Episode No.127 – The Starter Motor

Episode No.127 – The Starter Motor

Welcome back, today a real
treat, the starter motor.
These are all the parts of the starter
and the engineering in there is so tasty
that I’ve included a knife and fork.
A knife and fork is perhaps
my favorite little bit
of engineering on this car,
elegant in its design
and extremely reliable.
So today we’re going to build this starter
from its parts and then you’ll
know exactly how it works.
We’ll get to explore all
sorts of new mechanisms,
solenoids, planetary
gears, and sprag clutches.
So stick with me, we’re
covering a lot more
than just starter motors here.
Why do we need a starter at all?
It’s because an engine can’t start itself
and this applies to most engines
not just car engines, you might have seen
little starter trucks driving around
at airports starting up planes.
Or you’ve pulled the cord
to get a lawnmower started.
Now all internal combustion engines
need some help to get moving.
And that’s because only
one of the four strokes
in an engine generates
power as the third stroke,
the power stroke, the
engine could have stopped
at any point with its
pistons in any position,
and to start again we need to get back
around to a power stroke.
And that is the job of the starter motor,
to turn over the engine,
taking it through the unpowered
exhaust, intake and compression strokes,
until the power stroke is reached
and the four-stroke cycle
is able to run for itself.
Now the first cars were
started with a hand crank,
turning a handle attached
to the crankshaft
would take the engine through
those unpowered strokes.
But this was hard work, and
potentially dangerous too
because the handle had this
tendency to be unpredictable
so broken hands, ribs, legs were all risks
of starting your car.
It’s a great example of what
an extreme sport motoring
was back in the day, a strong likelihood
of a crippling injury before
you’ve even got moving.
So safety measures were added
but this was far still from ideal.
Various other methods of
starting were attempted,
using coiled springs or compressed air.
But from 1912, every Cadillac
included an electric starter motor.
– [Narrator] Cadillac
and a man named Kettering
found a way to start a car electrically
with a push of a button.
– Theoretically, this damn
thing oughta work now.
– The design of starters
has been fairly standard
since then, and certainly
since the 60s onwards,
starters were of a near universal design
like this one we have here.
And there’s been a bit of change lately
because we have stop-start
engines which use a different
type of starter, which also
functions as an alternator.
I’ll make another video on
that later as soon as I can
convince someone to
give me one to dismantle
but back to our classic starter.
The starter has a small
gear called a pinion
which interlocks with the teeth
on the outside of the flywheel.
Inside the starter is a
very strong electric motor
which turns the pinion,
which turns the flywheel
that turns the engine and
hopefully gets it started.
I will install this temporarily
so we can see in action
and then we’ll pull it to
bits and have a closer look.
(motor revving)
(soft music)
Normally, the starter fits
to the transmission casing
on this car so we’d have
a whole bell housing here
and we wouldn’t be able to see anything.
So I’ve made this bracket
and I’ve also cut away
a little piece of the starter housing here
so that we can see inside.
One thing that we can see immediately is
that the pinion gear is much
smaller than the flywheel.
Compared to engines, electric
motors turn very fast
but they don’t generate much torque,
that is, they don’t have
a lot of turning force.
And to turn the engine
over takes a lot of force.
And this small motor has
to move a load of metal
to get the engine started,
it’s got the flywheel,
the crankshaft, the harmonic
balancer, the alternator,
the power steering pump,
four connecting rods,
four pistons, the timing gear,
the camshafts and the valves.
And all in, I reckon that’s at
least 120 pounds or 50 kilos.
And, it’s compressing the air-fuel mixture
which makes things even harder.
And the engine needs to
turn over at a decent speed
to get it started, any
less than about 100 rpm
and there’s not enough
suction or compression
created inside the cylinders.
Now our flywheel has 112 teeth,
and this pinion has eight.
This reduction in gearing
means that on this engine
the pinion turns 14 times for
every turn of the flywheel
or in other words, the
torque from the motor
is multiplied by 14 times.
Let’s take a look inside the motor itself.
Even after the reduction in gearing
between the starter
pinion and the flywheel,
we still don’t have
enough torque to actually
turn the engine, so inside the
motor is a planetary gear set
which further reduces
the speed of the motor
and increases its torque.
Planetary gears are super cool
and this is the first time
that we’ve come across
them, so let’s go on
a little detour and
