By what mechanism does contact with concrete drain a car battery.?

Question

I know about the observed behavior that when a car battery is set on concrete for any time over about ten minutes, it is drained of all electrical charge and cannot be recharged.

What I dont know is how this works. What is the physics behind this fact?

I've heard many explanations, but I have never heard a solid ligical reason why this happens.

Answer 1

I think that this is just an urban legend (Unless the concrete conducts enough heat away to make the battery freeze).

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Continental Battery Storage:

Q: Will a battery rapidly self discharge if placed on concrete?
A: No, placing a battery on concrete will not cause it to discharge any faster than any other surface.

Q: Can I store a battery indefinitely?
A: No, a battery will self-discharge slowly over time. Allowing a battery to sit in a discharged state will ultimately lead to severe positive grid corrosion and battery failure. An unused battery should never be allowed to sit over 6 months without a recharge.

Answer 2

Thats a stupid myth, it drains the same as if it were on a table and on concrete, if you leave a battery sitting somewhere for a couple weeks it will die because it slowly loses it charge heres how: he lead acid battery in your car does not create electricity like an alkaline battery: It can only store electricity through a chemical reaction.* Inside the battery are a series of lead and lead oxide plates submersed in an electrolyte containing sulfuric acid and water. When discharging the plates react with the electrolyte creating lead sulfate. Normally this is in a form that's easy to break back down into lead, lead oxide, and sulfuric acid. After several discharges the lead sulfate will crystallize, making it too difficult to break down by a normal electric charge. Even when the battery terminals aren't connected there is still a chemical reaction taking place between the electrolyte and the plates. Since the battery isn't being charged the lead sulfate is left in suspension longer, increasing crystal growth. There are several trickle chargers on the market that keep current running through unused batteries, slowing this process. If the battery isn’t too badly damaged the crystals can be broken apart with high-voltage “shock” charging. Once the battery has enough crystals to stop functioning the lead can be remelted and made into a new battery. This lead can be recycled endlessly, so the metal your car battery may have been in several other batteries before you used it. So once an older car battery is fully drained its very hard to return it to its natural original state unless you were to charge it for a couple weeks so it could retain its memory.

Answer 3

Ray: You can completely ignore your grandfather's warning these days, Dave. In his day -- even in your father's day -- most battery casings were made of hard rubber. And because of the porosity of that material, battery acid would sometimes seep through the rubber and create a conductive path through the damp concrete, draining the battery.

Tom: But that can't happen today with plastic-cased batteries (which we've been using exclusively since about the time Nixon said, "I am not a crook"). That's because molded polypropylene (a k a plastic), as you probably teach your students, Dave, is not porous at all.

Ray: And here's the interesting thing, Dave. The cooler the temperature, the slower a battery's rate of discharge. And because concrete is often cooler than its surroundings, leaving a modern battery on a concrete floor might actually make it last LONGER than leaving it on a wooden pallet! How do you like them apples?

Answer 4

Generally, car batteries have a very thin layer of battery acid surrounding their outer shells, due to leakage. When you put a battery on concrete, the acid on the shell provides a path to ground for the current, which will drain the battery in a matter of minutes, depending on how bad the leak is.

Another theory I've heard is the temperature fluctuations provided by a heatsink (ie. concrete) causing a density gradient in the acid solution. It can block a current for some time, but probably not permanently.

Answer 5

I have heard that it is because concrete tends to conduct heat away from the battery, speeding up the reactions in the battery causing it to lose charge. What doesn't make sense to me is that concrete isn't a great conductor.

Good question.

Answer 6

This question was put to the "Car Talk" brothers...Click and Clack...:) Check it out...who say it can't happen with today's plastic battery. However, the older ones made with rubber could form a circuit using sweated battery acid.

Answer 7

Well, it depends on the age of the battery. The older the battery, the more likely the containment of the battery acid is faulty. Cold surfaces cause the structural integrityof the container to fail, and thus the chemicals leak.

Answer 8

This doesn't happen. Urban myth, I guess.