battery drain

[moltuae]

A latching relay is a good idea.

You can actually make most regular relays latch simply by using the relay to power itself through it's own contacts. Unlatching is a little more complicated this way if you need a toggle action since you need to break the latch circuit somehow. However it will unlatch, and remain unlatched by default, when powered off.



You can also buy relays with a mechanical latch mechanism, which usually employ permanent magnets. They have the advantage that they require no hold-current but they're pretty rare:
https://en.wikipedia.org/wiki/Relay#Latching_relay

[tc4332]

Mark,
You are correct but you need two push buttons, by using the one I suggested you only need one switch. Press once to set, press again to release.

The push button that I used was like this:-
http://www.maplin.co.uk/p/230vac-3a-...h-switch-n26ka

[moltuae]

Absolutely agree Ray. Like I said, it's not really suitable if a 'toggle' mode of operation is required. In that case your suggestion would definitely be best.

You can still use a single momentary button for a simple relay latch circuit though, but only if turning the power off (via an ignition-switched live, for example) is suitable for the application.

[oldnick]

being rather involved in projects at home pro tem , I rarely get out in the car so it sits in the garage

recently had a thought for seeing what happens to the battery as it drains

took a cigarette lighter adaptor from my box of bits , piece of twin wire to a double connector block , popped it on the dash with my digital meter

now if my theory is correct if you open a door the only thing that is activated is a couple of interior lights so I sit in , shut the door , plug in my device and measure the voltage [ on a D2 the front cigarette lighter socket seems to be constant live ]

after a minute or so the lights go out and the the voltage starts to rise , arriving at a steady state after a couple of minutes ..as one would expect

in my case the voltage is decreasing by a steady .02 volts/day

has anyone else checked this voltage drop ? is it what one would expect from a system that has no faults ? the bigger drain that I have previously suffered seems to have disappeared

[Goran]

According to this website, for a lead-acid starter battery, the difference between full voltage and empty voltage is 12.65-11.89=0.76V

http://batteryuniversity.com/learn/a...tate_of_charge
If your battery loses 0.02V per day, assuming its all linear for simplicity, that means your battery would fully drain in 0.76/0.02 = 38 days

If your battery capacity is 95Ah, that is equivalent to a constant current of 95/(38*24)=0.1A

That sounds pretty good to me. My last S8 had a parasitic drain of 0.4A, my current S8 has roughly 0.15A

[oldnick]

thanks to the kindness of Andrew [ HP sauce ] who donated his old VCDS lite kit I finally got around to finding which sensor was stopping the parking aid from operating ....another job that needs doing while the weather is so nice

whilst the VCDS was plugged in I took the opportunity of running a general scan and up came 01134 ....faulty alarm ; not a surprise , never had one since I got the car but I applied the rule of rhett butler !

but then a bell rang ! more a coup de foudre actually !

when unsuccessfully trying to stop my parasitic drain I discovered that removing the aerial/radio/CD stack fuse in the boot did the job ; replacing the fuse and disconnecting each item separately however did nothing ....how could that be ? I gave up !

but what if that fuse also feeds the alarm ? presumably there is a rechargeable battery ...could that be on the same fuse ?

is that possibly the source of the drain ? constantly trying to charge a defunct battery? is it ok to just disconnect the alarm which is a known failure ...anyone know the effect of doing that ?

[HPsauce]

I've run without an alarm for ages while trying to repair/replace them. Not a problem.
PS Glad the old kit had some use.

[oldnick]

thanks again andrew ; have removed the alarm and will see if that stops excessive parasitic drain

strange to say while researching fixing the alarm I have since come across comments that faulty alarms are a common cause of parasitic drain !!!
perhaps a sticky is called for ! would have saved me some labour if it turns out to have been the cause of my problem ...we shall see