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New LifePO4 Lithium
House Bank
Battery Charging
System Philosophy
Trickle Charging
Start Battery
Solar Charging System
Alternator Repair Lightning Issues

Last Updated: 04/27/2021

The electrical systems and issues on our previous boat, a 1980's CSY monohull, were quite different than those on our 2004 St. Francis Catamaran.  For all the great stuff I did over the years on the CSY electrical system, go to this link:  CSY Electrical Systems.

Future Additions:
Alternator-Regulator Engine Charging

Lithium LifePO4 Battery Upgrade

About a year ago we embarked on replacing our 13 year old Gel batteries with lithium LiFePO4 cells. We quickly learned that:

  • The technology and supporting equipment are still evolving
  • There are many opinions regarding how best to do this; even the experts’ opinions differ
  • Competition among sellers of lithium equipment is stiff
  • Not everything you read on seller websites is true, and some things that are not there are true
  • In order to create a successful installation you must have an open mind and never stop learning about what others have done

Our installation is optimized for a catamaran cruising full time overseas. We have little tolerance for jury rigs, marginally safe practices and unnecessarily high cost equipment. However, we do believe in developing the skills to make repairs ourselves with adequate spares and proper tools. Below find our thoughts on how to complete a DIY installation after significant research, money expended and living with the new technology for about 6 months.

When we purchased our St. Francis 44 catamaran 6 years ago, the house bank consisted six 6-volt Sonneschein Solar Block 200 batteries, for a total of 600Ah. They are very high quality German Gel batteries. In early 2020, locked down by COVID, and with our gel battery bank now 13 years old, we decided it was a good opportunity to upgrade to new Lithium technology. The Gels might have lasted several more years according to Gel literature, but given our location in SE Asia, and the time we had available, this was a good opportunity to upgrade. Had we not had the years’ time to study the technology and make the many necessary electrical system changes, we would not have taken this on. It is not a project for those not willing to spend time learning how to do the installation properly.

Lead Acid (LA) vs Lithium Differences: Because of the vast differences between these two types of batteries, great care must be taken to not make a mistake during installation. A number of different lithium type cells are available now, but LiFePO4 is the best choice for cruiser batteries.

  • They do not require a full charge on a daily basis; in fact, charging to only 90%, or even less, will give a longer service life.
  • No absorption or float charging is needed, only bulk.
  • 80-90% of LiFePO4 capacity is useable on a regular basis vs a maximum of 50% for lead acid under ideal conditions.
  • Cell level monitoring, balancing and disconnect control are required with LiFePO4, whereas only total battery voltage is typically monitored for charging with LA.
  • There may be some significant electrical system modifications required for a proper lithium installation, including purchase of a reliable battery management system, installing separate charge and load busses and use of separate inverters and chargers.
  • Charging with an alternator requires some modifications, either by making sure the alternator is externally regulated by a smart alternator regulator, or using battery-to-battery charging, with a lead acid battery connected to the alternator.
  • Finally, long term storage requires placing LiFePO4 cells at about 50% state of charge (SOC) and in a relatively cool location for best longevity.

LifePO4 Cells:  After a great deal of research, we settled on prismatic style aluminum cased LifePO4 cells, purchased directly from RJ Lithium in China. There are a number of other Chinese cell companies we could have bought from, but other friends locally had researched this and were buying from RJ. Their reputation was good, the sales representatives were responsive, the website was informative, and they offered a 5 year warranty.

Our 8 Cells

One bank of 4 cells

We purchased eight 3.2 volt 271 Ah LifePO4 cells, for about $150 each plus about $35 shipping each for a total house bank capacity of 540 Ah. (Note that the shipping cost was excessively high due to the peculiarities of shipping into the Philippines).

Of that 540 Ah, about 450 Ah are useable, so we increased our capacity by about 50% over our Gel battery bank's useful capacity. And we were able to remove 390 lbs of Gels and replace with only 100 lbs of lithium cells. Later we discovered that we could have bought similar well regarded EVE or Lishen cells for about $100 each and modest shipping cost, sourced locally here in the Philippines, but without the longer warranty.

