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Discussion Starter #1
Well I don't know where to post there but here goes:

A buddy off mine since back in high school is on a fire dept. on the left coast. They had a problem with their headset controller switching channels when they started the fire truck. The measured 9V when cranking, the headset manufacturer said "There's your problem, that's too much drop".

So I set out to measure what the battery voltage looks like during cranking. I took a 8 megasample digital scope and captures the event, then went looking for dips and spikes.

The 1st shot is a 400 HP Detroit in a fire engine. Note that the battery got down to 7.8 volts. The waveform you see from divisions 3 to 6 corresponds with the "cranking" rhythm during cranking. At 6 divisions, the engine had started and you see the starter free-wheeling. Then the guy starting it takes his finger of the button, and you see the alternator kick in and the battery voltage go up.
I was very surprised to see no spikes over 16 volts on the system (you have to zoom in to see them). I had been told years ago that you would see 100V spikes for a microsecond or so. BTW, I was sampling every 250 nS.

The next shot is a spike form the charger that is in the truck when it is on shore power. It has a 17 volts spike, but it is very short lived. I upped the sampling time to 5 nS for this.

The 3rd shot is starting a F350. Very similar patterns to the big engine, but it took about .75 seconds for the alternator to kick in. Minimum voltage was 7.6 volts for a brief instant.

The "broad width" of the trace is brush noise during starter, and alternator whine at the end.

So if you ever wanted to see what's going on when you crank, here it is.

Pete
 

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Pete,

Do you think we would see similar results on our JD Yanmar engines?
 

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Most fire trucks that I have seen and worked with (only 2 :lol:) had 2 batteries for starting (Big MoFo's) and also another battery for all the electronics. That way they had a constant voltage to work from and also an "emergency" battery if the other two went dead that they could still have a radio, lights, or gps.

I too am suprised not to see a voltage spike to 16 or so volts. I would think a normal car would/does have that?
 

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Pete, Did you add a capacitor to the radio unit to prevent the momentary low peaks?
 

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Discussion Starter #5
The big fire truck had 2 BMF truck batteries, no 3rd one for the electronics. I don't know of any in this area that have the 3rd battery. Might be an "up north cold" thing...

I suspect the F-350 curve would be very similar to what we would see on our tractors. Maybe someday I'll drag all this stuff out and put it on my 4520. I'm _guessing_ that the smaller the battery, the larger the "brush noise" from the starter, but I don't think it would get too bad.

I have no capacitors on anything, this was the raw "12 V (lighter)" on the truck. Now the radios and other electronics on the truck were all connected up, they might have added some filtering but I suspect it would be minimal. Some trucks disconnect the radio stuff when cranking, some don't. I know the iPads I'm putting in the trucks are on the same circuit as the thermal imagers and flashlights. Those are directly on the battery. I _suspect_ that the starter and solenoid go directly to the battery, and everything else also runs radially to the battery. So the battery is the "filter" for the starter noise. I need to research battery impedance vs. frequency next...

Part of my "take" on this is that a simple filter circuit could protect your electronics in this type of environment. A coil, TVS (power zener diode) and small cap (15 to 100 uF) would provide protection against any spikes from relay coils and the like. The starting process is not going to produce so much energy as to wipe out the TVS protection diode. I'm probably going to make a little filter circuit board for "stuff under 2 amps" that wants to be powered off the battery. A goal in all this is to be able to use the off the shelf $5 "12 V to USB" converters for portable electronics in a "battery and engine" environment. The ones I've looked at are rated "12 to 24 V". Taking them apart, they use a 40V max switcher IC but have a 47 uF 35V cap on the input. There is _no_ TVS protection diode, which surprised me. So the off the shelf switcher (that are rated 12 to 24 volts) are probably good up to about 28 volts (or two 12 volt batteries).

We had on of the off the shelf units blow, so I'm just trying to figure out why. I still need to take the broken unit apart. The one I reverse engineered was a different one that I'm using everywhere.

Pete
 

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Have you checked the batteries to make sure they are in good condition? 7.8V seems low to me.

I work on fire trucks and we had an Oshkosh TI airport crash truck that had issues with its control tower radio. Turns out one of the 4 batteries was bad and causing problems. Truck ran fine and started fine, especially since it was always plugged into shore power in the hall, but that one bad battery caused havoc similar to what you are experiencing. Truck would roll out of the hall and not realize that the radio had switched away from the control tower frequency and the driver didn't realize he was missing vital directions about taxiing aircraft!

In the end we replaced all 4 batteries. I always replace batteries as a set. All the radio problems went away.

I would install new batteries and try your test again or run the test on a unit that doesn't have radio issues and see how low it goes.

We had also previously switched out the 2 two big 8D batteries for 4 smaller Group 31s. More total cranking amps and more stable voltage across the 4.
 
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Discussion Starter #7
tnx for that info. We've shifted from "change 'em when they have problems" to a schedule. And I agree with the change them all at once- if you don't you seem to ruin the new one and are changing stuff out/diagnosing more often.

Am working on "battery to 5V USB" power supply, it will be able to work under bad voltage and bad spike conditions.

Pete
 
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