Batteries wired in serie

I still can’t keep this straight but back to cars. My diesel Olds had two batteries with the positive to positive and neg to neg, connecting the two, so I assume it was still 12 volts to the starter and not 24 like I once thought. Once I went down to start the car at work and got nothing, just a spark at one of the batteries. Cables were tight etc. Never understood what happened but went and picked up two new batteries and all was well again. I guess I always assumed one was bad and drew the other one down. Don’t suppose it matters after 20+ years but was always surprised how with two batteries, they could both just go dead so fast without warning. Back to lawn mowers.

Bing- check this out- https://books.google.com/books?id=oM8DAAAAMBAJ&pg=PA54&lpg=PA54&dq=diesel+olds+two+batteries&source=bl&ots=ElLLK8ZQEr&sig=wEVQIKsqwOqIICn20Bt5Uy2x5XI&hl=en&sa=X&ved=0ahUKEwjducH0y4LPAhXBHB4KHYZ3A0UQ6AEIKzAF#v=onepage&q=diesel%20olds%20two%20batteries&f=false

still is 12V but doubles the amperage.

Thanks. There was only one time the car failed to start and needed to be jumped. We were at scout camp and it got down to below 20 below over night. I had gone out and started it and warmed it up too but I should have driven it I guess. We almost gave up and called a truck trying to jump it but it finally fired. No power in the woods to plug it in and that hill was supposed to be 4 wheel drive only but I made it fine. Them were the days.