Alternator does not charge at idle? (Grove RT-60S)

I watched the voltmeter gauge closely in my RT60S crane.

I have noticed that the battery, with the engine at idle, only has 12.5 or so volts.

If I rev the engine up, then voltage goes up to respectable 13.5 volts.

So, as it is, the crane does not charge the batteries very well.

I had a similar situation with my 30,000 lbs forklift a while ago, the only way I could resolve it was by making a custom smaller diameter pulley to speed up the alternator. I made it on my lathe.

Here, I am wondering "what is normal" and whether a change of alternator could improve the charging situation.

I do happen to have a spare rebuilt alternator, so cost wise it would not amount to much money.

Any comments? Is it normal to not charge at idle?

thanks

Alternator does not charge at idle? (Grove RT-60S)