Balancing and Busses

1 Dji 0128 — Dave and Sherry McCampbell aboard their St. Francis 44 Mark II cat Soggy Paws in the Solomon Islands.

Editor’s note: Below is Part 2 of an article providing information on choosing and installing a LiFePO4 lithium battery system aboard a cruising sailboat. Part 1 appeared in our 2021 Ocean Voyager annual issue on page 8.

Recently we removed 390 pounds of GEL lead acid batteries from our catamaran and replaced them with 100 pounds of lithium (LiFePO4) cells. The LiFePO4 batteries have almost twice the useable capacity and take up half the space. Our cells came from one of the many well-respected lithium factories in China. Most of these have been in business for ten years or more so have earned their reputation in a crowded marketplace. Lithium forum discussions, length of warranty and price (cruisers can’t afford cheap gear) will often tell the real story. Below are details of our experience replacing our 13-year-old Gel batteries with LiFePO4 cells.

As mentioned earlier in Part 1 of this article, careful study of trusted resources and forums specializing in lithium batteries will greatly improve your knowledge of the technology and provide advice on purchase options, testing and installation. Also, there is some good practical information on YouTube videos.

Before settling on a lithium battery manufacturer, a couple of basic decisions need to be made. These include:

Whether to use preassembled batteries or individual cells
Cell configuration (prismatic or cylindrical)
Cell case construction (aluminum or plastic)
Quality of the cells (grade)

See Part 1 of this article for details on the first three.

Grades. Cells are available in A, B or C grades based on capacity and internal resistance, which affects run times and cycle life, physical condition and several other factors. Grade A cells will have a QR sticker indicating they meet all factory data sheet test criteria including size, weight, capacity, internal resistance, etc. B and C cells that do not meet test criteria, have less run time and cycle life, or other issues, and do not have a sticker or it has been scraped off at the factory. Grade A cells will perform better and usually offer a multi-year warranty. Used cells, often from the electric vehicle market, with some capacity remaining, are sold at a big discount with no warranty. Serious cruisers should only buy grade A cells from a reputable dealer with a good warranty.

Purchase. Final purchase cost depends on a variety of things like cell characteristics, source and destination locations, shipping, taxes and duty. Cells or preassembled batteries for a DIY project can often be sourced direct from the factory at a substantial discount. But shipping, handling and duty may be more expensive. As an example, the cost of our eight 271-amp-hour, 3.2v individual prismatic grade A cells with aluminum cases from a Chinese factory was $150 US each. Connectors were extra. Cells like ours are available from multiple sources. Prices for this size cell vary from about $100 to $300 US each, depending on the many variables described earlier. Cells are also available as 12v nominal batteries in packs of four cells inside custom plastic boxes. These have external terminals and possibly a battery monitoring system (BMS) inside for additional cost.

Shipping. Cells shipped into the US need special certifications for safety and handling. Shipping price, even for a small number of cells, by sea freight is often quoted as the cost for a minimum of a cubic meter or pallet. Individual cells or batteries should be insulated from others using small boxes within a sturdy outer box, all protected with good cushioning insulation. They should be arranged only right side up, never on their sides, and the external boxes should weigh less than about 70 pounds. They are classified as Dangerous Goods UN3480 Class 9. Boxes must be marked and labeled correctly on the exterior. Cells without proper certifications could be rejected and returned. So, purchase only from a reliable company with a verifiable track record.

International shipping regulations do not allow air transport. Shipping for our cells (about 100 pounds total) from the factory in China to the Philippines, via Hong Kong in two months, was about $35 US per cell. Shipping cost from China to other worldwide locations varies widely. See Sidebar 1 for a partial list of well-known lithium brands.

After considerable research we bought our cells from a large reliable Chinese company, Shenzhen RJ Energy, which has been in business for more than 15 years. They specialize in LiFePO4 technology and produce thousands of grade A cells monthly. They have numerous certifications to support their large export and government business. Their advertised warranty is five years extendable to 10 years. All cells are carefully tested and results labeled on each cell before leaving the factory. There are a number of other big factories in China with a wide range of lithium offerings and similar production reputations and facilities. So it is worth shopping around. Many export to dealers in the US and elsewhere who resell at an increased cost, but usually with more local customer support.

