Warning its a long post but it contains all you need what to do with an
alternator if you switch to LFP bank. These are the 4 ways how to do it.
why we need to care about the
alternator is that former lead had a resitance in high kilooom figures but new LFP has an very low internal resistance of a few milliohm close to a short and the alternator
regulator therefor runs it wide open till it self distructs.
the key to get the alternator under control is temperature control, and the way to do this is
current output control, the rest will follow automatically. With lead due to high resistance the alternator is limited in
current delivery but with LFP not as described above and how much you need to limit current depends on each individual install and specs of your alternator.
Import spec is current
delivery and what stator temps it can withstand, normal one max 130 degrees celcius, then the isolation lacquer of the coil will be destroyed. 110 degrees if max output with reduced lifespan 95 degrees for optimum lifesapn and max output compromise.
You have 4 ways to do that.
1) modify your existing alternator and add an external
regulator WITH temp control
sensor as if you modify you loose the internal one. The
sensor is placed on the case best somewhere in the pulley facing front part but next step measure the temp difference between stator temp (the hottest part that needs limitation with an IR temp tool) and what your sensor measures, substract that
delta from 100 degrees celcius (or from 140 degress if you have a seldom alt that has the 170 degrees lacquer) and you have the temp parameter to put in your regulator, it delivers you the max current in your
installation dynamically. In case of a sudden disconnect you have to make sure the BMS cuts the field wire first and add a surge protector.
The way to get around modifing your alternator is to use Nordkyn VSR200 regulator that works via the sense wire without any modification to alt. VsR200 works with a lot alternators that have internal temp regulation like Mitsubishi 115A found on all newer
Volvo D1/2/4 and partly 6+small
Yanmar or the Valeo 125A found on various manufaturer. Be careful it must be originals the aftermarket ones mainly don't have this temp regulation!!! In a well ventilated
engine room the 115A Mitzi delivers 85A continuously with the VSR200, in reality most heavy duty small case alternators in that spec range won't do more or maybe max 20A+ for 2000Euro invest...more you only really get with large case alt that most likely won't fit in the original location and be up for 3000Euro+ if you don't
DIY everything.
2) if your alternator has internal temp regulation (like original!! 115A Mitsubishi or the 125A Valeo) then you can add a splitting diode like
Victron argofet and connect to
lithium house and to a lead starter
battery. The lead present the dump load in case of a disconnect and also limits current output of alternator and the voltage drop on the diode makes sure that even at 14.7V alternator output only 13.8V or less gets to
lithium. The internal temp protection protects your alternator and dials it back when needed. The Mitsubishi 115A internal one is set to very conservative 85 degrees. Like this you can charge a lithium to about 95%, the rest your
solar can do from time to time. The alternator get cut off from bank when BMS cut the charge bus due to end of charge parameter reached or you can add a relay just to do that for the alternator, would add a manual switch to control the relay so you can disconnect it from the LFP manually too due to any reason, eg just to charge the starter faster.
3) connect your alt to your starter and use a DC2DC
charger. If you have no glue about real temps at the max output capabilities of your alternator i advise max 40% of its rating and add 10A charge for the starter, that together shouldn't exceed 50% of the rating. This limits an 80A alt to 40A and most likely avoid burning it out. I say most likely because you have actually no glue at what temp your alt is really working and if you are in the limits or actually could go to 70% of rating because your
installation is well ventilated and the alt has enough self couling capacity. This 50% rule/adivise is based on vast experience of lot installer and the typical alternator specs used and takes bad ventilated
engine rooms into account but if you are in the approx 5% where this
rule won't
work if a) your alt has an internal proper working temp control you are save, if b) no internal temp control like most older alternators that brings me to solution 4.
4) this is for all alternators without or insufficent internal temp regulation: install a temp probe at your alternator and log temps. Use a very hot day, all close up like its in normal operation. Now log in idle, your motorsailnig
RPM and typical
motor rpm at 33% or 1/3d current load of alternator capacity, so a 90A one on 30A and log temps at startup and all 10min till 2h of operations. Measure at 2h point with an infrared temp tool the stator temp so you get
delta of your probe and stator. Add that delta to the temp results. Did your alternator get to 100 degress, if yes thats the current you need to derate it too. If not add gradually 10A load and do the same process then above. If you have derating you can do 2 ways. Way 1 buy a DC2DC
charger that fits the derating, thats 100% safe compared to 3). Way 2 is the
cheap but complex way: additionally measure field current/voltage at cold startup with
motor rpm and at the 2h operation point. Thats the delta you have to derate the field current by a resistor, means you have to inline the resistor (i used 10w one) into the field wire and isolate/ seal it 100% with shrink tube. That has the disadvantage that during the cold startup phase of 15-30min depending of your installation less output of the alternator but its safe in permanent operation. Then use a splitting diode with lead starter like in 2) or connect it directly to LFP with a surge protector device and a diaconnect relay in the dield wire for the BMS. The resistor in the field wire limits the alternator also in max volt put out and limiting it to 30 till 50% should limits voltage to below 14.4V but you need to check that. This needs a BMS that controls the charges sources and just disengage the alternator at 95% SoC like eg a electrodacus, REC, Batrium, TAO BMS.