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william.p.dube(at)noaa.go
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 Posted: Tue Mar 07, 2006 11:32 am Post subject: Battery cranking power, Plus starter comparision |
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Figure about 300 amps for the starter current.
30 feet of 2 gage is about 5 milli-ohms. The battery is 12 milli-ohms.
Add in 2 or 3 for the contactors and the terminals and you are up to 20
milli-ohms.
At 300 amps, the starter is getting just 6.8 volts, even on a warm day
with a fully-charged battery. (This may not be enough voltage to run an
electronic ignition, by the way.)
>>> Series-Wound (SW) versus Permanent Magnet (PM) starters <<
The torque curves and voltage versus RPM are quite different for a
SW motor and a PM motor. The PM motor is simpler, so we will start with
that.
The torque of the PM motor is directly proportional to the current
supplied. The "back EMF" (**) of the PM motor is directly proportional
to the RPM. The internal resistance is typically slightly lower compared
to a SW motor of the same size. PM motors are typically more efficient
than SW motors. PM motors are typically lighter than SW motors.
The torque of a SW motor is proportional to the square of the
current. The back EMF of a SW motor is proportional to the rpm times the
current. The internal resistance is typically slightly higher than a PM
motor of the same size. The zero RPM torque is typically MUCH higher
than a PM motor of the same size.
If the wires are long, the battery is weak, or the oil in the engine
is thick, it is easy to "fall off the curve" of a PM starter. If you do
not supply it with enough voltage to push the current through the
windings to make the torque required to turn the engine at all. If it
spins at all, it will likely spin fast enough to start the engine. It is
more of a "yes or no" operation.
The SW starter is more forgiving. Since the torque is proportional
to the current squared, you can more often generate enough torque with a
weak battery and/or long wires to get the engine (with thick oil) to at
least turn slowly. Once the engine turns, however, the back EMF
(proportional to both current and RPM) pinches off the torque quite
rapidly. Thus, it may not ever be able to spin fast enough to actually
start the engine if the conditions are unfavorable. It has a much
broader "almost enough" zone of operation than the PM starter.
Bill Dube'
(**) Back EMF is the voltage the motor will create internally as it
spins that opposes the applied voltage. All motors are generators, after
all. :
)
Skylor Piper wrote:
| Quote: |
One thing that I haven't seen mentioned yet is the
fact that the Panasonic LC-RD1217P has a higher
internal resistance than the Odyssey batteries do.
Panasonic literature states 12 miliohms at 77 degrees,
while the Odyssey PC680 is 7 miliohms. This may not
sound like much, but during high current draw when
cranking the engine, this can make a considerable
difference in voltage drop at the battery.
Skylor
RV-8 Under Construction
Firewall Forward...
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khorton01(at)rogers.com
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| Posted: Tue Mar 07, 2006 1:11 pm Post subject: Battery cranking power, Plus starter comparision |
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On 7 Mar 2006, at 14:21, Bill Dube wrote:
| Quote: |
Figure about 300 amps for the starter current.
30 feet of 2 gage is about 5 milli-ohms. The battery is 12 milli-ohms.
Add in 2 or 3 for the contactors and the terminals and you are up
to 20
milli-ohms.
At 300 amps, the starter is getting just 6.8 volts, even on a warm day
with a fully-charged battery. (This may not be enough voltage to
run an
electronic ignition, by the way.)
|
Great post - thanks for the info.
I believe the voltage drop in the battery contactor and the positive
lead from the starter is one reason why Klaus Savier recommends that
his EI be connected directly to the battery. This way it is not
subjected to some of the voltage drop that the starter gets, so it
should be seeing a slightly higher voltage than the starter. Every
volt counts.