appreciate their beauty.
A planetary gear, or epicyclic gear train,
is so called because it has planet gears
orbiting around a sun.
They have a couple of benefits
but one of the big benefits
for us is that they’re
compact, so compact that
from the outside we wouldn’t even realize
that there’s a gearbox in the motor.
They’re also strong, stable and efficient.
In our system we have a sun gear,
which is driven by the electric motor.
Around the sun gear are our planet gears,
we have three planet gears but
we could have more or less.
Three just happens to be a nice number.
The planet gears mesh on
their outside with a ring gear
which in our starter is fixed.
So as we turn the sun gear, the
planet gears all roll around
the ring gear and orbit around the sun.
To get power out of this system,
we use the orbiting motion
of the planet gears.
A carrier with small shafts
runs through each planet gear,
linking them together, and
taking the rotation out
at a greatly reduced
speed and a higher torque.
One of the benefits of this
system is that we have lots
of teeth meshing at once,
if this was just a pair
of differently sized gears,
then just a couple of teeth
would be taking all the force.
This way the force is spread
across multiple points,
and the gears can be made from
weaker, cheaper materials,
we’ve got a nylon ring and sintered gears.
Calculating the gearing
ratio of planetary gears
is supposed to be complicated, tricky math
but it’s actually really, really easy.
Just go on Google and you
type in planetary gear ratio
calculator and you’ll
find someone else has done
the hard work for you.
Standing on the shoulders of giants.
Now from this calculation,
I can see that our planetary
gear ratio is 6:1 so six
rotations of our motor
gives us one rotation of the carrier.
So the gearbox reduces
the speed by six times,
and then we’ve got the ratio
of the pinion and flywheel
which takes things down another 14 times.
So spinning our motor 84 times
will turn the engine once.
It’s very common to find
electric motors that incorporate
a drive set like this,
because motors are most
efficient at high revolutions.
So you’ll find gear sets
like this inside drills,
power tools and most electric motors.
To turn this motor we
need a lot of electricity.
If you get into a car with
even a slightly flat battery,
everything will seem fine until
you go to start the engine.
And that’s because the starter
motor is the single biggest
user of power in the car.
The radio, the headlights, the fuel pump,
none of it even comes
close to the power demands
of the starter motor.
The label on this starter shows
that it will take a current
of 400 amps, depends on the load.
On a cold day, with thick
oil it might take 400 amps
but normally it’s probably
100 amps, maybe 200 amps.
So how does this motor
compare to, for example,
this beefy mains powered compressor motor.
Or to a kettle?
We can’t compare the current
figure, that 400 amps
because a low voltage like 12 volts
requires much more current
than a higher voltage
like the 240 volts that this motor takes.
We need to calculate power,
and power is measured in watts.
And it’s actually very
easy to work out power,
it’s voltage times current,
and it’s a useful formula
to know for all sorts of things
around the garage or in the house.
So the voltage is 12 volts,
and the current is 400 amps.
So 400 times 12 gives us 4,800 watts.
Or 4.8 kilowatts, and that’s a lot.
A kettle takes about 1.2 kilowatts
so this starter can put out
the power of four kettles.
And this huge motor takes 2.2 kilowatts
which is surprising, this small motor
is double the power of this bigger one.
And while we’re comparing
these two motors,
notice the difference in design.
This bigger motor is
covered in cooling fins
because powerful motors get hot.
And even though the starter
motor is more powerful,
it doesn’t really have any cooling fins.
That’s because it’s intended
to only run for a few seconds,
not for hours like this beast.
A starter is designed for bursts
of high power and not endurance.
The current requirements of
a starter are going to vary.
A starter for a bigger
engine will need more power.
Diesel engines have higher
compression so the motor needs
to work harder there too.
Starting a warm engine with
thin oil is much, much easier
than starting a cold engine
on a cold day with thick oil.
And a big diesel truck
might well take 1,000 amps
for its starter motor.
But even at 400 amps, getting
the current to the starter
motor needs a big fat supply cable,
the thickest cable in the car generally.
A cable will run straight from
the battery to the starter,
and if you want to find your starter,
then just follow the thickest cable
off the positive terminal,
and the return path
to the battery because this is a circuit
is made via the engine itself,
the negative side of
our motor is connected
to the metal motor body which
is connected to the engine.
The battery’s negative terminal
is attached to the chassis,
