off by > 20 seconds, makes sat acquisition difficult.

Consider this, if the Solo spends 2 weeks in the supply chain after manufacture, it's backup battery will be pretty dead by the time
it gets to its buyer, and won't be able to reach full charge unless extraordinary measures are taken. Partial charge will
effect cycle life and be terrible if you can ever get it recharged enough to matter.

And that assumes the the battery is fully charged when it is installed in the factory.

This MAY be why new owners see 3-4-5 minutes to get enough sats to fly.

The result of this is that the GPS module will loose its time/Almanac/Epheremis and do a cold start almost every time
the Solo starts up. (1.6 volts is at the minimum necessary to maintain SRAM integrity)

TTFF for the u-blox 7 is specified as 28 and 29 seconds for warm and cold starts with 1 second for hot start. This is
for the first 3D fix of 4 sats. Solo wants >=7 and a HDOP of <2 so TTFF should not be relied on to indicate how long
it will take for the Solo to be ready to fly. In addition, these factory tests were performed with the GPS signal provided by a simulator,
so no interference, and signal levels all at -130dbm which is certainly not reality. I've had 9 sats sitting for 10 minutes and not be
ready to fly. Why? the HDOP was 4 due to.... whatever... bad geometry, high Kp, phase of the moon...


I have modded my Solo and replaced the MS621FE with a non-rechargable CR2032, also removing the diode from the charge path so there is no possibility of charging that cell. Which might result in "Bad Things(tm)" happening.

http://data.energizer.com/PDFs/cr2032.pdf

With a capacity of 240mAh, at a 15uA discharge it should see a lifetime of 1.5-2 years.

This is "engineering accuracy" and only pretends to be at best +/- 20%, but is certainly good enough for this seat of the pants analysis.

At the same time I did this mod I noticed that the copper foil shield was very close to/touching the antenna/lna components. I took
this as an opportunity to add a small plastic box around those components as shown in the photos "just in case". Whether or not this
is a cause of problems remains to be seen.

My Solo came from Amazon with 2 day shipping, but was the last one they had for almost a week, so who knows how long it was in
transit from the factory in China.

u-blox specifies 28/29 seconds for a warm/cold start, having the Almanac/
Ephermis/time has made a difference to my modded Solo.

I believe that 3dr made a mistake in designing the GPS in line with the u-blox design guide, which was intended for "always on" applications where the small battery will not be discharged too much and will have ample time to recharge during normal operation.

>>>The Solo is NOT the same use case. Especially considering what initial shipping times from China might be.<<<

My u-blox 6 & 7 modules all have the same battery as the Solo and they are all always dead. The u-blox 6 can store
the data in an eeprom, unfortunately the u-blox 7M can't and has to rely on the backup battery. The u-blox 7N used in the
Solo can store some data in eeprom, but it can't keep time when the battery is dead, or do a warm start.

IMHO it would be better to have a 1-2year non-rechargable battery that guarantees Time/Almanac/Ephermis are retained during long power down times (like shipping from China), (the Almanac may still be ok, but the Epheremeris won't) rather than one that will discharge during shipping (or idle time) and never be able to be fully charged again. My tests seem to prove this out.
The ephemeris and clocks parameters are usually updated every two hours, while the almanac is updated at least every six days.
 
Well then, a little more than my request for a battery part number.... You've been holding out or where was this originally posted?

Very interesting discovery you had and explaining a work around. Kind of connects with some of the snippets related to the RevB gps module, IIRC was something to do with capacitors and battery backup.

I like the CR2032 idea and you make a convincing point as it has a larger capacity. I can handle swapping out every year, no problem. Would be nice to have a quick lock after a week of sitting, now that the battery is discharged to the present level and unlikely to recover enough to be of use as intended.

Obviously, thanks for the share. I can't recall this being discussed previously, but now it will be.
 
I have modded my Solo and replaced the MS621FE with a non-rechargable CR2032, also removing the diode from the charge path so there is no possibility of charging that cell. Which might result in "Bad Things(tm)" happening.
Can you point out the diode to be removed?

Nice fit with the battery holder...
 
Wow. That's awesome. What kind of difference have you noticed with the solo getting a satisfactory lock? Also, you might want to purchase the new RF shield from 3DR, which replaces that useless crappy copper foil.
 
I have a 3DR new shield, but have yet to put it in. My plastic "cardboard" mod works ok.
I have 3 Solo's 1 with the GPS mods one pure stock (RevA GPS) and then there is FrankenSolo.

Sorry RichWest, it was never posted anywhere.
/me pleads for mercy

As to how much difference it makes, well, if the Solo sits for > 6 days, all that is valid is the time (if the chintzy 32khz clock hasn't drifted toooo much) and it's 30-60 seconds on average.
At times less that that I usually get 4-6 sats immediately. Then more till a max of 12-13. I can usually tell if the clock has drifted to far as I don't get anything for ~30-45 seconds.
Remember the Ephemeris is only good for ~2 hours.

The more you fly the better your GPS battery will feel, so fly often!!
 