Drop-In LifePO4 Batteries: We do not recommend installing what is commonly referred to as ‘drop in lithium batteries’ on a cruising boat. They are better suited to coastal power boats and outboards since the cells and most of the electrical components are secured inside the battery box and are inaccessible if there is a problem. Also, typically the BMSs provided use small internal relay components that must control the high currents usually found on cruising boats. They are also frightfully expensive compared to a DIY installation using individual cells. And you will still need to do most of the electrical work as with a DIY system. If you can’t take the time to do a proper DIY installation yourself, we recommend buying quality Gel batteries to install into your existing electrical system.

Cell Matching and Balance: Factory matching of cell capacity and internal resistance is important to being able to maintain cell voltage balance. The charging curve is very flat in the middle between 3.20-3.4v per cell, and then goes nearly vertical (the knee) as the cell approaches full SOC at about 3.45v per cell and above. Likewise, as the battery is discharging, the voltage stays very flat until dropping off rapidly below about 3.15vpc as the battery reaches 10-15% state of charge.

Charge Curve for 4 Cells in Paralell

Charge Curve for 4 Cells in Parallel
Charging at 50 amps for 18hrs and 40 minutes
(small downward spikes are when we paused charging for the day)
Note vertical voltage rise at approx 3.5v (14.0v in a 12v system)

LifePO4 Cell Discharge Curve

This curve reflects one cell being discharged from "full" at 35 amps continuously.
Of note is the flat curve until the cell reaches 3.15, and the rapid drop-off after that.

If the cells are not perfectly matched in capacity and internal resistance, when charging one cell can get to 3.65v per cell well before the others, and end up in an overvoltage situation that will destroy that cell before the total battery voltage reaches an alarm condition. Likewise, when discharging, if one cell is slightly more depleted than the others, its voltage can drop rapidly to below 2.5v, again while the overall battery voltage is still at a safe level.

Also, cells that are not well matched will have slightly different charge curves that might cause an automatic balancer to balance the wrong cells during charging.
Well-regarded lithium factories produce thousands of individual cells a week. During initial testing those that match advertised specifications are graded as A cells. Those that do not become grade B cells. Both are sold on the open market. Grade A cells are usually a bit more expensive, have QR and testing stickers intact and usually have warranties. Cruisers would do well to always buy Grade A cells.

Configuration: We originally planned to configure the cells into 2 4-cell "batteries", with a 1-2-Both selector switch so we could switch between batteries if we ran into a problem with one cell.  However, once we started wiring up the BMS, we found that trying to control the separate Loads and Charging busses with two BMS's got way too complicated. We also found that rearranging the cells from an 8-cell bank to a 4-cell bank, if we ran into a cell problem, to be pretty simple.

So we ended up with 4 sets of 2 cells in parallel to make 4 "supercells", then those 4 cells in series for a 540Ah at 13.2v bank. This also had the advantage of requiring only one BMS instead of two. Whether this is called 2P4S or 4S2P configuration depends on who you ask. It is written both ways all over lithium literature and the internet.

Compression box: Most literature and experts on LiFePO4 recommend using a ‘compression box’ to house the cells. Really this should be more of a Limitation box that prevents the cells from chafing against each other, terminals being damaged by connector tension as a result of expansion during charging, and sliding around as a bank in rough weather. Also, according to EVE Energy, one of the largest lithium cell manufacturers in China, their aluminum case cells require compression in order to reach their full cycle life. So we built a compression box to store the cells using epoxy coated ¾” marine plywood and 3/8” SS threaded rod. Between the cells we used 1mm Formica to prevent the sides shorting together. The box is strongly secured, has a Lexan cover, and is mounted under our main cabin settee only about 3 feet from the BMS and busses.

Our Battery "Compression Box"

The LifePO4 cells in the compression box

Battery Box, Closed Up

Battery Box, Closed and Secured

Battery Management System:  A good BMS is a critical component of a LifePO4 system. Unlike most lead acid battery charge sources that monitor overall battery voltages, a good BMS can monitor individual cell voltages. So a BMS can watch the voltage and state of charge of each cell, and make appropriate decisions regarding when to disconnect charge sources in case of an over voltage event (HVD) or disconnect loads in case of an under voltage event (LVD) in order to prevent cell damage.