Initial testing. After receiving shipped LiFePO4 cells or batteries it is important to visually inspect them for damage and testing stickers indicating a grade A cell. Then if you want to ensure your cells are as advertised, it is necessary to perform charging and load tests. From these you will be able to learn each cell’s capacity, overall performance, charge curve and internal resistance. You should have a quality bench power supply, load tester and logging capability for this work. See Sidebar 2 for a list of equipment we later bought.

During shipment and storage, cells should be at mid-state of charge, not fully charged. Buying fully charged pre-balanced cells may not a good idea if they have been stored for a while. So we first charged each cell individually to 3.6v, the top of the recommended charge curve for our cells. Later we discharged them individually to 2.8v. Using the logging capability of our multimeter, we were then able to see the discharge curves on our computer, with high and low voltage “knees”, and calculate the true capacity. We used 3.6v to 2.8v for discharge because those are safe voltages representing almost all of the amp hours (capacity) available in a cell.

Top balance. Cruising boats wanting adequate battery capacity and charging capability to offset the daily amp hour load will want to initially “top balance” the cells. This gets all of the cells at equal voltage at the top of the charge cycle. We did an initial top balance by using our solar panels to first charge our eight cells in two packs of four as 12v batteries back up to 13.8v, fully absorbed. Then we disassembled the packs and connected all the cells in parallel. We used the bench power supply and our more accurate multimeter to charge them all together equally to 3.60v. Finally, holding the voltage constant at 3.60v, we let them again absorb, down to 0.1 amp charge rate indicated on the power supply. The goal is to get all the cells almost fully absorbed, at exactly the same voltage, at the top of the charge curve.

Balancing. Since all cells cannot be produced exactly equal in capacity and internal resistance, expect each cell to perform a bit differently over time. Because of this, there may be a slight difference in cell voltage at rest, and more when within the charging knee. Cells with lower capacity or higher state of charge (SOC) will reach the charge set point first and that should trigger a charger shutdown, therefore not allowing a full charge of the lower voltage cells.

Typically all cells in a battery at rest should have voltages within 15mv (.015v), but ideally less. If not, it is time to use a cell balancer or other equipment to correct the balance. Balancers are either active or passive and come in a wide range of configurations, balancing current capability, and quality of construction. As mentioned earlier, small balancers may also be included within a BMS. Passive units use resistors to just reduce (shed) the voltage of higher voltage cells. If no balancer is available, incandescent bulbs or even long wires can be used in place of resistors.

Active balancers can be either manual or automatic. They take some current from the highest voltage cell and feed it to the lowest cell, to bring the bank into balance faster and so the BMS doesn’t have to dissipate any power itself. If at or near the system’s high voltage set point the greater the balancing current and the faster this balancing can occur.

Balancing can also be done manually using a bench power supply and load tester to adjust cell voltages as needed. Careful monitoring in millivolts at the cell level with an accurate multimeter is important. If cells are properly top balanced at the start, are not damaged and carefully charged and discharged, additional balancing should not be required for a long time. But close monitoring of cell voltages on a frequent basis is still required.

Dual busses. Installing lithium batteries in a boat will require some electrical system modifications and the ability to protect the batteries in case of problems. Voltages critical enough to permanently damage LiFePO4 cells/batteries are below 2.50v/10.0v and above 3.65v/14.6v. So a three-tier disconnect system is commonly used to protect equipment and the lithium cells.

1. Most electrical equipment will have low-voltage disconnects that shut off equipment loads from batteries. Quality charging sources should have high voltage set points to control charging voltages so they don’t get too high. However, neither of these can see individual cell voltages and may not be appropriate for LiFePO4 cells.

2. If these fail, or a single cell surges high or low, forced shut down of individual devices should be arranged through the BMS.

3. Finally, if both of the above fail, a full battery disconnect from the busses should be available using the BMS.

For this reason, it is best to route all loads through a “load buss” and charging sources through a “charge buss.” This dual buss system then facilitates installation of two separate disconnect relays, controlled by the BMS, between those busses and the batteries. They will provide high or low voltage disconnect of the batteries from the busses, even at the cell level.

