Drop-In LiFePO4 Batteries – Be an Educated Consumer
- Thread starter Maine Sail
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Did you read the article I posted? Functionally all generators work the same, a coil of wire moves through a magnetic field. Sometimes the magnetic field is static, sometimes the wire is static. The external alternators on cars and boats have a static magnetic field and spin the wire coil. On motor cycles, it is the opposite, the wire coil (stator) is static and the magnetic field is rotated by putting magnets on the fly wheel or crankshaft. The difference is significant.
In an alternator the out put is controlled by varying the field current, on a motorcycle with a fixed stator the current is controlled by the speed rpm of the magnet, higher rpms, higher output. To regulate the current to the battery the rectifier provides a connection to ground so that there is always a load on the stator and the voltage spike won't occur. An alternator, like ours, does not have a constant load, so when the load dumps from the BMS or for some other reason the voltage spikes causing the diodes to heat up and burn out.
Interesting useful discussion, just one somewhat unimportant point to make.
Rectfier diodes have a forward current rating where they can handle a lot of current but they also have a reverse bias voltage rating. That reverse voltage might be 400 volts and if this is exceeded, the diode breaks down and current flows. Remember power is current times voltage so at that high voltage, it doesnt take much current to produce damaging power. Its the big voltage spike that reverse biases and causes breakdown in some of the diodes in the rectifier that causes the damage.
This is correct. BMS dump should really be called BMS disconnect since its just a switch that disconnects the actual battery from its terminal. If the current is being produced in a coil and the load is completely disconnected, the current instantly goes to zero. The basic inductor equation was given in the quote and the rapid current change is a large di/dt producing a large voltage spike.
Rectfier diodes have a forward current rating where they can handle a lot of current but they also have a reverse bias voltage rating. That reverse voltage might be 400 volts and if this is exceeded, the diode breaks down and current flows. Remember power is current times voltage so at that high voltage, it doesnt take much current to produce damaging power. Its the big voltage spike that reverse biases and causes breakdown in some of the diodes in the rectifier that causes the damage.
Just to reiterate, yes, that's an excellent and informative article. I'm saving it. Thanks for posting.
Any external multi-phase regulator can and will turn the alternator off. This happens every time that the regulator switches from absorption to float. On my last cruise, my battery topped out a couple of hours before we reached port and the regulator dropped to float. For those last two hours, the BMS app showed negative current as all loads were supplied by the battery and none coming from the alternator. The field voltage was 0-volts.
As for storing the battery at a lower state of charge that is not a thing that any BMS is designed to do because it cannot set voltage. It only turne the battery off when out of spec. This can easily be done by what you pick for your float voltage in your charging source. On my motorhome, when I am in storage mode, I set float to 13.15v which is about 50% SOC. Then when I am actively using the battery, I increase the float to my normal 13.4v which will maintain the battery at about 95% SOC.
You probably were not drawing power from the alternator because I suspect the battery voltage was higher than your float set voltage. For an FLA installation, the regulator is certainly not off when it's in float, it's just regulating to a lower voltage, and you can still damage diodes by a sudden disconnect.
Yes, I know that. I was imagining what a more highly evolved BMS and regulator combination would enable.
The regulator was not off, it is always on and monitoring the voltage. What I said was that because the set voltage was below the resting voltage of the battery, the regulator reduced the field voltage to the windings to 0-volts. Thes mean that there is no magnetic field, so the alternator is producing 0-current, and all loads are supplied by the battery. The same thing happens when my alternator case thermostat hits the high temperature set point and the alternator shuts off so that it can cool down. This would be true for FLA as well as my LFP batteries.
FWIW, is the current output of the alternator is very low, there will be no boom with a disconnect because per Ohm's law, v-ir so if i = 0-current, and r is infinity, v = 0-volts.
There are BMS's that will communicate with charge sources but that is not really needed as long as your charge parameters are set to appropriate levels. The job of the BMS is to be a safety cutoff in case the cell parameters are not met. The charge control is on each charge source and should be set to appropriate levels.
