This means that if they sat around on the shelf for a few months, they lost much of their charge. Secondly, the self-discharge rate of a NiCad battery is around 15–20 per cent per month. But you had to be careful-completely discharging a NiCad battery also damaged it. Ensuring the battery was properly discharged before recharging it went some way towards preventing this problem. This was caused by the formation of large, rather than small, cadmium crystals during the recharging process. Firstly, they were prone to something called the ‘memory effect’, where the batteries would ‘remember’ previous discharge levels and not recharge properly. However, if a lead-acid battery is allowed to discharge too much, or is left too long before recharging, the coatings of lead sulphate form into hard crystals that can’t be removed by the charging process.ĭuring discharge, the reaction at the anode is the creation of lead sulphate, along with some hydrogen and electrons, as lead reacts with sulphate from the electrolyte solution: The Pb 2+ ions then pick up two electrons and are re-plated onto the anode as neutral Pb.Īt the cathode, the Pb 2+ ions give up two electrons to form and react with water (H 2O) molecules to re-form neutral lead dioxide on the cathode, and some bisulphate ions that go back into the electrolyte solution. The lead sulphate coatings are dissolved and forced back into the electrolyte as Pb2+ and SO 4 2- ions. That’s why when you’ve left your car lights on and the battery’s gone flat it’s advisable to drive around for a while after getting the jump-start to give the battery time to charge up again.Īs the battery charges, the chemical reactions described above that produce the electricity are forced backwards. Lead-acid batteries are rechargeable-the ones in our cars charge up using a little generator connected to the engine, called the alternator. ![]() This also produces lead sulphate, which again forms crystals, this time on the cathode. The electrons travel over to the cathode via an external circuit, where they, along with bisulphate and hydrogen ions from the electrolyte, react with the lead dioxide cathode. The lead at the anode reacts with the bisulphate from the electrolyte, freeing up some electrons, and producing lead sulphate, which forms crystals upon the anode, and hydrogen ions which go into the electrolyte. The electrodes are placed within an electrolyte solution of sulphuric acid (H 2SO 4), which is made up of hydrogen ions (H +) and bisulphate ions (HSO 4). In fact, both electrodes (the conductors through which electricity enters or leaves the battery) contain some lead-the anode (positively changed electrode) is made of lead metal (Pb) and the cathode (the negatively charged electrode) is lead dioxide (PbO 2). Image source: Steve Rainwater / Flickr.Īs the name implies, these batteries have some lead in them. Lead-acid batteries are commonly used to start car engines. These were invented by Gaston Planté in 1859. ![]() When was the last time you had to pull out your crank handle, insert it into your car’s crankshaft and give it a good turn to get the engine running? Never? That’s because we have lead-acid batteries connected to our car engines that provide that burst of power the engine needs to get going.
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