Home-  Sailboat-  Submarines-  ROVs-  Metal Working-  Other Stuff -  About Us

Why Build a Sub
   Submarine 101
   Wet Sub
   Planing Wet Sub
   Dry Ambient Sub
   Submarine Yacht
Design Evolution
Cutting Aluminum
Building the Hull
Kort Nozzles
Battery & Chargers
Engine & Jet Drive
   Engine Tune-Up
   Hull Cooler
   Wet Manifold
   Engine Mounts
   Engine Box
   Jet Pump Valve
Helm Controls
Ballast Sled
Landing Gear
Trim Tanks
Wiring Harnes
Tow Truck
Rules of Thumb



Battery and Chargers

Ballast sled & battery box.

Optima AGM "Yellow Top"


There are a total of 8 batteries that ride inside the ballast sled. Six of these provide 36 volts to the thrusters and helm controls, and two provide 12 volts for starting the engine.

My budget nearly drove me to using lead acid batteries but "Doc" the creator of the Bionic Dolphin had sold me on Optima AGM batteries.  He has had them down to 150 feet, completely exposed to the pressure.  He only sealed the terminals with "E6000" urethane sealant available at Ace Hardware.  Doc recently retired one of these batteries after 9 years of service. DC Battery at has good informative pages about the Optima. At $150, they are 3 times what a car battery cost, but the reliability and ability to safely flood the battery compartment with sea water is a real safety advantage.

There are 3 types of  Optima batteries: Yellow Tops are for deep cycle or when you plan to slowly use the battery power such as with trolling motors.  Red Tops for starting batteries when you want all of the power all at once for starting the engine, and Blue Tops which fall between starting and deep cycle. I'm using Yellow Tops for the thrusters and Blue Tops for the starting batteries, since the starting batteries will also power the linier actuators and lights.

You also get more amp hours from deep cycle batteries since the power is used over a longer time but they also provide less Cold Cranking Amps (CCA) for starting. Below are the details for Group 34 Optima batteries.

Group 34 Optima batteries
Model Amp-hours CCA Pounds
Yellow Top, Optima D34 55 750 42.9
Blue Top, Optima D34M 50 800 38.4
Red Top, Optima 34 50 800 37.9




All of these batteries are group 34 and measure 10" x 6.8"w x 7.8"h. and will fit the box. There are models with side terminals or both side and top terminals but I avoided those due to the open top design of the battery box.

Interstate Batteries carries Optima locally and DC Battery at also had good pricing.
Interstate Batteries of Tulsa  918-610-0007
Part #SC34DA is the Yellow Top, Optima D34:   165.69 qty 6
Part #SC34M is the Blue Top, Optima D34M:      156.95 qty 2
Total with tax: $1,320 (Fall of 2007)
Just for comparison; Interstate also has a deep cycle, 65 Amp-hour lead acid battery for $82.95


Amp-hours measures the total amount of energy that a battery can deliver over 20 hours at a constant rate of discharge, before the battery voltage drops too low to be useful. However if you actually drain a battery beyond it's allowed depth of discharge or DOD you will damage the battery. The recommended DOD for lead acid batteries is 50% and most gel-cells can go to 80%.  With 6, 12 volt, 55 amp hour batteries, I have at total of 6 x 55 = 330 amp hours at 12 volts. After I connect 3 of them in series to get 36 volts, then I have 55 amp hours at  36 volts.  Then I take those two sets of 3 batteries each and connect them in parallel, and I end up with 110 amp hours at 36 volts.  To protect the batteries I don't want to discharge them more than 80% so 110 amp/hrs x .8 = 88 amps per hour if I were to use them over a 20 hour period.  We'll actually use them over a 4 hour period and since you get only about 1/2 of the available amps when you drain the battery very fast, then we actually have about 44 amps/hour so need to keep the total amp usage to 176 amps.  (44 amps/hour for 4 hours = 176 amps)  An amp meter described in the wiring section will monitor the actual amp usage.

The 6 Yellow Top batteries are divided into 2 sets of 3. 
Each set of 3 is connected in series to get 36 volts, then the
2 sets are connected in parallel to give one 110 amp hour,
36 volt supply.

Runtime is highly dependent on how much throttle is being used, but if my Minn Kota 101 pound thrust motors draw a maximum of 46 amps each the the total draw would be 92 amps and a 110 amp-hour supply can provide power for 1 hour 11 minutes (110 amp-hr / 92 amps).  Minn Kota offers this formula: (.85 * Amp-Hr Rating) / Motor-Amp-Draw = RunTime which would be (.85 * 110) / 92 = 1 Hours.  In reality 3+ hours is more likely because full throttle is reserved for stopping and not cursing. 

The actual usage of the 36 volt supply will be monitored by a "Watts Up"  amp hour meter.  There is more about that on the Thrusters page.

