Monday, July 13, 2020

Lithium (LFP) battery update

I kind of left everyone hanging last year when I put my LPF batteries in service.  In fact, I don't think I even reported that they had gone on line.  Well they did, and they have been running flawlessly for the past 18 months.  I absolutely love them, and will never build a lead-acid house bank again unless there is some extenuating circumstance.  They just work SOOO much better.  No more worrying about when they last reached full charge.  No more long drawn out absorption cycles, and when needed, no more long, lightly loaded generator runs.

"Andy" asked a couple of questions in the comments for this post on the BMS design, and I thought I'd answer them here since they are of general interest to anyone messing with this stuff.

The first question was about cell balancing, and whether I had automated it somehow.  The answer is, just the opposite.  What I actually did is completely counter to every guide on how to commission and run an LFP battery bank, but I did it in the name of science.  I had heard recommendations stressing the importance of regular balancing, making it sound like the batteries would fail spectacularly if you let them drift even the slightest.  And I heard reports of people who had run for months and months without having to re-balance.  So I decided I'd find out for myself.

But before getting in deeper, let me review quickly what this "balancing" thing is all about.  To make any battery bank, you need to wire some number of individual battery cells in series to get the voltage you are looking for.  My system is nominally 48V.  Lead battery cells are 2V each, so you need 24 of them in series to get 48V.  LFP cells are 3.2V, so you need 16 of them in series to get 48V (or close enough).

Batteries get charged by the current that you push through them, and when a bunch of cells are wired in series, each gets the same amount of current.  If all the cells are at exactly the same charge level, then as you apply a charge current, they will all fill up by exactly the same about, and they will all reach full charge at exactly the same time.  That's in a perfect world, and I haven't seen one of those yet, so let's look at reality.

Reality is that variations in each cell's construction and chemistry causes each to charge/discharge by slightly different amounts even though they see the same current.  So in reality, when charging, not all cells will reach full charge at the same time.  With lead batteries, we just ignore this and keep on charging because a little overcharging is pretty benign.  So the cells that reached full charge first you just allow to overflow a bit until all the other cells are full too.

But LFP cells are not at all tolerant of overcharging, and can actually be damaged quickly and irreversibly if over charged too much.  There are a variety of ways to deal with this, but my system simply monitors individual cells, and sends an alarm when any cell starts getting too full.  And if any really get too high such that damage may occur, my BMS disconnects the batteries.  As you identify cells that are more charged, or less charged than the average, then you need to do some combination of adding extra charge, or bleeding off some charge from individual cells.  That's balancing.

OK, now that you understand balancing, what did I do.  Well, nothing.  I just put the batteries together and started the system.  Then I monitored it looking for alarms on cells that were too high or too low.  I got none.  In fact the cells all operated within about 5mv of each other, and that continued for about the first year of operation.

But slowly I did see some drift, and after a while I got my first alarm.  My chargers charge to a nominal cell voltage of 3.45V, and my "high voltage" warning alarm goes off at 3.50V.  No action is taken other than to send me an email.  Now, 6 months later, I get warnings pretty regularly, especially if it's a really sunny day and the batteries have charged quickly.  Here's one from today:

Bank  55.519V, On line

Min Spread Max
Present  3.424  0.087  3.512
Min/Max  3.110  0.148  3.555

Cell Voltage Status
1  3.441 Ok
2  3.455 Ok
3  3.455 Ok
4  3.428 Ok
5  3.500 High
6  3.460 Ok
7  3.466 Ok
8  3.446 Ok
9  3.488 Ok
10  3.509 High
11  3.500 High
12  3.486 Ok
13  3.424 Ok
14  3.484 Ok
15  3.459 Ok
16  3.512 High

This is at it's worst, just before the charger switches to float.  This whole process of warnings triggering, then going away lasts maybe 2-3 minutes.  As you can see, cells 5, 10, 11, and 16 are high.  Also notable is that cells 4 and 13 have fallen behind the pack.  These are at the point where I want to take some action.

You can balance my adding charge to low cells, removing charge from high cells, or a little bit of both.  My plan is to address this step-wise, and the first will be to add charge to the low cells (4 and 13).  That will catch them up with the rest of the pack, and will also partially address the cells that are high too.  With the low cells brought up to around the nominal end-of-charge voltage of 3.45, the overall pack will reach it's final voltage sooner, and stop charging before the high cells get as high as they are now.  Then I'll monitor, and continue to add charge to the lower cells until they all get back into a reasonable range of each other.  Now there's no reason why I couldn't drain some charge from the high cells, except I have a charger, and not a suitable load.

One question is how much charge to add, and I really don't know.  Each "cell" is actually two 180Ah cells bolted together, so 360Ah.  My power supply can put out 6A, so I figured I'd start with 6A for 1 hrs, so adding 6Ah.  Then see how much it changes.

OK, that's a long answer to Andy's first question.  Now on to the second, which was whether I am doing any sort of Ah counting to figure out state of charge.

The answer is no.

