Hi Iris- What you've figured out about series and parallel is exactly right. So if you were using standard little dry cell batteries (about 1.5 V) you'd need to string about 8 or 9 in series to get the 12V (really more like 13+V) of a standard car battery. The problem is that the total charging capacity of those little batteries (even D cells) is small compared to a car battery. Probably you could drain them without charging up the car battery enough to help. If you took a bunch of those 9-battery series chains and ran them in parallel, you could get more charging capacity, but it would be cheaper and simpler to buy an extra car battery.
As you say, once things get started the battery should get charged up by the alternator system, if it's working right, especially if you plan a long trip with few stops. So you might have thought about trying to start the motor off one series string, without first charging up the car battery. The problem with that is that standard cells can't supply much current before their voltage stops to drop, even for little bursts like those needed to start a car. So weíre back to the idea of getting an extra car battery, or a jump start.
The sorts of batteries you can throw together out of coins etc have very poor current-delivery ratings, so they wonít help.
As for how to tell + from -, there batteries often have + and - markings or various other conventional signs. But the general-purpose way is to use a voltmeter. A combined volt-ohm-amp meter with a digital display is actually very cheap (maybe $15 for a decent one) and a lot of fun to play with. Having one is a great way to start on figuring out how home and car electrical systems work.
Of course none of this is in time for today's crisis.
Some batteries require you to add water every now and then (not many new ones -- they're called "maintenance free" and are sometimes sealed so you cannot). Instructions should be written on the battery. You can probably check the level of fluid in the battery by lookin at the side or on a gauge if it's designed to have its fluid topped off.
It might take a while to recharge a battery with other sources, like the dry cells you mention. Some people hook their batteries up to a "trickle charger" which takes its time charging the battery and is either powered by plugging it into the wall and I just found one on the web that was powered by solar cells.
I once had a small car with a tiny battery and some thin jumper cables and tried to give a jump-start to a big pickup truck, which did not work. You need a lot of current to turn the starter motor in a big vehicle.
(published on 10/22/2007)