I thought I had a bad Interstate battery. Took to Walmart where they ran their rather extensive test, charge, run loads, etc. They said battery is good. What has been happening lately is half day of fishing one battery reads 12.67(?) and other 10.29(?) or less. Have a Motorguide 74# 24volt tm. Using a ProSport 12 which I leave plugged in all the time. HELP!!