i have a 34lb thrust minn kota. can i use a car battery, i think its a deep cycle car battery is that is even possible. i have been using these small brick sized batteries but it takes 4 of em to keep me funning for 4 hours

not sure about the car battery  but i doubt it  will work better than a marine deep cycle

Cars don't use deep cycle batteries and don't waste your money on a "car battery".  If you are refering to a Deep cycle Marine or Trolling motor battery, that's what you need to be using.   The best of them is the Trojan SCS 225.   A good cheap battery is Wally Worlds big yellow battery.  Unless you run a lot on high, these will run you most of the day, if you mainly keep the TM at a slow casting speed.

I use a car battery in my small 12 foot boat that i use on private lakes that i fish with no trailer launch. The car battery is lighter and since i just borrow the boat whenever i want i cant see buying a deep cycle for it. On a good charge it will last around 4 hours. But the power level really drops off after about an hour and a half or so.

There's a reason a car battery is lighter than a deep cycle.  The plates inside are thinner and the casing is thinner as well.  Neither of these attributes lends itself to a marine application.

Car batteries aren't meant to be drained and then recharged over and over again either. They're also not intended for intermittant or prolonged on/off cycles of high current draw. You'll ruin the car battery quickly.  You're wasting your money.

well today i bought a marine/rv battery. when i hooked itup to the boat the boat ran SO slow in 5, but when i hooked it on the car battery i got, it was crusin. well, the marine batt. wasnt charge 100%, maybe 80-85% and it had a fish finder attached to it also, but shouldnt the motor still run good?

Was there a good connection between battery and motor?

I've never noticed a drop-off in the performance of my motor when the battery is a little drained (down around 60% according to my charger).

What are the numbers on the battery you bought?

There are many types of batteries for different applications. I will however only speak on the ones mainly used in marine applications.

A car/starting battery, as already stated is not capable of being used as a deep cycle for prolonged periods due to the size of the plates. The plates are not only thin, but are porous like a sponge and are very soft. They are a combination of lead and calcium. The reason the plates are so thin is so that more plates can be placed in each cell, thus giving maximum surface area contact with the electrolite/acid. This is how a starting battery is able to give so many CCAs. (This could also be the reason for the difference in the speed you noticed between the two batteries. Higher amperage available to be drawn for shorter periods with the starting battery.) It is also the reason for having such a low percantage of discharge. With repeated discharges below their reccomended level, the plates begin to fall apart and settle to the bottom of the cells within the battery. Recommended discharge for a starting battery is 2-5%. (Regardless whether it is liquid, gel, or AGM.)

Hybrid batteries or Marine/RV batteries are a combination of charecteristics of a deep cycle and starting battery. They will normally have an amp hour rating (AH) and a MCA rating. (Marine cranking amps.)The plate composistion while thicker, is still soft and porous. This allows the plates to have more surface area for MCA (Available amp burst given at 32 degress as opposed to 0 degrees for CCA.) Plate composistion is still similar in the fact that most hybrid batteries have the lead calicium make up. The thicker plates allow for the battery to be drawn down further without damage occuring. Most hybrid batteries are able to be drawn down repeatedly by 60-65% and recharged without causing damage to the plates or cells as long as there is proper water levels in each of the cells. Water/electrolite levels do not apply to Gel, AGM or maintenance free batteries, as there is no way to check or replenish.

A true deep cycle battery has plates that are not just thicker, but solid and normally rough. This is what allows the battery to be discharged to such extremes where other batteries would fail. This plate composition does have a draw back. This type of battery is suspect to loosing water faster than other plate composotions. These batteries are normally used in a commercial setting where they can be carefully monitord for water loss. (Back-up universal power supplies, telecommunications devices, etc) There are some deep cycle batteries that are made from the lead calicium combination. Their amp hour, while still longer than a hybrid of equal class will fall slightly short of the solid lead plate batteries. Both styles of the "true" deep cycle can be drawn down repeatedly to apx 80% and recharged without harming the battery in any way. These batteries can be liquid, gel or AGM.

Edits were for spelling and content.