Class T fuse smoking on high current charge

[Adamberti]

Had a few issues today, and just looking for a sanity check on what I experienced and observed and identified as issues.

I was charging my system from zero percent SoC today, and the Class T fuse started to smoke! I could feel the heat coming from the holder and the wire on the battery side of the fuse. At the time the Cerbo GX was reporting 90 amps flowing into the system - this was a first for me. The battery has never been this low and never had 90 amps going into it - but I believe it should handle that just fine.

This is a marine installation, and I've built a 12v 304aH battery from 4 Eve cells (LF304). BMS is Rec-BMS and charger is Victron Multi-Plus Compact inverter / charger. There is a CerboGX providing the brains to the system. I've been away from the boat for 4 months, and through some shore power outages and unintended power usage and configurations, the battery found itself 'dead' - I think it was around 8.5V - it's been this way for about 4 weeks.

My understanding is when Shore Power returned, the MultiPlus doesn't power up and charge because it's waiting for a message from the Cerbo - but the 12v side of the system is so dead, the Cerbo is offline and therefore the system can never turn on. Is that correct? I read that you could disconnect the network cable and it should work. I disconnected from the Cerbo and there was no change. I didn't try disconnecting from the MultiPlus as that requires opening it up.

I got a 12v trickle charger powered from Shore to help feed the system. It's a 6 amp charger. Knowing I could be here a long time, I also flipped my DC to DC charger that charges the engine starter battery to use it to help charge the lithium bank. After not too long there was enough volts to boot up the Cerbo, however the BMS didn't want to come online (maybe not enough volts), My understanding is the Cerbo wouldn't tell the MultiPlus to charge because it couldn't talk tot he BMS and be sure it was 'allowed' to charge. Is that correct?

I turned off DVCC to get over this restriction an and rebooted the system. Once back up, the charger went full steam ahead and was charging at 90amps. With enough voltage coming in the BMS was now online as well. After 5 minutes or so I could smell some electrical burning. Upon investigation I could see some discolouration of the Class T fuse. It's a 300 Amp Busman - sold by the local chandlery (https://www.eaton.com/content/dam/e...circuit-fuses/class-t/bus-ele-ds-1025-jjn.pdf), installed into a BlueSea Holder. I moved a temperature sensor closer to motor the temperature, and as I went back to my computer to see readings, I noticed some light smoke from the fuse. At this point I quickly stopped the charger system. I turned DVCC back on and rebooted again. After reboot the system went back to charging at 30amps (as restricted by the BMS - I've never changed this setting up until now).

So I'm looking for a sanity check that everything is sized properly. A 0.5 Charge or Discharge on the cells would be 150 amps. We we only at 90, so I think I'm ok here. The wiring from the battery to the fuse block is 2/0 gauge - Also ok?. The same size going from the MultiPlus through a Lynx Distribution. The only point of heat seemed to be on the battery side of the Class T Fuse.

I had done a bit of a push and pull on on the fuse and wire connections and nothing was lose. However, when I pulled off the fuse I noticed a few things:
- Discolouration of the mounting bolt for the battery side of the fuse.

Upon removing the fuse, I noticed the other side of the fuse was not tight at all, the nut could be hand removed. Behind this there was some white on the fuse holder:


If the connection on the left side was poor, would I expect to see over heating on the right side? I think this makes sense.

The BMS has previously been set to limit charging at 30amps, from factory settings. I'm about to replace the alternator on the boat engine with a 90Amp Balmar in order to charge this battery faster, so I'm just looking for reassurance that this is setup and sized properly, and this all comes down to a loose nut!

Thanks for hanging in and reading to the end!

 

[ScrodusGargleRotten]

If the connection on the left side was poor, would I expect to see over heating on the right side?

Poor connections lead to arcing. Arcing makes heat. The arcing would be on the fuse side left and the inverter side.
The other side would be warm, but there is next to no thermal mass in the fuse itself and wouldn't transfer enough heat to discolor it, especially given the loose side looks better in my opinion.

Lets see what others say. I have been known to be bat chit crazy.

Are you absolutely sure the loose side was the side that looks better?

There could be 2 problems. The battery side was loose for longer and the heat transferred over the fuse many, many times, to undo the bolt thru heat/cool cycles. but the real problem is on the battery side.

 

[42OhmsPA]

Clean it up, apply some No Ox ID Special A, torque down appropriately (you can flip the fuse to see if it lays flatter ), run it and monitor.

 

[Adamberti]

Are you absolutely sure the loose side was the side that looks better?

Yeah, definitely sure. I removed the battery side nut first and then when I went to do the other side there was no friction on the torque wrench to 'catch'. I used my fingers and it just turned.

