Upgrading our house battery system, part 2; life is better with lithium
In part one of this series, I explained our motivations for upgrading to a lithium house battery bank, my choice of all Mastervolt components, and the pleasing results of early testing. Now let’s talk about what makes lithium different and what you need to consider if you too plan an upgrade. Active battery monitoring and control are critical to protecting other power components, for instance, and there are other important install details. Plus, I’ve got more real-world testing results to share.
It’s said that a lithium battery bank should be thought of as a whole system, like an engine, with several important accessory components. So switching to lithium power storage is not simply a matter of swapping batteries. Every boat built in the last 50 years, for instance, has chargers, alternators, and distribution components meant to support the various lead-acid battery types, not the decidedly different lithium iron phosphate (LiFePo4) charge and discharge characteristics discussed in part one.
Moreover, a LiFePO4 battery install should also include a battery management system (BMS) and safety relays to ensure normal and safe operations. If the individual battery cells are out of balance, for instance, the BMS can balance them. If the battery gets too cold or hot, or it’s hit with too large a load, the BMS can trigger the safety relays to shut it down until safe parameters are restored. Relion Energy has an easy-to-understand blog post about the whys and hows of lithium battery management systems.
But the differences in discharge and charge behavior also make lithium batteries more efficient and in many ways friendlier to the equipment on board, which also impacts the economics of a lithium conversion. My recent Lithium Math discussion dove deep into this subject and I hope you’ll take a few minutes to understand how the tech differences affect the overall cost. But the most important points to understand are that lithium batteries deliver more usable power from similar specs and do so with more consistent voltage.
Types of LiFePO4 batteries
For our discussion, I’m going to break down the various LiFePO4 battery options into three basic categories:
- System-integrated batteries — These are purpose built batteries designed to work as part of an integrated system with charge sources, monitoring, and status displays included. Victron and Mastervolt are the two major manufacturers making products in this category.
- Drop-in batteries — Drop-in batteries are designed to replace existing 12v batteries with minimal changes. All of the reputable drop-in batteries will have a BMS built in, but it’s very unlikely that BMS will communicate externally with charge, safety, or display systems.
- DIY batteries — There are many sources of individual LiFePO4 cells, BMS modules, and other components to make your own 12v battery. The configuration of these can vary widely depending on what you select, as can their ability to communicate with other on-board systems.
Each of these three options comes with plusses and minuses. As you might imagine, one of the biggest variables is the cost per watt hour. DIY batteries are typically the least expensive, but come with a need for more knowledge, skill, and work from the boater. Additionally, building a DIY battery properly can require some tools like bench power supplies, quality electrical meters, and more. Plus, if not well built and installed, DIY batteries can be dangerous.
Drop-in batteries are a good compromise on cost per watt hour and total install cost. As the name implies, they are intended to drop into the place of another 12-volt chemistry and work without major modifications, though upgraded charge sources my be needed to get the most benefit from the new bank.
Drop-in lithium batteries also don’t usually communicate with other components in the DC system. Their built-in management systems are designed to protect them from out-of-range ambient temperatures, over-charging, or overloading by disconnecting them, but some components, especially alternators, can be damaged by a sudden battery disconnect.
Second battery installed
When I installed the first Mastervolt MLi Ultra 12/5500 battery, I also prepared for the second battery when I installed the first, so finishing the whole 11,000 watt hour bank was a pretty quick affair.
My previous lead-acid bank consisted of eight 6v batteries in parallel and serial. I paid a great deal of attention to that bank’s cables and eventually selected method 2 from SmartGauge’s excellent tutorial on how to wire up various battery combinations. While that strategy pulled positive from one end of the bank and negative from the opposite end, method 3 — using positive and negative cable runs that are each exactly equal length from each battery to a single bus bar or terminal post — was said to be preferred, though possibly not worth the hassle. I agreed given eight batteries, but with only two to deal with this time, the optimal method 3 was easily achieved. In theory, this should ensure that each new lithium battery charges and discharges at the same rate.
Mastervolt uses a 500-amp Blue Sea Systems ML-RBS remote battery switch connected to and controlled by each MLi battery. The remote battery switch acts as a safety relay that the MLi’s BMS can use to disconnect the battery if anything goes wrong.
To complete the connections I connected the battery negative by 4/0 battery cable to the pre-battery monitor shunt bus-bar and the positive from the battery to a T-class fuse, the T-class fuse to the safety relay, and the safety relay to the positive bus-bar. The control systems also need to be connected for each MLi battery. These consist of five control wires for the safety relay and a Cat5 cable for MasterBus. There’s more to come on MasterBus in my next installment.
