ZTS has meters that test under load. However, the readout is not a voltage, but expressed as a percentage, i.e., 40%, 60%, 80%, 100%. Also, the only battery chemistries it is calibrated for are NiMH and alkaline. If you are using Lithium or the rechargeable Li-Ion 9 volts you have to learn how to interpret the readout. For example, an I-Power Li-Ion comes up to 80% on the alkaline tester when fully charged; less than that is not trustworthy, we've found.
Admittedly, life was simpler when all we had to worry about was 9 volt alkalines. In those days a tongue test would do it. (I only used the lip, never the tongue.) Nowadays, there are several different battery chemistries to manage, each with its own output characteristic, and a simple voltmeter is probably not sufficient. In an effort to manage and minimize our contribution to the waste stream we are using more rechargeables which seem to require closer attention.
-----------steve nelson
Downtown Los Angeles