Charging & Monitoring with LiFePO4

LiFePO4 batteries require very different charging parameters than lead acid. Bulk charge only, then stop. Do not float charge. Do not store them at 100%. They like to stay between 98% and 10% SOC, and storage around 50%. This implies your charging sources (alternators, solar, wind, hydro, shore power) need to be able to bulk charge and then stop, which means they must all be programmable. Also, when you are returning back to your slip, you want to be able to cut off all charge sources so so that you end up leaving the boat with the cells less than completely full.

Sparrow’s Charging & Monitoring Diagram attempts to take all this into account.

As for the batteries themselves, I started to look into what the Battery Management System (BMS) actually does for the LiFePO4 pack:

  • High voltage charge cutoff– Sparrow’s charge sources will all be programmable – the alternator regulators, the solar charge controllers, and the hydro converter/controller – and as a backup I will install high voltage cutoff relays. Finally, the battery monitor will monitor bank level voltage with a high voltage alarm, so the BMS functionality is utterly redundant.
  • Low voltage cutoff – The battery will be monitored with a low voltage alarm. The alarm is preferred to just cutting off the battery so the autopilot doesn’t just stop, which potentially could lead to really bad things (crash gybe?). An alarm will give me time to deal with the situation proactively. Note that this use case is unusual – most sailboats will be left unattended for some length of time with electrical loads running (say, refrigeration) and in this case one would want a BMS or low voltage relay to cut the loads to protect the battery.
  • High temperature cutoff – I will monitor the temperature with one or two separate temperature alarms such as the Extech TM20.
  • High charge current protection – the solar and hydro sources will be well below the battery charge capacity. They max out at roughly 50 amps each, while the bank can absorb 200amps without difficulty. Alternator charging alone will be below battery charge capacity. I will just plan to not run the engine with hydro & solar pumping at the same time.
  • High discharge current protection – there simply aren’t enough loads – even with everything running – aboard Sparrow for this to happen.
  • Cell balancing – Cells will be initially top balanced, and I will build a board with  cheap voltmeters on each cell so I can quickly see if there is a problem anywhere. If there is a problem, I can disconnect one of the 4P 100Ah “super cells”, manually connect the cells in parallel to rebalance them, and re-install the super cell. I’ll just lose 100AH for a while. I may also carry 4 active cell balancers and connect them if a super cell goes out of balance. After Sparrow returns, I plan to balance the cells manually once a year & things should be fine. In the meantime the cell voltages are being monitored and there is a high temp alarm if something goes crazy.
Much more detail here: Sparrow LiFePO4 Management Notes This all appeals to me more than a “black box” BMS as the battery current is not running through any device that could introduce failure, and I have much better control and detailed monitoring at the cell level. However, Sparrow’s use case is unique in that the pack will basically be attended 100% of the time. If left unattended for any length of time, a low voltage cutoff would be desirable, and not everyone would be willing to manually balance their cells every so often. So when I get back I may add a low voltage cutoff relay, or even a BMS depending on how things go.
This analysis has lead me to purchase raw cells, and all together the 400Ah LiFePO4 bank with monitoring, and backup protection relays will come in well under $2,000 with shipping. Call it 320AH usable, meaning at least 600Ah of AGMs would be needed for the same effective capacity. The AGMs would cost at least as much up front with far less life, less charge acceptance, less stable voltage, and weigh 350 lbs more.

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