I've had some trouble with my 350z lately with starting issues, now I don't get to drive it very often and it's had a flat battery that's normally jumped ok however I came off holiday to find it was completely flat, I tried jumping it but it wouldn't go tried again another day off a jump pack and fired into life

I checked when running and it had 14v so it's charging
I then checked with the engine off and had just over 12v after a good run but slowly it was going down I checked every hour and it was still going down, after about 4/5hrs it was around 11v

Is it time to replace it to get the reliability back?

Have you actually charged it up? Running a car will only put a little bit back into the battery, it won't properly charge it. Stick it on a slow charger overnight and hopefully it should be a lot better.

get rid of it, and get a replacement. If the car is kept in a garage, and has long periods of non-use, then i would be tempted to disconnect the battery to stop it getting drained by clocks/sensors etc

prolonged inativity kills car batteries. Add to this many heat cycles, especially cold ones, will kill a battery.

If you dont want to buy a new one, and it can be serviced, try using the battery refresh tablets from halfrauds or similar. They have worked for me in the past.

When I worked in a garage. battery alternator issues were 80% battery due to short journeys and no use. The other 20% were diode packs on alternators.

Chances are it's goosed.

Using the alternator to charge a bettery from flat isn't a great idea though. They're spec'ed to provide enough current to run the engine and top up the battery. Charging a flat battery can overload them, especially if done repeatedly.

New battery needed

Batteries like life is complicated
Your battery will have damage that has reduced its charge capacity but it may not be completely goosed.
You need to charge the battery slowly ie over 12 to 24 hours at a low current rate then either get it tested with high discharge rate load tester or do a starting voltage test.


Everytime you take charge out of a battery the amount of charge it can hold even after recharging is reduced.

In everyday use with a well charged only a very small amount of damaged is done by a cold start followed by enough running to recharge.

As a rough and ready guide after winter cold start the engine has to run for 20 minutes to replace the charge used by a single cold start

However the deeper the battery is discharge the more damage is done, so if it has left than 70% of a full charge then significant damage is done to the charge capacity of the battery even after full recharging. If you do this often enough a 65 ah battery will eventually only have a 15ah storage capacity.


Another rough and rule is if you discharge a battery over a short period of time, you can recharge at a high current rate provided you put it on charge immediately
But if you discharge the battery slowly (or delay recharging) you have to recharge at low current rate over 12 or more hours.

When a car is in storage It is really best to regularly charge the battery or float charge it.
If you attempt to charge a battery that is nearly flat at high current rate (ie a boost charge or from the cars own alternator) the battery voltage after charging/running will initially appear ok but will slowly fall off.

If you know a car has a battery that is quite low on charge to avoid damaging the battery further it is wise to put it on a low rate of charge for a few hours before you try and start the engine.


[Edited on 6/11/13 by britishtrident]