This looks like a good page...

http://www.batteryuniversity.com/partone-13.htm


"Simon#" wrote in message
...
There are a couple of products around to maintain a car battery.

They work is different but similar ways, the most complicated is the
accumate (http://www.accumate.co.uk/OM3CHARGING%20ALGORITHM.htm), however
one of the most popular products is the Airflow
(http://www.airflow.uk.com/battery-conditioner.htm#2)

This seems expensive for what it does "The unit will charge a battery from
a
minimum of 9 volts to 13.8 volts and then turn off. It will then monitor
battery voltage until it drops to 12 volts, through natural drain, and
then
turn on again to re-charge and exercise the battery to 13.8 volts"

Has anyone built such a device? Can it be acheived for less the £40 / US$
70

Simon# wrote:

There are a couple of products around to maintain a car battery.

They work is different but similar ways, the most complicated is the
accumate (http://www.accumate.co.uk/OM3CHARGING%20ALGORITHM.htm), however
one of the most popular products is the Airflow
(http://www.airflow.uk.com/battery-conditioner.htm#2)

This seems expensive for what it does "The unit will charge a battery from a
minimum of 9 volts to 13.8 volts and then turn off. It will then monitor
battery voltage until it drops to 12 volts, through natural drain, and then
turn on again to re-charge and exercise the battery to 13.8 volts"

Has anyone built such a device? Can it be acheived for less the £40 / US$ 70


"Trickle" means low charge current. The description you posted says
nothing about charge current characteristics; I assume the box is a
charger, not a trickle charger.

Speaking only about a charger, yes one can be built for not much money.
Back in the 70's I used the circuit from a Motorola app. sheet to build
a car batter charger. The ehart of the thing is a UJT oscillator. When
the osc. runs, it fires an SCR that is in series with the battery. Osc.
gets its power from the battery; osc. will not run if batter is hooked
up backwards, so reverse charging is impossible. But if the batter is
FLAT, the charger will not charge it (one drawback). The circuit is
simple, comprises a few resistors, a disc cap. or two, and (for my
application) 4 stud rectifiers. Total cost was probably less than
$5.00.

Oh yeah .... the transformer. It's a multi-tapped, "high current" model
that I got for free from an electronics surplus warehouse because I
bought a bunch of other stuff at the same time. The tranny probably
cost $40 new. G And, like I said, that was in the 70's.

I still use this charger. In fact, it's keeping some SLA batteries
topped off as I write this.