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A few days ago it was -20 and my car wouldn't start. It cranked for a few seconds then slowed down. Tried to jump start it but had the same results.
I checked voltage on the battery posts and it was 12.3, when the car is cranked the voltage drops to around 6. The battery rebounds to 12.3 after a short time.
Does this sound like my battery or starter? I would think if the battery was bad it would still start with a jump. Any input will be appreciated.
I would agree with the assessment of a bad battery, especially if the battery is over or nearing five years of age. Just the average life span of a battery. As described, it is doing one of the things which can go bad with a battery, that being dropping voltage when under significant load, yet showing just fine when not. As stated, get it load tested to make sure.
As for it not working while attempting a jump start, realize most available jumper cables on the market are not of sufficient gauge to provide the needed amperage to start a car. Normally, these are considered "booster cables", which provide some power during starting, but their main goal is to actually recharge the battery for a short time until there is enough power in the battery to allow it to start the vehicle. Since your battery already looks good when not trying to start the car, it's not going to absorb much more power. When you turn the key to start the car, the high load causes the batter fault, and the booster cables are not enough to get the engine going.
To me it sounds like a bad battery. One common problem is that cracks develop in the internal connections. They work fine when there is no or little load on the battery, so when you measure with your voltmeter it shows good.
But put a load on the battery, especially a heavy load like starting the engine and it fails and the voltage drops.
Take the battery to a shop and have it load tested. Most do it free of charge. That will let you know if it's the battery or something else.
