Experimental Physics and
Industrial Control System

The info you provide is confusing.
If your record is a longin, its value is an integer (not a string, not an error message, ...).
In your protocol, the out line is what the computer writes on the bus to fetch the status of the device.

Base on what I understand, you probably should use a binary out (bo record) for each of your batteries.
You can pass the battery number in the OUT line as a parameter to the protocol

record(bo,  $(prefix):$(number):BatteryOn")
{
    field(DTYP, ...
    field(out, "mydev.proto getStatus($prefix,$number)) dev)
}
record(stringin, $(prefix):$(number)Message")
{
}

with protocol
getStatus {
  out "Battery On \$2";
  in "OK";
 @mismatch {                                         
      out "Battery On \$2";
      in "%(\$1:\$2Message.VAL)s";               # Write on another PV                        
 }
}

The mismatch should not be required if you know all the allowed values for $number.
If that's the case, you just dbLoadRecords with the allowed $number parameters

I hope this helps.


--
Emmanuel Mayssat
http://www.epicsqt.org