Battery

[spolij]

How long should a battery stay charged (enough to stat} if the car is not used. A daily driver but not used for a week or so? Nothing on in the car. Garage kept.

[Daryl Chesterman]

John, the following article addresses your question for a more modern car, and also talks about what to do and not do to keep a battery charged.  The main thing is to have no parasitic draw on the battery while the car is not being used.  The age and condition of the battery will also affect the length of time it will maintain a charge.
A battery will self-discharge on its own--more rapidly if the top of the battery is dirty than if it is clean.  If you don't believe this, you can take a bare battery (no cables connected to it) and using a multimeter set to millivolts, place the red lead to the positive terminal, and touch the black lead to the top of the case, and you will get a reading.  Do this same test on a battery that has a dirty top, and you will see that the self-discharge is more.  To be sure that you have no parasitic draw on a battery in a parked car, disconnect the positive cable from the battery, and using a multimeter set on milliamps connect the leads between the cable clamp and the positive terminal of the battery.  The reading should be zero, unless you have a clock, or a newer car with computers, electronic radio presets, seat memory, burglar alarm, etc., all of which have a slight current draw when the car is not running.

https://www.jcwhitney.com/blog/how-long-can-a-car-battery-sit-unused/

Daryl Chesterman

[35-709]

Be SURE a trunk or glove box light is not on, also a clock that has failed can also continue to drain your battery if left connected, depending on the type of failure that occurred in the clock. 

[savemy67]

Hello John,

I have a 7 year old Deka battery.  I have left my car alone with the battery connected for up to 4 months without starting the car.  The car starts easily even though the voltage reads just below 12 volts.  The battery then charges while the engine is running.  That is my situation.

There is probably no precise answer to your question.  As Daryl indicated, age, condition, manufacturer, weather, alternator/generator performance, and other factors will determine your battery life.

The only draw on my battery when the car is not running is the clock.

Christopher Winter

[35-709]

Yes, I didn't word the clock part of my response correctly.  Usually the draw from a clock is pretty inconsequential but some clock failure modes will drain a battery overnight as happened with a '72 CDV I once owned.   

[gkhashem]

I have one car not stored in my temperature controlled garage and all I do is leave the battery disconnected. It will start the car after sitting that way from mid November to mid April.

[Caddy Wizard]

If there is no drain on the battery other than a clock, it should remain strong enough to start a car for 2-5 months.  I had one that was only good for about a week or two. Turned out that there was a trunk light that was staying on and killing the 6v battery.  It took a while to find the drain, but once we found that drain, the battery has remained strong more or less indefinitely.

[TJ Hopland]

Seems like knowing a year and model would help.   Some years and models have common causes of battery drains.

[gkhashem]

Also disconnecting the battery saves the clock. The clocks die when voltage drops enough to burn the contacts. (weak battery or very cold conditions)

Another reason to DISCONNECT the battery at the negative terminal. Save your clock if you restored it.

[bcroe]

As said above there is no one answer.  Batteries start to self discharge
as soon as the charging source stops.  The rate of discharge varies hugely
between batteries.  Certainly disconnecting any load will reduce the loss. 

The fact that your battery started the car after winter does not mean
there was no discharge, just that there was enough left for the start. 
It might be interesting to measure the state of charge after the winter. 
Lead acid batteries do not tolerate a state of discharge well, and a long
time at a significantly discharged state is the most likely way to damage
them, though it may not be immediately apparent. 

I got tired of buying batteries every spring, after failing to take steps
I thought would help preserve them.  I bought enough small battery
maintainers (NOT trickle chargers that can overcharge) to cover every
engine starting battery I had, which included things like a tractor and
a generator set.  No need to switch off or disconnect batteries, just plug
them in.  I have adapted the economical and robust EC3 2 pin polarized
connector (used for model batteries) to most apps, getting rid of those
marginal clip leads.  I included a fusible link in the job, and a dust cover
(extra connector).  Handy for a timing light, etc as well. 

I use dirt cheap Horrible Fright maintainers, but do not kick the plug
out of the outlet, or THEY can run your battery down.  Bruce Roe

[spolij]

Thanks all.