and the engine is connected to the chassis
with one or more earthing
straps as we’ve seen before.
Of course the earthing
straps need to be just
as thick as the starter supply.
At 12 volts and big currents,
there is big huge voltage drop
along the wires so the
cables to the starter motor
should be as short as possible.
Most cars have their
battery in the engine bay,
but in this car, the
battery lives in the trunk
because that improves the
weight balance of this vehicle
because it tends to be
front heavy with the engine
and the gear box and all the
rest in the front of the car.
So there’s a long thick cable running
from the battery to the starter.
Okay, so we have the
current into our starter,
and the motor is generating enough torque
to actually spin the engine.
But right now we don’t have any way
of turning the starter on and off.
What we need is some sort of switch.
There’s a challenge here
because we could haul
this huge supply cable
up into the dashboard
but then we’ll need a massive switch on it
like something out of Frankenstein.
– It’s coming up.
– And big currents require big switches
and chunky metal contacts.
Routing this wire up into the dashboard
would also double the length
of the cable that we needed.
We just talked about voltage
drop and really wanting
short cables between the
starter and the battery
so before drivers began
worrying about their camel skin
seats and hand-brushed aluminum interiors,
a big clunky switch was
exactly how the starter
was operated, often a
big metal foot switch.
Nowadays, we want an elegant little key,
or even a push button that
can activate the starter.
And that’s achieved by
splitting the electrical supply
into two separate circuits,
a starter motor circuit
which carries the heavy
currents to drive the motor,
and a control circuit
which uses lighter wires
that control the heavier circuit.
And to do this, we use a solenoid.
The principle behind the
solenoid is that a small current
is used to turn on a bigger current.
And if you’re familiar with relays
then it’s exactly the same idea.
A solenoid is a simply
a tight coil of wire
that generates a magnetic
field inside itself
when it’s powered on.
That magnetic force will pull a piece
of iron into its center.
A starter solenoid uses this exact design
and here’s one that I’ve cut open.
You can see that there’s a
coil of wire running around
inside and inside that coil
of wire fits a plunger.
And when we turn on the
current to the solenoid coil,
this terminal is a
positive and like usual,
the casing of the solenoid
acts as the ground,
as we turn on the current,
the plunger is pulled in
and it’s pulled in with
quite a lot of force there
as you can see, when
we release the current,
the plunger pops back out.
Now at the end of the
solenoid is a push button
which is just inside the
coil and when the plunger
is pulled into the solenoid,
it pushes on that switch,
which connects the heavy
current to the starter motor.
On the back here, we have two terminals.
One terminal brings power
from the battery’s positive
terminal, and the other terminal
runs to the starter motor.
And when the button is
pushed, this big hefty piece
of copper bridges the
two copper terminals here
and that takes current to the
starter, the starter runs.
In some cases, the plunger
will directly bridge
these terminals but having the push button
that we have here and the
separate area where the contacts
are made keeps any sparking or arching
in its own little area here.
We’ve seen the two big terminals
on the back of the solenoid
and there’s a third smaller terminal here
and that third, small terminal
is used to turn the solenoid
on and off, it’s called the
S wire or the S terminal,
the S being for signal and the
wire from this small terminal
leads up to the ignition switch.
And as we said, with most
electrical systems in the car,
the solenoid is grounded via its casing
so we only need one wire to control it,
a current flows down the signal wire,
in around the solenoid coil
and then back to ground
through the chassis of the vehicle.
It’s quite common for
there to be two coils
inside a solenoid, a strong
closing coil that pulls
in the plunger, and a second
weaker coil which holds it
in position because it takes
a lot more energy to pull
the plunger from outside the coil into it
than it does to hold it in position.
When the contacts for
the starter are made,
a switch also de-energizes
that pull in coil
so it saves a bit of
power from the battery
that’s not needed when the
starter is actually cranking.
But don’t worry about that
because if the solenoid coils
aren’t working then you’re
replacing the whole solenoid
or probably the whole starter assembly.
If you’ve ever tried to start
a car with a flat battery,
then you’ve probably heard
a repeated clicking sound
(clicks tongue) that’s
not a clicking sound.
Click, click, click,
click, click, click sound
and the engine doesn’t start.
That clicking is the solenoid
moving or the plunger moving
