Starter fault

[spg handyman]

Hi got a anoying problem on my 2011 trend 115bhp mwb
When i turn the key to start position i get click and lights dim and then it turns over quickly and starts.
I have for checked volt drop on earth and live both ok . But i have noticed that the battery voltage drops to 8.4 v which is low . If i connect a boost pack it is still the same .
I have renewed battery, starter and ignition switch .
Any idea's
Thanks in advance
Steve

[spg handyman]

Tried another battery on now getting fault less often and seems ok with boost pack .
What is the correct battery for a 2.2td fwd ? As it appear there doesn't appear to be enough cranking current .

[ned]

Fit the biggest battery you can fit under the seat, Bosch 110 batteries are good, also fit the highest CCA battery you can get, have you only got 1 battery under the seat?. N

[spg handyman]

Only got 1 battery under the seat . Did wonder about putting a second 1 under there in parallel.
The one I've got is a 720 cca but it just the initial hit as the starter engages and then it spins over fast and starts .
Steve