Gear: Batteries

Battery Uses

In the end, power is king on a sail boat.   It is the most important resource on board.  Your comfort, safety, and basic needs are all based on the power you have available.   In fact, you are your own little power plant serving your own little world, and the more you can produce and store, the merrier!  Of course, cost and weight stand in your way to true happiness.  Here are some of the things that batteries help you enjoy on a sailboat:

1) Water
On a sail boat, water comes from power because we make our own from separating fresh water from sea water.  To do that, we need a watermaker, which runs off of electricity (see Watermakers) . So, if you want to take more showers, or clean the boat, or make food, wash dishes, and not have to go to a dock to replenish water or cart jugs of water in the dinghy, you need power to run a watermaker.


2) Saving diesel fuel
Making a passage requires yet more power, especially if you are a parallel hybrid  with a second electric motor.  The more batteries we have, the less diesel fuel we will burn.  With a 33 kWh battery for long passages, with 26.4 kWh usable, we can motor at half throttle doing about 5 knots for around 3 hours.  While out for a day, we regain about 24 kWh back from solar, so we can get another 2 hours of motoring.  That works out to 4 hours a day of motoring on electric without burning any diesel!  Most single day trips will only need that or less, so no diesel will be required.  In our Patron Crew area, we go over a breakdown of all kinds of length trips, we made up a data sheet to compare a diesel only boat, a hybrid boat, or a combo hybrid boat. 

Parallel Hybrid Electric

3) Air Conditioning
Keeping us cool in hot places is not the only use for an Air Conditioner.  It is also a heater for when we are in cold climates, and it conditions the air, meaning, takes out moisture, which cuts down on mold, a constant problem on boats due to being in the wet marine air.  However, all that takes a lot of power!  Most boats either don't run their Air Conditioner much unless they are at the dock.  Some will run their generator to use their A/C.  On S/V Lynx, with a large enough battery bank and solar array, we can run A/C far more often and all night, when needed, without turning on a noisy, diesel fuel burning, generator.

Air Con

4) Refrigeration
On most boats, this is the largest power draw simply because it runs all the time.  Your A/C draws more power, but you don't have to use it constantly.  Your refrigerators must stay cold.  If you are a couple on board a boat, you might get away with a smaller amount of refrigerator and freezer space; however, we plan to have four to six people on board, most of the time, and eight to ten occasionally.  That means we need 3 to 5 times the amount of refrigerator and freezer space compared to a typical couple's boat.  That also means we need 3-5 times the power to keep them running.


5) Electronics
This includes a lot of things, like charging all the modern gadgetry we all use, phones, underwater lights and strobes, cameras, microphones, laptop computers, etc.  And we will have a lot of these things due to video documenting the entire adventure.  It also includes running our boat electronics, like radar, chartplotters, etc.  Then there are all the lights we run at night.  We are also getting rid of propane, so we have to power all the electric burners, ovens, mixers, etc. in our galley.  We have a dive compressor that runs off electricity.  All this takes power, lots of power..



So, at first glance, it would seem that we need to add a crap load of batteries, like 100 kWh worth! Unfortunately, that's not possible for a few of big reasons, weight, cost, and ability to charge without burning endless diesel.

Even assuming you can afford a lot of batteries, you still have to charge them and handle the weight, especially if you are on a catamaran.  Due to these limitations, we want a 22 kWh battery for the our performance boat.  We will also have an additional 11 kWh available from the electric dinghy batteries, which we may tie into the main boat on long passages to get 33 kWh.

Assuming we only discharge to 15% SOC and charge to a maximum of 95% SOC we get 80% usable power.  The reason not to go from 0 to 100% is to increase longevity and reduce heat, though LifePO4 cells could easily use 100%, if we desired. 

Next, let's take a look at why charging also limits our battery size.


Performance Catamaran Batteries

With a performance cat, like our Solitaire 1520, this boat will be more efficient at moving through the water, by sail or motors.  Therefore, it does not need as much battery power to go the same distance as a comfort oriented production cat, we figure, about 1/3 less. 

A maximum 80% draw from our main 22 kWh bank is about 17,600 kWh of usable energy.   If we put eleven 435 watt panels we get 24,000 watts a day.  This matches fairly closely since we will be using house power while charging.  Therefore, we may use 80% of the main battery bank and easily recharge that amount back by solar the same day. 

If it is cloudy, our first top options are to plug into shore power or run our diesel engine/generator.  The third option, which is even more viable on a performance cat since we will sail at a higher after speed, is using the electric motors in regeneration mode.  That can return around as much as 48 kWh watts in a 24 hour period (if we are sailing in strong winds doing 10 knots).  That makes up quite a bit of power. 

