Bought a new starter battery for my 50ESL73R Johnson and it started right up. I had some separate coolant issues, took the lower unit off, serviced and replaced. All back together, Icranked it over and the starter spun but briefly made some contact with the fly wheel, but didn't start. After that, the voltage on the battery went below 12 amps, then to 9 amps, and continued down from there and there was not enough juice to even to get a "click."
Question, is their a short somewhere in the system that is draining the battery that quickly?
If there is a short somewhere draining the battery quickly something will be getting hot , or at least very warm but you do not know for sure how well the battery was charged when you fitted it
If you have drained the battery as low as 9 volts, when measured with no load, you have done it no favours.
Get the battery on to a good multistage battery charger with a maximum output current adequate for the size ( Ah capacity) and charge it until you can measure about 12.7 volts if the battery is allowed to stand with no load or charge for 12 hours.
ideally then monitor the rate at which the volts fall. They will fall quickly at first but should take days if not weeks to fall below 12 .6.
If you cannot get the battery up to 12.7 rested or it wont retain a good charge it is probably not going to be much good although some battery chargers will to some extent "rescue" a deeply discharged battery.
When you reconnect the battery take great care that you do not reverse the polarity , as that will blow the diodes in the rectifier. It would be sensible to measure the current flow if any when you connect it. Either connect it cautiosly with an ordinary ammeter in circuit or measure the current with a clamp type ammeter that can measure DC amps.
If you find any measurable current ( more than say a few milliamps ) you will need to investigate the reason for it. If you measure a high current, disconnect again immediately and then investigate.
