Can anyone recommend a battery replacement for this poor excuse of a cycling computer?

Assuming a linear depletion ...  this isn’t the case.  

I have been so frustrated with this device I've almost binned it on multiple occasions. 

Basically mine does not last overnight, if I do not have it on charge overnight before my morning ride it will be on about 20% and its been like that from new. 

The replacement batteries look like they are lithium so I will give that a go first. 

I have been so frustrated with this device I've almost binned it on multiple occasions. 

Basically mine does not last overnight, if I do not have it on charge overnight before my morning ride it will be on about 20% and its been like that from new. 

The replacement batteries look like they are lithium so I will give that a go first. 

Same as mine 

If I do not leave it on the charger overnight I can not use it in the morning. Charge it the day before and its dead by morning or at least just enough that as a pedal away the screen goes off. Sigh 

I also have had the battery bug as well (I only run heart/cadence sensors, no wi-fi, phone is not connected, back light on 15s and only use GPS). My device is ~3 years old, I ride ~2.5K/year. For most of my local rides I don't use the navigation feature, so the device would last me on my 3 hour rides with remaining battery life dropping to sub-30% towards the end of the ride. However the battery gets depleted even faster when using the navigation - on my recent 5 hour ride, the device went into battery save mode half way through the ride and basically became useless.

I have talked to Garmin support about a 1.5 years ago when I first noticed that I don't get full 15 hours out of the device and was told that a software upgrade that was coming up was going to address the issue. And I believe there was an improvement on the battery life after the software upgrade. But after this week-end's longer ride where I needed the navigation and it only lasted me for ~2.5 hours, it really pissed me off. So I talked to Garmin Support and said that I expect lithium battery to degrade over time, but not to the point that it looses over 70% of its capacity in a 3 year span. They offered me to pay $110 for a refurbished device to fix the issue since the device is out of warranty. My point to them was that my device is working really well function wise, I don't have software/phone connection issues, screen looks as when it was new, no cracks/scratches etc. It is the battery that is the issue, so it is really not a break/fix type of a scenario we should be dealing with, it is the manufacturing defect/faulty battery batch, since so many people are experiencing that issue. The other thing that the support person told me was how they arrived with that 15 hours battery rating and I couldn't believe what he was telling me (he claimed that he was in their testing lab): he said that the device gets fully charged and they put it in a timer mode. I let him know that we're buying this device to be used as a cycling computer, not as a stop/watch. I put his statement under question whether it is actually true as it really sheds a gray light on the company. It is hard to believe that such respected company as Garmin would do something like that to come up with that 15 hour battery rating for the device, which is a miss-representation to say the least. Very frustrating that they even have people like that in customer support.

I gave up on 820(real PITA) and purchased 1030 plus. Much better unit but still Garmin didn't fix Weather alert since over a year for iOS phones.

I agree that the 820 did not live up to expectations.  I never had battery issues with mine, but I returned mine due to the Halo effect on the screen and paid $110 for a rebuilt unit.  It runs perfectly fine, but I only use it for backup since I upgraded to an Edge 1030, which is a great computer.  As for Wx alert, I don't use that feature since my normal rides are done in 3-4 hours.  However, I highly recommend the Wind Field data field that you can download from Garmin IQ apps and install on your Garmin unit.  You can set a wx alert in that if you like.  But the WF data field will give your real time wind direction and speed, plus other wx readings to choose from.  The free version will update data once per hour, the $5/yr version will update every 10 minutes.  The Wind Field Pro version 7.0 is $12/yr subscription and updates every 5 minutes and gives you several options for the wx source.  I highly recommend for $1/mo .  I have it in a data field on my first screen and have been using it for a couple of years.  Five of us in my cycling club use it.

In fairness, I have to give an update here. Shortly after that post I lost the unit. I bought an 830 as the only available replacement ready for my race two days later, and the 830 is an improved unit. About a year later though I found the 820, where it had been hiding all that time. The battery was completely flat, of course. I charged it, then handed it on to my partner who uses it now. We went for a 6 hour ride yesterday, and my 830 came home with plenty of battery. She turned the 820 off for two of our stops, and had it on battery save mode for the last 1:40, but did get about 5:20 recording time out of it and it was still showing 6% when we got home. So if anything, it has got better after a year of total neglect and then two more years of use! (Mind you, in a few weeks we set off for a long trip, so I'm still looking for a new battery, hence coming back to this thread)

Since my previous posts and considering aftermarket batteries,  I currently use a small cylindrical power bank that I velcro to a tube on my bikes.  I run the Edge on its battery until it starts getting low then turn on the power bank and run off that as the Edge charges.  I can't use this in the wet, so if wet, I just rely on the fully charged 820 battery.  So far this has worked well for me.  If the 820's battery deteriorates further, I might replace it then...or maybe just bite the bullet and get a newer model (840?), or possibly a different brand.