In the past year of living with 560Ah (7.2kw) on my sailboat and 920Ah (11.8kw) of LFP. I have had 1 over-voltage protection (OVP) cutoff event on the boat which was on the first charge for testing purposes. I have had less than 10 on the drop-in batteries in the motorhome which were all during startup when I was in the process of getting the cell balanced. On both, I have no worries about diode boom because the alternators are connected to both batteries for charging and isolated for loads. If the LFP ever cuts out, the FLA will still be connected.
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Yes, that's what I said.
That's not necessarily so, because we are dealing with reactive components, notably the inductance of the alternator, so you can't analyze this is purely resistive terms. If you suddenly disconnect the load from a circuit with current flowing and an inductive circuit element, you will produce a voltage spike; even at low current. How big a spike, and will it be big enough to damage diodes, I don't know.
Wow, you have a lot of LFP! I'm jealous. Not in my budget at this time, Someday, I hope.
The LFP's are game changing. We just got back from a weeklong cruise and spend 3-days on anchor and never needed to fire up the generator to charge the battery. On the last evening, I did run the generator for an hour because Sue wanted to use the microwave and my invertor is not big enough. I also wanted to make sure that the 2kw was big enough to handle the 60A charging I have. It worked fine for the chargers but could not handle the full 60A plus microwave. I reduced the charging to 30A and the microwave was fine.Yes, that's what I said.
That's not necessarily so, because we are dealing with reactive components, notably the inductance of the alternator, so you can't analyze this is purely resistive terms. If you suddenly disconnect the load from a circuit with current flowing and an inductive circuit element, you will produce a voltage spike; even at low current. How big a spike, and will it be big enough to damage diodes, I don't know.
Wow, you have a lot of LFP! I'm jealous. Not in my budget at this time, Someday, I hope.
We have owned this boat for 25-years and on every cruise I was needing to run the engine or generator solely to make power by the second day in order to keep the FLA batteries above 50% SOC. Every day of every cruise I would need to find a way to make at least 100Ah or more of charging. It was always my first concern of the morning and last of the evening, unless it was blowing a Hoolie.
I now have 3 cruises of 7-10 days each and have not needed to make power other than just what I got from moving from place to place.
The secret to no boom is some FLA that is connected into the charging circuit to give the current somewhere to go in case of a OVP cutoff. On the boat, they are isolated with an ArgoFET and on the motorhome, they are isolated with a combiner relay that only opens when a charge voltage is present.
I am not familiar with an ArgoFET
Is that correct, or did you mean it closes when a charge voltage is present?
The ArgoFET is a FET based battery isolator. It works in the same way as the old diode based isolator in that it is a one-way valve for current. The difference is that it uses FET's instead of diodes so there is no voltage drop when charging. Here is a link to the Victron site. mine is the 200-2 but they also have 3-battery versions.
On my motorhome, the charging relay is controlled by the Spyder system which is a CAN system for all 12v loads in the coach. For charging, it checks the voltage of the batteries vs the alternator and when the alternator voltage is something like 0.7v higher than both batteries, it closes the normally open relay.
Hayden - how, or did you, test the ArgoFET setup for a disconnect? I understand the theory, but wonder if it has passed a practical test. If you did test, how did you do it without risking the alternator?
Thanks,
Mike
The other big challenge to going with LFP is preventing the alternator from cooking itself in its own juice so to speak. None of them are rated for 100% full power duty cycle and will generate too much heat especially at lower rpms. This can be overcome easily by adding thermostatic control to the alternator. On mine, I did it by connecting the regulator to a digital thermostat that will turn off the regulator if the alternator case temperature gets too high. If you do not have an external regulated alternator, you can install a simple thermal switch inside the cast as shown in this video.
Let's Dissect an Alternator! - YouTube
Let's Dissect an Alternator! - YouTube
I have not "tested" it but have disconnected the LFP from the charge sourse several times to top of the AGM. In the JK app, I turn off charging which is the same as an OVP event so that the LFP is not pulling the output voltage down due to the large capapcty of the LFP bank. I run it this way for about ten minutes to allow the AGM to get topped off after starting so that it is not at a partial charge state.
The basic principle is that the AGM cannot be disconnected from the alternator by the one-way valve of the ArgoFET so there is always a 100Ah battery for a current soke. It is no different than changing the 1-2-B-O switch from both to 1 or 2, it just does it automatically but does not allow for back draining one battery to the other.
That aspect I have tested and when the engine is not running, there is no current flow between the two systems.
