I am getting ready to build a 40' cat and I am researching electro propulsion. My delema is how many battery banks do I need. Of course there has to be one for the house electrical but do I need to separate two others specifically for the two motors or will one big bank do the job? Or should I have one for the house and one for the motors? Keep in mind I will have solar and a backup generator. At this stage I'm planning for 3-5 hours of run time but that will be extended by the solar and generator in hybrid mode.
I always like to have a backup. When I converted my Nonsuch 30u to EP in 2008 I kept my house bank separate from the propulsion bank. I did this because:
1) I wanted to have two ways to power emergency equipment like a VHF radio, AIS etc.. and not have all those eggs in one basket. I was an early adopter and it was a leap of faith to go with EP at that time.
2) Since the house bank was already wired up it was easier just to leave it in place. "If it ain't broke don't fix it" was my thinking.
I would still do the same today. Though I do occasionally tap into the EP battery bank when at anchor to power refrigeration and/or laptop use if the weather has a few rainy days and drops solar charging for the house bank enough. Nice to have the extra power and not have to fire up the generator.
As far as having two EP banks. I think you could go either way. having two separate banks would give you some redundancy should one bank have some type of failure. My experience is with AGM batteries not lithium but, I find the EP system to be very reliable.
With two separate EP banks you could:
1)Operate off of one and keep the other in reserve
2)Operate off of one and use the generator, solar to charge the other bank.
EP systems are very easy to change to change and modify. You could start with two separate banks and easily make them into one big bank or visa versa. Depending on how you find which way works best for your needs.