[Rhoderman]

I'd say it sounds like a weak battery, or more likely a bad ground connection.
Where the battery cable attaches to ground is usually rusted or corroded. Also, your engine should be a good contact to the frame (no loose bolts). The absolute best way to connect the negative is directly to the starter-gen bracket if you can. It takes more cable, but will spin the starter with gusto.
If you think it's a bad battery, boost it with your jumper cables attached to your vehicle. If it doesn't spin a lot faster, your old battery was probably fine. Leave your car/truck off when you're boosting it though - you just want the extra battery capacity for this test. Disconnect the booster cables before starting your car/truck.