How long will a brand new 12v battery hold its charge when stored outside of vehicle.

 

6 months maybe. The self discharge rate is about 4-6%/month.

 

6mths at 6% per MTH is 36%.

 

A fully charged battery is somewhere near 12.6 volts....my wife's BMW which isn't driven much won't start when the battery is below about 11.2-11.4 volts.

 

Eric gave the figure for a flooded battery.

There are also AGM batteries. The pure lead variety of AGM will last 2 years outside the vehicle at 70°. Shorter in hotter storage, longer in colder storage.

If the AGM does not say pure lead, then the storage life will be about 12 to 18 months.

 

If a new battery lasts approx 6-12mths outside the car,is it a good idea to put it on charge every mth.

 

You also have the option of putting a 'trickle charger' on it to keep it ready to go.

 

Probably better to leave it on a trickle charger / maintainer.

 

Use this chart, anything in yellow or below is questionable on whether it will start an engine. I'm not fond of trickle chargers because over time they will ruin a battery due to evaporation. A trip to the store if a battery needs replaced is much simpler for me and the warranty will remain in effect longer during ownership.

 

There are trickle chargers that are designed to be used for extended periods. I have never heard of the evaporation issue on a sealed battery.
https://www.carsdirect.com/car-repai...rickle-charger

 

I’ve never had a battery able to start a vehicle less than 12.1V. A fully charged battery should be reading above 12.75v

BMW computers don’t like to see voltage less than 12.2-3v or they start doing all sorts of crazy ****, I’m sure many modern electronic vehicles are the same. Olds dinosaurs like ours can handle closer to 12v, compression and starter draw depending.

My dad will leave his cars in FL with the batteries unhooked for 7 months sometimes, and they start at the turn of the key. His ‘91 Maxima went two years, he hooked up the battery and started. Temperature and climate play a role.

Me? The ‘84 GMC sits outside and goes months sometimes without a start, when I hop in and turn the key she starts every time. The Olds I’ll leave on a trickle charger as it’ll see even less drives during the winter. I just replaced the Olds battery after 7 years, with an unknown short to ground, and the GMC is approaching 8. So that battery maintenance plan works for me... YMMV.

 

That is true in two circumstances...
1) With lead-antimony batteries--production of those stopped by about 1980.
2) With trickle chargers that give too high a constant amperage. This will ruin any battery.

To ensure your battery stays healthy during non-use, connect a very low-amp trickle charger such as Battery Tender Plus or a smart charger such as CTEK. Either of those types can be connected for extended time without causing damage or water loss on post-1980 batteries.

 

I've owned our unmodified '07 Duramax now for about seven years, it is not driven daily and is used only for hauling purposes, emergency vehicle necessity and often driven on Sunday's to service for gp maintenance purposes. Four years ago I replaced both batteries with yellow top Optimas. I had noticed that my rate of discharge seemed excessive and suspected a parasitic power draw, especially when I've been on extended work trips. After doing a amperage draw check, I discovered that it was not excessive and put the batteries on isolated deep low amperage charges. After doing this the truck started with the enthusiasm that you'd expect with two batteries. I finally got smart and used a trickle charger that I had that was not being used and have had no issues since. Last weekend I purchased a new 12 volt battery for my lawn mower which sits directly beside the front of our truck. I realized that I could very easily swap the connector from the charger between the two and built a second molex connector for the mower. Two for one! I need to find my second charger to put on my 6 volt '40 coupe!