Paul
I will tell you why, because everyone went along like sheep, what do suppliers sell ? Leisure batteries.
I have got 3 vans with twin pumps, we have some massive jobs where the pumps pump for 8 hours continueos with no problems.
I have fitted systems for all the local window cleaners the same way, yet again no problems.
The people who have took my advice on this forum, yet again no problem.
Its not what most people say, its the people who say the right thing that matter.
You dont need an extra battery and you dont need a split relay.
Take this advice or leave it, I aint bothered, just letting you know how it really is.