I agree that marine use is closer to RV/conversions that we would use. But as far as battery usage I don't really see the difference? I mean honestly - all batteries do is store energy, and supply energy when required. You can look into the whole usage equation, all manner of current draw over x amount of time, ad infinitum... but when it comes down to it it's still just energy in/energy out.
Okay, I'm reading through the Compass Marine article now - thought I would take notes as I go... I am going to just put some thoughts down to try and make sense of this. So I may correct myself as I read on. Hopefully I don't just come out of this more confused.
I did not know about compression packs for lithium batteries/cells. I would always have strapped the cells together to hold them rigid in some manner, it doesn't look like any compression is actually applied, just that the cells aren't loose. Which I would have done anyway, but good to know the reason for it (other than not having cells loose & bouncing around everywhere).
BMS
The article states that the main purpose of Battery Management System is to avoid over charging and over discharging at a cell level. No mention of balancing being a primary function of BMS. In my previous plan I was thinking - ok, let's just set the correct safe voltage cutoff levels on the chargers? Then there can't be any high/low voltage events? He does mention a good point about the BMS being there to catch any system errors, for instance if the charger reboots and loses it's programming. Yes, that would be very bad... But what if the BMS fails as well?? You can keep building protection into the system, redundancy on redundancy on redundancy... and where do you draw the line? He goes on to mention a manual reset contactor as the last ditch/ejector seat protection for the cells. Except that this also acts on individual cell voltage. Well that does seem like a good idea, if we have this then why the need for other "insurance policies" for your insurance policy??
I'm still thinking at this point - "ok, why is all the cell-level monitoring needed? If there were no current draw from individual cells, then they would all remain at the same voltage??" So I haven't yet seen a reason to monitor cells. But an ejector seat contactor on the battery bank seems like a good idea. And then we have it...
"I DO NOT LIKE OR BELIEVE AUTOMATED CELL BALANCING IS NECESSARY FOR A WELL EXECUTED FRACTIONAL "C" SYSTEM !!!!"
Shunt-balancing, it doesn't sound too good to me, and he admits that it's rarely done correctly:
Picture four 5 gallon buckets filling with water for a moment. When one becomes full, ahead of the others, a small spigot dumps some of that excess "overflow" into the less full buckets but if the fill rate is fast enough it can't handle all the excess "flow".. This is sort of how "shunt balancing" or a diversion of current balancing system works.
However it is possible to do it "properly" with a very well designed system, at
low current. Which is what how his setup works, except with a manual balancing procedure. But then - guess how many times he's had to balance? On 500+ cycles his system has shown
no balancing necessary! Side point - if there is the need to balance cells the preference is to do this on a bench using a power supply with independent voltage and current control - and this would need to be done every 5-6 years for a coastal cruiser, or 1-2 years for a full time cruiser; based on his battery bank setup & performance. (Not a difficult process at all.)
So then I come back to my original question - why is cell balancing necessary??? Well... even his experience shows that it absolute is not needed! But this comes back to the most important point, no argument on this - that it is essential to have a properly designed system, which in my mind still comes down to the basic principle to bottom balance the cells once upon installation, and then never, ever draw current from any individual cell. He does mention numerous times the need for "safe and sane charging levels". Absolutely - no doubts there! And again these safe and sane charging levels are the same as recommended by EVTV. Compass Marine (CM) notes that high charging voltages above 14.2V (3.55V per cell) are what result in the need for balancing, and it becomes a catch 22 cycle. The voltage limit agrees with what EVTV recommend - 3.5V.
One thing that stands out is that his setup and recommendations apply to "fractional C" systems. I'm not 100% sure on what this means, but I can only assume that is means that charging & discharging are done at less than 1C (a fraction of 1C). So for a 400Ah battery bank, in order to classify it as a "fractional C" system you would not charge or discharge at rates greater than 400A. Is that right? If so, that would most likely apply to all RV requirements - but it is where EV usage may differ? Perhaps in discharge, maybe if the system is not designed correctly (too small of a battery bank)? But probably not in charging as there are no high current charge stations available. So this leads me to believe that the recommendations from EVTV are also based on a fractional "C" system? (Perhaps? Maybe only partly, for charging?)
Interesting tidbit now - CM mentions one case from the EV crowd:
I was conversing with a gentlemen who emailed me who is an avid DIY EV enthusiast. He, like me, read and read for years before jumping in. Once he got his cells he began experimenting in his home shop to confirm, and put what he was reading, into practice. Long and short is he chose not to have balancing BMS and relied solely on one bottom balance when his cells were new. Yes he did have LVC and HVC protection in place just not a BMS that balances. He also chose safe & sane charging voltages.
At his first EV car show all he heard all day was that he was going to ruin his cells; "You have to have a BMS that balances!", "You must have cell balancing!", "Hey guys here's a fire waiting to happen!". One particularly obnoxious electrical engineer berated him for nearly 40 minutes while himself admitting he was on his second set of prismatic cells in a few years. "Wait until you get a few hundred miles on those and they are junk." To which the guy responded "A few hundred? These puppies only have 33 THOUSAND miles and are just getting broken in.". The electrical engineer walked off with his tail between his legs..... A lot of LFP is simple common sense...
