AGM and Flooded battery combination

No wing nuts !! Perdy much tells the story IMO. How ridiculous !

Take a look at these Marine forums

http://www.yamahaoutboardparts.com/
http://forums.iboats.com/forum.php
http://www.walleyecentral.com/forums/index.php
http://www.thehulltruth.com/index.php
http://www.crappie.com/crappie/forum.php
http://www.bbcboards.net/forum.php

Please remember
Outboard engine charging systems are VASTLY different from Cars, Trucks and RV charging systems. They tend to be more similar to Tractor charging systems than any of the previous.

Check your battery cable connections they are major contributors to failed charging systems on outboards.

The Yamaha charging system consists of a simple 3 phase stator. The flywheel is called the rotor because it contains the rotating magnetic field. The coils feed a 3 phase sine wave A/C voltage to the rectifier/regulator. There it is rectified by a full wave bridge rectifier and fed to the regulator side what power is not used is simply shunted to ground. A simple brute force design that is prone to failure when the starting battery is disconnected.

It is a 2 stroke. Thanks for posting this I never would have thought about it. Neither did the PO because wing nuts are on the terminals now.

2002 is not an EFI engine - it might be a 2 stroke
Either way the warning applies

Wing nuts do not get tight enough and tend to come loose over time
Standard nuts with locking washers are recommended

If the starter battery comes loose while the engine is running - you bought yourself a new Rectifier/Regulator assembly

A switch with a disconnect aka OFF position for the starting motor is bad news as well - if you happen to switch if off while running - once again you bought yourself a new Rectifier/Regulator

It is a 2002 90 HP Yamaha outboard. Thanks for the heads up. I will research in more detail. Why no wing nuts?

A 90Hp outboard engine????

If it's a Yamaha Outboard I suggest you continue reading my warning if another brand - I can not speak from experience but suggest you ask that specific brands Marine Forum members - Outboard engines are VASTLY different from RV charging systems

If it's a Yamaha EFI engine - check what Yamaha says about AGM batteries
They are very specific that only flooded lead acid batteries be used as starting batteries
Clearly stated in their manuals
NO Sealed Lead Acid batteries
NO Maintenance free batteries aka sealed Lead acid
NO AGM Batteries are to be used as starting batteries
NO Wing Nuts are to be used at all for battery connections

The Rectifier/Regulator assembly is know to be a poor weak design and many times when it fails it takes along the ECM module - Both together run $250 or more for the parts alone

Be very careful

Use only a flooded lead acid battery as a starting battery
The AGM is a great Trolling motor battery and can be topped up from the outboard charging circuit with the appropriate connections

The Yamaha F90TLR charging voltage is typically a constant 14.8Vdc @ a max of 25amps - if you plan on using it to charge more than the single starting battery install a fuse to protect the Rectifier/regulator or plan on purchasing a new one within a year

I have three Yamaha Outboard EFI engines and Yes I have a F90TLR

Suggest you check out the many Yamaha Marine Forums
RV Charging circuits with high power engine alternators, Solar systems, converters or inverter/chargers are vastly different from outboard engine charging circuits

The differences are minimal with the pair you have chosen.

Of all the battery companies LIFELINE recommends a very fast recharge for THEIR AGM batteries.. Optima is not lifeline, Neither is Deka, MK or Sears.. Only LIFELINE is LIFELINE.

The differences are minimal though for initial charge and storage split 'em.

You can get a marine grade 4 position battery switch

A/Both/B/OFF.. I suggest using one of these. You can choose one, the other, or both,, You can remove one for charging while the other floats the boat. This might be your best all around option.

My take on it is a little different than already posted.

You have the flooded battery. You have the AGM battery. Both are bought and paid for. The chemistry is virtually the same, both are lead-acid batteries.

If you want to use them together, go for it. The worst thing that will happen is you might shorten the life of one by a few months. On the other hand you can use what you've got without further expense, labor, or aggravation. I would do it.

I would not hesitate to use, and charge them in parallel.

Thanks for the quick responses. Smkettner, Thanks for the actual experience and explanation which is what I wanted to do.