We don't plan to spend much time in marinas, and burning diesel in unattractive because we don't want to cut down on paying for and finding a place to obtain diesel fuel.  So, our goal for a battery is 33 kWh total battery bank (22 kWh main plus 11 kWh tender batteries).  We should be able to recharge the main on a sunny day just from solar and if we sail, have enough added from regeneration to also charge the tender bank and run house needs.

Charging 1520


Six 5.5 kWH 24vdc batteries, two each in series to get to 48v, then in parallel for 33 kWh total.

Besides charging limitations, going beyond a 33 kWh (total) battery starts to become a problem in weight.  Catamarans are sensitive to weight, especially performance cats, like SV Lynx.  Weight slows them down more so than a mono hull.  Our performance cat has about 8300 pounds of LCC capacity to work with. 

Because of that limitation we reduced from 33 kWh main bank size to 22 kWh, cutting 1/3 the weight.  The Tender will have another 11 kWh of batteries.  When needed, we can tie the tender batteries into the main bank for long ocean voyages, and increase the storage to 33 kWh.

We are planning to go with a new battery company that we have been working with and use six of their 5.5 kWh batteries.  Each will weigh 100 pounds for a total weight of 600 pounds.  As you can see, going any larger would be too heavy!

However, even if we were not going with electric motors, we would still want to add a decent size battery bank for all the other uses (air conditioning all night, electric cooking, water heating, etc.)  Therefore, we would still want a good size lithium battery bank.  In fact, switching to a Parallel Hybrid system in one hull and an electric motor in the other will cut over 300 pounds from our boat weight.  This is huge, especially on the performance boat where keeping her light really pays off.  Plus, our parallel diesel engine acts as a 12 kw generator, yet  eliminates the need for a separate 600 pound generator on board!



Not all lithium batteries use the same chemistry.  Some are more energy dense, meaning less weight and size for a given amount of stored power, while others are safer from fire.  We considered three chemistries that are all currently available and applied the Goldilocks principal, choosing the one that was 'just right'.  Below are the three choices we considered.

1) NMC:
Also known as lithium-manganese-cobalt-oxide batteries, or NMC, lithium nickel manganese cobalt oxide batteries, these are made of several materials common in other lithium ion batteries.  This chemistry is the most energy dense of all the batteries we considered, but also the most volatile when it comes to thermal runaway and causing a fire.  A fire on a boat is serious issue, so we decided to go with something safer.  So, in Goldilocks terms, this porridge is too hot.


2) LTO
The lithium-titanate-oxide (LTO) battery has three advantages, it is very fast to charge, can be recharged 7,000 times, and they are incredibly safe from fire.  That sounds like a great combination, until you learn that they are also extremely heavy and large.  So, in our Goldilocks terms, this porridge is too cold. 


3) LFP (or LifePO4)
Enter LifePO4 (lithium iron phosphate), the porridge that is just right.  They are not as energy dense as NMC, but better than LTO,  They are very fire safe and tests by a fire department show that even punctured, shorted, over-charged, and otherwise abused, they still don't catch on fire in tests.  In the end, their combination of middle ground weight, decent cost, energy density, and fire safety make them the best choice for S/V Lynx.



Now that we know the chemistry we want, it's easy, right?  Just buy LifePO4 batteries.  Not so fast; there are a whole lot of brands of LifePO4 batteries out there so we had to decide which is best for S/V Lynx.  Below are a list of the options we considered.

1) ReLion (48v)

100ah per battery
24 kWh (total)
5 batteries to get to 500 ah total
Recharges: 7000
7 year warranty
Weight: 165 lbs. (825 lbs. total)
Size: L 29.1" x W 19.6" x H 15.7"
Total square inches for 500ah pack: 14,316
Cost per watt: $1.00


ReLion are a well respected brand of batteries used on many yachts.  They have a decent warranty of 7 years, though not as good as Battleborn (see below).  We only a few concerns with these batteries.  First of all, they are too heavy.  However, even more importantly, they are very expensive; in fact, they are the highest cost per watt of any battery we considered.  In the end, we decided that they are just too expensive for our needs.  A 33,600 kWh bank would cost about $33,600!