What you said makes sense about arcing - that would cause the heat and discolouration. I was thinking that maybe there was some resistance on the battery side because of the fuse (just guessing). But I can also confirm the lack of thermal mass - the left side was quite cool.

I'll get some conductive lubricant tomorrow per @420Heysus and see how she goes.

 

[robbob2112]

Have you ever come back and retorqued/checked all your connections? Ifnot you are overdue

 

[Skypower]

I acetone, Scotchbright all mating surfaces, dust off and apply a small film of No Ox ID A special. I never seem to run out of a tub of OCD

 

[Backyard]

I've seen connections that were loose that sort of welded a tiny spot from arcing. This made a decent enough connection that it didn't present any issues. But it was fragile and was hard to spot. Could be what happened to your left side

Right side may have just been high enough resistance from a loose, dirty or oxides that it didn't make any good connection and got hot. Both sides where probably not good connections but 30 amps wasn't going to show much issues. Only when you got 90 amps was the connections not good enough.

Like mentioned above. Clean all the mating surfaces and put something like Noalox on it before reassembly

 

[Adamberti]

Thanks all. Yes I’ll do some physical cleaning of the connections before lubing it up.

And good point on checking other connections - If there’s one loose one…. There’s probably more.

 

[BatteryBrewer]

Just finished my 3p16s install and checked the fuses with a FLIR...they're rather warm as well at just half their rating.
I verified all connections and they are tight (but not lubed yet).

This is 100A current per 200A-rated fuse (RS308-HB 3H) continuously for 90mins:



I will try to repeat with 150A next week. Not sure how hot is acceptable but the datasheet states maximum 85°C ambient temperature.

 

[Adamberti]

Just finished my 3p16s install and checked the fuses with a FLIR...they're rather warm as well at just half their rating.
I verified all connections and they are tight (but not lubed yet).

This is 100A current per 200A-rated fuse (RS308-HB 3H) continuously for 90mins:

View attachment 285323

I will try to repeat with 150A next week. Not sure how hot is acceptable but the datasheet states maximum 85°C ambient temperature.

Thanks for sharing that. I’m mostly interested in the wire being hotter than the lugs. Perhaps because the wire is insulted it’s retaining more heat?

 

[Backyard]

Thanks for sharing that. I’m mostly interested in the wire being hotter than the lugs. Perhaps because the wire is insulted it’s retaining more heat?

Most metal, especially shiny silver colored has a lower emissivity for the same temperature as many other materials. The insulation will emit more IR for the same temp. His scale is a little off too.

Bottom left of the screen shows the emissivity setting at 0.95. adjusting this to show the correct temp on the lugs will make the insulation look like it's on fire. Metal will often need to be coated with some paint, tape or other material to make it easier to read.

That image above is a classic example of not being able to trust the temperatures indicated. The ceramic or porcelain of the fuse is probably pretty close to the indicated reading as well as the insulation of the wires. The metal on the other hand is way off. Looks at the chart I posted below. Bringing the metal into calibration will throw everything else off. The temperature the flir camera shows should be taken with a grain of salt unless it's set for that one specific materials emissivity. It's better understood as an IR camera that shows you how much IR is coming off different materials. Not it's actual temperature. Great tool for someone who understands the limitations. Poor in someone who doesn't

 

[Backyard]

Emissivity calibration chart

Aluminum Foil 0.04
Asphalt 0.93
Black Body, matte 1.00
Brick, red 0.75 - 0.93
Chrome, polished 0.05
Concrete 0.85 - 0.94
Copper, polished 0.02 - 0.05
Fabric 0.87 - 0.98
Galvanized Pipe 0.46
Glass 0.92 - 0.95
Granite 0.45
Gypsum 0.85
Ice 0.97
Iron, polished 0.14 - 0.38
Iron, rusted red 0.61
Marble, white 0.95
Paper, white 0.68
Plastic 0.84 - 0.95
Quartz Glass 0.93
Rubber, black 0.95
Sand 0.90
Skin, human 0.98
Snow 0.80
Soil 0.92 - 0.95
Steel, oxidized 0.75
Steel, polished 0.07
Tape, electrical, black 0.97
Tile 0.97
Water 0.95 - 0.98
Wood 0.86 - 0.90

 

[Hedges]

With current flowing, check voltage drop across each part, like from crimp terminal to busbar of fuse holder, etc.
That will show problems before they even warm up enough for IR.
You could pierce insulation to check resistance between terminal and strands. Seal after? That's what auto repair books say to do (road salt corrosive so worse than your environment unless on a boat.)