Flight Sketch Mini battery life and removal?

I have 3 FS Minis (one beta, two from the Black Friday sale) and love them, so much better than the Estes altimeter. Almost lost one on Saturday when my rocket got stuck high in a tree, but fortunately got it down.

Two questions for other users:
1) How long does the battery last for you? I'm only getting about 3 flights per battery, does that seem normal? I've had a few flights where the rocket sat on the pad for a long time and/or it took some time to find the rocket after the flight, but still the battery life seems short. Fortunately the button battery is cheap.
2) Is there an easy way to remove the battery? I'm having to tug on the white plastic case quite hard to pull it out, perhaps I'm doing it incorrectly.

Thanks,
Bill

Three flights? Good heavens, the thing will literally run for days on a fresh cell. I routinely turn one on during a flying session and leave it on for several hours straight.

Getting the tray out does take more force than you expect. But then, you want it to say in even though the altimeter is subjected to some pretty crazy shaking about at ejection....

It’s a slightly larger version of the same tray/holder used on the FireFly and MicroPeak. The easiest way to get it open for me is to use a small flat head screwdriver to pry first in one corner (put in, twist between plastic and metal), then go to the other side and do the same. Then “just” pull it out.

Russ uses Renata cells and they can be had from Amazon for a little more than those Chinese knockoffs.....

"Three flights? Good heavens, the thing will literally run for days on a fresh cell. I routinely turn one on during a flying session and leave it on for several hours straight."
You are having better luck than I am. The first time, I assumed I must have forgotten to turn it off, but then it happened twice again after I was certain I had turned it off (3 flashes on the LED means off, right?).

How much voltage is needed for the Mini to work? I put in a fresh battery, downloaded a file, and noticed on the display that after just a few minutes the voltage had dropped from 3.3 to 3.2v. After one flight, it was down to 2.9v.

Well, since those are nominally 3V cells, dropping from 3.3V to 2.9V isn't all that surprising. You're changing out those cells way too soon.

I've had the "how low can you go" discussion with Russ since I've had them out on cold days and seen intermittent voltages down to ~2.5V.

Here's most of the email he sent me when I asked that question:

Russ Parrish to Bernard Cawley via email said:

The flash memory is the limiting component as far as voltage goes. It is only rated down to 2.3V. It will likely continue to operate a little below that but not guaranteed. The voltage displayed will noticeably fluctuate with load since the cell has a relatively high internal resistance. The Bluetooth advertising interval has a random component to help avoid collisions with other devices so the voltage displayed is also somewhat random. The biggest load is erasing the memory when it is armed so if it can be armed, it will probably complete the flight if there is not too much delay before flying.

At room temp, the discharge curve gets pretty steep after about 2.5V. The cell also has a significant temperature dependance so is is normal to see a drop of several tenths just moving outside this time of year. The data sheet for the cell is linked below:

https://www.renata.com/fileadmin/downloads/productsheets/lithium/3V_lithium/CR1225.pdf

The average load for the Mini is close to the 15k Ohm curve.

The forward voltage of the LED is 2.0V it should be getting dim at the lower levels too.

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Yes, three quick flashes after holding the switch down for a few seconds is "off". The state indication of the altimeter via the LED is an area that could use a little improvement. When Russ implements the full set of indications required by the FAI altimeter spec, which is something on the "to do" list as I understand it, it will be easier to tell if you've forgotten to turn it off or not than killing the app on your phone and then restarting it to see if if it finds any active FS Minis.

I was at a launch on Sunday and wasn't the only one with FS Minis there....and we were seeing each-other's units on our phones, too.

BEC said:

Well, since those are nominally 3V cells, dropping from 3.3V to 2.9V isn't all that surprising. You're changing out those cells way too soon.

I've had the "how low can you go" discussion with Russ since I've had them out on cold days and seen intermittent voltages down to ~2.5V.

Here's most of the email he sent me when I asked that question: The flash memory is the limiting component as far as voltage goes. It is only rated down to 2.3V. It will likely continue to operate a little below that but not guaranteed.

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I don't change the cells until the unit stops working. But if each flight drops the voltage by around 0.4v (e.g., 3.3v to 2.9v) and the Mini stops working when battery voltage drops below about 2.3v, then 3 flights per battery seems pretty normal.

billdz said:

But if each flight drops the voltage by around 0.4v

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Its not necessarily a linear relationship

billdz said:

I don't change the cells until the unit stops working.

