DIY Electric Car Forums banner
1 - 8 of 8 Posts

·
Registered
Joined
·
201 Posts
Discussion Starter · #1 ·
I have a relay (KUHP-11D51-12) which says it has 20A contact ratings. This relay is used to connect my 12v aux battery to either the DC-DC converter (when the key is on and I'm driving) or to a battery charger (when the key is off and I'm charging). See attached schematic. In other words, the aux battery charging current (max 12A) can flow through this relay.

It was difficult to get a good picture but this relay failed. You might be able to see that there is a black wire inside the relay that is burnt (it's hard to make out through the smokey residue on the inside of the plastic enclosure). This fried internal wire connects pin 9 to the moving switch contact (the "arm" which can switch between 6 and 3).

Fuse F8 is a 15A fuse.

So there should never have been more than 12A flowing because that is the max rating of the charger. The charger didn't fail. The 15A fuse didn't blow. But the 20A relay is cooked.

Why is that?

I can try a 30A relay (T92S11A22-120) but I would have thought 20A was good for 12A max current.
 

Attachments

·
Registered
Joined
·
7,791 Posts
Hi hale,

I'm looking at this on a cheap phone in sunlight so could be missing something. But it appears you have the other contact in the relay breaking the high voltage side of the converter. I see only a 240 VAC contact rating on that model. So I suspect it is not DC rated to the level of your main battery voltage and it experienced an arc-over failure.

major
 

·
Registered
Joined
·
201 Posts
Discussion Starter · #5 ·
I discussed this with an EE at work. He's seen this before where, even though the ratings are not exceeded, if there is enough current such that the wire gets warm (which is the case here) over a long period of time the heat eventually breaks down the insulation. I've got something in the neighborhood of several amps flowing for hours per day, nearly every day, over the course of a year and a half. So there was never an over-current situation which is supported by the fact that there appears to be no indication of failure on the contacts. Only that small (braided) wire is cooked. So I'm going to just replace that wire with a few pieces of teflon-insulated, solid copper wire.
 

·
Registered
Joined
·
7,791 Posts
OK. Bigger device and inside, and nice new pics. So now I see your converter output runs thru that part of the relay. Your evalbum claims 53A rated. What's F3 fuse rated?

major
 

·
Registered
Joined
·
201 Posts
Discussion Starter · #7 ·
Ah, right, thanks for pointing that out -- it was so obvious I missed it (I was focussed for some reason on the aux charger only). Yes, the DC-DC output is rated at 53A max and runs through that same little wire inside the relay. F3 is 30A (and hasn't blown).
 
1 - 8 of 8 Posts
Top