Using 12V battery instead of power supply

[Gizmo1977]

There's this digital keyboard/pad/multiFX/ whatever device that runs with a PS that says 12V 1500mA.
It would be interesting that this whatever could work autonomous among the crowd for some time in the show.

I have been given lead batteries before but would want to know the theory/limits behind.

From those 12V 1600mA PS specs:
1) I need a 12V (true/false) unless specified from the. manufacturer.
2) According to the salesman I don't compulsory need a 1600mA battery but the figure determines the time range. If I used a 12V 1600mA would it last ""60"" minutes, ""30"" with a 800mA one... and so on? ("""""" mean theoretically/lab and I should assume it's not that linear, and a bit exponential in the first minutes). I always thought (using PSs, perhaps thats the difference) that one would need at least the specified mA.
3) READINGS: If I measure voltage in the battery terminals (stand alone) it will draw a different value from readings if the device is connected (true/false)
4) TOLERANCE (from 3): While I use the battery I can see the voltage dropping. As long as when it's fully charged it's beyond 12V (12,4 or so): Is there any compliance in Europe that would tell me what are safe limits over and under voltage? Not over X% or below Y%?
5) UNDER VOLTAGE: I have always assumed that if you are over voltage tolerance you can damage the piece of gear. In case you are under voltage: would the device simply not work (or at least not as it should) could it damage the device (a digital one) in terms of EPROMs, presets, patchs, etc. or it would just stop working?

Thanks in advance, mind my English and sorry if the question is confusing or assumes things can be easily answered.

[Philbo King]

Batteries are rated by Ampere-Hours. In this case, (as examples for your case) a 1500 mA-H 12V battery could be expected to last an hour, while a 4500 mA-H 12V battery could last 3 hours. But that's not necessarily a guarantee, especially if you got it from a sketchy source like Temu.

[Gizmo1977]

Thanks, Philbo!!

[Sam Spoons]

I use a 12VDC Li-Ion battery pack to run different devices* with no problems. Run times are sufficient so I haven't actually tested how long it lasts.

Li-Ion batteries are best as can run down to virtually zero without problems (and, in a battery pack the BMS will ensure it shuts down before risk of damage), sealed lead acid batteries also work fine** and are cheap but are bigger and heavier and can only be run down to around 50% so you only get to use around half the rated capacity.

* Mostly a ZT LunchBox Junior guitar amp and a Tone Dexter preamp.

** I have modified an Epiphone Valve Junior with a 12V SLA and an inverter built in.

[Gizmo1977]

Thanks, Sam!

The guy at the store (an on other place I visited) offered me an AGM, perhaps just to get the most cost effectice solution. I wouldn't care if it was more expensive as long it is safer or offers any improvement. That 12V 1,3A amp battery does the job (it is intended to be used just for 20 minutes or less).

The thing I'm worried about is about voltage.
When I measured the volts from the terminals of the battery (out of the box, keyboard off) it showed 12,6V.
Leaving the probes ln the terminals while the module was playing music it went down to 11,80V after 30'.
I'm assuming readings are always lower when the device is ON.
Then I full charged it and the readings went up to 13,40V. As it is +11% I waited until it dropped to 12,5V which is 4%.

[Sam Spoons]

You probably need a voltage regulated battery pack rather than a raw battery, a nominally 12V Li-Ion battery will reach 14.8 volts when fully charged and shut down at around 10V. I use one of these https://www.ebay.co.uk/itm/354725270795 which has proved fine.