Start with something simple. Hopefully you have a volt meter. Attach the meter across the battery and observe the meter reading. Should be a minimum of 12.6DCV if fully charged. If not fully charged, attach a battery charger to fully charge the battery. Next try to start the unit, watching the meter. If the reading drops below 10.6DCV your battery is bad and needs to be replaced. If you have no meter or charger take the battery to an auto parts store and have them do a load test on the battery. If you need to replace the battery make sure to get one with a minimum of 325 CCA.