Terrapin battery wiring and charging upgrade

In the Spring of 2019, I spent about 7 days upgrading the battery and charging system of Terrapin.  For those wishing to do something similar, I have written a detailed description of my upgrade.  Although I have a good foundation knowledge of basic electrical circuits, I needed to familiarize myself with marine electrical systems, particularly as they relate to the cruising sailboat. While there is no one right way to wire a boat, I was able to focus on a series of best-practices that allowed me to design a good charging system.

My main sources were:

  • The Boat Owners Mechanical and Electrical Manual, by Nigel Calder
  • A great web site operated by Rod Collins at Compass Marine
  • Several knowledgeable folks on Cruisers Forum

I actually purchased my Balmar alternator and regulator from Compass Marine.  Customer service was great.  He gave me great advice prior to purchase to make sure I got the right alternator/regulator combination for my needs and provided help during my installation.


My goal was simple: to create a battery and charging system that would sustain long term cruising away from the dock.  To do this, I also carefully considered the charging needs of Terrapin, the existing systems and the typical electrical usage of the crew (Laura and I, and the occasional guests). 

Existing equipment:

  • House battery bank: 4 6-volt deep cycle lead acid batteries, 216 AH each, wired in series/parallel to produce a 12V bank of 432AH. 
  • Two stock Hitachi Alternators, internally regulated, 50A.
  • A Powerline battery isolator
  • Two engine starting batteries, 12V automotive cranking batteries.
  • Solar panel, 150W, PWM controller
  • Air X wind generator
  • A Freedom 20 battery charger from Heart Interface

New equipment installed:

  • Balmar 6 series Alternator, 120A, including MC614 regulator and serpentine belt kit.
  • Victron Battery monitor, BMV 712 Smart.
  • Fusing: Blue Sea MRBF terminal fuse blocks
  • Xantrex Echo charger for the start battery
  • Tools: FTZ Battery Lug Crimp tool and FTZ battery lugs.
Blue Sea fuses
Victron BMV 712
Balmar alternator

This was not a complete re-do as most of the existing components and charging systems worked, and the batteries themselves were in good shape.  This was also phase 1 of an anticipated two-phase upgrade.  The main aspect of phase I was an alternator upgrade and a re-wiring of the battery and charging system.  This was made a little more complicated in that Terrapin (a catamaran) has two inboard diesel engines, two alternators, two starters, two starting batteries and a four-battery house bank. The house bank were relatively new deep cycle lead acid golf cart batteries. 

Many would argue that it is better to upgrade to as much solar as possible and that it is inefficient to run the engines solely to charge the batteries.  I agree, but Terrapin is a small cat and there is limited room for additional solar. My existing panel is at the stern, above the dinghy davits. There is room for additional panels on the coach roof, but part of that area is always in shade due to the boom and the stack-pack, or the main sail if it is up.  Nevertheless, I do plan to add a panel up there.  While I don’t plan on routinely using my engines solely to charge the batteries, I am usually running the engines at least an hour or two every day while cruising, usually to get in and out of an anchorage.  Of course, if there is little wind, then I run the engines a considerable amount more.  Thus, a high output alternator makes sense to make sure the batteries get as much charging as possible when motoring. 

Based on all my reading, my specific goals were as follows:

  • Upgrade one of the alternators to a high capacity alternator with an external regulator.
  • Run all charging to the house bank: both alternators, solar panel, wind generator, shore power battery charger/inverter.
  • Get rid of one of the start batteries and wire the one start battery to both starting motors of the engines.
  • Add an echo charger to keep the start battery charged.
  • Re-wire the battery switches to allow engines to be started from either the house or start battery, and allow power usage from either battery.  With this set-up, the start battery essentially becomes an emergency battery, normally isolated from house loads, but accessible if needed.

The lead acid (LA) battery was invented in 1859 and represents the earliest type of rechargeable battery still in widespread use.  In the boating community, the lead acid battery still dominates but they are slowly being replaced by lithium ion, specifically lithium iron phosphate (LFP) batteries.  The type of LA batteries that serve has the house storage bank for boats is very different from your typical car battery.  They are referred to as deep cycle batteries and are designed for repeated discharging and charging. The most popular batteries for this purpose and the batteries we have on Terrapin are 6V golf-cart batteries, wired in series to produce 12V.

The LA batteries work well if you take care of them, but they have several limitations.  They are very heavy, the usable current is only 30-40% of the total amp-hour capacity, they are take a long time to charge to full capacity and they have to be topped off with water periodically.  LFP batteries overcome these limitations making them a much better option on a sailboat, but they are considerably more expensive.  Because my bank is in good shape, I decided not to go with LFP yet.  That will be phase 2.

When away from the dock, the amount of charging available on a boat is often limited.  In brief, the number of amps removed x the number of hours (amp-hours) must be replaced by charging.  On top of that, as the battery gets to about 80% charged, it begins to accept less current (amps) making it harder and taking much longer for that last 20% of charge. As a result, many LA batteries on boats never get fully charged, and die an early death.

