re lead acid batteries part the lead acid secondary cell releases energy electricity with the following chemical reaction pb pbo h so pbso h lead and lead iv oxide and sulfuric acid produce lead sulfate and water heats of formation deleted the heat of reaction at c is therefore kcal per mole pbso produced note that lead sulfate is not very soluble grams per grams water at c and it will thus precipitate out of solution where the reaction is occurring or the cathode positive terminal of the battery i am almost sure it is the positive terminal where the precipitate forms but i may be wrong oh well i don t have a corroded battery to corrobate and i don t feel like thinking through it right now the major problem with this is that the reaction takes place in an acid solution pbso is soluble in an acid solution and will not precipitate out also h so is in a water solution as h and so thus the heats of formation of pbso and h so are for the most part irrelevant what is important to notice here is that the reaction as you knew it would be is exothermic or energy discharging as it turns out the reaction is indeed exothermic heat producing more about this later what actually happens to make the battery completely useless is this we re talking lead acid batteries of course the battery slowly self discharges as this discharge takes place two things happen the level of pb ions in the acid solution increases i e the lead and lead oxide plates are dissolved the level of h ions in the acid solution decreases i e the solution becomes less acidic or more like water if you like now as the post to which i am responding correctly stated pbso will precipitate in a water non acid solution when the battery dies i e is fully discharged we end up with a high concentration of pb and so in water so pbso forms in the solution and falls to the bottom of the battery of course this happens in varying degrees the more discharged the more precipitate forms the precipitate forms a conductive layer on the bottom of the battery if there is enough of the lead and lead oxide plates left to touch the precipitate more common in a newer battery a dead short results i have seen products in automotive shops to correct this condition but they are for the most part useless they can dissolve the pbso but cannot restore the lead and lead oxide plates properly you may have some success with these products for a newer battery stuff deleted to understand why lead acid batteries do indeed discharge faster when stored on concrete as opposed to wood or earth dirt one should recall lechatelier s principle which can be paraphrased as anything subjected to some stress will act to move to a more comfortable position here are the thermal conductivities of a some selected materials stuff deleted this is where lechatelier s principle comes into play removing energy from the exothermic reaction will drive the reaction further to completion if the reaction normally occurs at room temperature keeping the battery at that temperature requires the removal of any heat produced a concrete surface is a better heat sink than a dirt or wood surface store a battery in the corner of a poured concrete basement and you have surfaces removing energy which pulls the reaction along this stuff is just made up by the author and is completely invalid in fact the discharge reaction takes place at a higher rate at higher temperatures a logical consequence of the above argument is this if you really want your car to start lower the battery temperature to to pull the reaction along we all know from experience at least those of us in canada do it gets cold up here that this is not true if we want to start our car on a really cold day we warm the battery besides which there is not enough energy released through self discharge to appreciably raise the temperature the air would amply dissipate any such heat whether the bottom of the battery was insulated or not this is of course irrelevant since you would want the battery to be cool during storage just keep the battery in a cool dry place and keep it charged
