On Wed, 23 Oct 2019 13:32:48 +0100, "dennis@home"
wrote:

snip

5W for 24 hours would be 120 watt hours; at 12v = 10 AH - quite a drain on
a small battery,


What do you think the capacity is for a car battery under a very small
load like that?

Erm, roughly the rated value at that load, given it's normally the C20
one.

Even the cheap nasty ones you used to get would be around 80AHr for that
small a load.

Nope.

An "80AHr" one in a modern car would probably be 160AHr for such a small
load.

Whilst it might be more if the load isn't on the C20 rating for that
capacity, it's unlikely to be double for 'that' (~.4A) load.

The capacity is usually stated for a much higher drain.

Nope, it's stated at the C20 rate and they are the sorts of numbers we
are talking about here.

https://d3i71xaburhd42.cloudfront.ne...-Figure1-1.png

Most batteries now have capacity in how many amps you can draw while
starting.

Yes, Cold Cranking Amps or Marine Cranking Amps. Not the same as their
std C20 rating.

When I was racing EV's, because the race I entered lasted one hour and
the average draw 25A, the 'Reserve Capacity' of the batteries was of
interest to me and I built a discharge jig to test batteries for just
that. Fully charge all the batteries to be tested, put them on the jig
one at a time and select the best 2 that had the same characteristics
(then took the rest back, after fully recharging them of course). ;-)

Cheers, T i m