Etrex 20 "battery Low" false alarms?

[mattc]

/GPS_Muppet

I went through 3 pairs of AAs in about 40km on Monday morning 
[and then found a petrol station stocking every other size of battery in creation!  ]

Now: they were a mix of Lithiums and Eneloop NiMH, all of which I had good reason* to believe had loads of charge.

Of course it's possible that I misjudged the charge level of all 6 cells, but I'd like to know if the eTrex might have any known problems with This Sort Of Thing. It was also pissing down, which may be relevant.

(It was completely happy with some standard Duracells stuffed into it).

Any suggestions?


(*won't bore you with detail)


\GPS_Muppet

[Kim]

You had told it it was using the right flavour of battery?  There's a setting for the battery chemistry.

(I did some measurements on my eTrex 30 - the battery chemistry setting doesn't affect the ultimate shut-down voltage, but will change the calibration of the battery level display and when the 'battery low' warning appears.)

[Phil W]

What Kim said.

With low battery warnings I just acknowledge them and carry on riding. I then change them when the GPS actually turns off. Did you keep all the batteries so you can see if they are still good to use?

[mattc]

Yes, I checked the battery chemistry setting (and even remembered to switch to NiMH when I put my last-resort Eneloops in!).

[ Intersting study, Kim.  ]

No, sadly the device DID turn off. I am used to getting a few hours after the "Low" warning comes on
_{(not sure exactly how long "a few hours" is, I've only done a couple of proper long rides with this thing.)} Perhaps I misnamed the topic, in hindsight!


I do plan to look at all the batteries cells*, but if there is an overriding issue out there, it would be really useful to know about it. I'm suspecting there isn't ... !



*They are in a big puddle** on the dining table, near to all the other crap I haven't unpacked properly from the weekend.
**As in "a puddle of knickers" (c) Sue Townsend, no water involved.

[Pingu]

I had something similar: https://yacf.co.uk/forum/index.php?topic=102928.msg2162834#msg2162834

Not had the problem since.

[frankly frankie]

If I'm using rechareables I have a battery strategy which simply means I never get to see a 'battery low' message.  Using a good pair of cells, I know they will run for 2 days, but probably not 3 (assuming 8 hour riding days on average).  So I carry a spare set and swap them out/recharge after every 2nd riding day.

If I did see a battery low warning I would expect the Garmin to switch itself off within the next half hour, and possibly much sooner.  For it to run for 'a few hours' suggests a mismatch in the battery type setup.   This in itself I don't think is harmful because the switch off voltage threshold seems to be quite high - around 2.1V (not tested this, but that's my guess) which is not so low that NiMHs would come to harm through over-discharge, unless the pair is badly mismatched.

[Kim]

This in itself I don't think is harmful because the switch off voltage threshold seems to be quite high - around 2.1V (not tested this, but that's my guess)

My testing confirms this:

https://yacf.co.uk/forum/index.php?topic=102928.msg2162706#msg2162706

[mattc]

If I'm using rechareables I have a battery strategy which simply means I never get to see a 'battery low' message.  Using a good pair of cells, I know they will run for 2 days, but probably not 3 (assuming 8 hour riding days on average).  So I carry a spare set and swap them out/recharge after every 2nd riding day.

I can see that has a lot of advantages. But it requires;
- carrying more cells-per-trip than my skinflint strategy. and
- the text in bold!

If I did see a battery low warning I would expect the Garmin to switch itself off within the next half hour, and possibly much sooner.  For it to run for 'a few hours' suggests a mismatch in the battery type setup.   This in itself I don't think is harmful because the switch off voltage threshold seems to be quite high - around 2.1V (not tested this, but that's my guess) which is not so low that NiMHs would come to harm through over-discharge, unless the pair is badly mismatched.

More useful stuff. Thanks.   [I have no faith in that "a few hours" estimate. My recollection is hugely unreliable.  I do expect to need new cells "very soon after" the warning, but don't expect the unit to abandon me 5 mins later e.g. in a town-centre ]

[frankly frankie]

When NiMHs run down they fall off a cliff.  That is, they maintain a steady constant voltage (of approximately 2.4V for a good pair) for 95% of their runtime.  Of the other 5%, 3% is when they are fresh off the charger and a bit 'hot', and the remaining 2% is at the end of the runtime when they start to dip below their plateau voltage.  It is important to stop draining them at this stage, which is why the Garmin shuts down, but even if it didn't the cells would be sucked dry (and possibly damaged) only a very few minutes later.

So - with the GPS set to 'NiMH' in setup, the warning is presumably triggered around 2.25V and by then the NiMH cells have less than half an hour in them.  Suppose, with the same setup, you have a pair of lithiums on board.  These run at a higher voltage than the NiMHs (around 2.8V for a pair, assuming no regulation**) but this steadily and gracefully diminishes, and at some point they dip below the NiMH curve and the warning is triggered.  However the lithiums still have plenty to give, and could still take hours to drop from 2.25V (warning) to 2.1V (shut down).

** I think there probably is some regulation, because the runtime of lithiums in a Garmin is prodigious - in 2007 and using a Legend Cx with lithiums on board, Sheila rode from Paris to Brest and back to Fougeres before she had to swap them out.  These days as a tourist, using an Etrex 30, she gets 5 touring days from a pair of lithiums.

So I carry a spare set and swap them out/recharge after every 2nd riding day.

I can see that has a lot of advantages. But it requires;
- carrying more cells-per-trip than my skinflint strategy. and

You don't carry a spare pair at all?  I would at least carry a pair of lithiums and hope never to use them, in the same way that I carry a 2nd spare inner tube.  Lithiums weigh next to nothing, and have a 'use by' date somewhere in the next decade.  Even if you do have to use them for a few hours, they can go back in the luggage and still be good to go in 5 years time when you need them again.

[mattc]

You don't carry a spare pair at all?  I would at least carry a pair of lithiums and hope never to use them, in the same way that I carry a 2nd spare inner tube.  Lithiums weigh next to nothing, and have a 'use by' date somewhere in the next decade.  Even if you do have to use them for a few hours, they can go back in the luggage and still be good to go in 5 years time when you need them again.

Yup, always have spares. In this case 2 sets of Lithiums - which I believed were 90% charged 

If I'd been further from home, on a tighter schedule, not carrying routesheet + maps as backup yada yada ... I might have put more thought into what spares to start Monday's ride with. I do generally carry virgin Lithiums, and this will be my Golden Rule on future big rides.

My problem is that being scatter-brained, I will use a good strategy like this, but screw it up by carrying 90% used cells instead of 90% charged one:

Even if you do have to use them for a few hours, they can go back in the luggage and still be good to go in 5 years time when you need them again.

So of course one should know one's limitations, as a great man once said ...

<adds to bottom of long TO-Do list: work out nice way of labelling sets of batteries in luggage>