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Friday, September 28, 2018

Lithium-based Power System

There is so much to say on this topic that I struggle with where to begin.  It's been an exercise in drinking from a fire hose, and everywhere I turn there is another hose blasting away.  So I'll just pick a starting point, and describe what our plans are for 6837, and how I'm proceeding from here to there.  That should set out the context, and I can write other articles later digging into particular aspects of Lithium power systems.  Make sure you have read this Lithium Battery Backgrounder before reading this article, or some of the terms and concepts may not make sense.  I'm hardly an expert, but learning fast, and my goal will be to distill it down to something more digestible, without at the same time dumbing it down.   So here goes...

First a little personal background. I'm a retired electrical engineer/computer science person, turned exec, but still a geek at heart. Our boating is long distance/remote cruising with extended periods of time away from civilization, so dependable systems are vital, and repair/workaround essential when the inevitable issues arise. We are building another Nordhavn and the plan of record is to use LFP,  but before pulling the trigger on LFP for the boat, I'm building a proof project in a slightly less demanding environment to test and assess LFP viability for the boat.

Now I already need to pause and explain something.  You are probably wondering what "LFP" is.  It's short hand for a particular type of lithium battery, specifically LiFePO4.  There are many different types of lithium batteries, and LFP is the only on that I (and most other people) consider suitable for use on a boat, mostly for safety reasons.  Some other time I'll dig into the different types and their characteristics, but not now.

I mentioned a test project, and that will be to build an LFP system for an off-grid house. In nearly every way the requirements are the same as the boat, but less consequential if there is a failure, and a lot easier to get parts etc when needed. If nothing else, the shipping address isn't changing all the time :-)

With that preamble, here's my current thinking for the LFP sub-system:

- I am building the system myself. It's not that I object to the available pre-engineered BMS (Battery Management Systems) systems, but I'm very concerned about the long term viability of nearly all of the companies and products. When some part fails in 3 or 5 or 10 years, I think there is a good chance that any given company I might choose will be gone, and I'll have to buy a whole new BSM. Victron is about the only company that I think has long term staying power, but the cost puts LFP into a price range where it isn't worth it to me. I want something that is long term repairable and supportable and will live the life of the batteries. So the plan is to an industrial Programmable Logic Controller (PLC) with off-the-shelf modules for monitoring and control.  Pretty much everything in the modern world is run by PLCs.  Manufacturing, lighting controls, building environmental controls, elevators, and many of the control systems on larger ships.  They are made by many very large companies, have highly standardized interfaces and protocols, are incredibly robust and reliable, and are relatively inexpensive.  And best of all, a lot of much bigger, much more important customers will be complaining way ahead of me if there are problems.

- The PLC "BMS" will be a monitoring and fail safe system, but not a balancing system. Instead, I'll follow the approach of operating the batteries away from their SOC end points, or "knees" in the voltage curves. When monitoring suggests an imbalance, I will have to respond manually to re-balance.

- For batteries I am using CALB CA180s. CALB seems to be the most reliable vendor, but it's admittedly a low bar, and I am open to alternatives. The 180 cells seem to be the largest that are readily available. Bigger cells means less paralleling, few interconnections, etc. The 400s are appealing, but don't seem to be available

- I'm planning on 360AH of LFP capacity operating at 48V, so about half of the FLA usable capacity that I currently have. That will last me 1.5 to 2 days without sun. If the batteries get too low, the generator will start and recharge in a little over 2 hrs. The generator may run more frequently, but for a lot less time in each run, and less in total. Right now, a full recharge of the FLA batteries takes just under 6 hrs.

- The battery configuration will be paralleled pairs of cells, wired in series for 320ah @ 48V.   Short hand for this configuration is 2P16S.  That's "blocks" of 2 batteries in parallel, designated by "2P".  Then 16 of those "2P" blocks are wired together in series, designated by "16S".  Put it together and you have 2P16S indicating 2 parallel battery groups, wired 16 in series, for a total of 32 batteries.

The other battery wiring option is two 48V strings of single 180Ah batteries (16S), with isolation switches for each string. So two strings - effectively 16S2P - totaling 360Ah at 48V. This later arrangement would be to provide for string isolation for diagnostics, and to be able to run on one string if the other is out of service.  I might convert to this arrangement later on, and it might be the preferred arrangement for the boat for increased fault resilience.

- For monitoring, I plan to instrument voltage and temperature on each battery, or pair of batteries if they are paralleled. Absolute values, and differences in values will lead to various warnings, alarms, and fail safe actions.  All of the thresholds are are adjustable so I can fine tune as I learn.  It will also let me start out with very conservative operation, then expand a little bit at a time while monitoring how everything works.

- I only have two charge sources to contend with at the house, and both are programmable. I plan to program each to essentially be a two stage charger, with a bulk voltage to perform charging, followed by a float voltage down at the battery resting voltage. There will be no absorb stage, with an immediate transition from bulk to float. I expect float will require some tweaking to prevent excess battery cycling and get solar to carry the loads once batteries are charged, but while solar power is still available.

- The only fail safe I'm currently planning is a main disconnect contactor that will be controlled by the PLC.  The only time the batteries will disconnect is after all alarms have been triggered, and if the situation remains uncorrected and the batteries are facing imminent damage because of over charge or under charge.  This should never happen under any sort of normal operation.

- I'll also have to adjust the Automatic Generator Start (AGS) stop/start.  The start voltage will be adjusted to match LFP voltages, but starting will probably have to happen more quickly once the low voltage point is reached.  LFP battery voltage drops very quickly when they reach their fully discharged range, and it will be important to get the generator running and charging started before the batteries plummet.  With the FLA batteries currently, I look for a low voltage level lasting for 15 minutes, or maybe even 30 minutes (I can't remember exactly).  With LFP I'll probably trigger a start after 15 seconds of low voltage.  It takes 45-60 seconds to get the generator on line, so that should be enough.

There must be more, but that's all I can think of right now.

Next up will be a bunch more detail on the BMS that I have built so far.

I welcome comments and suggestion?