Sail Life has a discussion of one option to solve some of your concerns. It’s a little mor DIY than drop-in batteries, but the BMS and external alternator regulator help solve the charging disconnect anyway -
LiFePo4 Refresh
- Thread starter jviss
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Thanks for the video. This guy mumbles and speaks too quickly for me, and he's a bit scattered. Also, his battery compartment looks like a bowl of spaghetti. But, there's some good info in there. I have to back up and replay several times to get what products he's mentioning.
I watch him every week so maybe I’ve gotten accustomed to his patterns. The battery system was also very much a work in progress at that point, thus the spaghetti. His finished work is much cleaner.Thanks for the video. This guy mumbles and speaks too quickly for me, and he's a bit scattered. Also, his battery compartment looks like a bowl of spaghetti. But, there's some good info in there. I have to back up and replay several times to get what products he's mentioning.
Here’s another perspective on blending lead acid with Lithium -
Your case of wanting to keep the Lithium bank at a partial SOC for extended periods makes blending with lead acid a little more troublesome. If you keep the lithium at a partial SOC the lead might be below full too, which would kill it pretty quickly. I guess you could look at the discharge curves and try to see if 80% on Lithium is acceptably close voltage to 100% on lead acid. If not you could treat the lead acid side as sacrificial, just there to protect the lithium, but that doesn’t seem very elegant.
REC and Battery Ballance X2 are two that I know of. I also understand that you can also get the control interface from most any BMS that has a RS485 port with some additional hardware, but I do not know how to do it.
Whoever came up with the term "Battery Management System" to describe the actual purpose of the device is responsible for a lot of the confusion here IMHO. The typical BMS is really just a CPS (Cell Protection System). It doesn't manage anything, it just keeps the cells from being put in a dangerous state. You could argue that balancing is management... maybe.
The BMS is kind of like the airbag in your car. Its just there to protect YOU (the Cell) from an external event, an accident (faulty alternator, solar charger). An airbag won't apply the brakes (but a Forward Collision Avoidance System might), it won't control your speed (Cruise control for that).
Now the alternator... Some BMS (Lithionics) have alternator field disconnects that will cut the field before disconnecting to protect the alternator. Balmar makes a device that will absorb the spike. There are many ways to skin that cat.
But all the features you want are available. The Multiplus has programmable assistants that can control relays based on a variety of inputs. You could use this to activate a water heater. The BMV712 has a relay available. You could use this to enable/disable the charge bus based on SOC. The Cerbo has a generator start/stop relay. Attach a USB cellular dongle and you can control your system from anywhere you have internet via VRM (Victron Remote Management)... no subscription fee either...
I would always advise a reserve battery bank. Even with lead, you should always have a backup. For me its the AGM start batteries. I have them connected to essential loads via a battery combiner. If for some reason the LFP disconnects, I'll get an alarm, but lights, autopilot, GPS, AIS, etc... will all still have power.
I am thinking a (relatively) large LFP bank, like 400+ Ah, that's kept at 50% SOC when I'm on the mooring, 80% when I'm on the boat, and 100% as I'm about to head out to another place; and, a fairly small LFP bank, maybe 40Ah, that's always kept at 80%, charged from the house bank, and used for starting.
It makes you think that maybe the house should be 24V and then stepped down via DC-DC converters for the main panel, charger for the starting bank, and so on.
It makes you think that maybe the house should be 24V and then stepped down via DC-DC converters for the main panel, charger for the starting bank, and so on.
You may have trouble getting the discharge current you need for starting from a Lithium bank that size. LiFePO4 is not especially well suited for high discharge current. For example even to 400AH bank suggested earlier says in the spec sheet it has a maximum output current of 300A for 3 seconds (200A continuous). Using that same ratio a 40 aH LFP bank will not come close to what you need for starting.
I think it depends on your usage and how long you’re comfortable running the engine to charge. Between refrigeration and electronics burning 100 AH per day is pretty achievable. That’s an hour of engine running per day to recharge. A bigger bank gives you more days before recharge is needed, but eventually you still have to make up the hours. Spend a few months on the boat and those engine hours add up. If you’re motoring between places anyway or you don’t mind the engine or generator hours then no big deal. But some choose to offset those hours with solar instead.
I highly doubt that is the case! I'm going to do some research, but it strains credulity that a 40Ah LFP bank won't start a garden tractor diesel! I have a hand-held LiOn pack that will start my diesel!
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As another data point, a BattleBorn 50 aH battery has these specs:
My 3YM30 starter is rated as 90 Amps no-load, and 250 amps loaded maximum current.
It’s not an LFP limitation, it’s the BMS. Here’s a 60ah start battery:
DL+ 12v 60Ah Dual Purpose 1000CCA Starter Battery Plus Deep Cycle Performance
Twoin pack, 12V 60Ah multi propose 1000 CCA lithium starting dakota battery plus for marine engine, automotive, deep cycle use. 4x longer 11year lifetime.
dakotalithium.com
Thinking some more, there are LiFePo4 batteries designed for cars and motorcycles. I found one for a car with 900A peak cranking Amps, 600 cold cranking Amps (-20ºC). And, one for a motorcycle with 380 peak cranking Amps.
So, you can have a an all-LFP solution.
The only issue I can see is charging the starting battery to 80% when the house bank is at 50 to 80% would probably require a DC-DC charger.
So, you can have a an all-LFP solution.
The only issue I can see is charging the starting battery to 80% when the house bank is at 50 to 80% would probably require a DC-DC charger.
Interesting. Unless you have external regulation I think typically you charge the AGMs with the alternator and use the B2B for the LFP (mine is set up that way). The APD should protect against a BMS disconnect, but won’t protect against overheating the Alternator or overcharging the LFP.
Interesting, indeed. I'm new to the topic of LFPs, but I have some thoughts. I thought one of the great benefits of LFP was that it could take advantage of all of the output of high-output alternators, and in so doing charge up very quickly, reducing engine run time for charging. If you're charging the AGMs with the alternator you lose this advantage. If you're charging LFPs with AGMs, what advantage is there at all? You're just shuffling energy from one store to another with the consequent transfer inefficiency.
Well, the system should be designed to protect against alternator overheating, especially with a high-output alternator; the smart regulator should do this; and, the BMS will protect against LFP overcharging, which is where the BMS disconnect comes from, no? In fact, I'm told some BMS's, or systems built with them, can avoid the disconnect by reducing or ending the alternator output (via field current control, presumably).
Not so?