The Wonder and Terror of Lithium Batteries
December 30th, 2019 by inavx
By. B.J. Porter (Contributing Editor)
Welcome to my Research Project
Lithium batteries are a wonderful technology. They are lighter, charge better, and are so efficient.
OH MY GOD YOUR BOAT IS GOING TO EXPLODE LIKE A GALAXY NOTE SEVEN ON A TRANSCONTINENTAL FLIGHT!
You’ll get so much more life out of them, they’ll last for decades and need so much less recharging.
THEY ARE SO COMPLICATED AND EXPENSIVE. No one really knows the technology. It’s really easy to screw it up and destroy your investment. And watch out for that Chinese garbage!
Seriously, you’ll never need another set of batteries if you do this right. You’ll have so much power.
YOU’RE PUTTING THEM UNDER YOUR BED? Are you insane? The technology really isn’t ready.
Welcome to the research I’ve done for the last few weeks. There is, to say the least, a lot of information out there about battery technology. And much of it is alarming, intimidating, and concerning. But most of it is pretty encouraging, if you’ve the stomach for electrical work and a bit of technology.
Apologies in advance for the length, there is a lot of material to cover here and even at that I’ll be skipping a lot of detail. Most of it is background for our choices and why we’re making them.
The Good Lithium
Lithium batteries come in a whole range of technologies and mixes. Some earlier lithium batteries were indeed risks for fire or explosion. Safety has been one of the many factors in developing more advanced lithium battery technology, as well as improved battery performance.
We are installing Lithium Iron Phosphate batteries, or LiFePO4, often abbreviated to LFP. LFP batteries are one of the safest Lithium technologies on the market. They are safer than many of the batteries in your house, pocket or purse now.
The Big Performance Win
The main reason to switch to LFP technology is the performance of the batteries. If you read the first part of this series, than you are familiar with the main faults of Lead-Acid batteries: taper charging, discharge limit to 50%, decreased efficiency at high loads, requirement to periodically fully charge AGM batteries, and limited cycle life. Specifically:
- LFP batteries do not taper charge and they can usually accept a massive charge as well..
- LFP batteries can be discharged to 20% without harming the battery. So fully 80% of the battery is usable.
- Not only can LFP batteries take massive charge loads, they can also handle large discharge loads without the same inefficiencies and loss of power of Lead-acid.
- There is no need to fully charge LFP batteries.
- Cycle life (complete discharge -recharge) in Lead-Acid batteries is quite low. 600-800 cycles to 50% discharge is good for many AGMs. Most LFPs are rated for 2,000 cycles – at an 80% Depth of Discharge (DoD).
So overall you get a battery that stores and releases more energy, is easier to charge quickly, is more flexible on charge and usage patterns, and has many, many more charge cycles of use.
How do we get to a three-day run between charge cycles? More usable capacity. How do we do that? Two ways: deeper discharge/better charging and more batteries.
LFP cells are considerably lighter and smaller than their AGM counterparts. Our current 660Ah 24V house bank weighs in at a hefty 792 lbs. For that, we get 264Ah of usable capacity. The new bank of LFP cells 720Ah 24V, but has double the usable capacity of 576 Ah. And they weigh 394 pounds – half the weight. If you look at usable Amp-Hours per pound, the LFP cells deliver power at about four times the rate (1.46 Ah/Lb) over the AGM bank (0.33 Ah/Lb). I could easily have fit more than 720Ah of capacity if I wanted to.
So you can replace your AGM bank with 1/4 of the weight in LFP cells, for the half the weight you can double your working capacity. More capacity equals fewer charge cycles, and more battery life.
The Lithium Risks
As mentioned above, the physical risks to your boat and your body using LFP technology are pretty low. Not zero, but no battery is 100% safe so we can stipulate some risks no matter which type of battery you use, and move on in the discussion.
