Thrusters and engine starting operate in pretty much the same way. They require a short burst of a lot of amps. LFP isn't designed for that. LFP can give up and accept larger current for recharging and discharging, but that's numbers like 100amps, 250 amps. Much larger than FLA and AGM can do. The Yamaha F300 requires 680 amps to start. Thrusters vary on size and such. Ours is small, but still fits the category. The windless is wired off the house bank and runs on LFP as the largest 12volt load on my boat (as far as wattage goes).
In my painful battery journey (I sulfated my house bank 90 days after taking delivery of my boat), using AGM, before I converted to LFP... I did join my thruster bank with the house bank. When I'd hit the bow thruster my cabin lights would dim from voltage drop. This is the reason they have their own battery bank.
When shopping for LFP, one of the key differentiators in LFP battery cost is how the top of the battery is made (the internal bus that connects all the cells together). What's it rated for, in amp draw. The less expensive batteries are usually 100amps, max. The Lithionics battery I have is rated to 150 amps continuous draw (from full to empty). 230amps as the max discharge current. I can pulse to 1200 amps for 1 second (think emergency engine jump-starting). But these numbers are specific to the Lithionics 320amp-hour GTX12V320A-E2107 battery.
LFP usually has minimal voltage drop, but it still does have voltage drop when hit with large loads. I use Sensar Marine for remote boat monitoring. It keeps track of house and engine voltage. I had to ask them for a software change which they implemented in short order for me. Whenever voltage drops below 12.6, they would send an alert to my phone since LFP usually is well above 12.6 volts. A weekend on the boat, and I got 11 alerts letting me know coffee was being made, my lunch was in the microwave, or dinner was in the microwave. It's common for me to see 12.2 volts from voltage drop running 1000-1500 watt appliances off the inverter which end up drawing about 140 amps at 12 volt DC off the house LFP battery bank. Imagine what the thruster(s) would do.
This is why neither Lithionics or Ocean Planet Energy would approve a design with the thruster load moved to the house bank. Thruster and Engine banks remain AGM on the boat. I put a DC to DC charge (engine to house and engine to thruster). I didn't need a DC to DC charger for engine to thruster, but I installed one so I have one on the boat to use as a spare should my house dc to dc charger ever fail.
Here's an illustration. In this photo we're making coffee, which is about 1500 watts ran off the inverter. Once coffee is made, voltage will pop up to about 13.3volts.
When we run high loads like this and am down around 30-40% SOC, I've hit 11.8 volts and had the inverter beep warning of low-voltage. 11.5 volts is when the inverter would shut off due to low voltage. I've never hit 11.5 volts, and I've ran my battery down to 20% SOC many times.
View attachment 27199
43% SOC, 11.9 volts, 150 amp draw.
View attachment 27200
www.letsgochannelsurfing.com