There are two types of BMSs. One uses small internal MOSFET relays to turn on and off charging and loads, the other can control much larger external relays. Since loads like inverters and windlasses and charging sources like alternators and generators might involve relatively high currents, cruising boat systems are better done with a BMS that uses external relays.

BMSs range in price from inexpensive/dumb $15 USD small open boards with no user interface, to high-quality programmable BMSs that will integrate seamlessly with existing smart Victron equipment for about $600 USD.

We chose a very smart programmable BMS made in Canada with a Wi-Fi interface called an Electrodacus SBMS0, for about $120 USD. This one is targeted at the DIY Solar community and can control multiple external relays with many adjustable parameters. It has a detailed monitor, a small balancer and excellent HVD/LVD protection capability.

Our Electrodacus Monitoring Screen

The Electrodacus SBMS0 also has some built-in historical logging, and with the addition of an optional wifi module, can provide a wireless MQTT feed to integrate into any Internet-of-Things (IOT) system.

A Node-Red-based Monitoring Screen for Electrodacus SBMS0

 Our Battery Management monitoring screen, using the MQTT feed
from the Electrodacus, and built using Mosquitto and Node-Red.
(more about this)

We are pleased with our choice of BMS, but it has involved a steep learning curve. (LifePO4 in general is a big learning curve!)

Wiring & Fuses: It is important to use proper wire sizes and fusing in a lithium battery project because these small batteries can produce huge amounts of amps if shorted. Although fires are rare with LiFePO4 technology, a shorted cell can melt tools instantly and cause sparking that can ruin your day.

Also, the display and balancing wires are typically 20 gauge or smaller, so you might need better crimping and stripping tools. For example, the ElectroDacus system uses twisted solid Cat 5/6 24 gauge wires for connections to the SBMS0. Our tool recommendations are below.

Relays & Shunts: When using a BMS capable of controlling external relays, care must be taken regarding which relays to use for which circuits. And shunts must be used in order to display current and State of Charge (SOC). Below is a list of those we used:

  • Solar charging: We use two of the ElectroDacus DSSR20 relays for the SBMS0 BMS to disconnect the panels from the 60a Morningstar Tristar-60 MPPT solar controller, if a HVD event occurs. They can each handle up to 20 amps of current, so are perfect for our 800 watts of 24v solar panels. For now, the MPPT handles the charging and termination. But we need the MPPT in this circuit anyway since it handles the voltage conversion from the 24v panels to the boat’s 12v electrical system.
    If the panel voltage matched the boat’s voltage there would be no need for the expensive solar controller, and the BMS could handle charge termination without any solar controller (one of the unique features of the Electrodacus design).
  • Shore Power Charging: We have a Sterling Power Pro 60 Amp AC charger, that we bought specifically because it has a "LifePO4" charge profile.  Unfortunately, that charge profile is a marginal charger for LiFePO4 cells, because the minimum absorb setting is 60 minutes, so if it is controlling the charging it will have to be manually shut off when the battery reaches Absorb voltage. It can be controlled by the BMS using a Fotek 40 amp AC-DC solid state relay (SSR) placed in the AC load wire before the charger. This allows the BMS to control the charger HVD by disconnecting the 30 amp AC wiring, not the larger DC wiring.
  •  Alternator Charging: We have two small 60 amp externally regulated alternators, one on each engine. The SBMS0 controls their HVD by disconnecting the ignition/enable wire from the regulator using 10 amp Fotek DC-DC SSRs. This is preferable to disconnecting the alternator field wire from the regulator because it safely turns off both the alternator and regulator. The Very Smart  Regulators (VSR) can be easily configured to handle LiFePO4 charging with both temperature and voltage sensors. (Our VSR's are the predecessor to the popular Wakespeed 500 Smart Alternator Regulator)
  •  All these relays are about the size of a matchbox and use about 5 milliamps or less when energized. Because we rarely need to use the AC charger or alternators, we also have small on-off switches in their control circuits from the BMS to turn the relays off when not in use. These relays require a small resistor in the circuit to prevent excess current getting to the SBMS0 in case of a short.
  •  As a second line of defense in case of over/under voltage problems, the SBMS0 also controls two high capacity Victron Battery Protect relays. These are placed to separately disconnect the charge and load busses from the battery in case of a problem that wasn’t handled by the charging sources or other relays.
  • Shunts: In order to be able to sense current and calculate SOC shunts are required. If one is used for the battery and one for the charging sources, charge, battery, and load currents can all be displayed. In the ElectroDacus system, both shunts are mounted on the positive battery cable. We used 100 amp 100 mV shunts from Reidon properly sized for our equipment.