(1) I had already played around with the idea of putting lead acid batteries inside a box that would stay at 1 ATM.  In other words the inside of the box would remain at the surface air pressure and it would need to be strong enough to withstand the pressure of the water as we descended.  I intended to build the box and then make the decision about the batteries later, but when I was building the box I used measurements for the Optima AGM batteries which are significantly smaller that their lead acid counterparts. So with that the final decision was made. The battery box compartments are a fraction over: 10 1/2" x 7 3/8" x 8 3/4" deep. It will fit some group 34 batteries, but not all!  Batteries are grouped together based on there physical size so any battey in a specific group can most likely replace another battery of the same group.  The group has nothing to do the the amp hour ability of the battery, just the size.  Here is a table containing the BCI (Battery Council International) group size specifications.  

(1) The battery box has room for
ballast lead in the bottom and 8
batteries on top of the lead.

(2) The box is rides on tracks so
it can be used to trim the boat.

(3) Power cables passed through
the lid and clay is added to make
a mold for epoxy that will seal
the cables.

(4) A cover goes on over the
cable entrance.


(5) Lid up-side-down ready for
epoxy to seal the cables in place.
The plastic glued to the lid will
help prevent accidental shorts

(6) Flipped back right side up,
opened up, and clay is replaced
by a lead block that is epoxied
into place.

(7) Additional lead plates were
fitted in beside the batteries.


(8) Heating plastic sheeting
to make battery terminal covers.
This is just 2 x 4 foot plastic
sheeting that is sold at Home
Depot for florescent lighting
diffusers.  We later switched to
using PVC cut from a S&D pipe
because the covers were much
more durable.

(9) The hot plastic is then
wrapped over a terminal and
allowed to cool. The edges can
be trimmed with a box knife to
make it look pretty.


(10)  Switching to PVC for the
terminal covers and cable
passage protectors between the
battery compartments

(11) Plastic insulation on the top
lid was replaced by carpet that
will also add a cushion.

(2) The Battery Box is also referred to as the Ballast Sled because the box is mounted on rails and it is moved forward and aft in order to trim the boat for submerged or surface operation.

December 9, 2007 and there are 3 inch icicles on everything outside.  Inside we are back to working on the ballast and battery sled.

Oh, by the way, when building the cables to wire 6 batteries into a 36 volt bank, be sure to just hook up one cable at a time.  This will prevent you from accidentally completing a circuit between the wrong terminals as I did. It's pretty amazing how fast the arc will melt the terminal connector.

(3) A couple rectangular holes were cut through the lid and 1 inch angle welded in place across the lid to form a box where the power cables enter the box. The holes leave about 3 inches of sheet on the outside edge.  The hole at the rear is for 2, 4/0 welding cables that will supply hundreds of 12 volt cold cranking amps back to the engine.  The hole toward the front as 2 -12 volt cables that swing around and come back to the relay box for the sleds winch,  2 - #6 welding cables that supply 12 volts to the equipment forward, and 2 - #6 welding cables that supply 36 volts to the equipment forward which is mainly the thrusters and the helm actuator box.

(3) Temporary clay dikes were added to keep the epoxy out of the center section.  (4) Next a cover is bolted on and the sled lid is flipped up side down.  The idea here is to leave enough slack on the cables so that the lid can be removed access the batteries without needing to break the seal where the cables pass through the lid.

(5) RTV Silicon Sealer is used to seal the gaps between the cables and to seal the opening where the cable bundles exit the box.  Once that is dry the remaining space is be flooded with epoxy.

(6) So now that the epoxy dried around the cables entering the sled lid, it was time to flip it back over, remove the cover, remove the clay dikes for the epoxy and fill the space with a chunk of lead that was epoxied into place. 

(7) The battery compartments leave about 3/8" of space on the sides of the batteries and there is 1/2" more clearence under the Yellow Top batteries.  The Blue Top starter batteries need the 1/2" to accommodate the 4/0 cables but the rest of the space can be filled with lead plates which will get the sled closer to it designed weight of 2000 pounds.  Making the lead plates is easy enough but time consuming like everything else.  You can see the details about making the lead plates "Working with Lead" page under "Making Lead Plates

After two nights of cleaning the shop thoroughly to remove all of the little pieces of lead debris laying every where I thing it is all gone an hopefully I only ingested enough to kill some of my brain cells storing information pertaining to Britney Spears.

(8) (9) I also wired in the cables for the battery chargers, and made battery terminal covers.  I had already glued plastic to the sled lid above each battery in order to prevent a short should a battery bounce enough to contact the lid, having seen what 750 amps @ 36 volts can do when accidentally connected, I decided it would be safer.  Yes, safer, a word I do not use much.  However replacing a battery would be expensive and that is close to my heart.

