Buy larger CCA battery or buy two & put in series

Currently l have an century 86MF which are 830CCA it seems to struggle cranking over, it has been suggested l should buy another & run in series. Buy 2 or buy bigger battery? Any suggestions

Check all the connections first..Clean up the the terminals and chassis contact points. Heavier gauge wiring might also help.

Two batteries would need to be in parallel, not series.

I think one bigger battery is the way to go. With two in parallel they have to be exactly matched otherwise one will end up doing all the work.

Also depends on your existing battery tray space.
One large battery is cheaper than 2-mediums, but the big battery will be heavy.
I changed to using all N200 batteries, modified the battery spaces to suit.
They have massive CCA but weigh around 60kg which is a big lift up to a digger etc.

I don't think the 86 size is cracked up to what they say. Used to run one in my 4 litre Dyna and was never quite up to the job. The smaller N70ZZ (Landcruiser size) did the same if not better job and was cheaper.. Next step up from an 86 would be an N150 - big bugger. As said before, your starter might be getting lazy and would get that checked first.

Pat, are you trying to start the Cummins in your W Model??
If so you'll need more than 1 86 battery..
I'd use 3-4 N70 Batteries hooked up in Parrallel.

Edit, up where you are I reckon 3 would be plenty..

Second looking at cable size and connections, if in doubt always go bigger on the cables and run the ground to that starter as well. Seen too many battery to chassis then chassis to engine with poor connections. Dual batteries are fine if they have an isolator/charge control to keep them both fully charged.

You would only need an isolator if you are running accessories off just one of the batteries, when they are not being charged.

With an isolator/solenoid set up you could use different size batteries and if 1 goes weak it does not pull the other/s down with it. Same with just replacing 1 battery in a bank, some will take charge better than the others.
I know they have been run in parallel and series parallel for years but that may not give the best battery life.

the problem with parallel batteries is that one or more will always fail with out you knowing until its to late because electricity always takes the easiest path
so the better battery gets charged while when discharging the weakest battery discharges first so what happens the weaker battery never really gets charged and looses charge first, there are ways to over come this but by far the simplest way is to buy one big battery or buy 6 volt batteries and run the in series both of these options prevent the above from happening