VSR FET Temp starts out at 50C!

[Soggy Paws]

We've been running two VSR's on our Catamaran for a couple of years.  Recently we put in LIfePO4 Batteries and hooked up a Solid State Relay (Fotek SSR-10D) in the Ignition Enable line of each one, to be able to cut off the charging (from the BMS) if something goes high on a cell.

We've finally gone out cruising (after a year and a half of lockdown/yard work), and when we enabled the Port Alternator, we smelled smoke within about 30 seconds, and immediately shut down.  Fortunately I was logging the text output of the controller and we can see in the log the Fault was a FET Temperature fault.

The log shows that the FET temp was around 33C until we went into Bulk Mode, and then it went to 50 and then -99.  Then the FLT;41 was logged.

Once we anchored, just looking at the regulator, I noticed that the Bat- wire was disconnected. (maybe due to me pawing through the wires trying to hook up the USB cord).

We reconnected that and with the alternator (engine) off, I plugged in the USB cord to fire up just the regulator (powered from the USB).  Now the FET temp is a constant 60C, without any charging going on.  We didn't have time to fool with it, so just didn't use that engine until we could look things over.

Firing it up today (with just the USB cable, no alternator running), the FET temp is a steady 50C, starting from the second AST we received (-99 in the first AST).  So we unplugged the USB and waited for it to cool off so we could check wires and things.  Lo and behold, it didn't cool off.  We finally disconnected (first) the Bat+ and Bat- (which didn't change things) and then the Alt+ and Alt- wires. As soon as the Alt wires were disconnected, it started to cool down.

So, have we fried our regulator??  Any advice on troubleshooting the issue would be great.  We have modest electrical skills and less modest electronic skills.

Thanks,

Sherry & Dave McCampbell

s/v Soggy Paws, in the Philippines

[Al Thomason]

Oh no,

 

Will – will pass on, BAT- disconnect, there is a high likely the regulator is damaged – specifically U7.    My suggestion:  Remove the reg and ‘Bench Test’ it using the setup in the reg guide.   A current limiting power supply is your friend here.   Look at the input to U7 and the output, I think if you are lucky you will see a PWM wave at the input and perhaps just U7 needs to be replaced.  But have seen ones that a bad ground flows back, even to the point of burning a hole in the CPU…

 

From the sounds of it though, the FETs might have been damages as well   --- ya, Floating Ground is the worst that can happen, sorry to say.

 

Give it a bench test and see what you learn.

 

-al-

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