2) Battleborn (12v)

100ah at 12v/battery (four in series to get 48v at 100ah)
24 kWh total
20 batteries to get to 500ah total
Recharges: 3000 (5000 in lab conditions)
10 year warranty
Weight per battery: 31 lbs. 620 lbs. total)
Size: L 12.75" x W 6.87" x H 9"
Total square inches for 600ah pack: 18,302
Cost per watt: $0.89


Battleborn are another well respected brand of batteries, also used on many yachts.  They have an excellent warranty of 10 years.  The good news is, they are decent in weight.  But we also have a few issues with these.  Firstly they only come in 12v, so we have to put groups of four into series to get to 48v, then arrange those in parallel to get to the capacity we need, requiring 20 batteries, which leads to a lot of connecting cables and hardware, which adds complexity and additional weight (not calculated here).  They also take up more space.  Next, they are too expensive, though a less than ReLion.  Therefore, we eliminated these as too expensive with far too many connections. 

3) Big Battery (48v)

120ah per battery
28.8 kWh total
5 batteries to get to 600 ah total
10 year warranty
Weight per bank: 210 lbs. (1050 lbs. total)
Size: L 21" x W 9.5" x H 31.5"
Total square inches for 600 ah pack: 31,421
Cost per watt: $0.52

Big Battery

Big Battery is the brand name that we considered due to the lower price.  I have seen a breakdown of these and like how you can access and replace LifePO4 cells, if needed.  They also have a 10 year warranty, which is as long as Battleborn.  They come in 48v versions, so they are better than Battleborn for our needs since we would only have to connect 4 in the main boat (vs 28).  Unfortunately, the included BMS isn't great, and some people are having to take these apart and replace the BMS units.  Still, they are much better priced than ReLion or Battleborn, coming in at .52 cents per watt.  That is half the cost of Relion and much better than Battleborn. 

If there were no other options these would be our choice due to the low price and access to the cells.  Fortunately, we have another option that solves the weight, size, and has a top notch BMS, and they come in at a better price.

4) Thermoworks Prototype Battery (48v, 230ah) 
Assembled in the USA

230ah per battery
22 kWh (total)
2 banks of batteries for 460 ah total
Cell Recharges: 4000
5 year warranty
Weight per bank: 235 lbs. (470 lbs. total)
Size: L 26.26" x W 19.57" x H 13.78"
Total square inches for 460 ah pack: 14,163
Cost per watt: $0.28

SafeLFP Battery

So, our main issues with all the batteries above are: price, weight, size, and safety features (we will get to the last one, below).  We have been working with SafeLFP Battery, Inc. to help them design and test a better, safer, marine battery to bring to market.  Best yet, they are lighter, smaller, and have more safety features! 

We planned to use their 230ah LifePO4, 48v batteries which come with Grade A, prismatic cells, housed in a waterproof polypropylene box, with a top quality JK BMS, with bluetooth connectivity.  Plus, they add some additional safety features not included in any of the above batteries. 

Unfortunately, we decided against them simply because they are too heavy, which makes them difficult to move around.  Now, compared to other brands, at the same energy density, these are lighter.  But 220 pounds is a lot to move around, when needed.  The good news is, these 230 ah batteries only run around would have only cost us 28 cents per watt, so they were less expensive than the other options by quite a margin.

As for warranty, there is only a 5 year warranty, but for what you are saving on a safer battery, we an live with that.

Now, as to the Thermoworks prototype battery safety features.  We added a top Battery Management System, along with a 200ah circuit breaker. A built in amp hour monitor, with shunt.  There is a passive cooling system with a large heatsink, as well as a digital display temperature monitor that will activate a fan cooling system if a set temperature is exceeded.  We have built in anti-vibration shock absorbers.  There is also a heating element for sub zero locations.  They include a spring compression system to protect the cells from swelling.  They have an external cut off switch.  There is also a BMS activation circuit and a Pre-charge circuit.  All this is built inside a waterproof case to keep salt air from corroding the batteries and other components.  None of the other options, Relion, Battleborn, or BigBattery, offer all these features and yet the Thermoworks Prototype battery is less expensive, lighter, smaller (than most), and in a waterproof enclosure.  Sweet.  However, for us, we wanted a lighter, smaller battery for ease of moving it around the boat as necessary, (see the other options, below).

