A system comprising: a first battery (31, 31') comprising: a first battery casing (32, 32'); at least one first battery cell (33, 33') carried by said first battery casing; at least one power measurement circuit (34, 34') carried by said first battery casing and configured to measure, upon being powered, a power level of said at least one first battery cell using an analog-to-digital converter without using a microprocessor; and a first Near Field Communication, NFC, circuit (35, 35') carried by said first battery casing and configured to communicate the power level measurement via NFC communication; and a mobile wireless communications device (36, 36') comprising: a portable housing (37, 37'); a second battery carried by said portable housing to supply power to the mobile wireless communications device; a second NFC circuit (38, 38') carried by said portable housing, said second NFC circuit (38, 38') comprising an active NFC circuit configured to generate an electromagnetic field to power said first NFC circuit and said at least one first power measurement circuit (34, 34'); and a controller (39, 39') carried by said portable housing and supplied with power by the second battery; the controller being configured to: cause said second NFC circuit to receive the power level measurement from the first battery via the NFC communication between the first and second NFC circuits based upon proximity therewith to thereby test a power level of the at least one first battery cell without the first battery (31, 31') being connected to an external load and without uncoupling the second battery from the portable housing or ceasing a supply of power from the second battery cell of the second battery to the controller. a first battery (31, 31') comprising: a first battery casing (32, 32'); at least one first battery cell (33, 33') carried by said first battery casing; at least one power measurement circuit (34, 34') carried by said first battery casing and configured to measure, upon being powered, a power level of said at least one first battery cell using an analog-to-digital converter without using a microprocessor; and a first Near Field Communication, NFC, circuit (35, 35') carried by said first battery casing and configured to communicate the power level measurement via NFC communication; and a first battery casing (32, 32'); at least one first battery cell (33, 33') carried by said first battery casing; at least one power measurement circuit (34, 34') carried by said first battery casing and configured to measure, upon being powered, a power level of said at least one first battery cell using an analog-to-digital converter without using a microprocessor; and a first Near Field Communication, NFC, circuit (35, 35') carried by said first battery casing and configured to communicate the power level measurement via NFC communication; and a mobile wireless communications device (36, 36') comprising: a portable housing (37, 37'); a second battery carried by said portable housing to supply power to the mobile wireless communications device; a second NFC circuit (38, 38') carried by said portable housing, said second NFC circuit (38, 38') comprising an active NFC circuit configured to generate an electromagnetic field to power said first NFC circuit and said at least one first power measurement circuit (34, 34'); and a controller (39, 39') carried by said portable housing and supplied with power by the second battery; the controller being configured to: cause said second NFC circuit to receive the power level measurement from the first battery via the NFC communication between the first and second NFC circuits based upon proximity therewith to thereby test a power level of the at least one first battery cell without the first battery (31, 31') being connected to an external load and without uncoupling the second battery from the portable housing or ceasing a supply of power from the second battery cell of the second battery to the controller. a portable housing (37, 37'); a second battery carried by said portable housing to supply power to the mobile wireless communications device; a second NFC circuit (38, 38') carried by said portable housing, said second NFC circuit (38, 38') comprising an active NFC circuit configured to generate an electromagnetic field to power said first NFC circuit and said at least one first power measurement circuit (34, 34'); and a controller (39, 39') carried by said portable housing and supplied with power by the second battery; the controller being configured to: cause said second NFC circuit to receive the power level measurement from the first battery via the NFC communication between the first and second NFC circuits based upon proximity therewith to thereby test a power level of the at least one first battery cell without the first battery (31, 31') being connected to an external load and without uncoupling the second battery from the portable housing or ceasing a supply of power from the second battery cell of the second battery to the controller.