When we did the electrical upgrade on our J/105, Kinship, we chose not to use lithium batteries, despite many potential advantages.  What are those advantages, and why did we forego them?

1. Lithium Batteries are Lighter

This is a big one for a race boat like ours.  We have two 80Ah Lifeline AGM batteries, each of which weighs 56 lbs.  By contrast, lithium-iron phosphate batteries storing 100 or 105Ah are 22-33 lbs.  In theory, we could shave around 50lbs off the weight of the boat by replacing our AGM batteries with lithium.

2. Lithium Batteries Can Be Charged Faster

Lead-acid batteries (including sub-types like AGM) charge very, very slowly after they get to around 65-80% state-of-charge (“SoC”), .  It can take more than five hours to reach 100%.  Lithium batteries don’t have this characteristic.  They can be charged to close to 100% much more quickly. This could be a big advantage for boats that do distance races.

3. Lithium Batteries Have More Useable Capacity

Most manufacturers of deep cycle lead acid batteries recommend only discharging them to about 50% SoC before recharging to maximize the life of the batteries.  (Although an occasional discharge to 20 or 30% isn’t the end of the world.).  Again, lithium batteries don’t have this characteristic.  Roughly speaking, most people consider the “usable capacity” of lithium batteries to be around 80% of rated capacity.  In practice, I think a lithium house battery of around 100Ah would get us about 2X the usable Ah in roughly the same physical space.

4. Lithium Batteries are Cheaper (in the Long Run)

It used to be that lithium batteries were really, really expensive.  In the past couple of years they have really come down in cost, however. As of today, I don’t see all that much difference in price between high-quality AGM and lithium batteries of the same capacity. However, because lithium batteries have more usable capacity and aren’t as affected by cycling, the cost per kWh delivered over the life of a battery is likely lower with lithium than lead acid.

Why I Nevertheless Used AGM Batteries

Given all these advantages, why did we nevertheless keep our AGM batteries during our recent electrical upgrade? In short, the lithium batteries that were available were difficult to integrate into a system that was safe, reliable, and redundant.  The products we found would have resulted in a system that met most — but not all — of those requirements.  Here’s why:

1. Most Lithium Batteries Can’t Start you Engine

Most marine lithium batteries cannot output the very high current (for a very short time) needed to start an engine. In our system, therefore, going to lithium would only mean replacing a single 80Ah house battery, because we’d need to retain an AGM battery for the engine.  Although the lithium house battery would be about 25 lbs lighter than the AGM house battery, we would lose the redundancy of being able to start the engine with the house battery if necessary. (There are some newer lithium models that can start engines, but they are missing some other critical features.)

2. Lithium Batteries Fail Hard and Fast

Lead-acid batteries are a simple electrochemical devices. Whatever their faults, they are can take a lot of abuse in the short run. Marine lithium batteries, however, are actually packages of battery cells and electronics. The electronics are there to make sure the very temperamental lithium cells are protected and safe. But, those electronics can also cause the battery to disconnect from the electrical system very suddenly. A sudden disconnect, however, can damage other parts of the electrical system. Or, the resulting sudden loss of electrical power could be dangerous for the crew because the navigation lights, AIS, and chartplotter would all turn off. A seamanlike lithium system, therefore, need to do two crucial things. First, it needs to communicate about an impending shutdown so than other systems (or as a last resort, the crew) can do something to prevent it. Second, it needs to protect the other systems on the boat in the event it happens anyways.

3. Many Lithium Batteries Are Missing Key Communications Capabilities

Most lithium batteries don’t have the ability to communicate an impending shutdown with a simple on/off DC voltage signal. We could use such a signal to shut down alternators and alert the crew before the lithium battery shuts down. Some “drop in” batteries don’t communicate this at all. Others can communicate via Bluetooth and an app or via Victron’s communications protocol. The first is not sufficient for a robust system and the second is useless in a boat without Victron power electronics.

4.  Lithium Battery Systems Require More Electronics

A lithium battery system requires more electronics than one that only uses only lead acid batteries. For example, a lithium system should include:

  • One or more relays, wiring, and a buzzer and/or light to alert the crew of an impending shutdown.
  • An Alternator Protection Device (“APD”), which, as its name suggests, prevents the alternator from damaging itself if a lithium battery were to suddenly disconnect.
  • A way to charge batteries of different chemistries, such as a DC-to-DC charger.

Roughly speaking, this would have weighed about five pounds and cost $300 more (just for parts) than the system we just put into Kinship.  So the total weight savings for our boat would have gone down to 15-20lbs.

