Frank Dinger wrote:

I was told that a car battery wouldn't last long anyway for this
application, so even when my car battery is 6-7 years old, it
wouldnt't last more than a year when trickle charged, while it is far
better economy to buy a leisure type battery
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A leisure type (deep cycle) battery is fine for a moderate load for a
relatively long time ,since the plates are relatively thick.
However they are not suitable for engine starting purposes ,since brief
demands for a high starting current cannot be met reliably over time.
That's why car batteries have relatively thin plates with a large area
capable of supplying the high starting current.

If a car is kept outdoors it is worthwhile employing a small solar panel
positioned near the south face of the car with its lead plugged into the
cigarette lighter socket. This will keep the battery in a healthy condition.
I have seen these solar panels advertised by the car accessories trade.

Frank GM0CSZ / KN6WH

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In the past I've maintained car batteries for many months using a small
(2" x 8") solar panel made for this purpose.

How much current could this put out? I don't know but I'm sure it's not
much. Nowhere near one ampere. It contained a diode and no voltage
regulator. This is the same effect I want to achieve with a small wall
wart.

If it's safe to float charge the battery at .01 C indefinitely, say 700
mA for a 70 AH car battery, then anything under 700 mA is also safe. My
200 mA wall wart is safe.

The question then is what is the self discharge rate, less things like
an alarm and clock?