In a previous post I inquired about my Garmin 800. The back , bought via ebay, will not work as it is a different size. Apparently there were 2 sizes. But it did come with a new battery. Here is were I need some advice. Are there quality batteries available? Not sure I want "trust" my unit to this potential knock off.

If your trying to decide between a knock off vs. an "official" garmin battery, I would probably stick with garmin. Battery life in some garmin units is not the best compared to others, so putting an copy cat may be a frustrating idea for even shorter battery life. However if there is company that has a better and pushes it out further I would go that rout. Don't think that does exist though.

I am just throwing out what I would do. I would stick with factory I guess.

What price savings do you get with the ebay vs. official?

I started looking for a official Garmin battery but am coming up empty. Garmin website is of no help.

To my knowledge, Garmin won't sell you a battery. They will let you send them your computer, and replace the battery in it for you, to the tune of $100. You can replace the OEM 800mah battery with a 1000mah off of eBay for $10.

Well, they really want you to buy a new Garmin, and they've got a fantastic system of price fixing in place. Garmin has established minimum pricing for refurbished units. The Edge 500 came out in December of 2009 at a street price of $199. Today, nine years later, a refurbished 500 is $149. Impressive.

​​​​​​If you brick it changing the battery, you can still get a new refurbished one for $100. But there's a very good chance you won't need to. This seems like the clear winner.

If the OEM battery is 800mAh then a 1000mAh battery either won't fit in or isn't 1000mAh.

There is a good you tube video that shows how to change the bstbatt. The fellow use a battery from New Power 99. The $ is $ 27+ shipping. More than other sellers but at least there is some recommendation.
BTW , I did receive one with the replacement back,that is the incorrect size, but for some reason I am not comfortable using this battery.

When I got into R/C racing the third time around, LiPo batteries had just started to make their appearance in the hobby. A 2S battery pack for a surface vehicle-- that is, one of the dimensions to fit in a standard NiCd battery tray-- was 2200mah. Now, the same size battery is 6500mah. Or just look at 18650 cells, all of an absolutely identical form factor, but available in capacity anywhere from 2000mah to 3600mah.

They get better at the chemistry, so it's absolutely possible to fit a 1000mah in the spot for an 800mah from nearly a decade ago.

Why not just buy a used original Garmin Edge 800 back cover and battery on eBay for under $20? From the product photos you can see exactly what you'll receive, including battery date of manufacture and mAh rating The battery appears to be 1100 mAh on the one I checked. BTW, The tool needed to remove case is a T5 torx.

My experience with batteries in the Edge 705:
I bought it new in 2008.
The original battery lasted 7 years.

I bought a replacement on eBay.
It lasted one year.

I bought another replacement from New Power.
It lasted one year.

I installed another replacement, which was still working until a few months later when the USB port went bad, and I quit using it.

From what I can tell, all the batteries available are the same one with the "Cameron Sino" label.
And once installed, the battery gauge in the display was never again accurate;
It might show fully charged when it really had only a partial charge,
or it might show nearly empty, but then run for several hours more.

So the good new is the replacement batteries might only cost a fraction of what Garmin charges.
The bad news is they have only a small fraction of the life of the OEM Garmin battery, and you never know how much charge you really have remaining.

your scenario would be my fear. I would probably try a cheap ebay one first and if it only went a year, I would ditch the whole thing and buy a new one.
I hope my beloved 520 goes at least 5-7 years before I need a new battery. If I can get to the 7 year mark and it started going bad I would just get the newer version of that model. I love my 520..its been solid!