The charging systems on Terrapin consist of 2 alternators, one on each engine, a solar panel and a wind generator.  There is also a Honda suitcase generator that can be plugged into the boat’s “shore power” inlet.  Of course, when at the dock, the shore power is plugged in and the boat’s AC/DC converter (Xantrex Freedom 20) charges the batteries to the max. After two seasons on Terrapin with prolonged time away from the dock, I realized my charging system was not adequate to keep the batteries fully charged.  The solution was to fire up the Honda generator every three days or so and run it for a couple of hours to get the batteries fully charged.  The solar and wind generator helped, but were not sufficient to supply the current needed.  I could add additional solar panels, but I simply don’t have the space on Terrapin to install the amount of solar needed to be fully self-sufficient. That leaves the alternators.  The stock alternators on Terrapin were automotive alternators driven by 18hp diesels.  These alternators were designed to supply small amounts of charge to a car battery after it starts the car.  This is inadequate to charge deep cycle batteries.  So, I decided to replace one of the alternators with a marine rated high output alternator, along with an external regulator to put out the voltage required for good battery charging.  Yes, Terrapin is a sailboat, but when out sailing, we are often running the engines 1-2 hours a day just to move the boat in and out of harbors or longer when the wind dies. So, the alternators should be able to handle the full charging needs of the house battery bank. Finally, there were two starting batteries dedicated to starting the diesel engines. These batteries were wired in a confusing way and shared charging sources with the house bank. 

Once I decided to upgrade the alternator, it became apparent that the wiring needed changing to make the charging more efficient, and take advantage of new technology.


The original configuration consisted of 4 6V deep cycle golf cart batteries, two in series and then paralleled to get 12v at 430AH, a pretty standard battery configuration. However, the heavy cables connected to the bank were at the wrong positions.  They worked, but were not optimal to deliver equal charging or equal loads on all 4 batteries.  There were two 12v start batteries with a complicated mix of switches connected to the starters as well as the alternators. There were two stock Hitachi alternators (55A) connected to the house and start batteries via a Powerline diode battery isolator.  This resulted in very poor battery charging from the alternators because the battery isolator had a significant voltage drop (see the Compass Marine web site for a good description of the limitations of diode battery isolators and voltage drop). The result was that the house bank never got fully charged when I was motoring, even for long periods of time.  There was one 150W solar panel and a wind generator that charged the house bank directly.  The fusing was inadequate and located in a very inaccessible place. The fuses were also original equipment (20 years old) and I didn’t trust them. 


The upgrade consisted of installing a Balmar 6 series alternator, (120A) and an MC 614 external programmable regulator on the starboard engine along with a serpentine belt kit.  I removed the Powerline battery isolator and rewired everything so that all charging went to the house bank, including both alternators.  Just because there are two engines does not mean one needs two starting batteries.  Starting an engine uses very little of the battery capacity, so a single start battery is all that is needed.  I removed one of the start batteries and gave it to Gary down the dock. This also removed about 75lbs of weight from Terrapin.

Another important feature of a good battery and charging system is a battery monitor.  I selected a Victron battery monitor.  The basic concept is that there is a shunt between the negative battery post and all ground connections to the house bank, whether charging or load.  The battery monitor then detects all current running either into or out of the house bank.  It also directly senses the battery voltage right at the battery terminals.  All of this information is displayed on a small display inside near the fuse panel. I can also connect with my smartphone via Bluetooth and read the information on an app. The original panel on Terrapin has an analog system that attempts to do the same thing, but it is not very accurate and does not keep track of the current in and out.  Why is this so important?  Consider a day on the boat, out sailing, away from the dock.  There will be several loads draining the house bank as well as sources of charging.  The refrigerator, nav system, autopilot, WiFi, and various gadgets being charged will all remove amps from the batteries.  At the same time, the batteries will be charging from the solar panels, wind generator and alternators if the engines are running.  At the end of the day, it is impossible to know the net amp-hours delivered or removed from the house bank. The Victron monitor keeps track of amps-in and amps-out and will give a picture of how discharged or charged the house bank is. Of course it will also tell you the battery voltage, but that is not a very good way of determining state of charge. It also allows one to know at a glance how efficiently the batteries are being charged from, say, the alternators.

If all the charging is now directed to the house bank, how do I charge the single start battery, you ask?  This is accomplished with a nifty little device called an Echo Charger.  Echo Chargers are good if the battery being charged does not need high amperage charging, like a start battery.  After starting two engines, the start battery is only slightly discharged and doesn’t require huge charging currents.  The echo charger puts a max of 15 amps into the start battery only if it senses a charging current is available.  The advantage here is that it does not require a heavy cable, only a 12-14 AWG wire. The echo charger also has circuitry to prevent over charging of the battery.

