What is an amp-hour rating, and how is it calculated? L.B.,
An amp-hour (Ah) is a unit of measure of a battery’s electrical storage
capacity and is based on the total number of amperes a battery can supply
in a 20-hour period of constant load. You calculate the amp-hour rating
by multiplying the current draw in amperes by the time in hours a battery
can supply it. For example, a battery that can deliver 5 amps over 20
hours has a rating of 100 Ah.
However, all of a battery’s capacity is rarely available, due to
factors such as age, low ambient temperature, and state of charge, so
as a rule of thumb, consider the real Ah rating to be 75 percent of the
I’m having difficulty cutting straight lines across pieces of plain-weave
(O degrees to 90 degrees) fiberglass fabric that I am using for a project.
Do you have any suggestions? A.M., via e-mail
Locate a single fiberglass strand that runs across the entire fabric.
Grab one end and pull it out while firmly holding the opposite side of
the fabric. If you can’t get a grip with your fingers, try using
a small needle-nose pliers. If you still can’t grab one, you may
have to cut a small slit in one side to expose more strands. Once you’ve
removed the single strand, you will see a clear path across for your shears
How important is
the acid-alkali balance of diesel-engine coolant? T.W., via e-mail
Very. Acid-alkali balance refers to the coolant’s pH, and an improper
one can result in degradation of certain metals within the cooling system,
even if the proper ratio of coolant to water is present. You can measure
your coolant’s pH by using a standard home pool-testing kit or litmus
Typical pH levels range from 1 to 14, and most engine manufacturers recommend
a cooling system pH between 8.5 and 10.5 (check your owner’s manual).
If the level is above 11.0, the coolant is alkaline and will attack aluminum,
copper, and nonferrous metals (those containing no iron). If the pH is
below 7.0, the coolant is acidic and will attack ferrous metals.
Because temperature affects pH—at higher temperatures pH is generally
lower—it is important to keep your cooling system at the proper operating
temperature. Check your coolant temperature gauge frequently, and maintain
the recommended coolant-water ratio.
Do you have any tips for removing drips, runs, and stray bristles from
a paint job that is already dry? A.S., via e-mail
If the paint is fully dry, fine sanding or a gentle scraping is the way
to go. Once you’ve removed the imperfection, wipe the area clean
and repaint. If the paint is still tacky, however, hold a piece of masking
tape at both ends and press it gently over the flaw, then pull it off.
When the paint is hard and dry, sand the area smooth, wipe, and repaint.
Need help with a maintenance problem? Write to Maintenance Q & A, Power
& Motoryacht, 260 Madison Ave., 8th Fl., New York, NY 10016. Fax: (917)
256-2282. e-mail: firstname.lastname@example.org.
No phone calls, please.
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