5) Thermoworks (Prototype) Battery (24v, 230ah)
Assembled in the USA

22 kWh (total for four)
2 batteries in series to reach 48v
4 total for the main battery bank
Cell recharges: 4000
5 year warranty (on the cells)
Size: L 22" x W 11" x H 13" (SKB 3i-2213-12BE)
Weight per battery: 100 lbs. (400 pounds total)
Total square inches for the pack: 8,389 sq. in.
Cost per watt: $0.30

Battery Tender

Four of these batteries will be used as the main battery bank of the boat, giving us 460ah.  When we add the auxiliary batteries (from the tender) we will have six of these for 660ah.  That will help with propulsion on the main boat on a long voyage where the tender is not needed.  Pairs of these batteries are connected in series, making each pair 48v.  We then put two of the pairs in parallel to create our main 460 ah bank on the catamaran.  We plan to go with these smaller designed batteries even though they are 2 cents a watt more expensive than the larger 48v Thermoworks prototype battery option.  The reason is the size and weight of the larger batteries.  These 24v versions are much easier to handle at 100 pounds and two are smaller than one of their 230 48v batteries (see above).

If we compare these to the other brand batteries, these 24v DIY batteries are even smaller  (than most), lighter, safer, and less expensive than anything other than their larger brothers from the same company.  They win in almost every category and are easier to lift, move, and work on than the larger option.  

Our Conclusion on Batteries

It really came down to BigBattery or the Thermoworks Battery.  The other batteries were just too expensive.  So, comparing the two remaining contenders, the BigBattery wins in one category, the warranty of 10 years.

However, the Thermoworks batteries are nearly half the weight of the BigBattery, while offering 90ah more capacity.  Even with the extra power, they take up 12,545 sq. inches less space.  They are also built in waterproof cases with a load of added safety and convenience features and a better BMS.  All of these things are incredibly important on a catamaran where reduced weight and size seriously matter, and the safety measures are critical.  Finally, even with all those advantages, the Thermoworks battery also comes in at less cost per watt.  Therefore, in the end, the 24v, 230ah Thermoworks batteries win, hands down. 


A BMS (Battery Management System) is critical for protecting the batteries and, therefore, our boat and crew.  A good BMS will monitor the low and high charge settings, shutting off either charging or loads pulling power, if the set parameters are met.  It will also monitor temperature, and shut down charging if the temperature is too low, or shut off the battery if the temperatures are too high.  They will also report the charge state of each cell via an App, as well as other data on your battery.  Finally, they will attempt to keep all the cells in balance.  With a LifePO4 lithium battery, a good BMS is essential.

BMS Options

1) Passive Balancing MOSFET BMS
A self-contained BMS that use MOSFETs (which control the power path of a Low-Voltage Battery Management System).
2) Active Balancing Capacitor BMS
A self-contained BMS that uses active balancing with capacitors.
3) Passive Balancing, Contactor BMS
A self-contained BMS that uses a contactor (a kind of magnetic switch to cut off power to or from the battery).
4) Per Cell Board BMS with Master controller.
Separate boards, with one mounted to each cell in the battery, these connect to a Master control. That unit then uses contactors or shunt trips to cut off power in an emergency state.
5) Master/Slave BMS
This system uses Contactors or Shunt Trip Circuit Breakers, where each battery in a bank has a separate monitoring device connected to a master control with the ability to trip a contactor or Shunt Trip Circuit Breaker.


We prefer choice #2, and the model is the JK BMS.  We like it because it has a good reputation on battery and solar forums plus it is jammed with safety features.   The JK BMS is a capacitor style, self-contained BMS for a 24 or 48 volt battery.  It has 2 amp active equalization, and low temperature cutoff.  This BMS handles up to a 200a load, even though we plan to only pull a maximum of 100a per battery (and BMS), giving us a 400a total for loads from the main battery bank.  The JK BMS has their own Bluetooth app which you may access to monitor or change settings on your batteries.  The active balancing should produce less heat than resisters used in a passive balancing system as that type of BMS burns off power from higher charge cells.

Tender Batteries

Thermoworks prototype Battery (24v)

11 kWh (total for two)
2 batteries in series to reach 48v
Cell recharges: 4000
5 year warranty (on the cells)
Size: L 22" x W 11" x H 13"
Weight per battery: 100 lbs. (200 total)
Total square inches for 230 ah pack: 4,194 sq. in.
Cost per watt: $0.30

Battery Tender

Since we are going with an electric outboard, we need 48v batteries for the tender as well.  We will be using the same size cells as the main boat, 230 amp, configured for 24v. That is why we need two of the 24v Thermoworks batteries, so we can put them in series to get to 48v.   On longer passages, we will hook the tender batteries into the main boat, through their own shunt, which will add their capacity to the main boat propulsion bank.

We will put one in the forward compartment of the tender and the other one in a stainless steel box back by the outboard, further protecting them from salt spray (even though the batteries are built in waterproof boxes). The two 24v, 230 ah batteries, in series, gives us a total of 230 amps at 48v, which is sufficient for our tender range requirements. The two batteries add up to 200lbs. 

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