How to convert your boat to a LiFePO4 house battery bank.

I get asked this all the time. It’s not that hard if you use the right products. First, be sure to use quality batteries. Cheap cells can be bought but should be avoided. Also when I say Lithium, I mean the safer LiFePO4 Lithium Iron Phosphate cells, NOT Lithium Ion. Do NOT use lithium ion cells on a boat or any 12v situation.

Just like Lithium Ion batteries, LiFePO4 battery cells are available in both Prismatic, and Cylindrical forms. Prismatic are also called Pouch Cells or blocks, Cylindrical batteries are called 18650 (18mm diameter 650mm long), 26700, or 32650, based on their dimensions. Both of these types of cells are usable in 12v application when they are wired 4 cells in series. Most of the time when you are buying a 12v LiFePO4 battery it is hard to tell which type of cells are wired inside the case.

Prismatic cells are normally used in cheaper batteries, just about all Chinese 12v LiFePO4 batteries are made with Prismatic pouch cells. These are ok to use on a boat and are much lighter and more energy dense than batteries using cylindrical cells. but the downside is that if there is a problem the pouch cells can swell in size, sometimes causing a mechanical failure of the case. For this reason, I personally DO NOT recommend using prismatic cells.

Cylindrical cells are used in more quality batteries such as Battleborn, Victron, Valance, batteries. Because of their higher cost, and weight, they are not commonly used, but the cylindrical internal casing are better at distributing heat and handling rapid charge and discharge cycles better than prismatic cells can.

In every situation LiFePO4 batteries make more sense than any other type. Over a 15-20 year life they are always cheaper than any other type of battery. They are lighter and take up less space, and you only need half of the amp hour to achieve the same usable amp hour of AGM or lead acid. Fewer and fewer people are still buying new AGM batteries ever 3 years, and they are making the switch to lithium.

I decided to write this post because there are SOooo many other articles written on how to do this and almost all are overly complicated, or just wrong. I even see “professionally” installed LiFePO4 systems have problems when the voltage goes low , causing the disconnects to shut-off, and then there is no way to charge them back up, or the full load of your boat systems get put onto the charger or solar charger when low voltage is detected. I’ve also seen alternators get fried by so called smart switches when voltages get high, or someone switches a selector switch by mistake with the engine running. KISS (keep it simple stupid) is safest way.

Fundamentally there are only 3 things to be concerned about when switching to lithium. (I will leave out cold temperature factors due to the fact that all sailors i know escape the cold climates before it becomes on issue)
1) Never over discharge them. An automatic low voltage shut off should be wired in.
2) Never over charge them. Control systems for alternator regulators and chargers should be set to never exceed 14.4v or 14.6v depending upon brand of batteries
3) They will take ALL the charge or discharge current you give them, so be careful! If you have 200amp alternators they will take it all! ( and could smoke your belts and/or overheat your alternators). And, if you have a short circuit they will eliminate what ever caused the short, and surrounding objects, into a cloud of smoke, maybe even your whole boat.

Also not all Lithium batteries are equal. Take extra caution to note the max charge amperage. This is called the C rating. The max charge rating is typically 1/2 of the max discharge rating. To calculate the max amps, you take the published C rating and multily it by the AmpHour rating. Example a 100ah battery that publishes a 1C discharge rating and a 0.5C charge rating, means that it can be charged at 50 amps. ( 0.5 x 100= 50) . Most cheaper lithium batteries can only safely be charged at 50 amps, no more. However, each battery wired in parallel multiplies the max charge current. For example if you have a 12v house bank consisting of 6 batteries wired in parallel (each with 50 amp max charge ratings) then you can safely charge the bank of 6 at 300 amps. 6×50=300

All quality marine LiFePO4 batteries have built in BMS circuits that maintain the internal 4 series circuits of 3.6v cells. Normally no other BMS is needed for 12v wiring unless you have a 24V, 36V or 48V battery bank, then you will need something like a Victron Balancer)

The first part of going lithium I have found is to first simplify the wiring. My experience has been primarily working on older boats that have had several creative wiring additions added over the years. Now is a good time to cleanup all the creative mess. Voltage monitoring of lithium battery systems needs to be exact, so you don’t want multiple devices sensing voltages from different connections throughout your boat.

