Not sure if these pictures will come through but a PO had built this box under the nav seat and installed 3 batteries there.  We changed the system so the batteries are under both game table seats.  It's a shorter run to the windlass and I am sure we will find a good use for that space under the seat. 

Hi,

I have two 115 amp AGM Rolls under stbd aft game table seat plus 2 more similar Rolls AGM under the front stbd game table seat. 3 house batteries 1 engine, all similar capacity and interchangeable if needed, trough the main switch, 1(1 for engine 3 house) , 2 (house batteries for both engine and house) 3 (engine batteries for all).
normal position is 1, 2 & 3 emergency situations.

After 30 years I own my C36, I have gone trough many battery brands, till now Rolls, by far the best.

Regards

A previous owner of my boat removed the nav seat entirely and made a large box with the top as the new seat. The top is an upholstered cushion with a wood bottom. There is enough room for two 6V 220Ah batteries and a couple tool bags, so there may be a way to fit 3 batteries in it. Unfortunately, I can't seem to find a picture of it, but I could get one this weekend if you want to see it.

Yes, the seat is still useable.  Unfortunately, instead of building a box to fit, they cut both the floor and the pedestal to build the box.  Now that we have removed it, we have to decide what to do about the 1/2 missing floor and chair base. 

It is a good space and it's nice to be able to use it for something but it wasn't easy to check the batteries with the way the lid fit and with the chair so tight on top.  Maybe a bit shorter of a box would have been a good idea but no doubt it was built to accommodate a specific battery size. 

Hi Mike,

We went the four 6v battery route and were able to fit "tightly" 4 batteries under the aft game table seat along with some components for the charging/monitoring system.

These are the Full River DC224-6 and with the combination gives us about 448 Ah which is nice, BUT if you do this you need to make sure that you have an alternator that is externally regulated, preferrably has a temp sensor, and would be best if it was high output (if you dont carry a generator on board or plan to be on the hook for more than one day without motoring). I learned this hard way after having this setup recommended to me without mention of the alternator needs. We smoked our alternator on one of our trips. Looking to upgrade to a high output alternator system soon. 

John

I have a 1984 Catalina 36 hull #260.  I modified my battery box ( stbd. game table seat ) to hold three (3) size 31 / 12 volt batteries ( these batteries are the perfect size and can be handle by a single person / appr. 70 lbs. each ).  All Three are deep cycle and can do any thing that may be required.  I then installed a digital splitter to proportion the charging coming directly from the altenator.  It charges the battery that needs it the most and the others as required.  I also installed a second battery selector switch so I could parallel two batteries for house use or keep separate.  We all know the shortfall of paralleling two batteries for any lenght of time.  If I were going to add additional battery capacity, I would do the box under the nav, station seat.  I personally would add Two (2) additional 12 volt batteries, a forth house battery and an isolated starting battery.

I attempted to attach this photo with my thread.

I have (4) 6volt batteries in place under aft game table seat and (1) 12volt under forward game table seat. Had to cut edge of seat to install 4th battery as shown in picture. Both positions required building a platform. You do have to pay attention to battery height with 6 volt batteries. I originally used West Marine which is basically a Deka battery, I replaced since this picture with Napa. This setup is approximately 430 AH for the house battery. This also solved a listing problem which is common with MK1.5. 

I was able to fit 2 GC2 size batteries (Trojan 6v 217ah) under the forward dinette seat, plus a 1000w inverter/charger. It's a very tight fit, but working great. Here's a picture with the batteries in place, and the inverter before I mounted it under the molded lip.

I built a couple of supports which conformed to the odd shape of the bottom. The batteries sit on these supports. Here is a picture of the forward one.

I am in the process of installing a third battery, adedicated engine starting battery.  I currently have 2 banks, house and auxiliary.  Each with 2 Lifeline GPL-31T batteries (12vdc/105 amps per battery).  I am adding a Lifeline GPT-2700T 12vdc starting battery.  I kept them all the same manufacturer as they will have the same charging characteristics.  The starting battery will join the two aux. batteries in the space under the aft game table.  Similar to Cliff Bassett’s arrangement   (https://www.catalina36.org/sites/default/files/forum/DSCN0866%20%28800x600%29resize.jpg).  My house bank is under the outboard seat of the dinette.  I have 2 battery switches, the original for the selection of either house or aux.  And a newly added starter and aux.  If the starter battery is weak or damaged I can still start the engine. 
My charging equipment is a 55amp alternator, a 2000w/100amp inverter charger and two solar panels of 171 watts combined.  The real-world charging capability of the solar panels is 6-9 amps on a sunny day. 
Why did I go that way and not combine the house and aux. banks into one 420amp-hr bank and just add a new starting battery?  There are a couple of reasons.  First, the existing batteries are of two different manufacturing dates purchased a couple of years apart.  Secondly, the useful, though, weak charging capability of the solar panels make large battery banks a liability not a benefit.  We anchor a lot and for extended periods as we cruise on the Great Lakes in the remote Northern areas.  The solar panels can keep up with our current usage, including using the inverter to make coffee (net draw 10amp-hrs) and make toast in the morning (net draw 6amp-hrs).  And final reason is: I prefer to conserve on the front end and have a reserve on the back end.
On the subject of battery combining:

1.  When combining batteries, they should be of equal type and date of manufacture for long term use. 
2. When combined in parallel the voltage remains the same and amp-hrs are additive.  Example:  2 12vdc/105amp-hr batteries combine to equal 12vdc/210 amp-hrs. 
3. When combined in series the voltage is additive and amp-hours remain the same.  Example: 2 6vdc/220amp-hrs combine to yield a bank of 12vdc/220amp-hrs.
4. Standard batteries (lead acid type) should not be discharged beyond 50% as a normal usage.  On rare occasions going beyond that is tolerable.  Lithium batteries can go down to nearly 10%.  Of course, the cost is very high.
5.  With 12vdc batteries in a bank, if one battery is bad, removing it, still leaves a 12vdc bank.  Not true with 2 6vdc batteries.

This is a long way around answering the original question, where are the batteries kept.  One last thought.  I store indoors for the winter and have used individual low amp trickle chargers on the batteries.  I disconnect the batteries entirely from the system and charge each with 1.5amp chargers.  I have kept the batteries for 10 years and they are still going strong.