Tony gave you some great advice, go with the largest amphour battery you can . Typically when you get a spin but no fire condition, it’s because the engine is not turning over fast enough for the C box to detect, so it doesn’t tell the ignition to fire. We use a 12V power supply to charge our tag batteries, a standard charger stops charging when the battery is around 12 volts. The power supply will charge the battery to around 14.5 volts , and it will start every time. It hasn’t harmed a battery yet, we typically run the same 6 batteries for an entire season ( we are a 3 kart team) and replace them during the off season every year.