# car battery and windshield wiper motor

Discussion in 'Electronic Basics' started by Pat Kilgore, Apr 8, 2005.

1. ### Pat KilgoreGuest

For an art piece, I would like to be able to anticipate about how long a
standard car battery would drive a car windshield wiper motor hooked
directly to it, if the wiper motor is activated for one cycle every ten
minutes. This is not in the manual.
Thanks for any input.

pat

2. ### Lord GarthGuest

The answer specifically depends on the motor. The demand of the motor
depends on the mechanical load. The mechanical load changes as the amount
of water & dirt and the drag of the rubber change, then there is the motor
speed
to consider. Also, pausing for 10 minutes between strokes is plenty of time
for
the battery to recover....

I would say there are plenty of undefined variables so the only answer is,
a while. Give it the acid test...hook it up and run it until the battery
reaches
10.5 volts. This is considered a dead battery, it is not good to deep cycle
a
battery to this point regularly. Deep cycle batteries can handle this
better but they cost more.

3. ### BanGuest

Around one week at least. 5s*20A = 100As each cycle. The battery has maybe
54Ah capacity of which you can use around 50% to not degrade it.
27*3600/100= 972 activations.

4. ### ChrisGuest

Warning -- as someone said earlier on s.e.d., there's a law that
requires at least one boneheaded obvious math error in any post
dependent on arithmetic to make a point. Caveat emptor.

Hi, Pat. By chance, a surplus wiper motor for 2000/2001 Saturns is
being sold at All Electronics. It's a 2-speed job, and the blurb says
it uses 4 amps for hi-speed at 106 RPM and less than 1 amp for low
speed at 41 RPM.

http://www.allelectronics.com/cgi-bin/category.cgi?category=400400&item=DCM-171&type=store

Not a bad deal for 16 bucks. Let's assume lo-speed. 41 RPM = about
1.5 seconds at 0.91 amps. Round it off to 1 amp * 1.5 seconds = 1.5
a-s (amp-seconds, a-s = amp * seconds).

Now, even a small car battery is good for 40 a-H (amp-hours), but even
for marine and deep discharge batteries, you shouldn't take more than
half of that. So lets assume 20 a-H capacity, which means 72,000 a-s
capacity. This will give you 48000 shots, and at 6 per hour, you're
talking about 333 days of use. Of course, as a practical matter, the
battery will self-discharge and go flat well before that, and the above
didn't take into account the work that will have to be done by the
motor. Short story, though, is you'll probably be OK for a month,
maybe two months with this motor, but not 333 days, or forever.

I would lean toward using a marine, R-V or wheelchair-type battery
instead of a car battery. Although they're somewhat more expensive,
they are made to handle deep discharges better. Also, if you ask
around, you can find batteries with better room temperature a-H ratings
(battery sellers and manufacturers have this data -- they just don't
want to publicize it for some reason). Higher a-H rating is better.

No matter what, stay away from used car batteries, unless you know it's
less than a couple of years old and hasn't been in a car with
electrical problems. Old car batteries are almost always a
disappointment, even for starving artists who have made disillusionment
their very sustenance.

Remember -- your 10 minute timer drives a relay, transistor, or SSR to
start the motor, and use a cam microswitch to stop it at exactly the
same place every time. (These are also available at
Allelectronics.com)

<Safety Man> By the way, try to get a sealed lead-acid battery, and be
sure to clamp or bolt down the battery well. Spilled acid can ruin
someone's day (possibly a kid -- be careful). Also, make sure the
terminals and any exposed wiring is insulated -- people use (and drop)
the darnedest things when setting up and taking down displays. It
might be better to get one of those batteries with the built-in
carrying handle, and install the battery after the display has been
moved.</Safety Man>

Good luck
Chris

5. ### Bob MastaGuest

As an addendum to what others have posted, I'd just like
to note that windshield wiper motors can have an
astonishing amount of starting torque. As a student
engineer at Cadillac Motor Car in the late 1960s, I used
a wiper motor to drive a test fixture (for something else
I don't recall now). In order to test the rotation direction,
I held the motor in my hand while connecting it to a big
bench supply. Big mistake! Jerked right out of my grip
and whacked me with the mounting ears of the motor
housing. So, 1) be careful! and 2) the starting current
may be a whole lot more than the run current, and in
your case you have a high proportion of starting current.

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

6. ### Pat KilgoreGuest

Thanks everybody for the replies. That's more than I knew about
What we have used in the past to control timing and switching is time
delay relays. http://www.ssac.com/catalog/TDR01A01.pdf(tdr1b12 is the
one i would use here) With this one current is always on and the timer
switches the relay on and off. Little pricey at fifty bucks but I didn't
see a switch timer combo I could use hooked up to a 12 volt battery at
cam microswitches, etc. You think I could do a lot better than \$50?

Pat

7. ### Lord GarthGuest

The only current that is on all the time is that which runs the timer. I'm
sure this is very low. Given that you don't work with electronics, this
\$50 part is probably a good approach.

8. ### ChrisGuest

Hello again, Pat. SSAC makes very good products. I would guess you
can save at least half of that \$50 by doing a home brew, but given your
level of electronics expertise, buying a packaged solution and avoiding
the hassle might be worth the extra \$30.

One question: You said you need one cycle every 10 minutes. What do
you mean by that? I guess I assumed in your OP you meant you needed
precisely 1.00 revolution of the motor. If not, the SSAC relay is a
great idea. If you need the motor to stop at the same place every
time, you're going to need a better solution, particularly a cam on the
motor shaft and a microswitch to sense the cam bump to stop the motor.
At least you don't have to worry about the power supply.

(By the way, I don't believe there's reverse-polarity protection on
those Solid State interval timers. If it's possible to reverse-connect
the battery terminals, you should think about using a protection diode
to keep from smoking your SSR. It's shown as "D" below. Use a 1N4002
for D, and a 1N5402 for D2 if the motor uses less than 3 amps.)

`
` + Battery -
` o o
` | Microswitch |
` | (Use N.C. Contact) D2 |
` | .---|<----o
` | -o | |
` | __--o-----------o _ |
` o------o | / \ |
` | o--( M )--o
` | | \_/ |
` | .-------------. | |
` | 1| |3 | |
` o----oRLY RLYo---' |
` | |COM SSR N.O.| |
` | 2| Timer |7 |
` o->|-o+ -o-------------o
` | D | | |
` | '-------------' |
` | |
`created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Every ten minutes or so, your SSAC TDR1B12 timer will close the relay
contacts for about 0.3 second, long enough to turn on the motor and get
the motor shaft cam off the microswitch. Since the N.C. microswitch
contact will now allow power to go to the motor, it will continue to
run until the motor comes around, and the cam pushes on the microswitch
again, removing power from the motor. D2 is necessary to absorb the
inductive kick of the motor when the microswitch turns it off. If you
don't have it, you'll get an arc on the microswitch which will reduce
its life. Simple & easy. Heck, windshield wipers used to be wired up
with a microswitch this way, except the dashboard wiper switch was used
instead of the SSR. That way the wipers would always stop down.

Post again if you've got more questions.

Chris