12V is probably short for 12 volts, the voltage used by regular car batteries. And since 12V can strike a spark, it can be used to start a fire.
Between 12V-14.2V is recommended, however standard charging units will optimize their charge to your batteries needs.
You just connect negative to negative and positive to positive. Click the link for a diagram.
The batteries you are looking for can be purchased anywhere and do not need to be special ordered. Some retailers I know of are Radio Shack and amazon.com.
Automotive batteries can be purchased at walmart for cheap. Many car parts sellers will also have some discount batteries.
Enough to last two or three days in all of your flashlights. If you have a 12V adapter, you can sometimes recharge rechargeable batteries from a car or truck outlet.
Four 12V batteries may not be connected end to end to obtain 36 Volts. But four 12V batteries can be connected end to end to provide 48V and push 36 amps. The actual answer to your question suggests that you drop one 12V battery and use three, not four 12V batteries end to end to obtain 36V.
without knowing exactly the types of batteries you are talking about but generally marine batteries are a higher quality battery and have higher amp hour ratings which basically means they can turn the stater for longer
AGM batteries are more expensive. AGM batteries can be purchased by batteries plus or even eBay. You should consult with your car dealership as well, to insure you are purchasing the correct one.
Virtually all automotive batteries nowadays are nominal " 12V " batteries, but will measure up to 13.6V when fully charged.
Yes, you can do this.
With the batteries in series, the alternator provides the same current, and therefore the same charge to all the batteries. The alternator can take no account of any current being drawn from the 12V take off point. Part of the charge current is being fed to the 12V load and not to the batteries. Therefore, the 12-24V batteries are being fully charged while the 0-12V batteries are being partially charged. Over a period of time, the difference between the charge levels will become more obvious.