Several folks have told me not to let batteries sit on concrete. Why is that? Is it because the cold concrete would cool the battery too much?
Steen Hvidd - Dolan Springs, Arizona
Your question is a frequent one. Many people have the impression that when batteries sit on concrete, energy "leaks out" or they are ruined. The short answer is that letting modern batteries sit on concrete does not harm or discharge them in any way.
However, this legend is historically based in fact. The first lead-acid batteries consisted of glass cells that were enclosed in tar-lined wooden boxes. A damp concrete floor could cause the wood to swell, breaking the glass inside.
The Edison cell (i.e. the nickel-iron battery) that preceded the rubber-cased battery was encased in steel. Those that weren't isolated in crates would discharge into concrete quite easily. Later battery cases used primitive hardened rubber, which was somewhat porous and could contain lots of carbon. A moist concrete floor combined with the carbon in the battery cases could create electrical current between the cells, discharging them.
None of this is a problem with modern batteries — safe in their hard plastic shells. In fact, concrete is generally an excellent surface on which to place a battery bank. The electrolyte in a battery sitting on an extremely cold floor with very hot air around it could stratify, causing damage from sulfation; whereas concrete provides good thermal mass to buffer any temporarily extreme temperatures in the battery compartment.
Energy can in fact "leak" out of battery banks — though in different ways. The first is from current between the battery terminals, caused by dirt, dust, and grime becoming carbonized (and therefore electrically conductive) from acid released from the cell. This is easily preventable. Use a clean rag to carefully clean the tops of the battery cases every time you perform your regular battery bank maintenance routine.
The second way happens to all batteries—it’s called "selfdischarge." Due to reactions within the plates, all lead-acid batteries will lose part of their charge over time. The warmer the battery compartment and the older the battery, the higher the self-discharge rate. An L-16 battery will lose 4% of its charge per week at 80 degF.
This brings us back to your original question, where you mentioned battery bank temperature. There are multiple electrochemical reactions going on inside any battery, all the time. Some are good (storing and releasing energy), and some are bad (self-discharge, sulfation). All of these reactions happen faster when the battery is hot, and slower when it's cold.
Cold temperatures don't damage lead-acid batteries unless the battery is heavily discharged and exposed to freezing temperatures. In that case, the electrolyte (which is mostly water when the battery is at a low state of charge) can freeze and crack the case. On the other hand, a fully charged battery can withstand -30 degF or lower without a problem.
However, since cold temperatures slow the desirable chemical reactions too, the amount of energy a battery can release at any given time is drastically reduced when the battery is very cold. That's why it's more difficult to start your car on a frigid morning. And it also takes more energy to charge a cold battery than a warm one — cold batteries are less efficient at both charging and discharging. At the end of the day, a good rule is that batteries like the same temperatures that humans do, between 60 degF and 80 degF.
Dan Fink, ForceField - Fort Collins, Colorado