REFLECTOR: How to protect the battery bus

Sat Feb 17 09:55:14 CST 2007

Excellent post.
I was chasing gremlins in by EFI setup about a year ago. Voltage drops 
all along the loop contribute to low performance at the starter. Once 
you get to the starter, all that current *has* to return to the battery. 
V=IR is the law. For each connector and wire in the return path, there 
will be a "voltage rise" that current has to work against to get to the 
battery.

If you have a big thermopile, this is a lot easier to illustrate. 
Perhaps a 1Kw heater or some other heavy load will do.
Connect the load instead of the starter.
Using your voltmeter, connect the ground of your meter to the negative 
battery post. For each connection along the way measure the voltage. 
Start on the ground wire to the starter and return back to the battery. 
This will tell you a lot about the quality of your connections and where 
you should pay attention to get the largest return on your 
troubleshooting time.

Terry Miles wrote:
>
> 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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