I plan on renting a boat on a small lake that allows you to use trolling motors. I have the spare motor just not the battery. I have got like 3 car batteries laying around. I was told that it would work but it would not be for constant usage. How much usage could I get out of a car battery vs. a deep cycyle? I really dont want to spend the money on a battery. 

Car batteries (cranking) are designed for putting out a lot of amps for a short period of time.  Deep Cycle batteries are designed to put out smaller amounts of amps over a long period of time.

 

You state that you have like 3 car batteries laying around.  Well it would be anyone's guess as to how long any of them would work for you because there is not telling what kind of shape that they are in.

 

If one of them was a good strong fairly new bettery it might go for several hours but then again depending on their condition they might not go at all.

 

The first thing I'd do it charge them up and check the specific gravity with a hydrometer.  If you don't own one they are only a few bones.  If you can get them charged up to 12.7 volts and get them to hold that for a while then they probably would work.  Even if that happens, I recommend that you take a couple of them with you when you go.

It depends on the condition of the batteries that are "just lying around".

 

If they haven't been maintained, they could be about useless.  Batteries that are not kept up to full charge will sulfate, and ruin an otherwise good battery.

Charge them all and bring at least two of them.  Wire them parallel (I think), positive to positive, negative to negative and hook your trolling motor up to one of the batteries.  They will probably last for a while, giving you time to save up for some deep cycle batteries and a trickle charger where you can leave them plugged in all the time when they aren't in use.

 

Oh yeah, on that note, invest in a paddle or better yet, a set of oars that fit the rental boats.  Going out in a rental boat with just an electric motor is asking for it.  I' recommend  an alternate propulsion source.

Batteries wired in parallel are only as good as the weakest link.  If I were you, I'd invest in a deep cycle battery, or a good set of oars, and a backup set of oarlocks.

The batteries came from a couple different cars they still work in those cars we just replaced them with better ones.

Get a deep cycle battery.

Take someone with you and two paddles, that way you have some help getting back to the dock.  Most likely with those old car batteries, you won't get very far anyway so you won't have far to paddle back. 

 

If you don't want your trip ruined, spend the bucks for one of WalMarts big yellow things.  Fully charge it as soon as you get it home from the store.  Then be sure to fully charge it as soon as you get back from the lake each time and either keep it on a good maintainer or charge it every six weeks or so that you don't use it.

Take someone with you and two paddles, that way you have some help getting back to the dock.  Most likely with those old car batteries, you won't get very far anyway so you won't have far to paddle back. 

 

If you don't want your trip ruined, spend the bucks for one of WalMarts big yellow things.  Fully charge it as soon as you get it home from the store.  Then be sure to fully charge it as soon as you get back from the lake each time and either keep it on a good maintainer or charge it every six weeks or so that you don't use it.

 Like a portable battery charger? the paddles are a no brainer, who rents a boat with no paddles?  I would just use an spare outboard if i had one that ran.