Originally Posted by
the evil twin
OK... few things to comment on in the quote.
At idle your Alt will be lucky to put out 40 maybe 45 amps so any load over that will cause the voltage sag you are seeing.
I reckon your Alt is fine, if you had dropped a diode/s on any phase then you wouldn't get 13.9 Volts at higher RPM.
On cold start at idle with glow plugs etc the load on the Alt is causing the voltage to sag to 12 ish volts due to high current draw.
The few revs generates a higher current output from the Alt so the voltage can now rise as well until the current draw is low enough for the Reg to do it's voltage limiting job at 13.8 to 14.2 nominal volts.
Cranker and Aux of the SBI12 will not be much of an issue unless the Aux is down on charge as well.
If the Aux is 80% or better then it won't pull much current.
If the SBI12 is energising within a minute or so and linking the Aux then the Cranker and Aux should be OK
If the SBI12 is energising/deenergising IE "cycling" then the Aux is drawing a lot of current so it is down>
The LiFePo current draw is limited by the DC/DC to 40 amps max BUT that is a fair wack if the battery is down even a little and the DC/DC wants its share of the load from the Alt as well as Glow Plugs, Cranker etc (etc meaning A/C fan, lights and anything else).
Bottom line is I reckon everything is probably OK and maybe do a recon with a smart charger or capacity check of the Cranker.
FWIW I used to try and have the same batteries (I like Caterpillar Batts) as Cranking and Aux and swap them across every 12 months or so and used to get 4 or more years life out of them easy peasy.
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