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:
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 a diesel engine and two electric motors. The more batteries we have, the less diesel fuel we will burn (up to a point, see Hybrid Propulsion). With a 33 kWh battery for long passages, with 26.4 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 22 kWh sback, o we can get another 2 hours of motoring, leaving some for house uses. That's is 5 hours a day of motoring on electric! Most single day trips will only need that or less, so no diesel is needed. 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.
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.
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.
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.
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 1500, 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 ten 440 watt panels we get 22,000 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 43 kWh watts in a 24 hour period (if we are sailing in strong winds). 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.
Three 11 kWH batteries plus two 6,480 kWh
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 designing our own batteries (see image right) to be as light as possible, but each of the six 5.5 kWh batteries will weigh 110 pounds for a total weight of 660 pounds. As you can see, going any larger would be too heavy!
However, even if we were not going with add on 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 our parallel diesel engine acts as a 10 kw generator and eliminates the need for a separate 600 pound 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.
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.
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
24,000 kWh (total)
5 batteries to get to 500 ah total
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: $1.04
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)
24,000 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
28,800 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 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 4 in the main boat (vs 28). As for price, they are much better than ReLion or Battleborn, coming in at .52 cents per watt, that is half the cost of Relion and much better than Battleborn. We only have a couple of small concerns. Firstly, they take up a lot 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, has a better BMS, and is nearly half the price. (see DIY Batteries below).
4) Lynx DIY (original) Battery (48v)
230ah per battery
22,000 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: $0.25
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 planned to use 230ah LifePO4, Grade A, prismatic cells. And we wanted to 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 came in at 470 lbs., total, which is much lighter than any of the other listed options! The 230 ah batteries would have only cost us 25 cents per watt, so they were 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.
NOTE (update): After building a prototype of this battery, we decided that the weight of these was still too much for a single battery. Each battery was too difficult to move around or take the cells out for any maintenance. So, we decided to go with a smaller design option (see 24v battery below).
5) Lynx DIY Battery (24v)
22,080 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: 110 lbs. (440 total)
Total square inches for the pack: 8,389 sq. in.
Cost per watt: $0.32
Four of these batteries will be used as the main battery bank of the boat, giving us 460 ah. When we add the auxiliary batteries (from the tender) we will have six of these. 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 this smaller design even though they are 0.7 cents a watt more expensive than the large 48v DIY option. The reason is the size and weight of the larger batteries. These 24v versions are much easier to handle at 110 pounds and two are smaller than one 230 48v DIY battery (see above).
If we compare these to the other batteries (above), these 24v DIY batteries are even smaller, lighter, safer, and less expensive than anything other than the larger DIY battery. They win in almost every category and are easier to lift, move, and work on than the larger DIY option.
It really came down to BigBattery or the Lynx DIY batteries. The other batteries were 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 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, not to mention the extra capacity and safety measures. Finally, even with all those advantages, the DIY also comes in 0.20 cents less per watt. That is a savings of about $6,224 over the BigBattery option (and even more of a savings compared to the other batteries). Therefore, in the end, the 24v 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).
We will post videos to show you how we built 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.
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.
Lynx DIY Battery (24v)
11,040 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: 110 lbs. (220 total)
Total square inches for 230 ah pack: 4,194 sq. in.
Cost per watt: $0.32
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 why we need two of the 24v DIY batteries, so we can put them in series to get to 48v. On longer passages, we will 'borrow' the tender batteries and use them to add to the main boat propulsion bank. Two of these batteries 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 tender range requirements. The two batteries add up to 220lbs.
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