inside the solenoid,
now with a good battery,
the solenoid moves once
to start the motor,
so it moves inside, clicks, the motor runs
and if you listen carefully
you’ll hear that click
before the starter motor starts running
and it drowns it out
with a whirring sound.
But when the battery is discharged
there isn’t enough power
to supply the massive current
that the starter motor needs.
And the solenoid doesn’t
need much power so it still
plunges, and that turns
on the starter motor.
As soon as the motor’s turned
on, it takes all the current
from the battery in an attempt to move,
but there isn’t enough
current to power the motor
and there isn’t enough current
left or isn’t enough power
left in the battery to
hold the solenoid close
so it’s forced back open by the spring
and the motor is disconnected.
But now with the motor
disconnected, there’s enough power
for the solenoid so it plunges
back in, energizes the motor,
the motor can’t run but
it takes all the power,
solenoid pops out and this
process just continues.
And that’s the clicking that you hear
as you attempt to start the
car with a flat battery.
So if you’re sitting in a car with a pack
of velociraptors bearing
down on you and the car
won’t start, you just
hear this clicking noise,
just think of the beauty
of this solenoid mechanism.
So far, we have a starter
motor that can be switched
on and off, and through
the reduction in gearing
it has the torque to turn the engine over.
And with this design we
can start the engine.
But the engine runs much
faster than the starter motor
and with the setup that
we have already discussed,
the starter motor would
be permanently meshed
with the flywheel and after
the engine has started
it would be driving the motor around,
in a situation called backdrive
where the flywheel is
driving the starter motor.
Now driving the starter motor all the time
would be a total waste of
energy but more importantly
it would tear this motor to
pieces because it’s designed
to do the driving, and not to be driven.
Take a look at a video of the starter
in action from earlier.
You can see the pinion
engages the flywheel,
then spins, and then
disengages the flywheel
when we release the ignition.
The pinion moves in and out
when the ignition key
is turned on and off.
And fortunately for us, we
already have something moving
in and out when the starter is engaged.
That’s the plunger inside the solenoid.
You might have noticed that
there’s a small white pin
on the plunger and it
hooks to a nylon lever
which I need, I’m going
to find this nylon lever.
This pin on a plunger
hooks to a nylon lever
that pulls the pinion gear in and out.
And this lever is a fork
that fits over a ring
on the shaft of the starter’s drive gear.
And the layout of the
plunger and the fork mean
that the pinion is moved
into mesh with the flywheel
just before the motor is turned on.
This is called a pre-engagement system
because the gears are engaged
before the motor runs.
As the pinion moves,
sometimes we’ll be lucky
and the pinion teeth will fit
into gaps on the flywheel,
the ring teeth here but we
can equally be in a situation
where the two gears clash
and the teeth bang into each other.
There are three ways this
situation is handled,
the first is with a small
bevel on the pinion gear here.
Which means that as it clashes here,
it will tend to be guided
just a slight rotation
to bring it into alignment so
if we’re slightly misaligned
here then that small bevel
will hopefully correct things.
The second way that this
clash of gears is handled
is through a slight
twisting in the pinion.
Let’s look again at the starter
engaging in slow motion.
Can you see there’s a slight
rotation of the pinion
as it moves along the shaft.
That’s achieved with this
helical twist on the splines here
on the armature, as the fork
pushes the pinion assembly
along, it rotates
slightly on these splines.
And combined with the bevel on the teeth,
it’s quite rare that
the pinion won’t engage,
but it is still possible.
And to handle this situation, the pin
on the solenoid plunger has a spring.
When the gears have collided,
and they’re not interlocking,
the plunger isn’t going
to make it all the way
to the motor contacts, so
the solenoid keeps pulling
and this spring is overcome
and it’s a very strong spring
allowing the plunger to
make it to the contacts
and switch on the starter.
As soon as the motor spins,
our gears will pop into mesh
and the flywheel can spin, it’s brilliant.
Now we’re almost there, but
there’s one final challenge
that the starter needs to engineer around.
What would would happen if
someone keeps the ignition key
turned after the engine has started?
At the moment, the starter
would stay connected
to the flywheel and it would
forced around by the engine
until we release the ignition.
I’m sure you’re all saying oh
I’ve got super fast reflexes,
I release the key immediately
when the engine fires up.
Well what about your gran?
She’s probably got super
