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Hemmings Motor News has been around since 1954. We're proud of our heritage, but we're also more than the Hemmings full of classifieds that your father subscribed to. Aside from new editorial content every month in Hemmings, we have three monthly magazines: Hemmings Muscle Machines, Hemmings Classic Car and Hemmings Sports and Exotic Car.

While our editors traverse the country to find the best content for those magazines, we find other oddities related to the old-car hobby that we really had no place for - until now. With this blog, we're giving you a behind-the-scenes look at what we see and what we do during the course of putting out some of the finest automotive magazines you'll ever read.

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Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

Posted October 04, 2021 9:32 AM by dstrohl
Pathfinder Tags: batteries motorcycles

For 99% of a 12 V lead-acid battery's service life, it sits on a metal shelf somewhere in your car, pushing electrons around with gleeful abandon. That metal shelf, an electrically conductive surface, never once gets implicated when mechanics go to diagnose dead batteries, and rightfully so. Yet many mechanics, professional and amateur alike, absolutely refuse to put batteries on a concrete floor without a block of wood underneath and continue to admonish others for doing so, believing that the concrete bears total responsibility for draining a battery.

In fact, somebody somewhere has taken one look at that lead photo and has already scrolled down to the comments section to excoriate me for featuring a photo of a battery sitting on my garage floor sans wood barrier. (For the record, the battery has been dead as a doornail for years. I don't even recall what it came out of now, and I've been too busy to properly dispose of it. Not like putting wood under the battery matters even if the battery were still viable, as we'll see in a moment.)

So why do people fear battery-to-concrete contact? The hard plastic cases that have been standard in battery construction since, idunno, the Nixon administration, were designed specifically to eliminate the possibility of grounding through the case. Some people, like CarTalk's Tom and Ray Magliozzi, have pointed out that, prior to the hard plastic cases, batteries had porous hard rubber cases (and even wood cases) that could conceivably leak battery acid to concrete floors and create a conductive path to ground. But it's been generations since that was the case, certainly long enough for the myth to wither and die, right?

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#1

Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/04/2021 3:34 PM

I do it to help prevent damage to the concrete. A scrap of plywood is my preference.

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#2

Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/04/2021 5:14 PM

I'll go with Tom and Ray. I used to listen to "Car Talk" on the way to work every morning. I sure miss those guys.

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Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/04/2021 7:02 PM

Click and Clack, the Tappet Brothers.

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#4

Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/05/2021 4:05 AM

I believe this stems from the dim dark past when the outside of NiCad wet cells was the metal case. AN old power station I worked in had racks of pine boxes holding the NiCad cells to obtain 240V DC. The 6V NICad batteries came in wooden crates but the 1.2V cells were held in place by rubber bushes that fitted over metal tabs on the outside of the batteries.

Old glass 2V cells used in telephone exchanges were held in pine cases to isolate them from sharp rocks on the concrete floors and provide some isolation from vibration and heat changes.

Wood gets wet so is not a great insulator but at 12V what does it matter if it is down to a few Kohms. Now they used poly battery trays between the battery and cars metal battery shelf to protect the metal eating acid. The batteries go flat because of internal self discharge which has just caught me out with the 24V excavator battery setup. 2 batteries on rubber mat in battery compartment with a battery isolator and one cell in each has become a short circuit.

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#5

Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/05/2021 7:48 AM

the block of wood was never about the LEAKAGE, but more the long term storage on a COLD floor.

the wood acted as a thermal barrier and kept the battery from freezing.

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Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/06/2021 12:34 AM

The electrolyte of a fully charged lead-acid battery will not freeze until approximately minus 76 degrees Fahrenheit (-60 C). A fully discharged battery can freeze around plus 32 degrees Fahrenheit (0 C). Not all cells will necessarily enjoy an equal state of charge.

In locations where the floor temperature drops below 32F (0 C) any discharged cell in that battery is at risk. Formation of ice may force that discharged cell's plates together in a short circuit, or create other havoc.

In some climates, a concrete floor may be warmer that the surrounding Winter air, so there are circumstances where a wood underlay is your enemy, some where it is your friend.

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Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/06/2021 7:36 AM

Ah I forgot about the people living in inhospitable areas for here the lowest outside air temperatures can be -5c at my house but the floors in my shed remain around 10c during the winter.

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#8

Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/08/2021 2:01 PM

Before batteries had handles, a 4x4 made the battery easier to pick up saving the back from strain before people learned to bend their knees....

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#9

Re: Why Do People - Including Seasoned Mechanics - Still Insist on Putting a Block of Wood Under a Car Battery?

10/09/2021 6:58 PM

The U.S. navy uses metal racks lined with wood to hold batteries for charging. The wood isolates the battery from the steel rack. The purpose is to prevent a spark that could ignite hydrogen generated during the charging process.

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