Initial Testing and Balancing: Not everyone will have the time or want to do the initial testing that we did. We had purchased the cells with an electrical engineer friend--a total of 24  271 Ah cells.  Our friend had the proper Load and Bench Power Supply equipment available, and we wanted to fully test that the capacity of the cells we received matched the manufacturer specs. 

We started by charging all cells up individually to 3.6 volts per cell (vpc). Then we discharged them individually to 2.8 vpc and graphed the resulting charge curve and capacity. Finally, we made a graph of the charge curves taking them back up to 3.6 vpc.  (see graphs at the beginning of this writeup)

 After removing the charge, each cell fell back to a rest voltage of about 3.35 vpc. All capacities were within 260-270 Ah, since we did not fully charge or discharge them. The charge and discharge curves were similar, but not exactly the same. Internal resistance was measured to be .076 ohm at 3.0 vpc.

Top Balancing:  Later we did what is referred to in the literature as a ‘top balance’. That is taking all cells in parallel to exactly 3.62 vpc and letting them absorb to about 5 amp tail current. This should get them all balanced to within a few millivolts (mv) of each other at charge termination. Although we did this a second time, we did not find it to be very effective at getting the cells closer than about 100 mv apart at charge termination of 3.45 vpc.

We found the most effective balancing technique to be using our bench power supply and adjustable load equipment to manually get the cell voltages closer. This consists of attaching the equipment to individual or a group of cells to raise or lower their voltage by adding or subtracting milliamps (mA).

Regardless of how the cell voltages look at rest, they will always have a much higher delta near charge termination, especially if well up the charge knee. After a week or so of adding and subtracting mA, cell voltage deltas can be reduced to 10-20 mV.

Then the small passive automatic balancer in the SBMS0 can be used to keep them in balance, if only allowed to balance above about 3.4 vpc. This is because at lower voltages, the cell voltages sometimes reverse which is high and which is low.

The balancing mechanism on our BMS uses resistors that are latched in during charging, when the cell voltage delta goes above 20mV, to suppress charging the highest cell(s). 

Our cells are now resting and charging with a 7 mV delta and 20-25 mV delta at charge termination. These figures are entirely adequate for cell balance.

Tools and Equipment: We purchased the following equipment to help us better do the installation:

• Long Wei PS1310F DC bench power supply. Amazon Link
• 20 amp 180 watt adjustable load  Amazon Link
• ISDT BattGo BG-8S 2 ea monitor balancer  Amazon Link
   (used for BMS prior to receiving Electrodacus)
• UNI-T UT-61E multimeter with logging  Amazon Link
• UNi-T UT-210E clamp multimeter  Amazon Link
• Titan 11477 ratcheting terminal crimper, yellow and black handles  [Amazon Link]
• Klein Katapult 11063W wire stripper  [Amazon Link]
• Klein 11074 blade for 16-26 ga wire  [Amazon Link]
• Manual large terminal crimper for #8 to 2/0 terminals
• Various wire terminals from 2/0 to 24 ga, heat shrink tubing, Cat 6 cable
• Assortment of 1 watt resistors
• Fotek AC-DC 40 amp SSR - 2 each with clear plastic covers and heat sinks  [Amazon Link]
• Fotek DC-DC 10 amp SSR -3 each with clear plastic covers and heat sinks
• Reidon 100 amp 100 mv shunts- 2 each  [Reidon]  [Amazon Equivalent]
• ElectroDacus SBMS0 BMS with Wifi  [Electrodacus]
• ElectroDacus DSSR20 relays- 2 each  [Electrodacus]
• RJ 271 Ah prismatic aluminum case cells- 8 each  [RJ Lithium]

Electrodacus and Control Wires
The Electrodacus and Control Wiring
Mounted Near Nav Station
Alternator Regulators and Solid State Relays
Smart Alternator Regulators and
Solid State Relays (for HVD)
Solid State Relays to Cut off Solar Panels
2 DSSR20 Solid State Relays to Provide High Voltage Disconnect for Solar Panels
To be added:
Picture of relay for Shore Power Charger HVD.