fast reflexes as well.
But anyway we only want the
starter motor to be driven
one way, the motor should turn the pinion,
but the pinion should not turn the motor.
Actually what we want to
happen is exactly the same
as on a bicycle, you know
when you’re on a bicycle,
power from your feet
should turn the wheel,
but the wheel should not turn your feet.
This is achieved with
a freehub on a bicycle
and we don’t even think
about it but it’s a type
of one-way clutch, force only goes one way
from the pedals to the wheels.
The starter motor also
uses a one-way clutch,
a design called a sprag clutch.
It’s this cylinder that
looks like a bearing.
It looks like a bearing because
basically that’s what it is.
There are a few different
types of one-way clutch,
but they all work on
pretty much the same idea.
The clutch is made up of an
inner race, and an outer race.
The inner race is connected to the pinion
and the outer race to the motor.
The two races are separated
with little wedges,
called cams, or sprags.
Turning the bearing one way
causes these little sprags
to rotate and get jammed,
locking the inner race
and the outer race together.
So the outer race is now
propelled around by the inner one.
This is how the motor turns the pinion.
But turning in the opposite
direction makes the sprags
rotate in the opposite direction too
and they don’t bind up.
The outer race is free to rotate
and since it isn’t
locked to the inner race
it can just freewheel.
So when the pinion is running
faster than the motor,
this clutch stops the
force being sent back
to the starter motor.
That’s not quite everything,
every car built since
about 2005, and many built before then,
will have what’s called
a starter safety switch,
or a neutral safety switch.
This switch prevents
the starter from running
when the vehicle is in gear
and the idea behind that
is that accidentally starting
the car in gear would cause
the vehicle to jump forwards
and potentially set
off driving on its own.
There’s loads of reasons why this is bad,
you might lurch forward a couple of feet
and hit a car in front, or a garage wall
or a kid left in the car could
start the vehicle driving
by messing around with the key.
The safety switch sits
between the ignition switch
and the solenoid, keeping the circuit open
unless the safety switch is engaged.
On an automatic vehicle, this
switch is on the transmission
and it’s closed only if the transmission
is in neutral or it’s in park.
Mechanical switches for
automatic vehicles also stop
the ignition key from
actually turning in the barrel
and those are called interlock switches.
With a manual vehicle like this one,
the switch will be on the clutch
pedal and it only actuates
when the pedal is fully depressed.
There is often another
switch on the clutch pedal
for the ECU and that
ECU switch makes contact
as soon as the clutch
is pressed even slightly
so if you’re rummaging
around down in the footwell
checking the clutch safety switch,
just be sure you’ve got the right switch.
Let’s take a look at the
starter system as a whole.
When you turn the ignition
key, that acts like a switch.
And current flows through
that switch, to some sort
of safety switch either
on the clutch pedal
or on the transmission, the
safety switch stops the car
being started in gear, and
with the ignition and safety
switch closed, power goes to the solenoid.
When the solenoid is
engaged, it pulls a plunger.
The plunger uses a lever which
pushes the pinion into mesh
with the flywheel, and
when the pinion is engaged,
the plunger moves a bit
further and switches
on the current to the starter motor.
The starter motor runs,
starting the engine.
And when the ignition key is released,
a strong spring in the solenoid
pushes the plunger back out
turning off the starter motor
and retracting the pinion
from the flywheel.
So we got through a lot there.
We’ll come across planetary
gears again on the differential.
We’ll meet solenoids on
various valves and actuators
elsewhere around the car,
although if someone talks
about a solenoid in relation
to a car they’ll almost
certainly going to mean
the starter solenoid.
And one-way clutches,
well they’re just cool
but they’re also found in
automatic transmissions.
And of course you know
absolutely everything
about starter motors.
Now the starter uses a load
of power from the battery
and we need some way to
recharge that battery
once the engine is
running, so next we’ll talk
about the alternator and how
it generates electricity.
Guys if you like detailed videos that go into the nitty gritty
Of how mechanical things work.
Then jump over to howacarworks.com
Where we’ve got a full video course that takes you from A to Z.
From starters… no… that’s S.
From alternators to…. something that begins with Z.
But basically we teach you everything that you need to know.
In mad detail, just like this episode about the starter.
So if that’s what you enjoy, you’ll learn it all.
Come visit us over there.

