deckofficer
Senior Member
BMS would equal Battery Management System?
Yes
BMS would equal Battery Management System?
Here are my thoughts regarding using LI as your house bank (essentially that is what we are doing).
I like the idea of a BMS - it gives you information on all the cells. The main way to protect your investment is to keep the individual cells within spec this means running them down too low and/or over charging - EACH CELL.
The BMS we installed (custom engineered from Balqon) has a remote display at the helm. It not only shows the pack voltage (total battery, in our case 48 v nominal), but high cell voltage, low cell voltage, charge/discharge rate in amps, cell temp, total amp hours available (and as a percentage), and BMS temp.
Additional pages on the BMS allow for manually connecting/disconnecting the battery from the bus - connecting/disconnecting charging devices - connecting/disconnecting load devices. It also "specks" via CanBus to the Elcon 3kW charger.
But most importantly, it will disconnect charging devices should ANY cell approach its upper limit (in our case 3.8v) AND disconnect the battery from the bus should any cell reach 3.9v - after this, the cell starts to damage itself. It works in a similarly manner on the discharge side, with disconnecting loads .1 v before disconnecting the battery itself. This is the only way the battery can keep from damage unattended.
One big surprise to me (not, I'm sure to the EEs out there), was upon connecting the battery to the bus, we kept welding contactors closed, thus preventing the battery from disconnecting from the bus. Turns out the capacitors in the inverters were causing a voltage surge. A simple resistor wired between the contactors and a delay relay to close the on/off solenoid (after the capacitors were charged) solved the issue.
And, on a boat, you are most likely going to have an inverter. And will want some sort of safety mechanism to disconnect the battery. Reading total voltage of the pack will not do the job by itself - all it takes is one cell at 3.0v to keep the reading of a 12v battery at 13.4v even though one cell could be 4.0v while the other two are at 3.2v each. This is not your grandfather's LA battery!
I used to work for the oil industry, and believe me they are not nice people.
For the "soft" start:
1. 100w/10ohm resister.
2. Wire the resister from the battery side of the solenoid to a relay and then to the bus side of the solenoid.
3. When the BMS key switch is turned on, 12v closes the relay allowing 48v from the battery to the bus through the resister.
4. Install a time-delay relay that starts timing when the remote BMS key switch is closed (turned on). When this relay closes (about 5 seconds after the initial relay), it actuates the solenoid which then closes allowing full current from the battery to the bus.
5. Since I have yet to build in a delay for the DC/DC converters, the resister gets warm but not unduly hot. It stays in the circuit as long as the key switch is on (all the time unless working on the battery or circuits), but the current is flowing directly through the solenoid at this time, not the auxiliary resister circuit.
This "soft" start is working well.
A cooking surface that is flat and doesn't get hot, doesn't bake spilled food into nooks, crannies, and inaccessible areas, instead it is a single wipe across the surface and done.
When looking at the cost of a boat/house bank conversion to LFPs on a ROI cost comparison basis you "must" include total system impact. For example, the incredibly low resistance of the LFPs can destroy alternators, so having some means to regulate their output to control thermal shutdown or burn up must be factored into design and cost. Seems the current thought is to add a smart regulator with temperature sensor to alternator frame that gradually reduces output as temperature increases. This same issue becomes important if the BMS actuates the bank disconnect solenoid while the alternator(s) are outputting power...your diodes die. On my boat (in progress lfp conversion) this involved leaving the agm starters batteries in line as a buffer for the alternators. The use of our existing Victron inverter/chargers involved agonizing hours(read billable) of programming questions and attempts that eventually lead to a solution. In summary, this project has multi-layered costs well beyond the purchase price of the batteries and a bare BMS. These "unanticipated costs" have rendered any direct ROI benefit very upside down (not with standing indirect intangible benefits).
Ebaugh
In the PNW we average very close to 4 engine hours per generator hour with genset currently at 410 and engines at 1750 hours. Without AC needs our situation is very different. Maybe your lavish boating E needs lifestyle is unusual, or you should do more swimming to cool off. How do sail boaters without AC manage in your cruising grounds?
Do you have high capacity engine alternators?