I have an 2003 Merida PC550 24v ebike which uses two 12v 9ah batteries. Is there a converter available to use one large 12v battery to get the same output? I know that I can use two 12v batteries in series but I just want to test it before I sell it.
From what I can tell, your Merida used two 12v lead-acid batteries in series to power a 24v motor.
Motor speed directly correlates with voltage, so if you ran your motor on 12v, you would only go half as fast, if at all. Furthermore, you wouldn’t be able to charge your 12v pack with the standard 24v charger.
Some people have replaced lead-acid packs with lithium batteries and used their own chargers. A seven cell system for example would be 25.2v nominal. But you might risk burning out the standard controller as max voltage with this is 29.4v while the controller only expects around 27v max from lead acids.
In any case, this is the wrong forum for further advice down this road. I suggest endlessssphere’s forum for more hacking advice.
There is a circuit called a Boost converter, which converts one DC voltage to a higher DC voltage, at the cost of using more than double the current.
The opposite is called a buck converter.
However this is electrical engineering and getting well off-topic for this site.
So to test your bike you could wire up two car batteries in series, giving you 24V. Not a particularly mobile setup, and any shorts will be a shower of sparks so think carefully if you do this. There's no significant danger to your batteries unless you short them or drop/crack them.
Frankly unless you think the bike was not working already, then two new SLA batteries of the right size and clip format should just work. And then you might choose to keep riding your ebike (they're quite trendy now!)