87 Replies to “Episode No.127 – The Starter Motor”

  1. Contents guys! There's a lot in here 🙂

    0:42 – Why is a starter needed?
    2:16 – Modern electric starter motors
    3:01 – How the starter turns the engine
    4:02 – Gearing between flywheel and pinion
    5:18 – Planetary gears inside the starter motor
    8:08 – Starter power requirements
    10:33 – Electrical connections
    11:48 – Starter switching
    13:14 – Solenoids
    16:22 – The clicking sound of a flat battery <– my favourite bit
    17:50 – Starter engagement system and solenoid lever
    19:46 – Ensuring the gears mesh
    21:25 – Sprag clutches
    23:32 – Interlock switches
    24:55 – The whole system CGI

    26:18 – If you made it this far you'll love my 20-hour video course. https://www.howacarworks.com/video-course

  2. Thank you for providing so much valuable information with such a great detail. Appreciate a lot and looking forward to buy the course soon.

  3. I wish you make all the series open source so people who can't afford to pay it; or people coming from a countries that does not have online payment services; like my case; can benefit.
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  4. I'm so glad I came across your channel from the interview you had with Learn Engineering. Thank you for putting so much effort into making this series!

  5. I have a question – Does electric motor turn pinion if solenoid doesn't work when we turn key trying to start engine? I hear something noise ( it sounds very quite, but it like noise of electric motor), when I trying to start a car, but engine doesn't crank

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  7. Great explanation maybe you could've included starters in newer stop start vehicles how it does or doesn't affect there durability

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  10. Please make many many more videos with this level of information. Awesome job!!! I love the background, and pretty much everything about how you made your video. I’ll definitely subscribe right now

  11. Make a guide on basic tools people should start fixing the cars with, no commerical bs just – "ratchet 3/8 , 1/4in , flex head/straight , set of wrenches, screwdrivers with impact heads , pliers , etc "

  12. Nice video. Did you know that up till the 80s the electrical wiring calculations were done for a nominal voltage of 8 Volts. Although the motors were stamped 12 Volts. This was because of the voltage drop in the open lead-acid batteries with those big sturdy plates inside. When the maintence-free batteries were introduced the max. Starter current rose dramatically because of the thinner (but more) plates per cell. They were mechanically protected by the gel. You can try this yourself. Put a "sealed" battery in a Vauxhall Kadett or Ford Capri. When you turn the key you'll fall to knees with laughter. It'll turn the engine at 20 rpsecond. Greetings

  13. But how does that starter motor gear get in contact with the fly wheel if it’s in a different position every time from it retracting because of the engine not needing it at the point?

  14. Great content mate.. love your work.. my friend runs a social media advertising business.. I can get you connected to him if you are interested in marketing your content to wider audience..BTW I don’t get any commission.. Your content is so good that I want your channel reach a larger audience group.. all the best mate. Cheers

  15. Awesome animation at the start of a "starter" video…
    The video is awesome. Great details.
    More videos please…alternators, ABS, breaks, transmission, etc…

  16. Guys, this is fantastic, seriously. The best I have ever seen on the starter. Keep at it… the subscribers and viewers will come soon enough with quality content like this.

  17. I can't put into words how phenomenally informative your videos are. This was carefully researched, scripted, and presented with incredible visual aids. Your channel has single handedly changed the game. Keep up the incredible work

  18. I can't put into words how phenomenally informative your videos are. This was carefully researched, scripted, and presented with incredible visual aids. Your channel has single handedly changed the game. Keep up the incredible work

  19. Amazing video! True genius is to explain complicated things in a way any one can understand. This channel certainly achieves it.

  20. Thankyou so much for making this video. This stuff is so helpful. Please cover all the topics for free, you can beat engineering explained and Chris fix

  21. This is by FAR the number one channel on YouTube explaining how a car works… or even better, mechanics related to com combustion engine in general.

    I don’t know what’s the plan, financing, team, incentives behind this channel but I hope there will be MANY more videos.

    I would be willing to pay money to get a full course from you guys.

    Thanks for creating this amazing contact.

    All the best!

  22. Thank you sir😊🙏
    Is beacause starting current is high for fraction lead acid battery is still used?? Plz answer m bit confused 😅

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