For what it's worth:
1) The need for a BMS to manage cells in a boat application is pretty exaggerated. I can speak to this at length, but suffice it to say that the typical boat with lithium batteries probably is only seeing 20-30 charge cycles per year, some much less (e.g. mine at maybe 5 charge cycles per year). Consequently, there is very little cycle-influenced drift to deal with.
2) Allowing a BMS to be powered and monitoring your cells full-time carries significant risk. Death by BMS is a very common thing. Don't believe me? Look at the self-discharge rate for New In Box 12v lithium battery replacements out there with built-in BMS in the sealed case. The shelf life is stated as 2 years (IF the pack was fully charged first) and most of this decay is due to 24/7 powering the BMS system. With those units, once the BMS detects the voltage has reached some low value, the BMS takes the battery down for good…so much for being a green alternative for lead acid. On my boat, I have 20 BMS cards that "can" manage 240 pairs of lithium cells. I say "can" because I only ever power them up maybe 1 or 2 times per year to check the cell status and to see if there's been any change to the lowest cell. I do engage them maybe annually or biannually to actually "balance" the cells.. I have about 5 pairs of cells that were noticeably lower than the rest (one of them 300+mv low). For those cases, I directly charge up the cell pair slowly thru the cell monitor lines to bring them up to the rest.. This way when I do engage my BMS cards I don't have the scenario where 235 cell pairs are being bypassed for hours just to bring them down to the state of 5 other pairs. I neither totally trust the BMS cards 24/7, nor do I want to over-stress them.
3) A friend of mine locally has put on maybe 5x the number of charge cycles as I have in just the past 2 years---to date, we have not checked his cells since we put his batteries into his boat and he does not use his BMS cards. Given the state of his cells when he got them and knowing how mine have behaved, I suspect they are still all tracking within 50mv of each other.
4) Read some of the latest study on Lithium Ion batteries. The truth is that you can bring these things clear down to 0v and they are not necessarily dead. One study reported a series of tests where they found that yes, if you reverse-biased a cell by as much as 1.0v there was irreversible chemical damage, but that to that point the typical Lithium Ion battery they tested not only recovered well by recharging but they held their charge, saw unchanged capacity and behaved normally. The danger there is that not all these are the same quality or same battery chemistry. In my case, my first nom. 42v lithium module was brought down to 3v over 1 month of accidentally being drained by its charger. My heart sank. But after bringing it home, charging it carefully and letting it float, it acted as normal and stored rated capacity. Cycle tests and 3+ years later and there's no issue with it.
Bottom line for me: Absolutely no regrets in going with lithium ion. Lots of lightly used or NIB car battery options out there these days. Mine are Enerdel modules from THINK electric cars. Lots of ways to go.
-Myles Twete, Portland, Or.
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
Sent: Thursday, March 7, 2019 8:57 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Re: Sailboat Props - Data Please
Excellent point, Fred.
My understanding of the advantage in USABLE capacity of LiFePO4 batteries over FLA or AGM batteries is the same as yours: 80% vs 50% respectively. Of course, that's the guideline to assume in order to achieve a reasonable number of cycles for each type. But LiFePO4 batteries provide a big advantage there, too, according to product literature, giving twice as many cycles as FLA or AGM when discharging only to 80% and 50% respectively.
So, to get the same usable capacity you'd have to use more FLA/AGM batteries. And to get the same number of cycles, you'd have to use twice as many sets of batteries. Looks to me as though, over the lifetime of the LiFePO4 batteries, they are actually more economical, even if the initial expenditure may be twice as high..
The fact that LiFePO4 batteries take up 2/3 the space and less than half the weight of FLA/AGM batteries seals the deal for me.
All that said, I'm told by the folks at ThunderStruck that most sailboaters implement their systems using FLA/AGM batteries owing to the lower initial cost, and likely also to the technical simplicity. I've read more than a few comments in various places expressing discomfort with needing battery management and battery balancing systems and all that, and/or a fear that making a charging/discharging mistake will spell doom for the lithium batteries, while FLA/AGM batteries can recover from one or two overcharging/overdrawing errors.
The diesel engine with accouterments that I'll be replacing weigh perhaps 375 lbs, but 8 - 6V 225Ah AGM batteries weigh 528 lbs! Add the weight of the drive motor and reduction gear, etc. and I'd be pushing 600 lbs of replacement equipment, while also taking up more of the engine compartment space that I do today.
Not very elegant! So for our situation, sailing on the Great Lakes where there are shore-power equipped docks in most transient ports, the advantages more than outweigh the disadvantages of having to take more care with the LiFePO4 batteries.
Posted by: "Myles Twete" <matwete@comcast.net>
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