(The above assumes the a boat’s alternator, regulator, and AC charger are up to the task of charging lithium batteries, which is not a given in an older boat but is true of the new equipment we put into Kinship.)

5. A Lithium System is Less Redundant

With our dual-AGM setup, if either the house or start battery is weak or dead, the other battery and all the associated wiring is totally capable of starting the engine and supplying all the house loads.  This is not the case for a system with an AGM start battery and a lithium house battery (that does not have engine-start capability).

In the final analysis, this is why I still have AGM.  I would absolutely spend $300 to save 15-20 lbs on a race boat, other things being equal.  But in a marine environment, I like the belt and suspenders of having two independent batteries, either one of which can start the engine or power electronics if necessary.

When I’d go to Lithium

The possibility of saving as much as about 50 lbs on a race boat is still pretty enticing.  I don’t think we are too far away from the products I’d need to make the switch, but for the record, I’m looking for lithium batteries that:

  1. Can reliably start a marine diesel engine, such as our Yanmar 2GM20F
  2. Store 80-105ah each
  3. Have a simple voltage-based indicator of impending disconnect that could be used to (a) switch off charging sources and (b) turn on a buzzer and/or light.

I don’t think its going to be long before manufacturers have such products, but until then I’ll be waiting.



















































































































Hitachi Alternator

Photo Hitachi 55A alternator in J/105

The stock, internally-regulated Hitachi 55A alternator had worked fine for us for daysailing, even when we sailed Kinship off a mooring ball for two years.  However, its performance during the 2022 Mac left a lot to be desired.  I suspected it wasn’t performing as well as it was supposed to (although I never had it tested).  I also learned that even if the alternator was working to spec, an externally- regulated alternator would be able put energy into batteries more quickly.

(Marine How To, the brainchild of marine electrician extraordinaire Rod Collins, was invaluable to me in understanding this and many other things related to this project.  Thank you, Rod!)



Guest “1, 2, Both” Battery Switch

The standard-issue “1, 2, Both” battery switch on Kinship was also an issue. I wanted to fix the electronics dropouts, and the only way to do that definitively is to separate the circuits for starting the engine and for house loads.  Also, separating them would also mean we couldn’t accidentally use all of the available energy for house needs and then be unable to start the engine.  In other words, it would be more resistant to human error.

Blue Sea Systems Voltage Meter

The voltage meter in the nav station was a poor substitute for a proper battery monitor.  While it is possible assess the state of charge of a battery based on the voltage across its terminals, the voltage that corresponds with each state of charge changes with the amount of load on the battery and several other things. Even worse, the meter at the nav station was affected by voltage drop in the wiring between the battery and the nav station.  We did get by this way for a few years, but it was probably about as accurate as wet finger in the air.  I wanted something I could put in the cockpit and tell the crew “If the battery SoC gets to 30%, start the engine.”

Promatic AC Charger

The Promatic AC charger, while over 20 years old, appeared to be working fine. It even had different programs for different types of batteries, one of which was pretty close to what Lifeline recommends. I decided to replace it anyways, though, because a newer one would allow programming that more precisely matched Lifeline’s recommendations.  It could also provide temperature-compensated charging. At this point, though, I might have been suffering from a bit of gear acquisition syndrome.

Photo of Promatic AC charger in J/105


I am not an electrician or electrical engineer, but I do run a solar energy company (Certasun), so I know a bit about electricity and safety.  The electrical system on Kinship was not safe, and certainly not up to current ABYC standards.  It was missing essential fuses, some of the wires were just big enough if you squinted, there were unprotected battery cables in the engine compartment, and many of the terminations were substandard.  All of this had to get fixed.  Its also interesting that none of this was mentioned in two surveys of the boat.

The New System

Now let’s look at what hardware I chose for the new system and how it fits together.

Lifeline AGM Batteries

Based on their reputation and age, I wanted to keep the Lifeline AGM batteries. However, I needed to make sure they had enough capacity to run the boat for at least six hours after being recharged by the alternator for an hour.  I started by creating an energy budget (based mostly on manufacturer spec sheets). We would consume 5-7 Ah / hour in normal operation:

Energy budget for J/105 sailboat

With the help of Lifeline’s technical manual, I was able to determine that one 80Ah battery would power the boat for 8-11 hours if coming off a 100% charge (i.e. from shore power) or 6-8 hours after being recharged by the alternator for an hour:

Table with charging times and run times for J/105 sailboat

We decided this was sufficient to give it a go with one 80 Ah house battery.  (We subsequently found my estimates were pretty accurate.)

