Lithium battery system thoughts

[JackConnick]

I guess the first thing I'd change is the old Moto Alternator. They just don't put out much output. You can buy a delco 70amp for about $65 online.

I added a small solar panel on my dodger that does a good job of topping up my batteries while at anchor. The only issue is that it needs to be mounted somewhere where it can't be shaded. But for a racer, there's little weight added.

I had a starting battery in reserve (I have 2x Group 27 flooded) that I ditched when it went bad and just carry a lith starting pack for emergency starts if all goes bad.

Jack

[edwardc]

LiFePO4 Charging

One more reason I am looking at the DC-DC charger is a big lithium bank can look like a dead short and be very hard on alternators.

Quite true. My Balmar 612 alternator regulator has a feature called "Belt Manager". It lets you dial back the maximum output of the regulator. It was intended to stop belt squeal when dumping full output into a highly discharged lead acid bank, but it is just the thing for keeping lithium batteries from cooking your alternator.

And this provides a great excuse to segue into a discussion of LiFePO4 charging.

One of the downsides of Lithium batteries is that they are extremely sensitive to over (and under) charging. A single episode can destroy your expensive investment. This is why the selection of a reliable, programmable, and "bulletproof" BMS is essential.

On the plus side, the charging protocol is much simpler than lead acid. It almost entirely consists of what would be called "Bulk" charging in Lead Acid. In this phase, you let the charge source provide as much current as it can until the cell voltages come up to the "knee" on the charge curve, around 3.35V per cell (this is why its important to have matched and top-balanced cells, so they all arrive there at the same time). At this point, the cells are within a few percent of being fully charged. Technically, you could go through a brief "Absorption" phase at this point until the charging current drops off to a low value, But its really not necessary, and skipping forcing the cells up to that last few percent will increase their cycle life.

And then you simply STOP CHARGING. No "Float" mode! Keeping Lithiums at a float voltage will greatly decrease their cycle life!! This is why you need a highly programmable charger, as many of the smart chargers' preset LiFePo4 mode has a float voltage programmed! If float can't be disabled, you can set it to a lower voltage than Bulk so it will never be a problem. This allows the charger to still carry some of the loads without charging the battery further.

The only tricky point then is to select a point to "Re-Bulk". Because of the almost flat charge/discharge curve, voltage does not provide a reliable indication of State-of-Charge (SOC). You need something that does "Coulomb counting" (Amp-Hours) to properly select when to go back into bulk.

And there isn't one stock answer for this. It depends on your use-case. From the standpoint of absolutely maximizing battery lifetime, a re-bulk point of 50% SOC or less is probably right, as Lithiums kept at 50% experience very little degradation.

On the other hand, when used as a "standby" power source that needs to be "at the ready" at full capacity, a value closer to 80% or 90% is probably indicated.

Sailboat usage is neither of these cases exactly, so I need to do more research to come up with a good re-bulk target.

My Balmar 612 can do some, but not all, of these algorithms. In particular, it uses voltage combined with simple timers instead of coulomb counting to determine when to end various phases. The WakeSpeed 500 is the only alt regulator I've found that includes a current shunt for doing current (and Amp-Hour) based calculations. And as a bonus, it includes a CANbus input, and The REC-Active BMS I have knows how to talk to it, including instructing it to shutdown gracefully in the face of an impending BMS disconnect.

Stay tuned for further developments.

[joe_db]

Looking through Amazon's offering, you get kind of a matrix.
BMS max current is 100 amps or 200 amps.
Some have low temp cutoff, some don't, and some say they do and don't.
Some have heaters and some don't.
Some have a bluetooth connection to the BMS.
Some have a plug in data link to the BMS.
Some have both and some have neither.
All of them will need Sawzall surgery to extract the cells if the BMS ever dies.
Some of them have USA based distribution and some don't.

Right now I am not even thinking about 100 amp max batteries nor ones without connection to the BMS.
My other cheap-ish play is to buy Doucan (sp?) cells and a decent BMS. That at least allows for a BMS swap if needed and a way to bypass the BMS in extremis.

[edwardc]

Joe,

That's a pretty accurate summary of most of my last year's research. It's why I chose the raw-cells/external-BMS path.

There's one other item for the list that Rod Collins (Mainesail) thought was pretty important. Some (most) BMS's use MosFets for switching, and others drive an external contactor. The MosFets tend to limit the currents to 50-100 amps, and are the weak link due to heating. The contactor versions can easily handle up to 500 A without breaking a sweat.

I selected the REC Active BMS because it tics almost all of these boxes. In addition, it is in a water-resistant housing with a water-resistant single connector. This makes it very simple to swap out with a spare in case of failure. And as a bonus, the manufacturer has been very helpful answering questions, responding within a day in understandable english despite being in Postojna, Slovenia. And the US distributor has a fairly active users forum, and also responds quickly.

[tritonyawl2]

Great Explanation

[QUOTE=edwardc;127610]Wow, a subject that's near and dear to my heart! I'm in the middle of what's turned into a years long project to convert our boat to Lithium batteries. Its a DEEP rabbit-hole!

That was a very helpful explanation of why i can't use a Lithium battery as a replacement in my car or my boat. Requires an entirely new system.

Thank you!