Electric vehicle
Download 3.49 Mb. View original pdf
|
| Table 3.3 Nominal battery parameters for nickel metal hydride batteries Specific energy Wh kg −1 depending on power Energy density Wh l −1 Specific power W kg −1 Nominal cell voltage V Amphour efficiency Quite good Internal resistance Very low, ∼0.06 per cell fora Ah cell Commercially available A good range of small cells, traction batteries difficult to obtain Operating temperature Ambient Self-discharge Poor, up to 5% per day Number of life cycles to 80% discharge Recharge time h, rapid charge to 60% capacity 20 min 46 Electric Vehicle Technology Explained, Second Edition Figure 3.9 A commercial NiMH battery, with integral cooling fans electrode, where it will react H+ NiOOH → Ni(OH) 2 This effectively discharges the cell – hydrogen is lost from the negative electrode and nickel hydroxide is formed at the positive electrode. The result is that this battery is subject to quite rapid self-discharge. An interesting feature of the cell, which can be seen by reference to Figure 3.8, is that the composition of the electrolyte does not change during charge or discharge – both water and OH − ions are created and used at exactly the same rate. The result is that the internal resistance and open-circuit voltage of the cell are much more constant during discharge than with either lead acid or NiCad batteries. Being backed by a metal layer, the cell has an internal resistance that is also a little lower – but not greatly different. The charging regime is similar to the NiCad battery, the current being switched on and off to keep the cell voltage between an upper and a lower limit. Like NiCad batteries, the NiMH battery can be charged within 1 h. Most cells can cope with an overcharge current of about 0 .1C, like the NiCad cell. As will be explained in Section 3.9, overcharging is necessary in a battery to make sure each and every cell is fully charged. Of all the new battery systems NiMH is considered to be one of the most advanced and has been used in a range of vehicles including the Toyota Prius. The market volume of NiMH batteries is still small, but with quantity production the price will drop. Download 3.49 Mb. Share with your friends:
|