The Fragile Knees
The charge state of Lead-Acid batteries can easily be checked by reading the battery voltage. A fully charged battery will read with high voltage, and as the battery drains, the voltage will slowly and predictably drop. The reverse is true for charging – the voltage rises with charging, and all the various “smart” chargers will adapt their charging behavior based on the read voltage.
The graph above shows the difference in the voltage states, and the “knees” in the graph of the LiFePO4 cell voltage as the charge varies. The problem is that LFP batteries can get really damaged by pushing them over those knees. Discharging an LFP down past the lower knee to 0% can damage the cells and reduce life cycles. Over charging past the upward knee can very quickly destroy the cells as well, resulting in the loss of your expensive batteries.
Traditional Lead-Acid chargers cannot cope with LFP charging needs, and must be reprogrammed to fake them into handling them properly. This is not always possible. But there is a way around this.
Charging and Management
Batteries are stupid. They have no inter-cell circuitry, no knowledge of their own charge state. The solution to avoiding premature LiFePO4 battery death then is to ADD a brain to the process. A Battery Management System, or BMS, becomes the heart and soul of your battery setup and prevents you from stupidly destroying your batteries through mismanagement.
In theory, you can avoid damaging the batteries by running over the “knees” as I described above. You could sit near the batteries with a Multimeter, checking the voltage of each cell repeatedly, looking for the first spike upwards when recharging, or the first precipitous dip when discharging. When you spot that spike, you can turn off the charging and all will be well. When you spot that dip, you can start the charging up to keep things from running too low.
But sitting with the batteries all day with a multimeter would get tedious.
In essence, that is what a BMS does. It constantly monitors every cell for voltage, and will cut off charging to prevent over charge. It also cuts off power out if the batteries are too low. At its most basic level, that “brain” of the system idiot-proofs the battery bank and prevents accidental destruction of the batteries. Although a simple BMS wouldn’t start charging automatically, more likely it would sound and alarm and flash some lights telling you to start the generator before you hit the lower knee. You really wouldn’t want your “need to charge” notification to be the complete shutdown of all electrical systems on the boat.
A more fully functional system can take it steps further than acting as a simple gate-keeper to power in/power out of the batteries. The system we are employing, the Emus BMS, can control certain chargers directly and preemptively using a protocol called CAN (Controller Area Networking – it’s used in a lot of automobiles and is the underlying infrastructure of NMEA 2000). It can also trigger relays, lights and displays, provide detailed information on the battery state, alert the operator when attention is needed, balance cells that are out of voltage (more on that later…some day), speak Bluetooth to allow battery monitoring on an android device, and even has an add-on module to accept a GSM cell SIM to enable SMS notification of battery state.
In theory, one can install and use a LFP battery bank without a BMS. But the risks to the batteries are big enough so most view it as foolhardy to do so.
Charging With the Oldies
If it’s not clear yet, there are some risks using “old school” charging techniques with your flash new LFP batteries. Charging “profiles” from “Smart Chargers” will kill your LFPs in short order. Many devices simply do not know about the “knees” and will try to charge by the old rules.
It all needs re-thinking and every device, no matter how old and stupid, needs to be brought into line and taught to behave or get replaced.
The cost is really the big nut when it comes to the downsides of LFP technology. There are three basic ways to approach LFP, and none of them will be anywhere close to the cost of just dropping in a new set of AGMs. Which isn’t cheap either – a single 4D sized 200Ah AGM costs over $750 USD. We need six of them to replace the house bank which could run over $5,000.
The Proprietary Install
The most expensive way to convert to LFP is with a complete soup to nuts solution from a single source, installed by professionals. It will work, but it will cost a LOT. This big dollar solution would be several times the cost of what we are doing. There are some good companies doing this stuff, but you need the budget.
The Drop-in/Prebuilt Assemblage
There are really two flavors of this, but they both involve pre-built batteries (a battery is just a group of cells, wired together). Many companies have made pre-built drop-in batteries which claim to have a built-in BMS on board. They also claim compatibility with existing charging systems, but there is no guarantee. The batteries are quite expensive, and results are varied and wild as it’s tough to validate claims about them. But without a comprehensive BMS and complete integration with the charging systems, it is difficult to see how this will really be stable and cost-effective.