We have consolidated our knowledge and experience (with contributions from others) for the Electrodacus SBMS0 BMS on a separate Electrodacus page.

An excellent summary of BMS features is provided in this document on DIY Solar:

Beginner's Summary of BMS Functions, Types, and Features

Good LifePO4 Reference Sources

For those contemplating a LifePO4 upgrade on their boats, here are the resources we found best to educate yourself before you get started.'s LifePO4 on a Boat
This is an excellent beginner's guide.'s Lithium Pages
This is a much more detailed and technical discussion

Stan Honey's Thoughts on LifePO4 Batteries on Boats (PDF)  [link] or [alt link]

Will Prowse's DIY Lithium and Solar on YouTube [link]

Bussed_As NZ's DIY Lithium and Solar on YouTube [link]

DIY Solar Forum "Beginner's Resources"  [link]

Battery Charging System Philosophy

This is a big subject and it is always interesting to see how others are set up also. Much of how you set up the boat depends on how you plan to use it. If you are dockside most of the time you will want a different system than if you are cruising and away from docks. We are currently cruising, often away from docks and also away from good (expensive) repair facilities.

It has taken me several years to get my new catamaran set up the way I wanted it... with all charging systems wired to the House bank and a trickle charge arrangement for the Start battery.  On the catamaran it is a little more complicated because we now have 2 engines and 2 alternators to worry about.

My system keeps the House and Start batteries physically isolated from each other in the normal use/charging situation. It allows multi step charging for the house bank and trickle charging for the start battery.

First, connect all your charging sources (alternator, 120v charger, solar, etc) on one "charge bus" and connect that directly to the house bank.

Run your charging sense and temperature wires to the house bank.

Connect your starting battery direct to the starter. Then connect the house bank to terminal 1 of the battery selector switch and the start battery to terminal 2.

Next connect all your loads to the common terminal of your battery selector switch. Add cutout switches and/or fuses as desired in the battery cables within 18" of the batteries, if you want to comply with ABYC specs.

Finally, connect the positive posts of the house and starting batteries together with 12 ga wire and a thermal circuit breaker and diode.

See Trickle Charging the Start Battery, for details, below.

So, if you leave the battery selector switch on 1--the normal situation--the starting battery starts the engine, and the house bank runs all the loads. And the two batteries/banks are isolated. If the start battery fails, you can temporarily place the switch on Both to combine them for starting.

Also, keep your house batteries away from heat while charging, as it is a killer. If in the engine room, for example, with temps over 115 degrees F you will lose 75% of your cycle life! And if you are using Lead Acid technology for your house bank, don't forget to equalize!

Start Battery Trickle Charging

At the 1998 SSCA Gam the Four Winds/Everfair owner, Bill Owra, gave what I thought was a very well thought out and much better wiring diagram for a house bank and a starting battery setup.  It featured, among other things, hands off trickle charging of the start battery off the house bank, an anti theft switch in the starter/starter battery cable,  using the battery selector switch to normally control only battery loads, rather than charging and loading, emergency cross connect for starting and house loads, and routing all charging source cables directly to the house bank so there’s no possibility of an alternator or other disconnect problem while charging.  It was the best layout I've seen and I've looked at many in the past 20 years of boat ownership.

I thought that the best feature was the automatic trickle charging of the starting battery from the house bank through a short 12 gauge wire with a diode (one way current flow) and thermal circuit breaker. 

The idea is that when the house bank is being charged from any charging source the diode will keep the voltage to the starting battery about 1/2 volt below the house bank voltage, and the thermal circuit breaker will break the charging circuit off if the charging amperage get too high. 

The starting battery can take up to about 15 amps if it needs it but with this system it normally just takes small float current.  I've never seen more than about 2 amps going into my starting battery from the house bank.

Battery Charging System Wiring 

This circuit provides a reasonable float charge to keep the starting battery always at 100 percent, just like in your car.  A true starting battery with thin, large surface area plates and a high CCA/MCA rating is great for starting your engine and will last as long as your house bank if kept this way. 

And it's all automatic with no risk of inadvertently disconnecting the house bank from its alternator charging source.  I also keep a couple of spare diodes and thermal circuit breakers aboard just in case.

