Designing a LiFePO4 Battery System Part 1 - LiFePO4 Cells

Our boat has a 12 V system, and we wanted at least 400 Ah of total battery capacity in the service bank.

For LiFePO4 cells this means four cells in series (resting voltage 3.3 V * 4 = 13.2 V) will do the trick to achieve roughly 12 V system voltage.

After carefully measuring the dimensions of the empty battery box on our boat we tried to find a match with the existing battery package sizes Winston produces.

Unfortunately the 200 Ah (and all larger) batteries were just a tad to high for the box, so we chose the 100 Ah model. 4 cells in parallel (called "4P") will yield 400 Ah @3.3 V, and four of these packs in series ("4P4S") provide the desired system voltage, while comfortably fitting the available space.

There will also be lots of free space left in the box, as the 16 cells are physically much smaller than the old Lead Acid battery bank. The space will be needed for additional electrics such as distribution, fuses and contactors.

The advantage of this 4P4S configuration is that the individual 100 Ah cells are quite small and lightweight (only 3.4 kg per cell), and installing them in a battery cage will be much easier due than heavier and bulkier larger models.

In addition, should one cell go bad later, the whole pack can be rewired to a 3P4S configuration, still yielding 300 Ah (with 3 unused cells as backup, then).

The downside is that a some pack-internal must be done to connect the batteries properly, and physically fixing the batteries in the boat might be a bit more work.

Lithium batteries need to be "balanced" in order to work properly. Without going into to much detail this means that it is beneficial for the health of the pack to keep all cells at the same state of charge in order to avoid over- or undercharging individual cells when approaching the top or low end of the pack state of charge. Protecting the cells from such a condition is the job of the Battery Management System, but it is beneficial to bring the pack in a balanced state to avoid premature shut-off of the BMS.

In the top picture you can see the test setup I created at home to balance the cells after I received them via mail order from our dealer LiTrade. Some initial balancing on the individual cells was done by the vendor before delivery, but this will typically not be sufficient after a 4P4S pack has been composed of the cells by the customer.

This is not an article about Lithium battery balancing (there is plenty information on the Internet). But I will summarize some of my steps here.

I decided to perform a top balance, because charging the battery to 100 % State of Charge happens much more often on a sailing boat than discharging the pack completely (in which case a bottom level balance would be more appropriate - such as in electric vehicles).

I arranged the cells in the final configuration, connected them electrically and labelled them so I could reproduce the configuration later. Then I discharged the pack a bit (about 10 Ah) to make room for top balancing.

Once this was done I recharged the pack with a constant 14.4 v (3.6 V * 4) voltage at a charging current of 5 A while keeping a keen eye on the individual cell pack voltages with a digital multimeter.

Whenever I noticed a disbalance (higher voltage of a single cell than the other cells) I manually used a 1 Ohm 25 W resistor directly on the cell terminals to temporarily shunt some charging current "around" the cell which had reached the ceiling of 3.6 V. I repeated that process with until all cell packs were at an identical voltage of 3.6 V and stopped the charging process. The whole process took about one hour to complete.