Battery storage and self-discharge

My lithium battery bank was removed from the boat in late October, 4 months ago. At that time, the batteries were charged to 100% and then disconnected and brought home. While they should be able to over-winter in the boat, I reasoned that 1) they were expensive to purchase and 2) they are rather light and relatively easy to carry. Spending the winter in a shipping crate in a basement, where the temperature swings are perhaps 5•C would certainly be better for them than in a fiberglass hull where they will be subject to wild swings in temperature, occasionally dipping into the colder side of -40•C. 

Anyway, the batteries were disconnected in to their 4-cell packs and brought home. They have seen no attention since. Last night, I decided to take a few voltage readings. I was pleasantly surprised to see that all 16 cells measured withing 0.001V of each other - 3.328V. Now I don't pretend that my cheap digital multimeter is capable of proving true readings down to mV precision, but since I was after relative voltage, the fact that it gave me the same reading for each cell gives me confidence in the data. They all settled on a reading of 3.328V. There were a couple of cells that hesitated between 3.327 and 3.328, but they did all ultimately settle on 3.328V. According to the storage discharge curve, that puts my cells around the 85% charge point. Right on track for 20% self-discharge over the 6 months of storage. 

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...

Spring 2015 Projects

Spring projects, 2015

So, at the risk of eternal optimism, here is a quick list of my projects for the spring of 2015.

Integrate an embedded wifi router into Fugu.

The VoCore is a very small (and very inexpensive), 25mm x 25mm router that runs OpenWRT (an embedded linux, specialized in wifi routers).  It is a standard Linux distribution, however and allows one to build all manner of existing software to it.  I already have a Brookhouse iMux, however it only support one wifi device and only runs in Ad Hoc network mode.  It is intended as a wifi cable replacement  The OpenWRT-based Vocore runs a full wifi router suite, with DHCP addressing, NAT, etc.  The VoCore does use nearly 1W of power compared to 0.2W for the brookhouse, but on a 5000Whr pack, this is negligible.  Especially on a day-sailor / weekender.  Here is what the VoCore will do for me.

Add K-Plex to VoCore

One of the tools I am adding to the VoCore is K-Plex, an NMEA routing package.  In other words, this little router will distribute NMEA data to any device connected to the VoCore wifi network.  In short, my iPhone, iPad, navigation laptop and any other wifi-equipped device my crew brings aboard, simultaneously.

Connect battery SoC data to VoCore

The VoCore also has an available USB port.  To this I am connecting my EV-Display battery state of charge meter from CleanPowerAuto.  The EV-Display outputs a fixed-format character string each second that includes a lot of information about the battery pack, such as voltage, temperature, amps, estimated amp-hours remaining, etc.  I can write a parser for this information on the VoCore that will allow three things:

1. Create NMEA strings using the XDR sentence for things such as voltage and current.  The hope is that these could then be sent through K-Plex to be displayed on a laptop or iPhone and become part of a general boat data distribution system.
2. Create a series of web pages that can be accessed by any client connected to the VoCore network.
3. Push this information to a server I control on the Internet.  This will allow me to check battery status remotely.

 Repackage the lithium battery pack

At the moment,  Fugu's propulsion battery is arranged in two columns of 8 cells, which are actually groups of 4 cells strapped together and strapped to a supporting plank.  This was so that I could easily (relatively easily) place the cells in the boat or remove them for winter storage.  After one season, I have come to the conclusion that I would rather have a single consolidated battery pack.  I am going to build a box out of 3/16" marine plywood and a 1/4" lexan top.  The cells will be in a 3-wide (3x 142mm) by 6-long (6x 61mm) arrangement.  Being a 16-cell pack, that will result in 2 5-cell columns and one 6-cell column.  The extra space of the two non-existent cells will be filled with the following:

• the 225A Class-T fuse, required to be within 7" of any source of power
• The EV-Display battery monitor
• The miniBMS.

Hoisting the pack in and out will require some engineering.  I will build a gantry-jig from some pocket-door track and cars, as well as some small pulleys (4:1 reduction) and line.  With this, I can lift the ~130lb pack in or out of the engine bay by myself.  To get it in or out of the boat, I can use the boom and main sheet block and tackle.  Failing that, two people could handle it easily enough.  Either way, this would be best done at the dock and not with the boat out of the water, in her cradle.

Running and nav lights

In 2013, I completely re-rigged and re-wired my mast.  However, since my electrical system was not yet installed, I never got around to filling the old through-deck holes and drilling the 5 (FIVE!) new through-deck fittings for the re-wired mast.  This must be done this year.  At the same time, the wires for my red/green running lights corroded through at the end of last season.  So these wires must be replaced.  It will be an uncomfortable install since the wires are run through the pulpit tubing, into a junction box in the anchor locker (?!) and then along the underside of the deck of the boat back to the main panel at the nav desk.
A number of years ago, I also acquired a new LED stern light.  this needs to be installed, and why not, rewired as well.

Install the pressure water pump

I divesting myself of all things diesel, I traded a Racor fuel filter for a pressure water pump.  I really ought to install this for this season.  That will also require that I reinstall my 90L flexible water tank and new hose to my galley and head sinks.  I will also have to install a pressure faucet in the boat. I will keep the foot pump, but I may switch it to raw-water.  Who knows.