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://www.kilohotel.com/rv8
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dan(at)rvproject.com
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| Posted: Tue Mar 07, 2006 1:40 pm Post subject: Battery cranking power, Plus starter comparision |
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| Quote: | I believe the voltage drop in the battery contactor and the positive
lead from the starter is one reason why Klaus Savier recommends that
his EI be connected directly to the battery. This way it is not
subjected to some of the voltage drop that the starter gets, so it
should be seeing a slightly higher voltage than the starter. Every
volt counts.
|
FWIW, I did not follow Klaus' instructions when I wired up the LSE Plasma II
on my RV-7. I didn't connect the power directly to the battery. Instead I
connected it to a fuse on my always-hot battery bus. That way it doesn't
suffer from the voltage drop across the contactor that you mentioned, it
won't die if the contactor dies, etc. -- but it's still wired in a central,
consistent manner at a fuse block. Just the choice that I made because it
seemed to suit my needs better. YMMV.
)_( Dan
RV-7 N714D
http://www.rvproject.com
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sbuc(at)hiwaay.net
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| Posted: Tue Mar 07, 2006 1:43 pm Post subject: Battery cranking power, Plus starter comparision |
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Bill Dube wrote:
| Quote: |
Figure about 300 amps for the starter current.
30 feet of 2 gage is about 5 milli-ohms. The battery is 12 milli-ohms.
Add in 2 or 3 for the contactors and the terminals and you are up to 20
milli-ohms.
At 300 amps, the starter is getting just 6.8 volts, even on a warm day
with a fully-charged battery. (This may not be enough voltage to run an
electronic ignition, by the way.)
|
I've been running small Panasonic batteries for about four years. Here
are the details:
1) O-320, carbed with two mags
2) AeroShell W100 (50wt)
3) Engine continuously preheated with sump heater Dec-Feb.
4) Insulated hangar in north Alabama
5) Skytec starter, but don't recall the model or type
6) Short battery cables
The small battery was installed as a replacement for the original
Concord. I wanted to move the battery forward of the firewall and also
incorporate 'Lectric Bob's method of replacing a cheap battery every
couple of years. The Concord was still working fine after three years in
service, matter of fact it filled in nicely for a friend last year who
had a battery die unexpectedly.
The Panasonic has worked well in my situation. The only time it failed
me was once on a January morning (~20F) when I managed to get the engine
flooded during engine start (no primer) and the battery gave it up after
several attempts to crank the plane.
In my opinion, the Panasonic battery is a viable option for conditions
similar to mine. If there is a need to routinely start a chilled engine
with electronic ignition and a high-current starter, the cheap battery
probably isn't a good choice. An Odyssey would no doubt give more
reliable service under those conditions.
When I first installed a Panasonic in my RV-6, there was quite a wide
difference in the cost of it and an Odyssey. But since the Odyssey
prices have fallen in the past few years, the rational for the Panasonic
isn't as strong as it once was. Even though I have gotten good service
from the cheap batteries, I may consider an Odyssey next time.
Details of my installation:
http://thervjournal.com/battery.htm
Sam Buchanan
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khorton01(at)rogers.com
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| Posted: Tue Mar 07, 2006 2:14 pm Post subject: Battery cranking power, Plus starter comparision |
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On 7 Mar 2006, at 16:37, Dan Checkoway wrote:
| Quote: |
> I believe the voltage drop in the battery contactor and the positive
> lead from the starter is one reason why Klaus Savier recommends that
> his EI be connected directly to the battery. This way it is not
> subjected to some of the voltage drop that the starter gets, so it
> should be seeing a slightly higher voltage than the starter. Every
> volt counts.
FWIW, I did not follow Klaus' instructions when I wired up the LSE
Plasma II
on my RV-7. I didn't connect the power directly to the battery.
Instead I
connected it to a fuse on my always-hot battery bus. That way it
doesn't
suffer from the voltage drop across the contactor that you
mentioned, it
won't die if the contactor dies, etc. -- but it's still wired in a
central,
consistent manner at a fuse block. Just the choice that I made
because it
seemed to suit my needs better. YMMV.
|
And, for the record, this is exactly how I've wired my LSE Plasma
II. It should achieve the benefits of wiring directly to the
battery, while being neater and allowing an easy way to fuse the LSE
power wire.
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://www.kilohotel.com/rv8
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