Diode loaction
 

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Diode loaction
I wouldn't know a diode from a capacitor, but looking at the underlying traces seemed the big black cube was it.... Thanks again for the info.

I did the cardboard mod as well and it has worked from the day it was installed. I did use a better copper shield however. But the start up time seemed related to memory of the Ublox, which sometimes was great and others just plain crappy. Just random.

And no mercy needed. Seems many hold back, myself included, since the dev culture seems too critical when revealing a pimple on their baby. I just want a solid bird, no harm in that, just tell me what's wrong and then let's fix it. Ain't nobody got time for that.

My bird tends to collect dust during the week these days, to firgg'en hot and humid. Getting ready for Fall flying weather, ends in January...;)
 
I just re-read your post, GPS Battery , and had time to absorb it. I really enjoyed reading it and learned several things that I had never understood. You Rock!

Just happen to have the parts that I can re-purpose Thanks again for the share.
 
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Dang cynfab... you just been sitting around, lurking in the background with all this info??
 
I really do not know anything of these things, I'm just a mechanic, but was able to meter the voltage at the gps battery while the bird was on. Here's what I found...

2.25 VDC with no supplemental power, I had turned the bird on several times since yesterday's reading of 1.65VDC.
2.60 VDC after about 5 minutes of power being on
2.50 VDC after about 10 minutes without power
2.39 VDC after 20 additional minutes without power
1.95 VDC after 1-1/2 hours without power

Just meaningless data, but allows me to understand further some why's and what's... I did mod a CR2032 battery and holder with leads last evening, so I'm ready to perform the battery replacement mod. Rather than jump out the window however, I've decided to order a replacement battery as supplied by the factory. I'm assuming my battery is near end of life based on the discharge rate indicated by cynfab's calculations and the data above.

I'm sure I'm being over-cautious for not doing the mod today. But seems logical to at least test with a known good battery as a bench mark first. If it fails to maintain, then I can perform the mod without reservations for creating a larger failure.... Worst case is a new gps module.
 
I have no patience...went ahead and mod'd the battery in...holy snap. I got lock just right after the bird fully booted, like within 5 seconds. Granted it was 5 minutes after I had shut the bird down testing the install in my shop. I can't recall getting that fast of a lock...ever. Probably screwed something up....

@cynfab Not much room under and around that area to mount the battery holder, mine ended up about two battery thicknesses and about the same footprint as yours. I hot glued it to the underside of the shield. My spacer mod covers the entire gps board and stands off about 4mm. I may need to rethink, what did you make your spacer/box out of?.
 
If anything just check the voltage on your battery. Definitely under a minute from the various flights and conditions. Seems strange no one has mentioned this before, other than slow gps lock....

I'm looking at CR2032 with solder tabs now, double sided tape and done.
 
@cynfab - Let me read between the lines after the very thorough analysis of the GPS battery above (Thanks!). If the casual sport flier/drone flier leaves his/her drone on the shelf for a couple weeks and the GPS battery was already weak, it will do a cold start and acquire data during the ~3-5 minute wait, correct? What happens if the battery goes totally south? Always a cold start? or will it not be able to get the GPS data at all?
Seems like this should be an externally replaceable battery (i.e. like the 2032) so the user can easily replace it.
 
Correct when the GPS cold starts it does not have Time, Almanac or Ephemeris. and needs to acquire that data from the satellites.
u-blox says a cold start should take 29 seconds, but that's under lab conditions and is only for 4 sats with unspecified DOP.
Getting what Solo needs will take longer, how long depends on many factors.

The battery is connected to the V_BCKP pin of the u-blox M8N or Neo-7N module.
From the u-blox Hardware Integration Manual:

"As long as power is supplied to the NEO-M8 module through the VCC pin, the backup battery is
disconnected from the RTC and the BBR to avoid unnecessary battery drain (see Figure 1). In this case,
VCC supplies power to the RTC and BBR."

So if the battery goes south or is of poor quality to begin with it will just cause a cold start each time.

RTC= Real Time Clock (keeps time)
BBR= Battery Backed RAM (holds setup parameters, Almanac and Ephemeris)
settings can also be stored in internal flash

A typical use case for the u-blox series of modules is in an "always on" situation like a cell phone or a GPS disciplined clock, in these situations, a small rechargeable battery is ok since off time is rare and there is plenty of opportunity for the battery to recharge in normal operation.
 
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Thanks for info. Like most typical R/C flyers who perform battery management for their transmitters, receivers, and flight batteries if they are flying electric, that management is a known part of flying. With the advent of more sophisticated electronics like we have in the Solo, there isn't anything (other than pilot forums) that would provide knowledge of the GPS battery. ( I don't think it is in their manual) So it would go un-noticed. If it is part of my equipment upkeep and I have several drones, I would need to power them at least 1x per week or so just to 'maintain' the GPS battery.

Most consumers (never having flown R/C or drones) might just shove their drone on the shelf and take it on vacation a few times a year or get it out when they have a family gathering or similar. So they will have trouble with power-up/GPS etc when they do go to fly.
 

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