Mod Details

PremiumYes Difficulty Mod ID1741 CreditAskold K For Linkhttps://www.evilution.co.uk/mod/451-ed-hv-mega-fuse.htm Copy to Clipboard

If you have a catastrophic high voltage electrical failure, the battery has a safety feature to stop all of that fire, sparks, screaming, running around and gnashing of teeth as everything in the local vicinity also sets on fire.

This fault could be a damaged HV cable that has earthed on the chassis, a short circuit, a massive electrical input surge or sudden internal electrical failure of the DC-AC inverter.

One night, I unplugged my car, got in, put my foot on the brake and turned the key to start the car. Despite being in park and me having my foot on the brake, the car lurched backwards a few millimetres and all of the error lights came on. From that point on, the car did nothing. I could smell something burnt so I pulled the emergency disconnect plug, pushed the car away from the house and went to work in another car.

That safety feature is nothing more complicated than a high power fuse. However, unlike all of the other fuses, changing it is a pain in the scrotum.

Mega Fuse

Unlike pretty much every other electric car, smart decided to put this fuse inside the high voltage battery. So getting to it involved removing the high voltage battery and opening it up. If the fuse blows, smart expect you to buy a new HV battery.

If you open the battery and stand at the connector side, look over to the right, just above the high voltage connector.

Between the 2 blue busbars is a push on plastic cover that pulls off…

…to reveal the mega fuse. Unbolt the Torx bolt from each side.

You can test the continuity across the fuse to determine if it has blown.

Getting a replacement was surprisingly easy and no, they aren’t available from smart.

It’s a monster. 225 amps, 450 volts.

Because the fuse has to disconnect a lot of current, it’s full of sand. That’s to quench the arc.

You can see that there was enough heat to melt the sand back to solid rock.

Replacement Mega Fuse

All of the data is written on the fuse. I did a Google search and found a few companies selling replacements. However, all of the sellers were in China when the manufacturer is Japanese so I just had a feeling that they were expensive fakes. I contacted the manufacturer directly and was ignored. Thankfully, Askold K from Go Smart Service steered me in the right direction for a genuine replacement.

As mentioned before, other electric cars have a similar fuse but they are much easier to change (including the 453 EQ). I was informed that some other EVs have the same fuse built into their HV service disconnect plug.

Thanks again to Askold for the vital information.