House Battery Bank. The amp/hour (ah) calculation can be very precise so it only makes sense to wire them so you know exactly how much current has gone in and how much you can use. To do this the best option I’ve found is a Victron BMV-712. Due to the fact that the battery voltage changes very little between 80% charge and 20% charge, a good amp hour meter is an absolute must have.

The first step is to be able to monitor your battery usage. To do this the BMV-712 shunt needs to be connected directly to the negative side of your house battery bank. NOTHING else should be connected to the house bank ground! NOTHING. On the other side of the Victron BMV shunt install a nice large buss bar. ALL negative connections are to be made at this buss bar after the shunt, including engine ground, start battery ground, bonding grounds, everything.

The positive battery terminal buss bar can have all the regulated charging sources connected via independently fused terminals. Items such as Solar charger(s), Wind generators, and if you have a “smart” inverter charger, the positive wire can be connected also. Note it is ABYC standard to have a disconnect switch for all chargers (alternators are excluded from disconnects).
Replace the wire on your alternator(s) with a new one that runs directly to this positive buss bar. Alternator wire needs to be ran from output of alternator directly to the House Battery bank.
The battery parallel switch between the start battery bank gets connected to the positive house bank buss as well via a fused connection.
Then connect a small fuse block to the positive buss bar for the individually smaller fused circuits “Always On” circuits like bilge pumps. Also make your sure your battery voltage sense connections for the alternator regulator(s) and any auxiliary controllers for solar, chargers, and/or the victron BMV-712 are connected to this always on circuit.

To be sure the house bank never falls below a safe voltage, an auto disconnect such as a Victron Battery Protect 220 is also wired via a fused connection to the house bank positive buss. This can be in line with your existing house bank battery switch, that feeds all normal loads onboard. All loads (except for the “always-on listed above) should be connected to the outlet side of the battery protect, including the DC-DC charger that keeps your Start Battery charged.
An exception to this are high current intermittent devices such as windlass, and electric winches. These should be wired to the positive buss with fuses since they normally exceed the 220 amp limit of the Battery Protect but their control circuits should be wired to the output side of the battery protect.

The victron battery protect guards against over discharge. A simple easy setup via the built in bluetooth app.

To guard against over charge; set your solar controllers and smart charger to shut off at 14.4v.
Only use externally regulated alternators that have temperature controlled regulators! As an additional fail safe for alternator outputs I use the Alarm Relay out terminal on the BMV-712 to trigger a NC (normally closed) relay shutting off the ignition feed to the alternator’s external regulator’s field wire when the house bank gets to 14.6 volts.

Start Battery(s). These are to start your engine(s), and nothing else. The only connection to your start battery should be the output wire from a dc-dc charger charging them from your house bank, the engine controls, the starter motor, and a battery parallel switch. There should be no connection to the start batteries from the alternators. Also these should NOT be lithium. The reason why you should NOT use lithium batteries for your start bank is because of the need to use an emergency battery parallel switch incase your start bank dies for any reason. If you had lithium for start bank, and you switched the parallel switch on, the amps could exceed over 1,000 amps depending upon the voltage difference between the two banks. Using an AGM or lead acid start battery easily negates this issue.

UPDATE; The Valence U27-12XP 138ah batteries are getting hard to find. Typically used ones you find are 10-15 years old, but can still have 90% of original capacity. What I like about the U27-12XP (besides the built in smart BMS) is that they can charge at 1C (138 amps) and discharge safely at 300 amps. This means a house bank of 11 x U27-12XP can be charged at 1,500 amps and discharged at 3,300 amps! Other, not comparable, batteries can be bought on Amazon but be aware of the maximum charge and discharge amp ratings. Example, If you buy a 12v 100ah battery that has a 0.5C (50 amp Max Charge) rating, it means if you wire 4 in parallel to get 400ah you can only charge them at 200 Amps, no more. The faster you can charge or discharge changes the price drastically so don’t just look at amp hour ratings when comparing brands. A good lithium battery bank can last you 10-25 years, so dont worry about saving money on the upfront costs.