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 combo hybrid with both diesel engines and electric motors.  The more batteries we have, the less diesel fuel we will burn (up to a point, see Hybrid Propulsion).  With a 44 kWh battery, with 30.8 kWh usable, we can motor at half throttle doing about 4 knots for around 4 hours.  Or, we could motor at 5 knots for about 3 hours.  While out for a day, we regain about 25 kWh so we can get another 2 hours of motoring, leaving some for house uses.  That's is 5 hours a day of motoring!  Most single day trips will only need that or less, so no diesel burned.  If you want to see 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 (like ours); just use the button below. 

Pod Motor

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 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 33 kWh battery for the production boat options and a 22 kWh for the performance boat.

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.


Charing our batteries

Let's start with the production cats.  A maximum 80% draw from a 33 kWh bank is about 26,400 kWh of usable energy.   Our primary source of charging the batteries are solar panels.  We are limited to a maximum of 5 kw of solar panels on the Salina 48 (see Solar).  5 kw of solar, on a sunny day, will recharge about 25 kWh of power to the battery.   Since our usable power is about 26,400 kWh, this matches fairly closely; we can use 25 kWh in a day and expect to recharge them nearly fully by solar the same day.  


Performance Catamaran Batteries

That all changes with a performance cat, like the Solitaire 1490.  It is just 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 the production cats, about 1/3 less.  A maximum 80% draw from a 22 kWh bank is about 17,600 kWh of usable energy.   If we put nine 420 watt panels we get 18,900 watts a day.  This matches fairly closely; we can use 80% and easily recharge the batteries to full by solar the same day. 

If it is cloudy, we would have to plug in to shore power, run our diesel motors, or turn on our generator (on the 1490, the diesels are the generators).  We don't plan to spend much time in marinas, and neither of the diesel options is attractive because we don't want to pay for and find a place to obtain diesel fuel.  So, our goal for a battery is 33 kWh for a production cat or 22 kWh on the performance cat, and no larger.

Charging 1490


Three 11 kWH batteries plus two 6,480 kWh

Besides charging limitations, going beyond a 33 kWh battery starts to become a problem in weight on the production cats and more than 22 kWh is too heavy on the performance cat.  Catamarans are sensitive to weight, it slows them down more so than a mono hull.  If we purchase a production cat with high load carrying capacity, we have a bit more LCC to work with, about 1,000 lbs over the performance cat. 

If we went with a production cat we would build three 11 kWh batteries for the main boat, and two 2.75 kWh batteries for the Tender, for a total of 38,500 kWh.  We are making them as light as possible, but each of the 11 kWh will weigh 232 lbs. and each of the 2.75 kWh will weight 103 lbs., for a total weight of 902 pounds.  You can see, going any larger would be even worse!

On the performance cat, we would have two 11 kWh batteries, for 464 lbs., plus the same two 2.75 kWh tender batteries weigh a total of 206 for a grand total of 670 lbs of batteries.

However, even if we were not going with additional electric motors, we would still want to add a large 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.  So, adding the electric propulsion will not increase our boat weight all that much, especially on the performance boat where the electric motors on part of the diesel engine and eliminate the need for a separate generator.



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 iron 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 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 not as fire safe as LTO, but still don't catch on fire in tests.  In the end, their combination of middle ground weight, 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, as well as a DIY option, and we had to decide which is best for S/V Lynx.  Below are a list of the options we considered, starting with the batteries you can just buy, then we will go over DIY batteries.   (Note that the comparisons are done for the production cats, the batteries for the performance cat would be 1/3 of price and weight).

1) ReLion (48v)

100ah per battery
33,600 kWh (total)
7 batteries to get to 700 ah total
Recharges: 7000
7 year warranty
Weight: 165 lbs. (1155 lbs. total)
Size: L 29.1" x W 19.6" x H 15.7"
Total square inches for 700ah pack: 20,043
Cost per watt: $1.04


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 $35,000!

2) Battleborn (12v)

100ah at 12v/battery (four in series to get 48v at 100ah)
33,600 kWh total
28 batteries to get to 700ah total
Recharges: 3000 (5000 in lab conditions)
10 year warranty
Weight per battery: 31 lbs. 868 lbs. total)
Size: L 12.75" x W 6.87" x H 9"
Total square inches for 600ah pack: 21,963
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.  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 28 batteries, which leads to a lot of connecting cables and hardware.  Secondly, 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)

170ah per battery
32,640 kWh total
4 batteries to get to 680 ah total
10 year warranty
Weight per bank: 215 lbs. (860 lbs. total)
Size: L 21" x W 9.5" x H 31.5"
Total square inches for 680 ah pack: 25,137
Cost per watt: $0.43

Big Battery

Big Battery is the brand name that we were seriously considering.  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 four (vs 28).  As for price, they are much better than ReLion or Battleborn, coming in at only .43 cents per watt, that is have the cost of Battleborn and even less compared to Relion.  We only have a couple of small concerns.  Firstly, they take up more space than the other options.  Secondly, a boat that purchased these batteries ended up replacing the Battery Management Systems themselves, to get a better model with more features.

