Battery keeps discharging

[synthium]

Hi everyone,
not sure if this is the right area to post. Please bear with my long (hopefully not too confusing) description.

My 300tdi defender 130 has a dual battery system (piranha isolator). The starting battery has gone dead flat as well as the 2nd deep cycle battery.
After jump starting it I manage to to drive to work (60ks away). There was enough charge to then start after work to get back home as well afterwards.
When I got home I could start the car 6 times in a row - no problem. I put the deep cycle on the charger over night.
The next morning the starter battery was completely dead again - as if it all discharged over night. I put the deep cycle back into the system and tried to jump of that but the car didn't start. I believe my battery charger is probably broken (it is very old ~20years and I havn't used it in a long time).

Anyway my alternator seems to be working intermittently (sometimes reads 12.5v, sometime 13.6) but regardless of this I am not sure why the starter battery seems to be discharging overnight. I am sure there is nothing left on. This battery is only 1 month old. Could there be an issue in the dual battery system which is causing some type of power leak?
any advice appreciated.

[drivesafe]

Hi synthium and to start with, when did you take the voltage reading and where?

Were they taken at cranking battery or elsewhere?

If the readings were taken at your cranking battery, more than 5 minutes after starting your motor then there is a very good chance your alternator is not working properly.

BUT, with two low batteries, the combined current draw of both batteries could be pulling your alternator voltage down.

Try measuring the voltage at your cranking battery after a 30 minute drive and with the motor still running but make sure all other appliances are turned off. Like headlights, A/C, sound system and so on.

Also, next time you get a jump-start, when you finish your drive, DO NOT continually start your motor, you just contribute the flattening the cranking battery.

By starting your motor 6 times, you would have needed around an hours driving time to replace the energy you used as your battery was in a low state in the first place.

The best way to test the reliability of your cranking battery is, again, after a good drive, when you get home, disconnect the NEGATIVE terminal of your cranking battery and leave it that way until you are ready to start your motor again.

If the battery is flat, then there is a good chance it is stuffed.

[synthium]

Hi the voltage reading was taken at the cranking battery just after I started the car from a jump start. I know regarding the continous start that I was contributing to depleting the cranking battery but I was home and wanted to see if it actually charged via the alternator - but will avoid doing that in the future and will follow the process you have outlined. Is there any possiblity that a loose connection could be draining the battery overnight?

[rar110]

I had the same experience. It was caused by the alternator (diodes apparently) draining the battery. Try disconnecting the alternator overnight.

[Lafia]

I had the same experience. It was caused by the alternator (diodes apparently) draining the battery. Try disconnecting the alternator overnight.

Ditto. Some nights I would lose more battery power than others. Sparky explained that this was consistent with faulty diode/s. Only sign was alternator output voltage was slightly down, which the first auto sparky I consulted should have spotted. Instead he diagnosed a stuffed battery. So I purchased a new battery but no change. Changed alternator and all sorted.

[synthium]

thanks for the assistance. I will check out the alternator.

[Jode]

thanks for the assistance. I will check out the alternator.
I'm no sparky but there's logic here: a diode is just a fancy non-return valve that lets electricity flow only one way. If the diode's faulty then you may get flow in the opposite direction, causing the battery to drain (I may be writing nonsense but it seems logical...).

Also, if the diodes are faulty you may be able to just swap out the voltage regulator rather than have to pony up for a complete alternator.

[bee utey]

I'm no sparky but there's logic here: a diode is just a fancy non-return valve that lets electricity flow only one way. If the diode's faulty then you may get flow in the opposite direction, causing the battery to drain (I may be writing nonsense but it seems logical...).

Also, if the diodes are faulty you may be able to just swap out the voltage regulator rather than have to pony up for a complete alternator.

Unfortunately the diodes in question aren't inside the regulator, they are spread out around the back plate of the alternator on heat sinks. Nowadays most alternator diodes are only replaceable in a complete set.

[synthium]

Thanks everyone for your assistance. I have replaced the alternator with a d1 100amp and things seem to be going good so far!

[drivesafe]

Thanks everyone for your assistance. I have replaced the alternator with a d1 100amp and things seem to be going good so far!

Hi synthium, have you taken any voltage measurements yet?