Look, it's simple. I have a fridge running 24/7, off a victron 3000 inverter, also on 24/7. On average the fridge uses a couple of amps an hour, the inverter maybe 1/2 amp or so. Thats say 60-65 amps of battery current/power call it what you will, in a 24hr day. Lets say with a bit of laptop time, shower pump and led lights - oh and a couple hours using my 240v stereo amp that I'm using 90amps a day.
My batteries are 4x100amp. So I can usefully use maybe 200amps?. Therefore they should easily last a couple days, maybe three if it's not been hot and I haven't been "living like that".
The alternator is 175amp (for the domestic batteries, the starter has its own). I would have thought that the alternator would have put out say 100amps even on tickover, hence I thought a couple hours would be adequate to recharge, given that on many days I have been cruising for a lot longer than 2 hrs.
I didn't have a convenient means of keeping an eye on the state of charge - now I do.
Thinking about it, I got into a bad habit - I frequently had to leave the boat to return to London and attend to my affairs, I would go late morning/early pm, and leave the batteries as they were, ie prolly not that well charged having been using the fridge etc over night. Upon my return always gave the engine a good run because I not only wanted power, but hot water. It had seemed wasteful to run the engine before leaving the boat and the tank of hot water.
We've all had car batteries go dead flat - leaving lights on, electrical gremlins, whatever. Sometimes you recharge the battery and its fine, but usally if its a bit old then a few days later it goes flat again so you buy another one.
These batteries were so new that I assumed they would have a bit of leeway in them, I was quite to prepared to replace them a little early ie 18 month, couple years. I didn't realise they were so fragile. Now, thanks to you all, I do. I was hoping someone would have a remedy or perhaps suggest I had a failing alternator or whatever, that's why I posted. Simples.
I apologise in advance if I've used the wrong units, mis spelt anything or upset anyone in any other pedantic manner
Have to go now as the laptop battery is getting low....