(A few notes:  First, in these calculations I assumed the alternator would only put out 50A, which might be conservative for the 70A alternator we ended up with — see below. However, there are a variety of reasons an alternator may not always put out its nameplate power.  Second, these calculations assume using the house battery down to 30% SoC.  Battery manufacturers usually recommend 50% to maximize the life of the batteries.  However, this is a race boat, so keeping the weight as low as possible is just as important as battery life.  Also, 90% of the time we are just racing during the day so it is only occasionally that we’d be going this low.  Finally, I already owned and paid for these batteries!)

Balmar 70A Alternator and MC-618 External Regulator

Knowing that the existing batteries were fine, I knew that I needed to find a way to get more energy into them quickly when using the alternator.  One option would have been to simply replace the alternator with a new version of the same thing, but I wasn’t sure if the problem was the alternator itself or simply that the alternator and its internal regulator weren’t capable of putting energy into the batteries quickly enough.  I figured that if I was going to the trouble of replacing things at all, I might as well replace them with things that would increase the capability to put energy in the batteries faster.  Therefore, I decided to replace the Hitachi with a Balmar 70A alternator and MC-618 regulator.

Blue Sea Systems “On / Off / Combine” Switch and Automatic Combining Relay

The stock “1, 2, Both” battery switch didn’t allow separating the circuits for starting the engine and house loads, which caused two problems.  First, the electronics would drop out and restart when we started the engine.  Second, we could accidentally completely drain both of the batteries with house loads and then not be able to start the engine, violating my goal that the system be resistant to human error.

I solved this by installing a Blue Sea Systems “On, Off, Combine” switch instead.  This switch separately switches two circuits on or off (one for house and one for starting).  In almost all circumstances, these would be the only positions used.  However, we can combine the batteries if we want.  This could be useful if the start battery is for some reason not able to start the engine on its own or if we want to risk draining the start battery to get just a few more hours of energy for the electronics without the distraction of running the engine (for example, one hour from the finish).

Since the engine battery and house battery circuits would now be separated, though, I needed a way to make sure they both got charged.  The solution was an automatic charging relay (“ACR”) from Blue Sea Systems.  This automatically combines the normally separate house and starting circuits when it senses that a charging source such as the alternator or AC charger is present. 

Balmar SG-200 Battery Monitor

I installed a Balmar SG-200 battery monitor to replace the simple voltage meter.  This is a much more sophisticated device.  It reports the actual state of charge of the house battery on a small display in our cockpit, so I achieved my goal of being easy for the crew to operate.  It also has a low state of charge alarm function.

Blue Sea Systems P12 AC Charger

As mentioned above, I’m not sure I really needed to replace the Promatic charger, but I replaced it with  Blue Sea’s very nice P12 charger.

A Lot of New Wiring

Replacing the alternator, battery switch, and AC charger, plus adding the regulator, battery monitor, and ACR required significant additions and changes to the wiring.  While I was working on all of that, I also replaced or upgraded a lot of other wiring or wire terminations.  Finally, I added:

  • Bus bars for the house +, start +, negative, and grounding/bonding
  • Wires to bring the alternator output directly to the house battery bus
  • An alternator disconnect switch
  • Fuses (and replaced some glass AGL fuses with easier-to-read ATC fuses)
  • Labeling on most wires

Here’s a line drawing of the electrical system (click to enlarge): 

Wiring diagram of electrical system in J/105 sailboat.

The Results

I met all of my goals for this project during the 2023 Mac we went about 11 hours before needing our first charge and then 6-8 hours between charges after that.  The instruments now stay on when we start the engine, and the system is certainly easier to operate and more resistant to human error.  Time will tell about reliability, but the system is certainly safer in the meanwhile.  And, I was sure not to do anything during the process that would violate the J/105 class rules.

Here’s a photo of the front of the battery compartment, where a lot of the new stuff comes together:

Photo of upgraded J/105 electrical system

The Value of the Journey

I spent a lot of time on my back and/or in tight quarters during the winter I did this upgrade.  And all of these little boxes, wires, and doodads added up to around $2000, so this project wasn’t cheap.  To a large extent, though, I just think about it as the cost of a hobby.

It’s a hobby with benefits though.  I met my performance goals of around 6-8 hours between charges.  This system is significantly safer than what it replaced, and more foolproof.  Finally, I have a much better sense of how the electrical system of Kinship actually works.  If we ever have a problem at sea, I’ll potentially be in a position to do something about it, whereas before I would have just been in the dark.