On Wed, 03 Mar 2021 15:36:46 -0500, Moshe Braner wrote:

Alas some of them use non-rechargeable coin cells, that you cannot keep
alive by maintaining the main power.

I thought I'd said that already, but what I should have emphasized more
clearly is that as long as the main rechargeable batteries have enough
charge in them to maintain a usable voltage *no current is being drawn
from the coin cell* so it degrades only very slowly. There *may* be a
diode to prevent flow into the coin cell while the main batteries are
good, but I bet a lot of electronics maker leave that out.

The proof of this is that the original coin cell in my first GPS II+ had
dropped far enough in two years to be flashing up #battery low' warnings
and this is what prompted to get it repaired. At that stage I was running
the alkalines flat and only replacing them before the next time I used
the GPS.

Now, almost 25 years later, both GPS II+ units are still going strong and
not warning about the coin cell getting depleted. It can be safely said
that this is due to the alkilines always being replaced when the battery
state indicator falls to 25% and never taking more than 30 seconds to
swap old batteries for new ones.

If this will work for Garmin GPS II+, then its extremely likely that that
it will work well in almost any other device that relies on coin cellds
to preserve memory while the main batteries are being swapped. If it uses
rechargeables, thin plugging in the charger should have the exact same
effect.

In particular, devices that are
(also) IGC-approved loggers (whether standalone loggers, varios,
computers or FLARM) seem to be REQUIRED (by IGC) to have a battery
inside the sealed unit - for the purpose of detecting any attempt to
tamper with the innards.

Sure, but that isn't problem provided you do as I do and never leave the
logger or whatever with its main batteries connected and containing
enough charge to hold their output voltage higher than the coin cell's
output voltage. In this condition current cannot flow out of the coin
cell.

They are built so that if/when you open the
case, the battery is disconnected and the IGC security memory is lost.

Quite, but not relevant. I'd be surprised if my logger (an EW
Microrecorder) doesn't have a coin cell in its sealed area. However, it
also has an accessible section where its set of NiMH main batteries live.
These can be replaced without damaging any seals - and will need to be
replaced if mistreated.

Ror 'mistreated' read 'let them sit all winter and only recharge before
the first flight next year'. I know a pilot who also has an EW
Microrecorder and did that. They ended up replacing the NiMH batteries
every couple of years. OTOH I've had my Microrecorder since 2012 and its
still using the original set of NiMH main batteries.

The only disadvantage of NiMH is that they have a high self-discharge
rate and so I need to recharge them every 4-5 weeks during the winter
(and the COVID Winter too).

Those batteries end up dead after a decade or so, and then you have to
have a factory rep re-seal them, meaning replace the battery and do
something or another to make the unit consider itself secure again.

Well, the coin cells in the Garmin GPSes have done 25 years so far and
the one in the Microrecorder is going fine after 9 years.

(The sealing sticker itself is just a warning not to open it.) Thus
these devices are ticking time bombs that, without factory-rep support,
will self-brick. Nothing lasts "forever"!

Sure, but the difference in lifetimes between something which is bunged
in the cupboard and neglected between flying seasons and a device whose
kept charged is quite dramatic.

And the inability of long-stored GPS units to figure out which "epoch"
they really are in is another form of planned obsolescence.

I think its more case of volatile memory (RAM) with a coin battery for
backup being more reliable and supporting many more read/write cycles
that the EEPROM that was available when the 1st gen GPS receivers were
built.

I have some very old Garmin GPSs that work just fine, except that some
of them are confused about the epoch.

That depended on the device. O lot of 1st epoch GPSes couldn't resync if
the epoch counter was zeroed by total power loss. IIRC my GPS II+ units
are in that group, which is why I keep them continuously powered except
for 30 seconds every 9-12 months whe n their alkalines are changed.

That's not a problem unless the device is also a flight recorder
(logger) since it will stamp your flight log with the wrong date
(off by about 11 years).

It can be more than that - some early receivers could not rsync with the
GPS constellation from epoch 1 onward if power loss zeroed their epoch
counter - I believe this design fault wasn't recognised until after the
first epoch rollover, at which point it was too late for recovery.


--
Martin | martin at
Gregorie | gregorie dot org