It is under the driver's seat.
Remove the seat. You will find the battery under it. Remove the negative terminal first, then the positive terminal.
Its basically stuffed and hidden in the middle of the frame, took me forever to find mine, trust me its in there.
Take a peek behind the left side cover. You might find it there- unless a gremlin moved it, then I have no idea where it might have gone.
TRY Honda dfw they are good.
Follow the cable rearward from the starter. It connects to the battery.
Find the battery first, look for the plus side it is the positive pole on the battery, You put the red or positive clamp from your charger on there. Then you put the negative which will be on the other side of the battery. Then you set your charger on 2 amp trickle charge, plug it in and let it charge.
To reset the code in a 2006 Honda Odyssey after changing the battery, you typically need to enter the radio or anti-theft code. First, turn on the ignition without starting the engine. Then, if the radio displays "Code," enter the correct code using the preset buttons. If you don't have the code, you can find it in the owner’s manual or by contacting a Honda dealer with your vehicle's VIN.
take out the Radiator and you will find it in a pipe that looks an L
Under the seat next to the battery. Remove plastic cover(2screws) and you will find fuses and battery.
Go on line and type in Clymer Manual - You can get a manual from there for around $25.00.
in the trunk,where you would find the spare tire
Go to the Honda Website and sign up on the Owner's Link. There you will be able to get the code. Click the link.