Another elec thread like this................... here we go. The answer will be the same as previous, that is "battery or leads/connections, what has been checked, how old is battery.
Firstly - what size/type engine? Because V6 won't drop voltage as much as V8 will.
People talking about electrics "love to compete" as I have found. They ignore facts/physics/elec theory, and it isn't helpful, so ignore anyone who tells you your battery needs to read a gazillion volts to start with.
My battery is perfect. When you turn the key on, it reads 12.5 volts (EVIC display). THAT IS NORMAL; in fact, a point or two under that is ok, as 12.5 is normal not under load, ign on gives slight load. Ok, straight away, everybody "But mine/my mates higher......" well, good for them. The Century battery site here says "a fully charged and stabilised battery will read 12.5 volts or over." That is good enough for me. If others think it isn't good enough, then................ I can't help with that, call Century. I quote the site because other forums here were telling people "If your battery EVER reads under 13.6, throw it away......" so they were ditching lots of PERFECTLY GOOD BATTERIES. The other reason I quote Century, is that no one listens to elec facts from people with trade/elec teaching experience. They all want to argue with "but mates said" - (mates that aren't elecs/can't fix the problem).
When I crank mine, it drops to 10 volts until it starts. That is NORMAL. If yours is a V8, higher draw, so 9.5 volts is probably normal. The slow cranking isn't. (Ijust went and cranked mine then to confirm. After sitting overnight, 12.5 volts ign on, 10 when cranking.)
So, there are 3 possibilities. Battery, connections, starter.
How old is the battery? Is it fully charged (if constant small trips/many starts, it may not be)? Has it been load tested?? In the US a battery load tester is about $15, cheap as chips, cheaper than the fuel to drive and get battery load tested at a shop. They are primitive and simple, but will work. Get one of those and try it, it will TELL you if your battery is no good. Battery HAS to be fully charged. And if it comes up as no good, then disconnect it, and check at battery terminals, to ensure it is battery and not bad terminal connection. Test at the battery, not at the jump point in engine bay. Electrically, they should be same point, but that is the point of testing as close to battery as possible.
Check battery connections in the boot (trunk). Either pull them off and clean, or test with a multimeter. They MUST be tested under load, otherwise pointless. I can tell you how to do that (or read many other threads where been discussed before) but load test battery first.
Finally, starter - if starter is drawing too much current, could be the problem. But check two things above first.
In the old days, we would say "Slow to crank? Try starting with gear lever in "N" rather than "PK";" as the Pk contacts get pitted/worn/dirty, wheras the "N" ones NEVER get used; but in these cars that is all elec signal voltages/very little current, and they hardly ever give trouble, but you could try that and see if any difference, just to eliminate it/takes 2 seconds.
Load test your battery first. Info about battery age/is it fully charged would help next time.
Good luck.
