This sidebar explains my choice of battery for the Prius battery current meter. The LCD panel meter needs a 9 to 12 V supply that is isolated from its input signal (from the current sensor in the SMR). Taking power from the car is not possible without a complex isolation and regulation circuit. The first version of the display used a 9V battery of the type you put in smoke alarms. This was, in my opinion, too big and heavy to mount on the dash with my slide-in bracket. So, four wires ran from the display - two for the signal and two for power (most telephone cords conveniently have this number of wires.) The battery and on/off switch sat on the floor. However, I didn't like the fat cord or the inconvenience of rummaging on the floor for the switch. I decided to put the switch on the display and mount a smaller battery behind the display so that I could reduce the number of connections to two and use a thinner cord. The MN21 battery appears to be widely available and is used in "keyless entry devices". It is cheap; Home Depot sells a pack of two for $1.77. It is a little smaller than an N size battery, but even so fits nicely in an N size holder (with a little modification as described above). So - small, light, widely available, cheap, fits in a holder - the only problem is it doesn't hold much energy. The LCD panel meter takes about 1 mA. Duracell publishes a typical discharge characteristic for the MN21 which shows it falling to 7V after 30 hours at 1 mA. Since this is the lowest voltage at which the meter will work, you can expect 30 hours of operation before the low battery indicator comes on. If I use the meter most of the time I'm in the car, I'd be replacing the battery once a month. After the novelty wears off, I'm sure I'll only use it when something interesting crops up, like a steep hill or a chance to accelerate hard. For me, this all seems fine, but if you want to use a 9V battery go ahead. You'll get longer service but you'll have to figure out where and how to mount it.
Above I said that the power supply to the LCD panel meter must be isolated from the supply. This has caused some questions, so I have to say that I'm not making this up. I read it from the rather brief document that came with the panel meter. Out of curiosity, I looked up the chip that is most likely the heart of the thing and it's pretty clear from the example applications that they're not joking. The ground side of the input signal is not connected to one side of the supply or the other. Instead, it is connected to a voltage part way between the supply inputs. Connecting it to either, as you would if you tried to power the meter directly from the car, would cause it to stop working (at best). This is true for at least one LED panel meter that I surreptitiously unpacked in my electronics hobby store. In fact, I've not yet found a panel meter for which it is not true. So, we're stuck with a battery unless we want to build a transformer-isolated DC-AC-DC converter. Go ahead if you want, but that's too much trouble for me; I'm sticking with a battery.