Electric vehicle
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| 3.3 Lead Acid Batteries 3.3.1 Lead Acid Battery Basics Until recently the best known and most widely used battery for EVs is the lead acid battery. Lead acid batteries are widely used for starting IC engine vehicles and as such are well known. However, for EVs, more robust lead acid batteries that withstand deep cycling and use a gel rather than a liquid electrolyte are used. These batteries are more expensive to produce. In the lead acid cells the negative plates have a spongy lead as their active material, while the positive plates have an active material of lead dioxide. The plates are immersed in an electrolyte of dilute sulfuric acid. The sulfuric acid combines with the lead and the lead oxide to produce lead sulfate and water, electrical energy being released during the process. The overall reaction is Pb + PbO 2 + 2 H 2 SO 4 ↔ 2 PbSO 4 + 2 H 2 O The reactions on each electrode of the battery are shown in Figure 3.4. In the upper diagram the battery is discharging. Both electrode reactions result in the formation of lead sulfate. The electrolyte gradually loses the sulfuric acid, and becomes more dilute. When being charged, as in the lower diagram of Figure 3.4, the electrodes revert to lead and lead dioxide. The electrolyte also recovers its sulfuric acid, and the concentration rises. The lead acid battery is the most commonly used rechargeable battery in anything but the smallest of systems. The main reasons for this are that the main constituents (lead, sulfuric acid, a plastic container) are not expensive, that it performs reliably, and that it has a comparatively high voltage of about 2 V per cell. The overall characteristics of the battery are given in Table One of the most notable features of the lead acid battery is its extremely low internal resistance (see Section 3.2.1 and Figure 3.1). This means that the fall in voltage as current is drawn is remarkably small – probably smaller than for any of the candidate vehicle batteries. The figure given in Table 3.1 is fora single cell, of nominal capacity 1.0 Ah. The capacity of a cell is approximately proportional to the area of the plates, and the internal resistance is approximately inversely proportional to the plate area. The result is that the internal resistance is, to a good approximation, inversely proportional to the capacity. The figure given in Table 3.1 of 0.022 per cell is a rule of thumb taken from a range of good-quality traction batteries. A good estimate of the internal resistance of a lead acid battery is thus R = no. of cells × 0 .022 C 10 (3.3) The number of cells is the nominal battery voltage divided by 2.0, that is six in the case of a 12 V battery. C 10 is the amphour capacity at the 10 h rate. |