REFLECTOR: How to protect the battery bus

[Terry Miles]

Hiroo,

 

If you showed us a drawing we could offer better advice.  The general rule
is to locate protection devices near the potential offenders.  The subject
line:  "How to protect the battery bus" has my attention.  Is that your
concern?  I'd go along with the advice already offered to use current
limiters for this purpose.  (Try B&C for products).  General practice is not
to circuit protect your starter motor, except perhaps a small CB on the line
from starter button to starter contactor if you like.  With regard to
ammeter needs:   And as I have learned-thanks to contribs on this
forum-those Hall effect ammeters placed near the alternator output are a
good way to go, and the method of choice if you are using Grand Rapids
engine analysis stuff. (I didn't know this either until this month and had
started to put in a shunt type ammeter device.)

 

For me:  My system is not flying yet:  I have placed 60a current limiters
both in the nose near the battery and on the firewall near the alternator
which is a 60a alternator.  I just had lunch w/ the Chair of the EE
department at Penn State and he agrees with two current limiter (nose&tail)
(batt&alternator) protection scheme.  I have a Hall effect gizmo for an
ammeter now.

 

Try the Aeroelectric list web site and skim thru the 300 page FAQ section
for this related topics (do a control + f to get a pop up window and then
type in the search word).  It is only as good as Bob Nuckols approach to
electrical design, but it makes for the nice consistent set of ideas to
accept or reject and then refer back to as you debate with yourself about
electrical design, installation, and trouble shooting.  That all said, I
wouldn't refute the logic behind Doug Holub's advice to you who is
protecting his fat wire.  

 

Here's some related physics and math if you want it.  Your battery has a
cranking amp limit.  If the starter motor is asking for more than what the
battery has in cranking amps, then what happens is the voltage starts to
sag.  Regards voltage demands and line loss:  2awg looses .156 ohms per 1000
feet.  If you had 30 feet for example.  That is .03 as a fraction of 1000.
156 times .03 is .00468 ohms lost.  Given a demand of 300 amps you multiply
300amps times .00468 ohms and get 1.4 volts lost into the length of the
wire. This is the voltage the starter sees at its input lug prior to
engagement.  Say you have a 12.4 battery, it can only deliver 11 volts to
the starter.  When the starter is engaged the V will sag to say 6 volts
aprox.  That's why any electronic equipment running at this point will drop
off line since most of that stuff needs about 8 volts or they shut off due
to their own internal protection circuits, but will pop back on when minimum
input voltage is sensed.      

 

If you are using 4awg, the volt loss to due to line length is greater.  If
you are using 00awg the volt loss will the lesser.   This does not include
any extra ohms lost to mechanical connectors, or other devices in the
line-especially diodes.  Unfortunately when it comes to engine start
matters, there is no gray area.  The engine starts or it doesn't.  IMHO any
number of starter systems out there that are deemed to be fine may really be
taxed to their absolute limits, but go undetected since the engine starts.
In those cases, too often it is said that the engine is hard to start, when
it is not the engine's fault.  

 

If the starter is stopping on you, it is possibly trying to push past the
inertia of that first engine compression (demanding more amps) and in the
process, the battery volts are dropping below that at which the starter will
operate.  If your problems persist, as another possible experiment, go buy a
couple of feet of 00awg and a big fat battery.  Set it on the floor next to
the engine and try to jump start it.  If the engine fires right up, it might
indicate that your starting circuitry has too much resistance as installed.
This experiment would also help eliminate, for now, any concern that
engine-start fuel/ignition problems are at play.  How big is your starter
switch?  SkyTec starters told me to use one rated at least at 30a.  It pulls
5 to 6 amps and they also said to use at least a #14 wire.  Remember the
holding coil and the pinion gear need to drink from the cup too during all
this.  Good luck.  Keep us posted.

 

Terry

 

 

 

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Hiroo Umeno
Sent: Friday, February 16, 2007 2:56 PM
To: reflector at tvbf.org
Subject: REFLECTOR: How to protect the battery bus

 

Last weekend, I cranked the fickle Franklin one too many times.

 

(Yes, I am still trying to find out why it simply will not run.  Long
story.)

 

On the third cranking, the voltage on the battery must have dropped
sufficiently that the starter motor stopped turning the prop.  I probably
left my thumb on the starter button a tad longer than I should have.

 

The shunt blew and the whole electrical system went (except the backup
battery system that ran the instruments (glad to know THAT works well).

 

That got me thinking.  Shouldn't there be a CB or something protecting the
starter line?  I've heard that the starter line is the only line that should
NOT have a CB on.  Yet I've managed to fry the shunt.

 

Any thoughts?

 

Hiroo

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