Everybody is quoting the capacity of new batteries, which is fine if you're always going to have a new battery.
When the battery is 3 or 4 years old, your new 40 amp hour battery will have a capacity of about 15 amp hours at best.
Even at 15 amp hours it will still do the job of starting the motor provided you use it every few days.
If you leave it a few weeks, even a small constant drain load is enough to drain it sufficient that it wont start.
Most cars now have a whole lot of stuff that leaves a small load even when turned off.
Things such as immobilisers, clocks, radios, can add up to about 25 milliamps.
Well,.. that's not much is it?
Only 25 milliamps?
25 milliamps for 24 hours adds up to 0.6 amp hours. Still not a lot.
For 2 weeks, that adds up to 8.4 amp hours.
For one month it's 17 amp hours. (double
)
On an old battery, that adds up to a 'no start'.
Not because the battery is dead flat but it has taken the top off it to the extent that it wont start.
(Yes,. I know,.. just buy a new battery.
)
