I have a 5 ton GVW flat bed trailer with dual axles and electric brakes. I rarely use the trailer. The brake battery does not get charged as a result. I thought it charges whenever the trailer electric is plugged into my truck and the truck is running but now I'm not sure it does charge. It's a 12 volt battery approximately 6" tall by 4" wide with flat blade terminals. Should it charge when my truck is running? Can I use a regular 12 volt battery charger to charge it? Appreciate the advice.