A good battery load tester is essential for anyone who tinkers with cars, boats, off-road vehicles and more. Just because your battery shows 12v doesn't necessarily mean it's good to go. Test your car battery under a load to tell for sure. The Harbor Freight Cen-tech battery load tester also can show the status of your car charging system. They cost only $19.99 or less with your coupon.
Harbor Freight 100amp 6v/12v Battery Load Tester:
https://www.harborfreight.com/automotive/battery-tools-accessories/100-amp-612v-battery-load-tester-61747.html