Impact of high constant charging current rates on the charge/discharge efficiency in lead acid batteries, for residential photovoltaic system applications

Abstract

The charging and discharging of lead acid batteries using Traditional Charge Controllers (TCC) take place at constantly changing current rates. These techniques do not permit the accurate estimation of energy input and energy output during charge and discharge processes. In this work, the main objective is to investigate the effect of high constant charging current rates on energy efficiency in lead acid batteries, extending the current range to 8A from 5A already reported in literature. We also present the experimental proof of the variation of electrolyte temperature with magnitude of the constant electric charging current. Firstly, a Constant Current Circuit (CCC), capable of charging the battery at current rates ranging from 0.5A to 8A was built and used to run experiments on two sample lead acid batteries, battery sample 01, the Vanbo battery and battery sample 02, a Winbright battery. Charge and discharge processes were conducted on these batteries through the CCC and efficiencies computed per current rate tested. Secondly, to compare the results of the CCC built with those of a TCC, the very batteries (battery sample 01 and battery sample 02) were charged and discharged through a TCC and their corresponding efficiencies computed. Thirdly, three constant charging current regimes (0.5A, 5A and 8A) were chosen within the tested current rates for which further electrolyte temperature monitoring tests were carried out, using two other lead acid battery samples of different health states. This was carried out on battery sample 03 which was a SIGA lead acid battery type with a compromised state of health and on battery sample 04, another brand new Winbright battery with relatively good state of health. It was noticed that, for the CCC, efficiencies between 62 % and 82 %, were recorded for the Vanbo Battery (Sample 01) while efficiencies of 53 % to 91 % were recorded for the Win Bright Battery (Sample 02). On the other hand, the efficiency of the Vanbo Battery (Sample 01) through the TCC was found to be 66 % while that for the Winbright battery (Sample 02) was found to be 79 %. As for electrolyte temperature monitoring, battery sample 03 with compromised health state registered higher temperatures for higher charging rates while battery sample 04 of good state of health registered no noticeable increase in temperatures even for constant charging current rates up to 8A. Since existing literature had tackled lower current values from 0.5A to 5A, this work therefore comes in with an extension of the current rates, testing higher current magnitudes and obtaining the same results with conclusion that, if the same energy is stored in a lead acid battery at precise rates, the charge/discharge efficiency of the battery increases as the charging current rate increases. As regards electrolyte temperature monitoring, the work provides further proof that even at charging current rates as high as 8A, battery electrolyte temperatures still remain within acceptable limits(below 50 °C).