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"Stops working" - how? I have occasionally had them freeze, typically after a flight. Pulling the cell out and putting it back in revives them. I have at least 156 flights (assuming I just counted my posted logs correctly) on these from the first betas to the current production version, in some cases with more than one in a model, and have only had to do this a handful of times, usually after a really hard landing (or in one case when the model landed in a puddle). I do have one beta that seems to be more prone to this than the others.

But if each flight drops the voltage by around 0.4v (e.g., 3.3v to 2.9v) and the Mini stops working when battery voltage drops below about 2.3v, then 3 flights per battery seems pretty normal.

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As noted just above, it's not linear at all. Look at the Renata data sheet linked in my quote of Russ' email. Also read (or reread) Russ' explanation of the varying loads and therefore varying voltages reported.

I think I've replaced a cell two or three times as a precaution across the flights noted above plus ground testing. Now granted they are not high power flights to over 6000 feet as the last flight you posted to the log was, and many of them were me flying on my own and so the models weren't sitting on the pad for more than single-digit minutes. But I've also had one left on for literally four hours at a stretch when doing a series of test flights as I did on 2/3/20 with no issues. Those sessions really suck down my iPhone's battery, though.

"Stops working" - how?
Would not turn on (no LED flash) when I pressed the on/off button.

"When Russ implements the full set of indications required by the FAI altimeter spec, which is something on the "to do" list as I understand it, it will be easier to tell if you've forgotten to turn it off or not than killing the app on your phone and then restarting it to see if if it finds any active FS Minis."
Good to hear, that would be helpful. I liked the slide switch on the beta which let us see with our eyes if the unit was on or off, not sure why he switched to the push button.

All 3 of my Minis are now are on a replacement battery, it will be great if they last 4 hours at a stretch. But no big deal if battery life is short, the batteries are cheap and the Mini is a great piece of hardware.

Anyone else have experience with Mini battery life? How long did the battery last?

billdz said:

"Stops working" - how?
Would not turn on (no LED flash) when I pressed the on/off button.

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Yeah....if you're not sure if it's on or off that can be frustrating. This is where I have pulled the cell tray out and put it back and gotten the unit to wake up again.

"When Russ implements the full set of indications required by the FAI altimeter spec, which is something on the "to do" list as I understand it, it will be easier to tell if you've forgotten to turn it off or not than killing the app on your phone and then restarting it to see if if it finds any active FS Minis."

Good to hear, that would be helpful. I liked the slide switch on the beta which let us see with our eyes if the unit was on or off, not sure why he switched to the push button.

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Smaller, lighter, and I had a couple of cases in beta where, with the unit in a pouch in with the recovery system, that an ejection event wiped that slide switch off the board (or broke it loose). The switch he's using now is just like the one on the FireFly—it may even be the same part.

All 3 of my Minis are now are on a replacement battery, it will be great if they last 4 hours at a stretch. But no big deal if battery life is short, the batteries are cheap and the Mini is a great piece of hardware.

Anyone else have experience with Mini battery life? How long did the battery last?

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Agreed it's great little unit....and I'm curious about others' experience as well.

But what I think you've experienced isn't the cell dying but rather it getting jarred loose and needing to be reset.

You should get far more than a few flights. What brand of batteries are you using? Have you been flying in cold weather? In very cold conditions, the coin cells will drop significantly in voltage to the point where the system will stop functioning. Most of the time it should wake up normally but a couple of people have reported having to remove and re-install the battery to reset it as Bernard noted. I haven't been able to reproduce this yet but it seems like depending on when it happens, the processor may hang in a higher power state and does not respond to the push button to wake up.

Also, I have heard rumors that some batteries are slightly thinner and more prone to break contact in the tray. You should see ~3V when the cell is new and it should quickly drop to 2.7-2.9V and stay there for a while. Try to squeeze it with the battery installed and see if the LED flashes like when it starts up. If so, it may have an issue with the battery contacts.

The early power switch, in addition to physical damage, had a couple of instances of getting bumped off in flight. That switch has actually been eliminated and the processor is hard wired to the battery now. The push button just signals when it should go into a sleep state and when it should wake up. I think the long term fix is an auto off where the system is shut down after a few hours of inactivity.

gtg738w said:

You should get far more than a few flights. What brand of batteries are you using? Have you been flying in cold weather?

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It was the original batteries that appeared to go dead. I threw them away so can't try the "remove and re-install" trick. I'm in Florida, no cold weather, above 70 degrees at all launches.