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 44 kWh battery.

Assuming we only discharge to 20% SOC and charge to a maximum of 90% SOC we get 70% usable power.  The reason for this 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

A 70% draw from a 44 kWh bank is about 30.8 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 (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 30.8 kWh, this matches fairly closely; we can use 25 kWh in a day and expect to recharge them nearly fully, only by solar the same day.  Charging a larger battery than that would require us to plug in to shore power, run our diesel motors, or turn on our generator.  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 solar limit leads us to want a 44 kWh battery, and no larger.



Four 11 kWH batteries

Besides charging limitations, going beyond a 44 kWh battery starts to become a problem in weight on the main boat.  Catamarans are sensitive to weight, it slows them down more so than a mono hull.  We are looking to purchase a production cat with high load carrying capacity, but even so, weight still matters.  We are building four 11 kWh batteries, for a total of 44 kWh.  We are making them as light as possible, but each will weigh 144 lbs., for a total weight of 576 pounds.  You can see, going any larger would be even worse!



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.

1) ReLion (48v)

300ah per battery
42,200 kWh (total)
3 batteries to get to 900 ah total
Recharges: 7000
7 year warranty
Weight: 388 lbs. (1164 total)
Cost per watt: $0.92


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 have one major complaint, 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 and heavy for our needs.

2) Battleborn (12v)

100ah at 12v/battery (four in series to get 48v at 100ah)
38,400 kWh total
36 batteries to get to 900ah total
Recharges: 3000 (5000 in lab conditions)
10 year warranty
Weight: 31 lbs. (1,116 total)
Cost per watt: $0.74


Battleborn are another well respected brand of batteries, also used on many yachts.  They have an excellent warranty of 10 years.  We have two issues with these, one is that 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.  Secondly, they are also expensive, though a a little better than ReLion.  However, they are still too expensive and equally heavy for our needs.

3) Big Battery (48v)

300ah per battery
45,000 kWh total
3 batteries to get to 900 ah total
10 year warranty
Weight: 600 lbs. (1,800 lbs. total)
Cost per watt: $0.42

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 ReLion.  They come in 48v versions, so they are better than Battleborn for our needs.  As for price, they are much better than ReLion or Battleborn, coming in at only 42 cents per watt, that's less than half of ReLion!  We only have one major complaint, they are still too heavy!  Still, if there were no other options, and we wanted a brand name battery, these would be our choice.

4) DeLong (48v)

200ah per battery
42,200 kWh (total)
4 batteries to get to 800 ah total
Recharges: 2000
3 year warranty
Weight: 209 lbs. (836 total)
Cost per watt: $0.27


Now let us take a look at Chinese batteries.  We just picked one, though there are a host of choices, yet they all have the same issues.  First, the good part, they are cheap!  They come in at only 27 cents per watt!  That is nearly half of Big Battery's price, which are the best of the US brand batteries we considered.  Now, to our issues, they have a poor warranty of 3 years (and it is from China).  They have miserably low recharge cycles.  And, as for quality, who knows how good the cheap BMS is that is protecting your batteries?  If there were no other choices, and we lacked enough money, we might consider these... but there is a much better option (see below).

5) DIY Battery (48v)

230ah per battery
44,160 kWh (total)
4 batteries
Recharges: 4000
2 year warranty
Weight: 227 lbs. (908 total)
Cost per watt: $0.23


So, our main issues with all the batteries above are price, weight, and quality.  We have done a study and believe we can build a better battery, for less money and make it lighter!  We plan to use 230ah LifePO4, Grade A, prismatic cells.  We will house them in a waterproof box, add a top quality BMS, along with some additional safety features not included in any of the above batteries.  They come in at 908 lbs., total, which is between 208 and 892 lbs. lighter than the other listed options.  They are also 23 cents per watt, so even less expensive than the cheap Chinese made battery, and a ton less than the US brands.  As for warranty, there is none (except for a 2 year warranty on the cells).  However, at this price, we could replace all of these batteries, two to three times compared to the US brand batteries, so that is our warranty.  Watch in Videos/Gear for future design videos for details on how we plan and make these batteries.


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 DIY batteries for the tender as the main cat as those are too large and heavy, so we designed a smaller DIY battery for the tender.

1) DIY Tender Battery (48v)

135ah per battery
12,960 kWh (total)
2 batteries
Recharges: 3000
3 year warranty (on the cells)
Weight: 94 lbs. (188 total)
Cost per watt: $0.28

Battery Tender

Much smaller than the DIY main boat batteries, two of these will fit next to each other between the RIB tubes.  We will put them inside a bench box, further protecting them from salt spray (even though the battery boxes are also waterproof).  This gives us a total of 270 amps for the outboard, which is sufficient for our range requirements, yet the batteries come in at 188 lbs., total.  There will be a video to follow about designing and another on building these batteries.

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