Early 2011 Macbook Pro faulty battery and high CPU - will a new battery sort this?

[JohnMcL7]

I've built an early 2011 Macbook Pro 17 which had a wiped drive and started with Sierra USB stick then updated to High Sierra and the newest update. The battery has failed and can't power the machine at all when off the mains, the status lights on the side flash when doing the battery check and it shows 'replace battery now' staying at 0% in the OS.

The problem is when the machine first boots the CPU sits at 100% due to the kernel_task hogging it, after some time and particularly when it resumes after sleep it drops to 0 and the macbook behaves normally. From doing a read around it looks likely it's the battery that's causing the problem although there's a few other possible causes, is there anything I can check that would confirm the battery as the cause?

Thanks,
John

 

[Audit13]

Without a battery, macOS usually throttles the CPU speed.

What is the CPU speed once it's booted into macOS?

 

[JohnMcL7]

Without a battery, macOS usually throttles the CPU speed.

What is the CPU speed once it's booted into macOS?

I've tried running Intel Power Gadget which is hopefully giving correct results, when it's powered on and running really slowly it shows a CPU max of 0.9, avg of 0.8 and min of 0.7 and the req core is 0.8. I'm not sure if it's due to waiting or putting it to sleep and waking it that the behaviour of the laptop returns to normal and those values stay the same apart from the req core goes up to around 3.0