The alternative “flavor” of this is to buy pre-made LFP batteries from a more known and reliable vendor like Victron or Mastervolt and work with their BMS, charging and other supporting systems. These batteries are crazy expensive, but should work well if you use all their high quality equipment.
Or the “Roll your own” battery bank for the somewhat less reverent. This is what we are doing on Evenstar. It requires that you order your own individual cells and wire them together into a battery, install and configure your own BMS, and integrate your own charging system. This is not a project for the faint of heart. If you can’t explain Ohms laws without looking it up or don’t grasp basic electricity it may not be for you. If you can’t crimp a cable or operate a multi-meter, it’s probably not for you. Even if you have a firm grasp on electrical systems, it still may not be for you.
Years ago I received an A+ in my “Electrical Systems” course at New England Tech, I installed all the instruments on the boat now, I’ve upgraded the charging and inverting systems on the boat, and I’ve crimped a LOT of cables. I’m mostly (but not completely) sure I can handle it. I’ve had to learn a lot, and immerse myself in some new concepts and ideas. It’s fascinating, and a bit intimidating. I do intend to detail our DIY install in later posts, so I won’t spell it out here.
It’s complex, and it’s a lot of work. At the end you get a completely custom setup tailored for your boat, for a lot less money. And like all DIY projects on your boat you know where all the bodies are buried, because you put them there.
The Return on Investment
Any of those approaches are workable, and your mileage will vary in your results as will your costs. But what you need to look at is the ROI to see if it’s worth the money for you. This technology is not for every boat or every boating application. It would be overkill for most weekend and seasonal sailors, for example.
In my last post I mentioned that we’ve spent an average of $2,200 a year on new batteries for the last five years. That figure does not include the diesel I’ve burned running the generator every day and the extra wear and tear for the 2,300 hours we’ve put on the new genset since we installed it in 2015.
So – if sticking with AGMs costs $2,200/year + diesel and shortened life…how long until the batteries pay for themselves if I change to LFP?
I figure it’s about five to seven years to break even. My outside/worst case budget on this project is $20,000 – though I hope to bring it in closer to $17K-$18K.
This upgrade should save me about 700-800 generator hours per year. Call that 1,000 Liters of diesel at $1.00 or so a Liter (prices vary wildly by location). It’s also saving me 3-4 oil changes on the generator (about $30-$35 each for oil & filter), and is extending the life of my generator by a couple of years every year.
If you assume 650 hours of generator time used/year, a 10,000 hour life on a generator that costs about $20,000 to replace, for annual cost savings by doing a LFP installation you can assume:
|Generator Life saved, $2/Hr
|$20,000 Replacement cost over 10,000 hour life, 650 hours saved/year
|Fuel Savings, $1.00/L
|Oil Change Savings
|3-4 oil changes
|AGM Annual Expense
|Not buying new AGMs in 2-3 years!
|Annual costs recouped by LFP install
With a $20,000 expenditure that’s a five-year return; I’m thinking seven because I’m probably overlooking a few things and being too optimistic. With the expected cycle life of 2,000 cycles, these batteries should last sixteen years, not the two to three we’ve gotten from the AGM batteries. If they last sixteen years, my annualized expense on the install is $1,250. Or a little more than half of what I spend on AGMs now.
Note that these numbers work GREAT for full-time liveaboards like us. If I were a seasonal coastal cruiser putting 50 charge cycles on my AGMs a summer and fully recharging them when I returned to my slip on Sunday night, they’d make zero sense. Keep the $20K, your batteries will probably last a decade.
But on the whole, if you live like we do, have our various requirements and limitations on power generation, and aren’t intimidated by the DIY approach it can work out well.
I hope… 🙂