Sulphur smell from battery. Dangerous?

[Onewheeler]

Don't forget that H2S deadens the sense of smell so it may reach fatal concentration without being noticed.

[Tigerr]

Update. Spoke to RCR, and an engineer advised to disconnect the boiling battery, isolate the leisure bank, and pull into Crick. There, very helpful staff provided contacts for batteries, and Neil, local to help with the installation. Within 40 minutes, Multicell delivered 4 replacements, and Neil removed the hot ones, installed the new first thing this morning. Plus, being Aquavista customers we moored overnight and used the facilities foc. 
Whole battery bank cooked, no doubt my own fault, but they are old, possibly wasted in the pre sale year, so maybe a failure was to be expected. 
Not used to good service but everyone has been extremely helpful. 
Next, Neil took time to check our our route back to Cropredy, and told us about the Napton closure. I thought I had managed the trip, but it looks like I missed this. What a mess we’d have been in had we kept going. 

Now, need to over winter the boat, with our mooring at Cropredy unreachable. Brilliant that Aquavista will transfer the remaining mooring contract from Cropredy to Crick, until the spring when we will continue onwards. 
Outstanding service, what a good team of people. They saved us from a load of woe. 
Gone back out for a couple of days on the amazing lock free stretch, before putting the boat in for the winter.