Notice how I made Starboard bases to secure each battery along with aluminum bars across the top to hold them down. Mastervolt does include two nylon tie-down straps for each battery, but I found them inadequate for the task, especially given the 125-pound battery weight.
Now that both batteries are installed, it’s time to tell MasterBus that the two batteries will be used together to form a single battery bank. This is done in Mastervolt’s MasterAdust software with a computer connected to the MasterBus network via a MasterBus to USB interface. Now that the batteries have been combined into a cluster, MasterBus can report on the two batteries as a single bank. This means that capacity, runtime estimates, energy consumption, and voltage measurements are all reported at the cluster level instead of on a per battery basis.
Monitoring the batteries
I’m a long-time user and fan of Victron’s BMV battery monitors and the closely related SmartShunt but, good as they are, those monitors are external to the battery and can only report what they’ve measured at the battery bank level. With MasterBus connected batteries I not only get the battery bank level information I discussed above but also detailed information on each battery like cell temperature and voltage, plus cell balancing activity. I also like the peace of mind knowing that any anomaly detected by the BMS will be reported via the MasterBus network and displayed on the EasyView 5.
Individual battery monitoring has allowed me to see interesting — though harmless, I think — differences between the batteries. As you can see in the graph above, for instance, at first the aft battery provides a little more of the energy needed, but after several hours and as the state of charge declines, the forward battery takes the lead. I’ve never seen the two batteries more than two-percent apart in total state of charge, so I have just chalked it up to slight variations in the batteries. And I’ll add that in most battery installs you’d have no idea what’s happening on a per battery basis.
During my early testing, I had myself convinced there was something wrong somewhere in the system. I was comparing the state of charge as reported to MasterBus to the SOC reported by the Victron BMV, and the former was consistently lower than the latter. But Mastervolt quickly set me straight; they calculate SOC based on usable energy, not total bank capacity. So, if you set the battery for a maximum 80% depth of discharge, it will report it is 0% state of charge when it reaches what I used to think of as 20% SOC.
This makes sense, but it’s also quite different from how many of us are accustomed to understanding state of charge. With my BMV-monitored FLA bank, I never wanted to go below 50% SOC because deeper lead-acid discharges caused greater wear. Mastervolt’s approach is more like what electric car and cell phone manufacturers do, telling you how much of the usable energy remains in your batteries, not what’s left on the way to flat dead.
There are merits to both approaches and now that I’m aware of it I have no troubles with how Mastervolt reports. In fact, I’ve changed my BMV by setting the battery bank “size” to the 640 usable amp hours rather than the full 800, and now the two state of charge sources are always within one-percent.
Real world results
My testing began with the boat still connected to shore power but with the inverter breaker off. That put a decent load on the new batteries, letting them run our boat’s refrigerators, freezer, microwave/convection oven, TVs, and computers. But during one of these rundown tests, I forgot that my girls had their weekly Zoom cooking class from the awesome Chef Laura at Oui Chef Chicago (not my wife Laura), and the oven had run for over an hour before I remembered that the inverter was running on batteries. If I’d made this same mistake with my old battery bank, it would have quickly resulted in dimming lights, low voltage alarms, and likely a low-voltage inverter shutdown. But with the lithium system, there wasn’t a hint of trouble.
With all dockside tests looking good, it seemed prudent to get off the dock for some real world testing. It’s hard work, but someone has to do it! As seen above, a somewhat cloudy day meant that we didn’t need air conditioning and so I shut off the generator as soon as we got the hook down.
At anchor we went about living our normally well-powered lives; we cooked snacks in the microwave, listened to multiple stereos, enjoyed March Madness games on the TV, etc. We started the generator for an hour or so to make dinner and in that time the batteries got back to 100-percent SOC, the MasterCombi 12/3000-160 inverter/charger having replaced all the power we’d consumed in the previous five or six hours.
At bedtime, I didn’t do my normal procedure of shutting down all non-critical loads, instead letting them run all night. When I woke the bank was at about 45% SOC and humming along happily. I started the generator to make coffee — it’s on my list to move the coffee maker to the inverter — and it took just over two hours to get the LiFePO4 bank to 100%. So, in 24 hours at anchor we’d run the generator for less than four hours, and that will be much less once I install the Mastervolt Alpha Compact 14/300 alternator so that the port main engine can top up the bank as we motor to the next destination.
