Well, I should be fishing today, but the boat isn’t cooperating.

I have an older Johnson 150 - 1980. Very dependable until the last month or so. I’ve put the battery on charge for a day and went to start and it’s sluggish. Now, this Johnson is cold natured, but once it starts, it’s good as gold.

I’m trying to start the motor and it does take some cranks. After about 2/3 minutes with a full charged battery, it doesn't seem to have enough to continue a good starting surge. I charged the battery over night. It doesn’t seem to have enough to get it going. I checked the battery and it read 12.60 which seems like this has enough. So I used a jumper cable from my car and the power was there. Shouldn’t 12.6 be enough to start the boat even if it’s cold natured? Once I put the car on it, I could hear the extra power. Does the car provide that much extra amps? Is there something on the boat going out that prevents the battery amps to be minimized? Why wouldn’t the normal battery just start the boat?

Very frustrated.
Any help is appreciated.