Battery monitoring system (BMS). As we learned in Part 1, a good BMS is the heart and brains of a well-designed lithium battery system. It will prevent damage to the cells if something goes wrong causing the cells to reach voltages above 3.65v or below 2.5v during charging or discharging. BMS-activated or separate audible and visual alarms are also useful for notification of significant voltage events, such as reaching a second- or third-tier set point. Some BMS’s are being designed to “talk” directly to popular solar controllers and chargers via NMEA 2000.

Choose this item for quality and features very carefully. Also, review its compatibility with other components like chargers, inverters and external relays. See Part 1 and Sidebar 3 for BMS overviews and some well-regarded BMS options.

Relay options. Relays, independent of the BMS, should be considered, especially if large amperages, as from an inverter, windlass or big alternator, are possible through either buss. Smaller amperages at half the BMS rating or less can usually be efficiently handled by most quality BMS internal MOSFET relays. External relays should be sized for the expected continuous loads and not have a large parasitic current draw while active. Magnetic latching relays have no parasitic loads, and only draw current while being activated, but are expensive and not compatible with some BMS’s.

There are many other relays and devices that could be used for battery disconnect protection. The BMS only has to be able to provide adequate current to close the relay and hold it there, or open it by cutting off the current. Examples include: alternator shut down by cutting off current in the field or ignition wires, but not the output, and battery temperature sensor wiring used to cut off solar or shore charging. There are many options for BMS’s that use internal relays and those that use external relays.

Charging & discharging. The charging regimen for LiFePO4 batteries is different from that of lead acid batteries. There should be little or no absorption and no float, or a very low set point for float. Typically, LiFePO4 batteries are bulk charged to the upper set point and then charging ceases until the next day. The house bank should be sized to allow cycling between something like 90 percent SOC down as far as about 40 percent SOC, but no lower than 10 percent SOC, to take care of the daily loads, if you want maximum service life. If primarily relying on solar charging, this is where properly sizing the house bank to provide power during a couple of cloudy days in a row is important. We plan to use about 13.8v as the upper set point for all our chargers. So, our solar charger will shut off at that for the day and not start again until the next day. Some BMS’s allow you to add extra charge cycles during the day.

If you use an alternator for charging remember that almost all of the charge will be in the bulk mode and this can over stress your alternator. So monitor it closely, use a heat sensor on the alternator and reduce charging current to less than about 70 percent of rated output.

Initial equipment load discharge limits should be set near the top of the lower knee, which for us is near 3.15v per cell or 12.6v for the battery. Alarm and load buss disconnect settings would be set just below this voltage. Use care in how you disconnect loads and charging sources, as some equipment disconnections can cause problems due to voltage surges.

Cell battery box. Prismatic cells mounted in a cruising boat, especially those with aluminum cases, need a strong box around them in order to prevent damage from movement. There are a number of ways to accomplish this including using a clamping plywood box similar to what is described in the two Resource websites in Part 1. Aluminum case cells require non-conductive separators, such as thin laminate or plastic between all cells, in order to prevent shorting the sides against each other. Boxes should also have a removable top to prevent accidental short circuits from falling conductive objects.

When attaching cabling to the cell terminals ensure they are clean and there are no washers inserted between the cell and wire end terminals. Finally, pay attention to ABYC fusing and wire sizing requirements for cabling, avoid weak connections, and wire no more than two batteries in parallel.

Final thoughts. As you can see, installing LiFePO4 batteries on a cruising boat is not an easy project. It is no place for guessing or cutting corners. There is much to learn and do before you will have a successful lithium battery system. But if done properly, with no magic smoke escaping and all electrons traveling in the right direction, the lights won’t ever go dim. Then you and your crew can concentrate on the more enjoyable aspects of cruising, like never missing a sunset. n

Dave McCampbell voyages with his wife Sherry aboard their St. Francis 44 Mark II Cat Soggy Paws.

LiFePO4 Brands

Battery Management Systems (BMS)

What to Look for in a BMS:
Beginner BMS Overview: diysolarforum.com/resources/categories/bms.6/
ElectroDacus SBMS0- 8S, $120 US
Orion Jr 2- 16S, $440 US
REC active BMS- 4S, $530 US
Batrium Watchmon4- 4S, $890 US
Chargery BMS8- 8S, $110 US
Overkill Solar- 4S, $115 US
Ant 7- 16S, $76 US