First, counter to what everyone says, I think battery bank voltage is a sufficiently close indicator of SOC.  People say it's a much worse indicator for LFP vs Lead, but I disagree.  My lead bank was 50.4 volts when fully charged, and 48.0V when is was 50% empty and the generator started.  That's a spread of 2.4 volts.  In contrast, my LPF bank is full at 53.6V, and ready for recharge at 50.4V.  That's a 3.2V spread and is 30% more voltage swing than lead.  The difference, though, is that in the mid area of charge, the LFP voltage doesn't vary much, so there isn't as much differentiation between 60% and 50%.

But the other side of it is, who cares?  In many ways, I don't care what my battery charge state is.  All that really matters is that if it gets low, the generator will start and run through a recharge cycle.  And if they get full, the chargers stop.  And all that is automatic.  So there is no action for me to take based on charge state, so why worry about it.

What I DO monitor are my every 6 hour health reports like the one above.  I do keep an eye on the current voltage, current spread in cell voltage, highest cell voltage ever seen, lowest cell voltage ever seen, and largest cell voltage spread ever seen.  That tells me about the health of the bank, which is much more important than it's current full/empty level.

Friday, June 5, 2020

Extended running gear

Early on in our build I talked about a few of the things we are doing a bit differently, and one of them is to extend the running gear.  When the 64 became the 68, the position of the prop and rudder weren't moved.  The 68 has enjoyed great success so this clearly isn't a problem, but early in our process I got to talking with Jeff Leishman (Nordhavn boat designed) about this and he suggested we could extend the gear by adding a strut.  They have done exactly this when going from the 86 to the 96, and the results were quite good.  By pushing the rudder aft it gets it further out of the "shadow" (turbulence) of the keel and into smoother flowing water.  The result is less cavitation, less turbulence, greater efficiency, and less vibration.  After a bit of hemming and hawing, we decided to go ahead with it.  Moving the prop further aft also results in moving the rudder further aft, and that should improve tracking and reduce susceptibility to broaching.  All sounds good, but of course only time will tell the full story.

Only a week or two after our visit in December 2019, the strut and shaft were installed.  The strut is Y shaped, anchoring to the hull in two locations, and the shoe below.  Normally the prop is right where the shaft exits the keel, but in this arrangement it's between 3-4 feet further aft.

Extended running gear "Y" strut

Extended running gear with shaft installed

Inside the laz where the steering gear is located, you can see the rudder post and supporting trestle is back nearly against the transom.   The structure forward of that (closer to where the picture is taken from) is the structure that the steering cylinders anchor to.  This arrangement makes it a bit easier to fully utilize the space in the laz, so another benefit to the change.

Thursday, June 4, 2020

I'm still kicking, and the boat build is still progressing

Wow, it's been 18 months since my last post.  How time flies.  And if you ever wondered whether volunteers are a good substitute for employees, now you know.  If blogging were my job, I would have fired myself long ago....

Here's what's been going on.

For the first 18 months of the build (June 2018 through December 2019) progress was painfully slow.  There were some issues at the yard and progress against schedules was at best 50%, and probably closer to 30%.  But the issues were identified and fixed, and much to everyone's delight things have come alive, and progress is amazing now in 2020.

Here's where things were at the end of 2019, which was my last visit.

In the first picture below you can see that the main deck is on, as is the pilot house deck and Portuguese bridge.

Here's the flybridge, still waiting to be installed.

 Most of the main deck interior is unfinished, but forward in the master stateroom there are some partitions, and furthest forward in the master head there is some joinery work starting to take form.

Master stateroom

Master head

On the lower level, the bunk room is coming along nicely.

Bunk room

Bunk room

Bunk room

Bunk room head

The utility room is coming along nicely too.

Utility room (laundry, freezers, storage)

And progress on the guest stateroom, but not nearly as far along as other areas.

Guest stateroom

Main engine bed is built and engine is temporarily set in place.

Main engine (Scania DI-13)

Wing engine bed built and engine set in place

Wing engine (Deere 4045)

20kw generator bed is built, but generator not yet installed.

20kw generator bed

In the laz, the 12kw generator is in place, but otherwise it's just piles of wires and hoses.

Laz and 12kw generator

And the Pilot house is yet to begin.

Pilot house

Here's the starboard walkway leaving to the Portuguese bridge, with the wing station in the corner.  This remote operating station provides an excellent view down the side of the boat for docking.

Wing station

On our last day we did a road trip to the granite yard and someone found granite that she loves.  It's almost the complete opposite of what we originally had in mind, but this was love at first sight.

Granite selected

Since this last visit, progress has picked up enormously.  I was scheduled for another visit in April but that of course got cancelled.  It's killing me to no be able to visit, especially with so much being accomplished.  But we are all dealing with the situation as best we can, and I think doing pretty well, all things considered.

Launch for sea trials is scheduled for September, but my best guess is that it will end up in October, and perhaps later depending on when a transport ship is available.  Launch doesn't happen until there is a ship scheduled since the boat goes directly from sea trials to the ship, and can't be left sitting around waiting.  But either way, I think there is a good chance we will have a boat in Florida by the end of the year.  Sometime in December is my best guess.