Emergency Start Battery Project – Catalina 36 MKII

By Bruce Landsberg

The stock battery arrangement of two separate 4D cells in the MKII is adequate, but not optimal. Combining the 4Ds allows double the amp hours available while under sail, or at anchor, before discharging any one battery to the critical 50% level. Using refrigeration and many of the other modern electrical conveniences can deplete a single 4D too quickly. However, by tying the two batteries together there is the possibility of depleting all the electrical power.

The solution is to add a separate starting battery that is used all the time or an emergency starting battery (ESB) that is used only when the house bank is completely depleted. The philosophy is different.

Normal Start Approach

Tom Senator, MKII tech editor, designed an excellent system that uses both the start battery and the house battery on every cruise and must supply on board charging.

Benefits: Totally separate system; battery switch for starter is always on, simple, can be used with boat on mooring.

Tradeoffs: Requires more hardware: echo charger and heavy gauge wire to echo charger, new battery switch, pulls more amperage from alternator and results in slower charging of house bank while under power.

Emergency Start Approach

This system uses the house bank for ALL electrical needs, including starting, and only uses the separate start battery in the event the house bank can not crank the engine. An engine start uses very few amp hours (a lot of amps but only for a few seconds – estimated at 3-5 amp hours – about the same as a VCR used for 2 hours). With a battery monitoring system, such as the Link 10, it is unlikely that one would deplete the system below the start level but a backup is there, if needed.

Benefits: Totally separate system, requires less hardware, uses existing switching, doesn’t require echo charger or associated wiring run. Does not add extra load to engine and allows faster charging to house bank. Shore power will charge all batteries.

Tradeoff: This system is not recommended if the boat is normally stored on a mooring or where shore power is not occasionally available – say once per month. However, if your circumstances changed, it would be easy to modify to the normal start approach.

Operation – In this setup, the stock 4 position battery switch is always set to Position 2 (for 2 batteries = house bank) when in use. Position 1 is for emergency start only.

Installation – I put the Emergency Start Battery, (ESB) under the forward game table seat adjacent to the battery switch but it could be located elsewhere. The main benefit is easy access for adding water and extremely short cable run. I used a basic group 24 starting battery. Note the red positive cable from the ESB going through the baffle to switch position 1. One black wire is the negative battery cable and the other “black” is actually cable sheathing covering the red 12 gauge wire to the 110V charger.

1. Add a shelf to support the battery and the battery box. There are number of ways this could be done. You could fiberglass in a shelf. I chose a simpler method – bolted a 2x4 to the interior liner and another to the wood baffle that protects the switches inside the settee. Bolts allow access to all items if required. Screw the plywood to the 2x4 brackets to form the shelf. Use 3M 4200 or 5200 for the third corner to provide some bracing against the hull. Paint all the wood before installation to prevent deterioration. This area is normally dry, but boats being boats… Leave the shelf disassembled until final installation. This allows access to the battery switch. I trimmed the rear baffle to the switch area to allow it to be removed vertically if access were ever needed.

ESB located under front game table seat

2. Negative Cable from ESB:

Run the negative cable (black) from ESB back to the negative bus bar. This is the most challenging part. The existing cable runs (PVC pipe) were full and so I had to fish this cable through between the hull and the liner. It took about 7 feet of 1/0 battery cable. I made up my own cabling purchased from West Marine – you can cut and crimp on battery terminals in the store. Very easy and much cheaper than buying pre-made cable. The store personnel were helpful in demonstrating the procedure and there’s no need to buy a big crimper or other tools that are seldom used. The usual wiring practices of using quality tinned wire, heat shrinkable tubing and appropriate connectors obviously apply.

Stock Battery Switch set to house bank normal position

3. Shore power charging:

Disconnect the red 12 gauge battery charger wire from one of the 4 Ds, clip off the connector, and splice a 7 foot length of red 12 gauge wire to run to the ESB. This should be done when running the neg. battery cable above. This will be hooked to the ESB positive terminal.

4. Paralleling the 4 Ds:

Disconnect Battery 1 (4D) from the battery switch “1” position and connect it to the “2” position on the switch. You have now paralleled the 4Ds. This also connects the shore power charger through the main battery switch so that both 4Ds are charged by shore power.

5. Adding Link 10 and Negative Bus Bar:

I used the Link 10 battery monitor for two reasons: It fits perfectly in the hole that holds the stock analog volt meter and it’s about $80 less than the Link 1000. Because the Link 10 requires all connections to be made downstream of the shunt, a bus bar makes it much easier to do a professional job. On the MKII, the 4Ds are held in place by two 2x4s. It is easy work to remove one of them and rout out a slot for both the shunt and the bus bar. If you try to mount them on top of the 2x4, the seat will not fit. Reinstall the 2x4 with the shunt and bus bar attached.

Link 10 replaces voltmeter in existing hole

2x4 securing 4D with shunt & Neg. Bus Bar under aft game table seat.

Parts list:

Shelf

1 2x4 x 3 for mounting the shelf – will be cut to size

3/8” (or thicker) piece of plywood – for battery shelf -cut to size

5 ¼” stainless bolts with washers and nylock nuts ( if you glass the shelf disregard this)

1 small piece of wood, cut to shape for gluing to hull with 3M 4200/5200

6-8 stainless wood screws

epoxy paint for shelf and braces

Electrical

1 group 24 starting battery

1 battery box

10 feet 1/0 tinned black battery cable - 7 feet from ESB to aft battery area, approx 3 feet cut into several pieces to tie together batteries, shunt and to bus bar.

2 feet 1/0 tinned red battery cable – from ESB to battery switch

7 feet of #12 tinned red wire to splice from shore power charger wire to ESB

Battery connectors and large heat shrink

Assorted Terminals, splices & heat shrink

Link 10 Battery monitor

Bus bar

Click for Drawing/Diagram (Word version)