I have a Optima yellow top 5+ years old. I left the trunk open for a few days and drained it.
Had it on a Battery Tender for a week and will only charge to 11.6 volts. Also tried charging it through a second battery...no joy.
Is this a paperweight now? These are really expensive now.....

You cannot charge an Optima like a normal battery. Battery tenders will NOT charge them. I found the best way to charge one it to put it in series with a regular battery on a 30 amp charge for a few hours, then switch it to 10 amp. There are specific types of chargers for Optima type batteries, but if you don't have one my method is preferred... Edit: I just read your post again, sounds like you tried it through another battery already, may have to charge it longer. One of mine sat for 3 years and I brought it back to life that way.

I am pretty sure you meant to say parallel not series.

I tried it in parallel. Is that incorrect? I used this method 2 https://www.onallcylinders.com/2016/12/13/3-ways-bring-deeply-discharged-optima-battery-back-dead/
I don't have a specific charger for AGM batteries. Maybe I will bring it to someone who does.
I really don't want to spend $300+.I think I paid under $150 for this one.

Schumacher and Duralast (Autozone) both have chargers and maintainers that work with lead-acid and AGM batteries.
I use them all the time.
The maintainers (aka "tenders") only charge 2 amps, kinda slow if the battery's got under 12.0 volts.

I revived an optima I left my fm convertor on and drained. It came back with a second battery.

I have the Autozone brand agm capable charger. It works on my dead agm to bring it back up, but that battery is old and won’t stay up though. Haven’t had to use it on my odyssey yet.

A Battery Tender is only keeping a charge on a charged battery. They are not a typical "charger" that will put more than 12 volts into it. The lifespan of yellow is 4-6 years according to their web site. Try charging it on a decent charger and see if it comes back up.

Yep, try series anyway...LOL..

My son has one in his truck Its at least 10 years old and been dead many times.He charged it and its still working like new.This is probally at least 5 or 6 times it has been stone dead and recharged and it works.

I bought an Optima charger for this very reason.

Are the cheaper AGM chargers on Amazon any good?
https://www.amazon.com/gp/product/B097D397M6/ref=ewc_pr_img_1?smid=A2ET2G27273OWR&psc=1

or do I need something like this?
https://shop.advanceautoparts.com/p...-battery-maintainer-150-33508/10245177-P

I think I paid around $50 for the Duralast charger I referenced earlier.
For the tenders/maintainers, I've also used Duralast (about $25) and Schumacher (about the same $).
Had a Black&Decker tender that quit working after about 4 years, but the other 2 still work fine after 10 years or so.

Grab a NOCO GENIUS10 charger.

https://no.co/genius10

They are approx $100. Home Depot sells them, Battery Warehouse, Amazon... Get best reviews and ratings. I love mine. Have the 10 and the 5.

GENIUS10 6V/12V 10-Amp Smart Battery Charger

10-Amp Battery Charger, Battery Maintainer, and Battery Desulfator
Introducing the all-new GENIUS10, one of our most powerful, highest-performing, energy-efficient, and compact chargers yet. The GENIUS10 is a 6-volt and 12-volt battery charger, battery maintainer, and battery desulfator rated at 10-amp for lead-acid automotive, marine, and deep-cycle batteries, including flooded, gel, AGM, and maintenance-free, plus lithium-ion batteries.

Meet the GENIUS10 - Similar to our G7200, just better. It's 17% smaller and delivers 115% more power. And it's simpler and easier to use than ever before.
Do more with Genius - Designed for 6-volt and 12-volt lead-acid automotive, marine, and deep-cycle batteries, including flooded, gel, AGM, and maintenance-free, plus lithium-ion batteries.
Enjoy precision charging - An integrated thermal sensor detects the ambient temperature and alters the charge to eliminate over-charging in hot climates and under-charging in cold climates.
Charge dead batteries - Charges dead batteries as low as 1-volt. Or use the all-new Force Mode that allows you to take control and manually begin charging dead batteries down to zero volts.
Simple to set up and use - Plug-in, connect to the battery, select a charge mode, and start charging your battery. A fully-automatic, worry-free battery charger for year-round use.
Restore your battery - Detects sulfation and acid stratification and restores lost performance for stronger engine starts and extended battery life.
Compatible with all types of vehicles - Charge and maintain cars, motorcycles, lawnmowers, ATVs, tractors, trucks, SUVs, boats, classic cars, hot rods, and more - you name it.

I have this one. It has worked great. I think I paid around $100 back when I bought it. Now $139 https://www.optimabatteries.com/products/digital-400/

A battery tender is worthless as a charger, they don't put enough current in to charge a dead battery up. They will charge a slightly discharged battery but they are meant to maintain a battery that doesn't see much use.

Now that you have it at 11.6 volts go buy a charger with an AGM setting and let it charge on a 6-12 amp charge. The optima brand chargers are great, I have the 4 amp smaller one and it does very good despite the low output of 4 amps. Even if your optima doesn't come back an AGM charger is good to have if your purchase another AGM battery regardless of brand.

Most chargers won't kick on if the battery is below 10 volts, that is the reasoning for hooking another battery in parallel. This would apply to any battery not just an optima.

There is a reason for that, it's called thermal runaway. A possibility when trying to charge a deeply discharged battery not designed for deep discharge. Now before someone pipes up with how they've done it for years with no problems, yeah I know I've done it too, but you aren't going to get sued if it happens and someone's house and car burns down. So the charger manufacturers but in limitations to mitigate that risk. What you do to defeat that is up to you.

Don't ask me why...
An Optima battery needs a special charging cycle..
Even though some smart chargers state they can handle AGM batteries..
I have Optima batteries in all (4) of my cars, typically I get 6-8 years, sometimes up to 10 years.
If an Optima battery is low, I bought an Optima charger, and it works great to bring it back up.
1 of my Optimas'(yellow top) I accidently left a small hood light ON and it ran the battery down, hooked up the Optima charger and the battery recovered just fine..

Just my $0.02...

Yeah just asked Jim about that. Sorry Jim to bring up bad memories.

I am debating on buying a Optima charger. Took the battery in this week to parts store and left it overnight.
Still dead 11.7 volts. I have had 4 Optima's 2 Red & 2 Yellows now down to 1 Yellow. I just hate giving them more $$$ for their crap. I hear Odyssey make a good AGM but pricey.
Funny my tractors have had the best Batteries. John Deer garden tractor finally replaced OEM battery after 22 years. My 38 hp Kubota still has original battery 17 years old.
None of my car batteries last much more than 5
Thanks for all the input.

Just buy it and don’t look back. One thing less to worry about.