A bit of a daft idea, but you could start it alternately off each battery. Needs two double pole relays, but that might be easy.

abt 18-24mth ago chap on a gopher in geraldton burnt to death- think he had lithium batts. i don't, but sorry, ur proposal is not safe enough.

 

abt 18-24mth ago chap on a gopher in geraldton burnt to death- think he had lithium batts. i don't, but sorry, ur proposal is not safe enough.

A lot safer than using a step down converter. What happens if the step down converter fails short and puts 24V on the motor?
Critical thinking tells me that the cause of the fire was someone inexperienced with lithium batteries.

 

re-my idea of using a small 12v charger-how do i stop it overcharging if batts normally floated on 24v intellicharger when not in use?
he prob had no idea, until it was too late. i have thought of doing it with fire/explosion proof battery case.
at least the converter is only briefly loaded, and it's likely less dangerous.
edit: adding a second parallel charger risks more imbalance, 2x 12v chargers in series with no common secondaries is only way to simultaneously charge 2 series connected batteries with different charges whilst not over/under charging one.

 

There is no problem at all with running the Motor directly on ~24-Volts,
as long as You reduce the amount of Torque that the Motor will be required to produce
by increasing the "effective" Gear-Ratio between the Motor and the Engine.

Trying to use a Rope is a bad joke.
It will FAIL miserably,
no matter what You "tweak", or how "special" your Pulley arrangement may be.

There are very good reasons why You have never seen this done before.
A Cogged-Belt, Gears, or a Chain will be required.
.
.
.

 

It sounds to me like he is planning on using a pulley to wind up the pull-rope starter. What could possibly go wrong?

 

It is generally considered bad practice to run anything of one battery in a series pair as either that one will end up undercharged or the other overcharged in time unless you recharge them individually

While you may be correct, it was a common practice in semi truck tractors for decades. 24V from two or sometimes even 4 12V batteries for starter motor, with the lights and other accesories running off of a single one of the 12V batteries.