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.
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.
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 typical draw of 60% from our main 38 kWh bank in a day is about 22,800 kWh of usable energy. We are putting ten 435 watt panels on S/V Lynx, so we should recharge around 21,750 watts a day. This matches fairly closely since to our usable battery capacity. Therefore, we can use 60% 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 average 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 want to cut down on paying for and finding a place to obtain diesel fuel. So, our goal for a battery is 38 kWh total battery bank. 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.
Worse case scenario (no wind, no sun) we can recharge from our diesel generator, putting 24kw back into the bank in 2 hours of run time.
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.
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!
We're adding one Vatrer 105ah battery at 48v, giving us a total capacity of about 38,000 watts.
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.
We choose to construct most of our own batteries, though Vatrer decided to sponsor us and sent one of their batteries to add to our bank.
1) Vatrer (48v)
105ah per battery
5,040 kWh
48vdc
Grade A LifePO4 cells
Recharges: 4,000+
200A Continuous Discharge (400A for 35 seconds)
IP67 Rated
Weight: 102.5 lbs.
Size: L 29.1" x W 19.6" x H 15.7" L19.69" × W 12.52 × H9.61"
Cost per watt: $0.35 (Normally, we were sponsored)
5) Thermoworks (Prototype) Battery (24v, 230ah)
Assembled in the USA
230ah
33,120 kWh
2 batteries in series (x3) to reach 48v
6 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. (600 pounds total)
Cost per watt: $0.26
Four of these batteries will be used as the main battery bank of the boat, giving us 690ah. 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 690 ah bank on the catamaran.
If we compare these to the other brand batteries, these 24v DIY batteries are even smaller (than most), lighter, safer, and less expensive.
It really came down to cost and safety. These batteries were the least expensive (by over $4,000 to the closest option)
They take up a lot less space, are built in waterproof cases with loads of added safety and convenience features and a top of the line 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.
But... if we didn't have the time or expertise to build our own, we would have gone with the Vatrer batteries since they are still a great deal at only $0.35 per watt.
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 choose the JK 300A BMS for 24v batteries. We like this BMS because it has a good reputation on battery and solar forums plus it is full of good safety features. The JK BMS is a capacitor style, self-contained BMS for a 24 volt battery. It has 2 amp active equalization, and low temperature cutoff. This BMS handles up to a 300a continuous load. That's more than we need as our batteries have a maximum output of 230a giving us a total of 690a continuous. However, with our Vatrer battery added, which can put out 200a continuous, we get a total of 890a continuous, and more for brief moments of 35 seconds, up to 1090a. Our electric propulsion motors pull a maximum of 1042a. So, in an emergency, like we are headed toward a dock too fast, we can use the full power of the motors. But, normally, while just motoring for long periods, we would only pull about 1/3 to 1/2 of the maximum, or 347a to 521a, well within the 890a continuous power our battery bank is capable of providing.
Zerojet Battery (48v)
105ah, LifePO4
5,376 kWh (total from two)
48v
Cell recharges: 4000
Size: L 28.5" x W 10.75" x H 9.1""
Weight per battery: 60lbs. (120lbs. total)
2 year warranty
Charging: Standard vs Fast Charger: 75/33 min.
Since we are going with the Zerojet electric outboard, we need 48v batteries for the tender as well. Their outboard can use their proprietary batteries (above) or we can use one of our own batteries, like the Vatrer 105a 48v battery if we purchase their Power Interface Module, which we are planning do to as this will give us the option of another battery as a backup to the Zerojet batteries.
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