Using float charging current measurements to prevent thermal runaway on VRLA batteries

In the past, several papers have discussed the relationship between the thermal runaway phenomenon in valve-regulated Pb-acid (VRLA) batteries and their float charging current. It has also been claimed that measuring float charging current is a proactive way to detect a large number of other nonhazardous VRLA battery problems before they can jeopardize the end-user´s quality of service. However, in the real world, measuring float charging current is very difficult to achieve because of the current level, usually in the milliamperes range. An effective measuring technique requires accurate reading of currents in milliamperes while sustaining the high charge or discharge currents of several hundred amperes. This paper presents experimental data showing that a thermal runaway situation can be prevented with the use of a new sensor based on a digital measurement technique. The test procedures used to induce thermal runaway situations on batteries are covered, as well as the float charging current digital measurement technique used in the experiment. A complete discussion on test results then follows. In conclusion, the authors discuss a different technique to compensate rectifier output voltage based on float charging current instead of battery temperature. This paper demonstrates that float charging current is directly related to the internal temperature of the battery and that the resolution of float charging current readings is higher than that of temperature readings. This leads to the possibility of improving the accuracy of rectifier output voltage compensation.