I used this electrical layout on our CSY, and have just (2018) installed it on our new catamaran.  Here are the components required:

Thermal Circuit Breaker and Schotke Diode

And links to online sources:

20amp Thermal Circuit Breaker

60amp 35v Schottky Diode (MBR6035)

And a picture of ours, installed.  I just mounted it on a piece of scrap aluminum, near the battery box:

TCB and Diode Installed

Solar Charging System

For the evolution of our solar system on Soggy Paws the CSY, see this page.
CSY Solar Charging Evolution

When we bought the catamaran, I sold off the old system that came with the boat and installed 4 new 200 watt 36v panels, and a new Morningstar MPPT solar regulator.  Though we really liked the Outback MPPT regulator that we had on the CSY, the Morningstar offered a way to monitor the system via computer connection.

Soggy Paws Solar Panels

Though the catamaran mounting doesn't allow rotation of the panels fore and aft, as we had on the CSY, we made up for it by adding another 200 watts of solar capacity.

Solar Panels Mounted
Solar Panels Mounted

As with the CSY system, on a good solar day, we are cranking out the amps, and our batteries are fully charged at mid-day on a sunny day, even if we're wantonly charging computers and stuff.  With the excess watts, we can handle several days worth of overcast days before we have to think about charging with other means.  Also, in the wintertime, when the solar days are much shorter and the sun angle is not directly overhead even at midday, we have enough power to run the boat at anchor without running any other charging system.


Alternator Repair

Parts and Tools Needed for
Alternator Disassembly

Parts and Tools Needed for
Alternator Repair

I have taken apart and rebuilt my own alternators ever since I paid about $100 for a rebuild down in the islands that did not work. The problem was that the shop installed 30a diodes in my 150a alternator that needed 50a diodes. Back then I was still trusting third world mechanics with some of the work on my boat. They are real simple to take apart, test and replace parts. See Nigel Calder's Mechanical and Electrical Manual. You certainly don't need to send them anywhere for repair if you have basic tools, electrical skills and first time instructions. If you have one of the older Delco, Balmar or Powerlines, it is fairly simple.

The older Balmar and Powerline alternators look the same, are both based on Delco alternators and most of the parts are the same including the cases. Parts are available many places in the US including some Alternator/Starter parts stores like Royal Battery in Florida and elsewhere. Used parts are available at auto junk yards and alternator shops everywhere. The small case alternators are built with capacities up to 150 amps, over that they use large cases.


Lightning Issues

Here is a bit of interesting info on lightning, its cause, prevention and cure from a discussion on an SSCA forum:

"Actually, lightning is very well understood in the scientific community. How it forms and how it reacts is also understood. The University of Illinois and others have many detailed scientific papers available on the subject. Exactly where and when lightning will strike is a variable as it is part of Mother Nature and there is a reason she is called "Mother Nature" and not "Father Nature."

As to lightning and boats, the issue comes in two parts - 1. Prior to the strike; and 2. During the strike.

Part 1. concerns how to reduce the probabilities of a lightning strike. Lightning is a two part event. The ionized "leader" sweeps an area below the generating cloud and an opposite "leader" sweeps back and forth up from the surface of the earth. When they connect a pathway is opened and the energy is discharged. To a boat the ground/earth "leader" is of most interest. As this "leader" oscillates around it reaches up "x" distance into the atmosphere. Whenever this leader encounters a vertical object like a tree, house, telephone/power pole, or the mast of a boat it can reach further up into the atmosphere and has a better chance of making a "connection." [same thing as tall men in a bar full of good looking women].

So reducing the "electrical" apparent height of your boat is good. This can be done with static dissipators such as the Forespar Lightning Master. (Which is a copy of the static wick principle used on airplanes and airliners). The sharp small "spikes" of the device "bleeds" off ions as they build up and electrically reduces your mast height to equal that of the ocean. However, to do this it must have a "significant" and good ground to the ocean. That translates to 4 square feet of flat plate copper in contact with the ocean and 2/0 welding cable between the mast and the plates submerged in the ocean. A static dissipator will not dissipate if it is not connected with the ocean.