Still, if there were no other options, and we wanted prebuilt battery, 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, better BMS, and is nearly have the price.  (see DIY Battery below).

4) Lynx DIY Main Battery (48v)

230ah per battery
33,000 kWh (total)
3 banks of batteries for 690 ah total
Cell Recharges: 4000
5 year warranty
Weight per bank: 240 lbs. (720 lbs. total)
Size: L 26.26" x W 19.57" x H 13.78"
Total square inches for 690 ah pack: 21,245
Cost per watt: $0.25


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 done a study and believe we can build a better battery, for less money that is lighter, smaller, and has more safety features! 

We plan to use 230ah LifePO4, Grade A, prismatic cells.  We will house them in a waterproof box (which none of the other options provide), add a top quality BMS, along with some additional safety features not included in any of the above batteries. 

They come in at 720 lbs., total, which is 140 to 435 lbs. lighter than any of the other listed options! The 230 ah batteries will only cost us 25 cents per watt, so they are far less expensive than the other options by $5,785 to $26,694.  Now, we are not counting our labor to build these Lynx DIY batteries, and you need a bunch of tools.  Yet, even adding in the cost of our labor and buying tools, they are still less expensive.

As for warranty, there is only a 5 year warranty on the cells with the Lynx DIY Main option.  However, at this price we could replace all of these batteries two to three times compared to the Battleborn or Relion batteries, so that is our warranty.

Now, as to safety, we are adding a top Battery Management System, along with a circuit breaker, amp hour monitor with shunt, a digital display temperature monitor that can activate a fan cooling system, built in anti-vibration absorbers, a heating element for sub zero locations, individual fused cells, a spring compression system to protect the cells from swelling, an external cut off switch, an audible and visual alarm in case the battery goes offline, 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 Lynx DIY Main battery is cheaper, lighter, smaller, and waterproof.

Our Conclusion on Batteries

It really came down to BigBattery or the Lynx DIY Main battery.  The other two options were both much more expensive.  So, comparing the two remaining contenders, the BigBattery wins in two categories, the warranty of 10 years and less labor since we have to purchase all the parts, assemble, and then test the Lynx DIY Main batteries. 

However, the Lynx DIY Main batteries are 160 pounds lighter than the BigBattery, while offering 10ah more capacity, and take up 3,892 sq. inches less space.  They are also built in waterproof cases with a load of added safety and convenience features.  All of these things are incredibly important on a catamaran where reduced weight and size seriously matter, not to mention the extra capacity and safety measures.  Finally, even with all those advantages, the DIY also comes in $5,782 cheaper than the BigBattery.  Therefore, in the end, the DIY battery wins, hands down, as long as you are comfortable with purchasing individual parts and doing the labor, assembly, and testing (which we are, videos to follow of the build).  


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 choose to go with choice #2, and the model is the JK BMS.  We choose it partially because it has a good reputation on DIY forums.   The JK BMS is a capacitor style, self-contained BMS for a 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 string (and BMS), giving us a 400a total for loads.  The JK BMS run about $165.00 on Aliexpress, so they are a reasonable price.  They use their own Bluetooth app, though it will not integrate into our Victron monitoring system.  Still, they are a great price and have a good reputation and we like having the active balancing which should produce less heat that the resisters in a passive balancing system burning off power from higher charge cells.

Tender Batteries

Since we are going with an electric outboard, we need batteries for the tender as well.  We don't want to use the same Lynx DIY batteries for the tender as the main cat as those are too large and heavy, so we designed a smaller Lynx DIY Tender battery.  On longer passages, we could 'borrow' the tender batteries and use them for the main boat propulsion, bringing us to a higher ah total.

1) Lynx DIY Tender Battery (24v)

11,040 kWh (total)
2 banks of batteries in series to 48v
Cell recharges: 3000
3 year warranty (on the cells)
Weight per bank: 103 lbs. (206 total)
Cost per watt: $0.41

Battery Tender

Less than half the size of the Lynx DIY Main batteries, two of these will fit next to each other between our RIB's tubes.  We will put them inside a bench box, further protecting them from salt spray (even though the battery boxes are also waterproof).  The two 24v, 230 ah batteries in series gives us a total of 230 amps at 48v, which is sufficient for our range requirements.  The two batteries add up to 206 lbs.  Unfortunately, the smaller batteries cost more per watt, about $0.16 more per watt than the Lynx DIY main batteries, but those are too large for the tender.  

Help us out

Subscribe S/V Lynx Youtube

Please subscribe to our Youtube channel and earn a chance to win a cabin for a week on S/V Lynx!  To subscribe, click the Youtube link (left).

Make your own web page with Mobirise