Batteries. Simple things, just a block of lead and acid that’s charged, discharged, and sits in the bilge most of its life. Or that at least was the limit of my knowledge until I started trying to specify one.
So, just how much power do I need? What’s the maximum discharge rate vs. quoted rate. What is the system voltage? How do I supply high current DC loads? What types of batteries are out there? And on and on went the questions. So after a bit of digging and a lot of learning, this is what I concluded.
There are three types of batteries you will frequently face:
Lead Acid traction batteries
Carbon Foam firefly batteries
Lithium Iron Phosphate batteries
Lead-acid batteries are similar to those found next to the engine in your car. They can be sealed or vented and in that respect, sealed are better. They do not like being kept at a low charge level, and indeed, the maximum discharge is typically 50% of the rated capacity. That’s a bit of a bugbear for a hybrid vessel. The consequence of deep discharge is sulfation and shortened battery life. Traction batteries are a little better in accepting deep discharge levels and are similar to those for a golf cart or forklift truck. Lead-acid will only accept a trickle charge and may take a day to recover fully. A single saving grace is they are cheap, but good luck with that down the road.
Carbon-Foam or Firefly batteries are similar to lead-acid batteries using the same primary reaction, but the anodes are different. Their discharge floor and overall stability are much improved. They will charge much faster, indeed quite quickly, to about 80% full charge (see graph below). They can cycle from a partial charge without damage. They, like their cousins, are also quite economical to purchase.
Battery charge rate and capacity.
Prices for Lithium are coming down though marine suppliers seem slow to respond so far (2021)
Lithium is the new(er) kid on the block. In our case, Lithium Iron Phosphate to LiFePO. They can charge quickly. They also have a vastly improved power density compared to anything using lead as an electrode, about 3 times better at 120Wh/kg – (Watts per hour per kg mass). One would believe they are the answer to everything, but that’s not true either. Available charge decreases at low temperatures. Some lithium technologies (Lithium Cobalt and Lithium Nickle Manganese Cobalt) suffer from instability during charge cycles resulting in thermal runaway. They also suffer from dendritic growths within their structure, resulting in internal shorts and again fires. All these issues can and have been researched and designed for, but the reputation of “Lithium” persists. Some yacht insurers do not like them at all. They are expensive at $600-1000/kWh but look more towards the life-cycle costs than upfront CapEx. (Note there are quite a few “Lithium” battery technologies in production, so not all Lithium batteries are identical.)
Our hybrid drive selection needs a large power storage capacity and fast charging. At the current level of technology, the only viable technology is LiFePO. So we researched examples in small marine applications. Two suppliers came to the forefront Corvis in Norway and Praxis in the Netherlands. Torqueedo also came up though they are more concentrated on the market’s consumer end with a marinized EV option. Both suppliers provide large power banks. Additionally, they have inbuilt charging and monitoring systems, cooling systems, fire relays, and most importantly, Class approvals for the application. They are modular and more economical than the one-off price often seen. We went for a modular system in the expectation of improving technologies. Our total installed capacity is 60kWh or about 2 hours of operation with a battery voltage floating 500 to 700 VDC.
Battery technology seems to be changing quickly. Electric Vehicles’ adoption is a significant force in play. There are some fascinating technologies now jumping from the laboratory to pre-production. Lithium Sulphur (LiS) is one where they have solid-state cells with a power density 4 or 5 times that of LiFePO. The applications seem to be military and aviation (it’s price-driven). How technologies will change the marine electrical landscape is anyone’s guess but change it will and soon.
Credit battery university.com
Having identified a viable solution, we decided to request a review of the order nearer the shipping date. That’s 18 months from now. Who knows what the rise of EV’s will bring to market in that timeline.