re lead acid batteries part in article apr cronkite ocis temple edu camter astro ocis temple edu carter ames writes i thought the first thread was hilarious so here goes another post some more background information on what has happened to my poor batteries one year i left the batteries in the garage the garage is unheated they were left in their places that they needed to be one in the riding lawnmower what a lux and one in each motorcycle the battery in the riding lawnmower was about years old and one of the cycle batteries was new and one was around years old upon spring time i discovered that all of the batteries were dead and needed to be recharged they all worked properly except for the older ones and they had to be replaced about halfway through the summer they had originally been stored in the garage and i think i remember my dad charging them in spring last year i decided to bring them in to the basement which has two parts one is dirt and brick and the other is concrete i brought in all three and discovered in the spring about days ago that not only were the batteries dead but when i put the charger on them the charger said hmmmmmmmm and the amp meter read around in other words it was pinned the internal circut tripped and i went on to the other batteries they were the same realizing that we still had gigantic sailboat batteries also lead acid i decided to put the charger on them these had been sitting for around and a half years on the brick and dirt part and noticed that they only needed a small charge around hours or so on trickle i tested them by using them at the local gas station for giving jump starts and they worked fine for around of em this leads me to believe one of several things i bought really tty batteries last year sorta true the concrete has something to do with the discharge of the batteries there was enough moisture to short out the terminals in the room where the concrete is the dirt room was able to absorb the moisture in the air better than concrete btw i can almost waterski in the dirt room any other ideas could be the aliens anything is possible 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 the standard heats of formation at c f of the various compounds are heat of formation compound at c kcal mole pb pbo h so pbso h 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 what is important to notice here is that the reaction as you knew it would be is exothermic or energy discharging if moisture with dissolved electrolytes acid rain condenses on the battery a conductive path between the terminals may form this will discharge the battery as the chemical reaction proceeds the reaction is reversible if electricity from an alternator or battery charger is put back into the battery lead acid batteries must change in chemical composition to discharge each cell has all the reactants necessary for the reaction the non reacting posts terminals of the lead acid battery are there to remove or add electric energy during a discharge or charge because the reaction is exothermic it has a tendency to happen though quite slowly under normal circumstances so while keeping the battery dry is a good idea it is not a total solution 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 thermal conductivity material temperature c f btu hr ft ft f ft diatomaceous earth natural across strata diatomaceous natural parallel to strata diatomaceous earth powder density lb cu ft concrete cinder stone dry pine across grain parallel to grain oak across grain maple across grain note that the thermal conductivity for dirt diatomaceous earth goes down with temperature also c is about k and the values for earth powder are on the same order of magnitude as natural earth the thermal conductivity of concrete is around a factor of greater than that of dirt and is times greater than wood it is these differences in thermal conductivity that cause the battery on concrete to discharge faster than that on dirt or wood at any instant the discharge reaction is occurring energy is being released either as electricity or heat thermal conductivity of air is about btu hr ft ft f ft during storage except for the trickle that passes through any condensate on the battery this energy is mostly released as heat the higher conductivity surface will remove the heat proportionately faster than the lower conductivity surface thus if you perfectly insulated a dry battery on all but the bottom surface at any given battery temperature above the storage surface temperature the battery on a concrete surface will be discharging about times faster than that on dirt 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 also if water evaporates from the battery that elevates the ratio of sulfuric acid to water this will tend to drive the discharge reaction jeff white jhwhit ulkyvx louisville edu how do i get the ketchup out of my pocket protector french fries