Then there is the issue of how to wire the switches to give maximum flexibility to start the engines from either the house or start battery, or to run the various loads from either battery, or isolate either battery in the event that a battery develops a short.  After doing a lot of reading, I settled on an ingenious and simple switch arrangement that accomplishes these goals.

To do all this, I bought a large long handled FTZ battery lug crimp tool needed to crimp the lugs on heavy 2/0 battery cable. This way I was able to make my own battery cables and do a neat re-wiring job. I was able to re-use two of the battery switches from the old set up.  Fuse options have come a long way in the last 20 years, and there are now many configurations to provide fusing right at the battery post, or blade fuse configurations for smaller wiring.   I used several terminal fuse blocks and some blade terminal fuses from Blue Sea Systems. 

Since a picture is worth a thousand words, here is the final wiring diagram I ended up with.

In practice, the system works very well.  To gauge the charging status of the batteries, I check the Victron Battery monitor frequently.  It will tell you the voltage of the house battery bank, or the starter battery.  The voltage should be in the range of 12.4 to 13.8 volts.  This doesn’t tell you the actual voltage of the battery because the voltage will vary depending on the charging or load on the system. The other thing to look at is the amps going in or out of the system.  Finally, the monitor will tell you how much the battery bank was charged or discharged since the last time it was fully charged.  The monitor is not smart, so it can provide misleading information on the state of charge of the battery, so the user needs to do the thinking.  A fully charged battery that has rested for a while, will show a voltage of about 12.8 volts.  It will also not take much current if you supply a charging source.  As you are drawing down the charge, keep an eye on the monitor and check it in the morning.  The battery voltage should never get below 12.1 volts.  Finally, you can check how many net amp-hours you have used in, say, a 24 hour period, i.e. the amps-in minus the amps-out.  This will give you an idea of how many amps you need to return to the battery bank the next day. 

Let’s say you see a net loss of 100 AH when you get up in the morning. As the bank is charging the next day, keep an eye on the amps going into the bank.  Early in the day, the amps being delivered (if the engines are running) will be between 75 and 100 amps.  Over time, the amps being accepted by the bank will decline as they become more and more charged.  When the monitor shows that 100 AH have been returned to the bank, it is fully charged, more or less. 

The starter battery is typically isolated from the loads, so that it will never be run down if a load is accidentally left on.  It is also isolated from the charging sources, all except the echo charger.  So, the start battery should always read 12.8-13.8 volts if it is healthy.  However, the switch arrangement allows one to start the engines from either battery bank, or run the house loads from either bank.

Below is a description of the use of the house battery bank/start battery switches.

House 1-2-Both switch
Start On-Off switch

Switch position                            

Start             House

On                1

This is the normal operating position. House and start batteries are separate.  All charging goes to house. The start battery only receives charging from the echo charger.  The only load on the start battery are the starter motors.

Off                1

Use this position if you want to cut off the voltage to the starter motors or to work on the engine (see below for additional precautions). Start battery is isolated.  House bank and start battery will receive charging.  Engines will not start.

On                2        

Use this position if the house bank is dead or shorted out, and you need emergency power, e.g. for nav instruments. The house bank is disconnected from load and the start battery is directly connected to both load and charge (except for the Balmar Alt).  The starter battery will receive charging from the port alternator, solar and wind, so this should get you home if the house bank fails. Beware that the start battery has low capacity. 

Off                2

There is no good reason to use these switch positions unless trouble shooting the system. Both the house bank and start battery will be disconnected from the loads and most charging except the Balmar. 

On                Both 

Use this position if the starter battery is weak or dead and you want to start the engines using the house bank.  This can also be used to “equalize” the start battery using shore power, or to provide extra charging to the start battery from the Balmar alternator. This attaches a “jumper” between house and start.  The start will receive charging from all sources, but will also be contributing current, along with the house bank, to all loads.  House bank will contribute current to the starters.

Off                Both 

Use this if the starter battery is dead or shorted out and you need to start the engines. This attaches a jumper between house bank and the starter motors, so that the engines will start from the house batteries. 

Off                Off

Use this position when leaving the boat for extended periods.  This will isolate the house bank from any potential battery drain from the house panel, and the start battery from any potential drain, e.g. from a shorted starter motor.  However, the house bank will still be connected to the Balmar alternator,  solar panel and wind charger. So the house bank will continue to be charged by the solar panel and the start battery will still receive charging from the echo charger.  There will be no power to the house electrical unless the boat is plugged into shore power. 


There is currently no way to isolate the Balmar alternator from the house battery bank.  I will be installing an Alternator Service switch in the engine bay for this purpose.  In the meantime, to work on the either engine, disconnect the negative cable at the Victron shunt near the battery bank, and turn the starter battery switch to off.  This will prevent an accidental short at the alternator, or the starter motor.

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