So the result of part 1. is to electrically make your boat height equal to the ocean. If you are anchored close to shore or in the close vicinity of other "unprotected" masts/boats your probability of being hit is significantly lower than theirs. If you are out in the middle of no-where/ocean by yourself you are now "even-steven" with the ocean as to getting a hit.

Part 2. is what can you do to prevent/minimize damage to the boat and its contents during a strike. Again, the 4 sq ft. of copper joined with significant sized welding cable to the mast(s) will provide a highly desirable pathway for the lightning's energy to get directly to its only objective - earth ground. If the mast(s) are not sufficiently well-grounded then the lightning energy will try to find an alternate path to the ocean. If the mast is not available to the lightning, then it will travel down the shrouds/stays to the bonding system and set up a "field" inside the boat that will "fry" most electronics and has been known to heat metal thru-hulls sufficiently enough to melt them out of the hull and you are left with 1.5" holes for the ocean to enter. Lack of any grounding to the ocean and you can end up with holes blown through the hull.

Side note: while your boat is floating in the water it is grounded. When you boat is "on the hard" (out of the water) it is not grounded and any lightning strike to your boat or to a neighbors boat will fry your electronics or other fine metal objects or more serious damage. It is advisable if you are going to leave your boat on the hard, in an area with probable lightning, to drive a copper or steel grounding stake into the ground beneath your boat and hook up a significant sized wire from it to your masts or boat grounding system.

There are many other esoteric factors available to folks wanting to get the "whole story" such as positive versus negative forms of lightning, high frequency vs low frequency lightning, etc. but for the boater I think the primary interest is minimizing damage to the boat, contents, and not curling the crew's hair. This is done by dealing with the before and during aspects of protecting/guarding your boat from the energy in lightning.

Exactly where and when a strike will occurs is not determinable - the same as the actual path of a hurricane or tropical storm is not totally predicable as there are too many variables in nature for even the powerful human built computer to input and resolve. So you can only do what you think is cost-effective to "lower the odds" that you will be the target".

Here is a link with a good discussion of lightning protection systems and the ABYC rules from a custom yacht designer dated 2007-09:

Here's what he says (comparing a 12' strip to a 1 sf plate):

"The ABYC suggests the use of a grounding strip, rather than a plate. The ABYC rule states: 'A grounding strip shall have a minimum thickness of 3/ 16 inch (5 mm), and a minimum width of 3/4 inch (19 mm).' A strip approximately one inch (25 mm) wide and 12 feet long (3.7 m) has nearly six times the amount of edge area exposed to the water, which will improve the dissipation of charges. 'The grounding strip, if used, shall extend from a point directly below the lightning protection mast, toward the aft end of the boat, where a direct connection can be made to the boat's engine'.

A grounding plate, if used instead, should be solid, rather than the sintered bronze type often used as radio grounds. The sponge-like structure of the sintered bronze plates may, in the event of a strike, allow the instant formation of steam, which could blow the plate apart, resulting in possible severe damage to the surrounding hull".

An excellent discussion on Marine Grounding issues (lightning grounds vs radio grounds vs electrical grounds) can be found in Stan Honey's article on Marine Grounding Systems.

My copper bar on the CSY was thru bolted to the hull on the port side running from about the forward main cabin bulkhead aft into the engine room. Two of the 3/8" bolts are nearly even with the mast so that I could get short runs of 2/0 wire to each from the mast without much bend. The connections need to be really solid, so I did the mast thru bolted connections while the mast was out of the boat.

If you Google boat lightning grounds/protection you will find much more info on this, including this useful tidbit on Kasten's site:

"The top-most end, or air terminal, should be a sharply pointed spike. Alternately, a wire 'brush' type terminal can be placed at the masthead, with the bristles pointed upward. There are several claims that a single spike is more effective than a brush for dissipating the charge built up by the boat".

We took both the spike and the bottle brush from the top of the mast on the CSY when we sold it, and mounted it on our catamaran mast.  My spike is 5/8" aluminum, about 18" long. I threaded the bottom so I could bolt it through a web in the mast cap.  Some pics of the CSY setup here:

Of all the scary things about cruising and boat ownership, lightning scares me the most. Not only because of the potential for huge repair expenses, but also because of the very real threat of loss of life. It is a serious subject worth careful consideration by anyone using a boat.