I have a Casio MA-120 keyboard (musical). Which requires 5 AA batteries. On back of the keyboard there are power source instructions which says:
CASIO MA-120 RATING : DC 7.5V --- 2.3W use BATTERY 1.5V X 5 or AC ADAPTOR AD-1
I have been using dry cell batteries like Duracell or so until now, but I'm looking for something like a rechargeable LiPO solution which I can plug in and use the keyboard. I have adapter too but I would like to make keyboard kind of portable too.
So what capacity LiPO battery I should use or there are any other options?
LiPO cells are ~3.7V, so you could use two in series to get 7.4V, which is very close to the keyboard voltage requirement. Depending on the arrangement and dimensions of the battery compartment, you might be able to fit a couple of small LiPO cells in. You could solder some wire to the terminals that normally contact the AA cells and create a connector that allows you to attach a LiPO pack. Just be sure to disconnect it when you use alkaline cells.
If you want to meet or exceed the running time of alkaline AA cells*1, select LiPO cells that have a capacity of about 2500 mAh.
One possible easy solution would be to use some rechargeable batteries such as NiMH size "AA" batteries. They are 1.2 volts instead of 1.5 volts like alkaline cells, which would yield 6 volts instead of 7.5. It's possible that the keyboard would work with 6V but you'll have to test to be sure. Failing that, add one more cell to get 7.2 volts and use an externally connected battery holder:
You might have to come up with a creative way to attach the battery pack to the keyboard, depending on how portable you need it to be.
*1 Based on discharge rate of 100 mA.