I've been thinking of replacing my deepcycle AGM with a car battery: I know it's practically heresy, but bear with me
my small offgrid system has a small 24v 100Ah AGM battery which I've been using for several years even before I got the hydro up & running. Now I have hydro continuously feeding ~300w into the battery (and I run my larger sustained loads when the solar is working), I seldom take more than a % or two from the battery - it's mostly there to provide the surge for the fridge & freezer.
It occurred to me that I could run large short duration loads (say a toaster (1kw)or kettle (2.2kw)) for a couple of mins without deeply discharging a 100Ah battery, but the amps drawn (90-100A for 2.2kw) would be a bit hard on the 100Ah deep cycle AGM.
So I'm thinking it would be much better to use a regular car battery which can provide 100's of amps for a short duration, as it will be recharging as soon as the short duration load is removed (much as it is in its 'natural habitat').
I've done some experiments with a couple of old batteries that were lying around and they seem happy enough, but before I run the system in anger (or actually spend money on new batteries) I'd like to have an idea as to what sort of regular DOD would be acceptable for car batteries?
I've been looking, but battery makers don't seem to provide dod vs cycles charts for car batteries (no surprise there). There does seem to be a common reference to 'not more than 50% dod' but that's a bit vague, and I wouldn't expect many cycles to that dod.
In it's normal application a car battery manages probably several 1000 'cycles' of starting the car but I'm struggling to put a figure on what DOD an average engine start represents (maybe <1% for a warm engine in the summer to 3 or 4% for a cold start midwinter?).
So to the question: Does anyone have any references to DOD vs cycles for a car battery? or suggestions as to what DOD would be sensible maximum to keep to?