[C320-list] Battery Capacity calcs

[Troy Dunn]

Daryl

Interstate claims 195 Ah for that battery.  You can find the full specs for
the SRM-4D here:

https://www.interstatebatteries.com/products/srm-4d

Just click on the specifications tab.  So now you have 390 Ah as your full
house capacity.  Now let’s check the rest of your math.

The 35% rule that Mr. Cote cites in his reference videos is designed to
market the carbon foam and LiFePO batteries to a broad market.   There are
some key assumptions he is making in order to come up with that figure so
you need to really understand those assumptions before jumping to the
conclusion that it is an appropriate figure for you to use.

First assumption is that you can’t draw an FLA battery below 50% or you
will significantly decrease SOH (state of health).  True.

Second assumption is that your charging circumstances are such that you
will seldom get back to 100% SOC (state of charge).   Could be true let’s
stick a pin in this one for a moment.

Third assumption is that during the time YOU own the boat you expect to
have to replace the batteries multiple times...not once more but at least
twice more.  True for you?

Fourth assumption the boat will somehow have a higher resale value if you
convert to carbon foam or LiFePO...ummmm....doubt it.

So, assuming you are planning to keep the boat a long time let’s visit one
more item.  AGMs.   AGMs are nice, they are 'spill proof', and virtually
maintenance free.   The trouble is that they are not super forgiving to any
kind of abuse, over charge them or accidentally run them flat and they can
rapidly lose SOH.  Carbon foam appears to be the only valve regulated
battery that may solve that problem or at least significantly cushion the
blow.

For me it all boiled down to that second assumption.  What would my house
bank utilization really look like?  How often do I motor?  What might a
realistic SOC vs. time curve look like for my house bank?  How big a house
bank could I fit in the battery compartment? Would I ever truly pull my
boat 'off the grid' or would I be using the charger at least once a week?
For me it sorta goes like so...

I'm not a live aboard, so my boat will always be at a full state of charge
when my cruise begins.   I have a 464 Ah house bank comprised of 4 x 232 Ah
GC2 batteries.  This leaves me with 232 usable Ah on day 1.   If I'm out
for a three day weekend, I can run on just the house and never start the
engine.  No solar or wind.   If I start the engine my alternator could push
a full .2C into the batteries, but I have the alternator backed off such
that I've never observed greater than 85 amps going to the batteries.    On
days where I do have to motor for a few hours my bank is generally above
90% SOC in less than three hours.   Even on perfect sailing days there
tends to be about 1 hour per day of engine running to get to and from
anchorage.

I think if you are a live aboard or your boat is on a mooring and you
struggle to get back to full SOC then carbon foam is definitely worth a
look.    If you are full time live aboard and cruising long
distance...maybe LiFePO4....maybe?   I still think it’s a reach.

Good luck.  If you do go to LiFePO4 keep in mind you need completely
different charging profiles for that which may mean new other stuff
depending on what your current charging situation is....

Troy Dunn
Hull #514
Wonky Dog