So - that's got do make you think, hhmmm??
Cell Balancing
It seems that in choose to use bottom or top balancing, the choice should be made based on whether you want to prevent over discharge, or over charging? Which would explain why the EV crowd use bottom balancing, because they use nearly, or all of their battery capacity and want to make sure a cell doesn't go negative. Whereas the RV crowd would be much more likely to use a bit here and a bit there, and would almost always have some charge source active (PV, alternator charger, shore power, etc.) Ok - so now I can see why top balancing would have an advantage in RV usage, so there is less risk of over charging. But even then, as we've seen in CM's experience - no balancing was needed anyway.
But I still don't understand how this mystery of top/bottom balancing actually works - if you ensure all cell voltages are equal at the top, then you discharge/charge the battery as a whole - the cell voltages should rise & fall equally. Also if you ensure all cell voltages are equal at the bottom, they should remain equal as the battery is charged/discharged as well?
Can anyone provide an actual explanation as to why there should be any voltage divergence between cells if they have been correctly balanced?? 
In my situation I will only ever get up to 0.5C maximum continuous usage rate. But I would be close to fully discharging the battery bank on a fairly regular (daily) basis during summer. Well at least in theory, as I'm sure that PV and alternator charging would mean it is never reaches 100% DOD. This is what makes me wonder if the EV method is better for me? But there's still the question of what
actually causes cell voltage to diverge? Well I'll keep reading and hope to find out...
Alternator Charging
CM gives the recommendation of Blamar MC-614 alternator charger, no better charger available for LiFePo4 batteries. Reason being that it has a dedicated voltage sensing wire which carries zero current so it can provide an accurate voltage reading (provided that it is wired correctly). Not sure if the Sterling charger has this - but something to check!
There is also a lot of talk about burning out alternators if they are running at maximum output bulk charging lithium batteries. This seems like a non-issue for RV builds, I guess this is where the equipment differs? But the Balmar charger even has the option of limiting alternator current. Note to self: check whether this is required.
Dedicated voltage sensing wire is also important on AC battery chargers, any units such as the Victron EasySolar inverter/charger that I am planning to use. However there are a number of manufacturers who fail in this basic requirement for a lithium charging system. CM names and shames the failures:
- Mastervolt
- Magnum
- Xantrex
There are only two manufacturers who "get it":
- Victron (Victron I/C's have dedicated voltage sense terminals right on the main unit)
- Outback (Requires FLEXNET DC & MATE Remote Control)
I knew there was a reason I had a really good feeling about Victron!
(Did not know that Outback is an American company. Thought it was Australian - "outback Australia" and all. Never heard anyone talk about "outback America". Haha
)
How Cell Balance Diverges
Okay, I said I might keep reading on and may correct my earlier comments - so it looks like I have found the answer to "what causes cell balance to diverge?"
Here is how I understand it:
The severe "upper knee" and "lower knee" high/low voltage curve of the lithium charge process means that a small variation in cell voltage at the "usable" flat & stable curve range is greatly exaggerated as the cell voltage moves into the upper knee, because as soon as one cell enters that range the voltage will increase much faster than other cells that are still in that "flat" part of the charge curve. And so you have one cell whose voltage ramps up to possibly (likely?) dangerous levels while the others are all safe, and the battery pack can register a normal, safe voltage.
Image:
http://www.pbase.com/mainecruising/image/155043664
So it follows that best practice involves limiting the charge voltage at 13.0-14.0V. Thus avoiding the likelihood of cell divergence. CM does state that if people kept out of the upper knee when charging, there would likely be no need for a balancing BMS, just HVC and LVC. Looks like the EV crowd don't have such wildly different ideas after all...
Capacity Testing
The case is made to do a bench test on your lithium cells, firstly to set a baseline before use - it's difficult to program an Ah counter when you have know idea what the actual capacity is. And secondly, in order to measure the capacity at regular intervals (yearly for example) - otherwise you will have no idea if you started off with less capacity, or your usage & mistreatment of the bank caused premature capacity loss.
You simply need a load, a way to track Ah's, and a way to track individual cell voltage so you can stop the test when the lowest cell drops to 2.8V. Sounds like a sensible thing to me!
Summary & Conclusion
Sorry for the wall of text, but I needed to try and make sense of all this. Well that Compass Marine article was quite informative!! Definitely worth a good read. So I've now come away from that with a few key points:
- A corretly setup lithium system is vital to long battery life
- Correct setup can mean nothing more than bench balancing once every 1-5 years depending on usage, as well as high & low voltage cutoffs
- Dedicated voltage sensing wire
- Do not over charge, do not over discharge
I don't think I've missed anything there? It really can be as "simple" as that!
I know that the article still recommends a BMS as insurance for HVC & LVC, but even after reading that info I still like the idea of minimising parasitic loads on the cells and ditching the BMS/cell monitoring. And he admits that he has not needed to balance the cells once in 550+ cycles. In combination with benchtop balancing once every year or two for a high usage bank, this seems perfectly reasonable to me.
The main question that remains is whether top or bottom balancing is going to be better for my particular usage. But this is enough typing for one post, I shall return...