The plastic is just a ceiling lighting panel diffuser. (10) When we later installed the battery sled into the hull we switched to using PVC cut from a 4 inch S&D pipe because it makes much tougher covers. Either cuts like butter on the table saw with 30 seconds of head from a torch they are ready to form around the terminal and cables.  Another 30 seconds later is can be removed and any excess trimmed off with a box knife or scissors to make it look pretty. If you get the plastic folded under the edge of the terminal it will keep them in place and they can be snapped on and off.

(11) I originally epoxied plastic to the top lid but the epoxy did not bond we to plastic and after an year it fell off. I should have used a flexible glue. So while fixing the glue problem I also upgraded from plastic sheet to commercial carpet which will insulate and provide a cushion.





The two 12 volt Blue Top batteries that are connected in parallel are primarily used for starting the diesel engine and they will be charged anytime the engine is running.  The 36 volt supply batteries are only charged where a 110 supply is available for the chargers.  There are charging systems for 36 volts supplies which can be powered from the engine alternator but the off-the-shelf solutions are expensive and they just don't supply enough amps back to the batteries to make them worth the time. 

DIY 36 Volt Battery Bank Charging Alternator

However there is a DIY way to charge multiple batteries from the engine.  This idea was sent to us by Richard Clifford-Smith. Obtain a 24 volt alternator, a 12V one would work but would need to spin quite fast, dismantle it and remove the regulator, connecting a wire directly to the field coil so that it can be connected to a custom regulator outside the alternator. Put it back together and mount on the engine as you would normally. You could have an alternator repair shop do this if you wanted, it's a simple job and shouldn't cost too much. The wire from the regulator needs to connect to some simple electronics that you could either build or have built for you. There are designs on the web. With the new regulator the alternator would then output a
voltage suitable for charging a 36 V battery pack in one piece or about 42 volts. You would connect the positive output to the battery + and the battery - to the engine chassis just as you do with the 12 volt system.

An alternator modified in this way could output plenty of current - you'd set it to whatever the maximum your batteries could take. That is the advantage of building a regulator is you can set a current limit unlike a normal alternator. Say that's 15 amps for a 55 Ah at 12 V battery and you would be effectively charging a 36 V 110 Ah battery so you could put 30 amps in and charge in around 4 hours.

Off-The-Shelf Chargers

Our way for charging the 6 batteries in the 36 volt system is to use 6 independent battery chargers. Even then it is a long process. The 36 volt thruster battery bank consist of  6, 55 amp hour batteries for a total of 330 amp hours.  If totally drained, a 15 amp charger would need; (330 / 15) * 10% = 24.2 hours to recharge the bank. 

Battery changers mounted on a
rack and cabled through a

The Dual Pros show
progressively fewer lights as the
batteries are charged and a
green light when done.

Both wiring diagrams result in a 36 volt supply, but to use 6
independent battery chargers it is necessary to connect the
batteries in series first and then connect the two sets in
parallel.  This allows each the battery charger to monitor the
12 volt output for only 1 battery.

I purchased 2, Three Bank Pro Chargers from  They provide 15 amps per bank or battery in this case, and cost $220 for reconditioned units.  They are $340 new, in Fall of 2007.  I also purchased 1, $84 reconditioned LP2, float charger for maintaining the 12 volt starter battery supply. The total cost of the 3 chargers was $520 plus shipping.  And I thought the batteries were expensive!

I was thinking about mounting the chargers in a 1ATM compartment on the top of the battery box with the charging leads passing into the battery compartment and an AC power cable passing to the outside.  However these chargers are oddly shaped and not very dense so it would be difficult to get much usable ballast weigh from the completed compartment.  Therefore the chargers will remain ashore and I installed a 16 pole bulk head connector to make it easy to connect the batteries to the chargers.  I was temped to build my own 16 pole connector but opted to purchase the $50 connector.  The connector has a nice locking seal so all I needed to do was make a blank top for it that will be locked in place to seal up the battery compartment when the chargers are not attached.

I built a rack for the chargers and have wired them to the batteries thought the 16 pole connector. In hind site a 14 pole connector would have been fine because the two 12 volt starting batteries are connected in parallel so they really don't use but one leg from the Pro2 charger.

It required a phone call before I could understand that you do not connect the 36 volt charger to the negative and positive of the entire 36 volt system, but you connect individual negative and positive leads from the charger directly to each battery that make up the bank.  There are 3 pair of negative and positive leads on each of the Pro3 chargers, so each Pro3 will charge 3 of the 6 batteries that are connected together in series and then the two sets are connected in parallel to make up the 36 volt supply.  The magic of the Pro3 is that it uses independent 12 volt chargers that connect to directly to each battery and are oblivious to the fact that the batteries are connected in parallel and then series so it is not necessary to have disconnect switches to physically isolate the batteries.