There have been several high-profile events with lithium-ion batteries including fires aboard Boeing 787s and a spate of hoverboard fires. But, while lithium iron phosphate batteries are lithium-ion batteries, they are a safer variety than those that made news. For instance, that’s a burnt lithium cobalt oxide battery (pictured above) which experienced thermal runaway inside a Boeing 787. Lithium cobalt oxide together with lithium nickel manganese cobalt oxide are responsible for the majority of the frightening events we’ve seen reported.
These other lithium-ion battery chemistries have more than twice the energy density of lithium iron phosphate, but they’re also inherently more dangerous because internal faults, external abuse, and overcharging or discharging can result in a thermal runaway. Fortunately, I don’t know of any marine use of these chemistries.
The FAA tested the safety of various lithium battery chemistries — available at www.fire.tc.faa.gov/pdf/TC-16-17.pdf — and the chart above shows the results I think most relevant to boaters. In this test, a heater forced one battery to overheat, while five connected batteries were monitored for thermal runaway. LiFePO4 was the only chemistry that didn’t propagate the failure to all five batteries. In fact, no other batteries failed and the FAA concluded that LiFePO4 is the safest of the chemistries.
I believe a solid case can be made that properly installed LiFePO4 battery systems are safer than the vast majority of lead-acid battery installations — including those on my own boats — which have few safety measures in place.
A good lead-acid install includes little more than an enclosure around the battery, a strap to hold it down, and over-current protection. In contrast, a proper LiFePO4 install has a BMS monitoring the health of each cell, the charge or discharge current, and the temperature of the battery. If any of these elements are out of range, my Mastervolt BMS can activate a Blue Sea Systems ML-RBS remote battery switch to disconnect a battery from the bus bar.
Meanwhile, a single failed cell in a lead-acid battery can easily cause overcharging in the rest of the cells. One result many experienced boaters are familiar with is the pungent, rotten-egg smell of hydrogen sulfide, a highly poisonous and explosive gas. So it’s usually the nose of a boat owner or dock neighbor that detects a failure and causes the charger to be shut down. But that doesn’t happen until the battery has already released toxic gases into your engine room. I’ve also experienced battery explosions from low electrolyte levels allowing the plates to be exposed.
I’ve heard rumors of trouble with insurance carriers when they hear lithium has been installed in a boat. I haven’t been able to track down any concrete examples. But, I did ask Menno Ligterink, Mastervolt’s OEM sales manager for RV and marine, about the safety of LiFePO4. He explained the many measures engineered into this system as well as the work they’ve done with outside safety organizations. Menno explained Mastervolt has worked with ABYC to help prepare technical information document TE-13 on safe installation of LiFePO4. Lastly, Mastervolt is currently working with UL to attain UL approval. Menno left me with this indication of his confidence in the MLi batteries he installed on his RV, “I put my batteries under my own bed.”
Cost / benefit analysis
In that recent Lithium math entry, I calculated a drop-in LiFePO4 battery cost at about $0.70 per watt hour. My new 5,500 watt hour Mastervolt MLi 12/5500 batteries can be purchased online for about $5,700, or about $1.04 per WH. In total, this system costs a little over $17,000, but unless you’re willing and able to do the install, there will also be labor expenses. It took me about 40 hours for the install and configuration, and combined with various possible extras like large DC cables, lugs, and bus bars I think this whole project could climb to somewhere north of $22,000.
That’s a major investment, and to justify that investment you need to get a major return. But my configuration is probably larger than the vast majority of boaters will need. In fact, I used the system with only one battery for a few nights at anchor and was impressed by how well that worked. I also think you could reduce cost by using a smaller alternator. Additionally, not all boats will need all their DC components changed. If I had a newer inverter or already had externally regulated alternators, I could have avoided those costs as well.
But even at full cost I believe this upgrade represents good return on your investment if you anchor with some frequency and value the ability to do so with minimal generator runtime, or don’t even have a generator.
There’s still more of the upgrade story to tell. I plan to detail the MasterBus capabilities and also explain how the Mastervolt BMS can be alternately integrated with CZone or direct to NMEA 2000. I’ll also show you how some of my system’s monitoring can be output to NMEA 2000 displays and how I can use that for the remote monitoring that Mastervolt has not developed yet. And there’s also the alternator and regulator to install — which may take some pro assistance — and a look at how they integrate with the other Mastervolt components.
In the meantime, I’ll just be here on Have Another Day enjoying the fruits of my labor so far, and happy to answer questions.