Im having to replace the deep cycle battery on my tm for my 8' flatbottom. Its an old one that was in my bass boat i kept when i replaced them last. Im looking at smaller, cheaper, and lighter options since i dont use it very often. Im looking at small batteries like this one http://www.amazon.com/gp/aw/d/B00A82A3RK/ref=mp_s_a_1_2?qid=1436469980&sr=8-2&refinements=p_85%3A2470955011π=AC_SX200_QL40&keywords=deep+cycle+battery&dpPl=1&dpID=412Hdo%2BNZ%2BL&ref=plSrch for it. Would only be runnning an old tm on occasion and a pirana depth finder. It looks like basicly one for an alarm system or power weels battery. Anyone have an idea on if this is worth it or should i bite the bullet and spend $90 on a regular deep cycle?

It would be good for the depth finder, but it would never run a TM for more than a very short time.

DO NOT go cheap on your trolling motor battery - even for your smaller boat.  ALWAYS go with the largest capacity you can afford.  If you've got to save weight, save it in other areas.  I'd go with a separate battery to run the depth finder.   For small boats, feature for feature, I don't think that you can beat the Hummingbird Fishing Buddy series.  Batteries are self contained in this model.  Skimping on your primary battery power will sooner or later come back to bite you.

There is no other weight to save. Its me, battery, clamp on TM, depthfinder, and a couple rods and a walmart sack with a few extra worms or lures. I use it 2-3 times a year max. Its 8' long and fits in the bed of my frontier with room on the sides between the wheel wells. The depthfinder is an old 'bird i took off bass boat when i upgraded it. I only use tm to move to around from spot to spot on a farm pond. Its not for use on moving water or big lakes. I motor over to a spot and drop a gallon jug of sand as an anchor. I just dont want to spend $90 on a battery to get 8 or 9 uses out of it before age gets it. It stays in my barn on a trickle charger rest of year.

Btw. The only problem with the battery weight is me carrying it from the truck to the lake.

Simple math(Don't freak out!)...Battery amp hours (Ah), divided by the load(A), equals run hours (h).or...

Ah÷A=h

Just need to know the load.

(By the way I use a similar battery 12v35ah wheelchair battery)

Its an old shakespeare wondertroll 606. Best i can tell its about 12 amps on high

If you're worried about weight the only real option is a dry cell and it's going to cost you.