More on battery repackaging

I continue to think about battery re-packaging...
I still definitely want to do one large battery box that contains all cells, main fuse, BMS and SoC meter.
I may even add a battery disconnect.  I recovered one when I stripped all the old electrical stuff from Fugu a couple of years ago.  That way, I can completely disconnect the battery pack when I need to service the propulsion electrical.  The only live terminals would be within the closed battery box.

To pack the cells in any kind of battery box will require taking into account one critical thing -- the cell terminals are not equidistant between rows and columns.  Terminal-to-terminal distance is 61mm in columns, and is 41mm between columns.  This means that I will need to do one of two things:

1. Obtain or fabricate some new cell interconnects with a 41mm hole spacing.  This will go with my existing supply of 61mm hole spacing connectors.  The advantage here is that it favours minimal pack width.
2. Add 20mm spacers between columns of  cells.  The advantage here is that I only need one connector size, regardless of how I interconnect the cells within the pack

But now, I am wondering if a 3x6 cell grid is the way to go.  Perhaps a 2x8 cell grid is better.
In a 3x6 cell pack, I would need two spacers which would add 40mm to overall pack width.
In a 2x8 cell pack, I would need only one.
Advantage 2x8 cell pack.

If opted for using a different cell connector (41mm instead of 61mm spacing), here is how it pans out:
In a 3x6 cell pack, I would need at least 6 such buss bars.
In a 2x6 cell pack, I would need only one.
Advantage 2x8 cell pack.


The 2x8 cell pack is considerably narrower than the 3x6 cell pack, but is 122m longer.
The dimensions of the available space are irregular to say the least.
So now, I really need to decide on which pack better fits the available space.
Both packs will have to be located behind the stuffing box in order to allow access to it for inspection and service.  I think I will just have to mock up both packs from cardboard and test fit them.


Regardless of pack geometry, I feel it is important to keep the positive and negative terminals of the pack relatively close together (but not so close as to pose a short-circuit risk)

Decisions, decisions...

Electric update

My 2014 launch was delayed until late August for several reasons, not all in my control.
The batteries didn't arrive until early July, then came  the cell-balancing process which took close to a week.
Then there was the hemming and hawing about battery pack geometry and how to secure the pack in the boat.
Then of course, once that was decided (but still isn't settled...) I had to fabricate the supports and install everything.
All of this had to happen without taking much time off work and without ignoring my family.
Ultimately, I did my first electric propulsion tests in mid-Septemer and moved from the service dock a couple of weeks before my birthday.  Fugu's first electro-sail was the weekend of my birthday, with my very patient family, who were rewarded with a glorious day-sail and lunch at anchor, out at the sandbar.

Real performance vs projected.

It turns out that my real performance is just on the better side of what I projected from Eric's numbers from Serenity.  So my range at 4kts is just about 15nm, 25nm at 3kts.  This is more than satisfactory.  For getting out to the club races, I can go nearly up to full throttle and make 5.5kts, do the race and have ample power for calmly coming back to the club -- we usually sail back anyway.

Varia

Since I switched from a left-hand to a right-handed prop, the prop-walk pulls the boat the other way, and unfortunately, this is away from the slip.  Still getting used to that one.  Maybe I'll practice backing the boat into the slip.  Other factors affecting prop walk are the pitch of the prop.  Since my prop went form a 12x8 to a 12x14, the amount of prop walk is significantly more.  On the plus side, the low-speed control of the shaft RPMs is amazing.  I can throttle right down to about 100 motor RPM which gives about 40 shaft RPM.  Ultimately, docking will become easier, once I acclimate to the new methods.

Another neat thing is the awareness of your speed you have with electric propulsion.  Since I can theoretically empty my battery bank on a single outing, I do pay more attention to my throttle settings.  With my old 1-cylinder diesel and even only a 3-gallon tank, I could go all day at full throttle and not worry about running dry.  My electric setup includes a battery state-of-charge meter that counts amp-hours consumed and is aware of the size of my pack and the usable amp-hours.  So I can show remaining usable amp-hours and match that with the day's sailing programme to ensure that I don't get stuck out there.  Slowing down a single knot is enough to nearly double my effective range.  It's pretty neat.

I am still nervous with leaving my battery pack charging unattended.  So when I get back to the dock, the first thing to do is plug in.  We can then take our time putting away the boat, perhaps have dinner at the club and then check in on things before disconnecting the charger and heading home.  A full charge takes about 5 hours from completely empty (20% state of charge).  Most days, I would come back with about 60-70% SoC so charging up takes significantly less time.  A benefit of LiFePO4 batteries is that cells are just fine with being left in a partial charge state.  As a matter of fact, they actually don't like to be kept at 100% charge all the time, so float-charging is a no-no.  The lithium batteries stay in "bulk charge" mode for much longer than lead acids -- up to about 90-95% charge.  As a matter of fact, when equipped with a well-balanced pack and a battery management system (BMS) that monitors each cell, the consensus is moving towards "dumb" constant-current/constant-voltage chargers that are controlled by the BMS.  This amounts to an output voltage configurable power supply where the constant-voltage is set to (cell-count * cell-charge-voltage) volts, in my case, 16 x 3.5V = 56.0V.  The BMS or some other monitor on the power supply would then wait for the current to drop below a threshold like (cell-capacity / 80), in my case this would be a CC of 100Ah / 80 = 1.25A.  Once the current going into the pack drops below the 1.25A level, the power supply could be cut and the pack would be considered essentially fully charged.  If along the way, any individual cell gets above the CV setting, the BMS cuts out the AC current to the power supply and throws an error.

Winterization

Since I have a lot of money invested in my battery pack, I am not keeping on the boat over the winter.  Each 4-cell group weighs only about 30lbs and can be lifted out easily enough, even at arms length.  The procedure for storing lithium batteries is to charge them up, then disconnect them and ignore them for 6 months.  So they are currently sitting in a crate in my basement, waiting for launch-day.  They are in their 4-cell groups and the BMS modules are still attached.  Current draw from these modules is very low, on the order of about 5ma.  So total draw from my 100000mah cells will be: 5ma x 24 hours x 183 days = 21960mah or about 22ah from my 100ah cells.  Regular self-discharge is also about 3%/month so I expect to lose another 20% in 6 months.  My pack should come out of winter at about 60% SoC -- just fine for lithium storage.

2014 was a short season.  Here's to hoping for a long and adventurous 2015 sailing season!

93 days to launch!