What kind of charger is needed for 840?

Def need a spec.  For the heck of it, I tried plugging it into the puniest charger I had lying around, 1A, and it charges OK. Also charges plugged into the USB ports of my computer.  I think the lowest output USB chargers used to have 500 mA.  I don't have one of those to try anymore.

I suspect the 2.1a charger is the issue.  Maybe it's worn or something physically isn't making the connections.

For some reason my Google Fu did not reveal the max current spec'd by Garmin.  I might have to measure that once my 840 arrives.

Typically, charging will take place at lower amperage and the upper limit is determined by the charging circuit in the device.  USB-C spec at 5v is maxed at 5 amps.  That means a 5A charger or a 10A charger will charge the 840 at the same rate.  No danger of letting out the magic smoke until the voltage changes significantly.

There is no mention by Garmin of the higher voltage/amperage that comes with PD enabled devices.  I think it's safe to assume that PD isn't a feature of the Edge series.

I have also tried wiggling cables (on both ends) when connecting Edge 840 to problematic charger, just to be sure it is not some weird connection issue. It isn't, no amount of wiggling changed anything. It really is a strange situation.

I know it is tempting to say that the problem is with the charger. However - and I must stress this - this multiport charger is in daily use for charging all my sport devices. Currently, I'm charging the following devices with it with no issue whatsoever: Epix (previously Fenix 5+), Edge 820, Garmin Varia radar, Stryd, Wahoo Tickr Fit, two different types of sport earbuds (Plantronics BBFit, Aftershokz), and occassionaly also Form googles as well as Polar HR monitor. Also, both my phones (old and new), that is Samsung A3 and Samsung S22 can successfully charge off it.

With such a wide range of devices, their connectors and probably also their charging currents, it is very unconvincing to simply say "it must be something wrong with the charger". Well, there may be something wrong, but now I'm thinking it is the combination of charger and the device that is somehow not talking correctly. Is it possible that they're not properly talking to each other?

So I decided to hook up the USB voltmeter I have lying around. Here are the pictures.

Above is a picture showing Edge 840 NOT charging while connected to the "problematic" charger. You can tell this by the fact that the battery icon doesn't show that charging is happening, and also by the fact that voltmeter is showing basically 0 amps.

The above picture shows Samsung A3 being charged just fine with the same "problematic" charger. It is proof that the charger is not obviously faulty or something.

The last picture shows Edge 840 being hooked up to a different charger (with nominal specs of 2.4A output per slot), which seems to charge the Edge just fine. The current does seem a bit low (0.2A), but I guess this is due to trickle-charging. As the picture shows, the Edge is 81% full at the moment the picture was taken.

So, what to make of all this mess? It seems that there is nothing obviously wrong with either the "problematic" charger (it charges other devices just fine) or Edge 840 (it does charge on other chargers). Cables are also not the issue. Just the combination of a specific charger and specific device (Edge 840) seems to not work. Can't wrap my mind around this, if anyone has any ideas, please share!

Nice work.

No ideas at this time.  Other than some USB cables use very small gauge wires, this limits current.  Good cables can be a challenge to identify.  Nobody markets their cables as the "smallest possible conductor that will appear to work".

I don't think lithium batteries are trickle charged.

I have a similar meter for USB, but I won't have a 840 until tomorrow.

Actually, in the pictures above I am charging with the Garmin original cable that came with Edge 840.

When you have your 840 at hand and the time to do so, please do share your measurement or Edge 840's charging current. I would be very interested to see the result.

Chargers and phones have controllers that send signals through the USB cable to negotiate the mode of charging. Over the years manufacturers such as Apple and Samsung have modified the basic USB standards to suit themselves so power supplies need to identify the device being charged and provide the correct voltage and current. Generic power supplies can cope with a lot of devices by pretending to be an Apple charger or Samsung or whatever but sometimes this process fails, and although it is common then to default to the USB standard of 5V at 0.5A, sometimes a device will not work at all with a particular charger and there is nothing you can do about it except use a different charger. 

samcek, in your photos the voltage with no current is 5.3 volts which is OK, but the voltage falls to 4.4 volts with only 0.28 amps drawn. This shows that the USB cable between the charger and the voltmeter has a high resistance, either because the wires are too thin (possibly a data only cable) or because the wires or plugs are faulty. This explains why the current is very low. Try a new USB cable rated for charging and you should see faster charging, but it will not make your 840 charge with the problematic charger.

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Those are exactly the readings I expect.

What happens when you plug the above directly into the 2.1a charger?

Here are the pictures of the "problematic" charger with different devices connected.

First, just a backplate, with all the charging specs. It is a 5-port charger from SBS.

Charger without any load. Note the displayed "Mode: SAMSUNG". Looks  like this is how the charger presents itself to the connected device.

Here's the culprit: Edge 840, with original cable, not charging. Note the "Mode: DCP1.5A" (whatever that means).

For comparison, here's the old Edge 820, charging with 0.38A at 5.4V. The Edge is at 71% of battery as you can see. Also note the displayed "Mode: QC2.0". If nothing else, this picture proves that the charger is capable of charging at around (just below) 0.5A, so, it is very hard to say that the charger itself is kaput. It may be worn, it was bought exactly 3 years ago. But still, that doesn't explain the fact that NO current is flowing towards the 840.

Just for fun, here's a picture of Tick Fit HRM charging with ridiculously low current of 0.04A. But still not zero!

I don't know what to make of all this. Especially with all the different charging modes for different devices. Edge 820 is using QC2.0, and Edge 840 is using DCP1.5A? Is this some sort of a newer standard?

If some Garmin representative is reading this - can you state what kind of charging standard is 840 supposed to be using?

I had the same problem when using my Ravpower charger (6-port unit).  I switched to my Anker units and the 840 charges normally.  I went through the same testing that you did with different cables.  I also tested various USB-C chargers and they all seem to work.  For some reason the Ravpower chargers don't work with the 840.