Switch for alternator to coach battery disconnect

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TANK CIRCUIT

Rectifiers (which are diodes rated for greater than 3-amps) hate transient voltage spikes with a purple passion. So do voltage regulator components. And every other solid-state component in a circuit. Including engine ECU, radios, you name it.

While chopping off a major load leaving a minor load (the chassis battery) isn't considered to be a true out-and-out LOAD DUMP it's ENOUGH of a load dump to cause problems, especially if the chassis battery is fully charged.

Here is a cut and paste excerpt of a description of LOAD DUMP


A load dump occurs when the load to which a generator is delivering current is abruptly disconnected. In automotive electronics, this applies to disconnecting a battery while it is being charged by the alternator. It is apparently well-described in this $65 SAE document; Wikipedia claims it can be "as high as 120 V and may take up to 400 ms to decay". This document claims a 12V system dump can be as high as 87V and 400ms long:

12V system 24V system
Us 65V to 87V 123V to 174V // maximum voltage
Ri 0.5? to 4? 1? to 8? // source resistance
td 40ms to 400ms 100ms to 350ms // pulse length
tr 10ms?? 5ms?? // rise time

end...


Deterioration (destruction) of solid-state components can happen instantaneously when subjected to severe voltage transients, and WILL BE CUMULATIVE when subjected to less severe voltage transients.

Here's an example...

145 amp alternator charging house bank at 90 amps
Charging chassis battery at 3 amps

The house bank is disconnected.

All of the load dump will be directed at the chassis. But the chassis battery is "full" presenting a relatively high resistance. The excess energy MUST go somewhere.

A ringing TANK CIRCUIT not only presents POSITIVE SPIKES. It also presents NEGATIVE ORIENTED SPIKES.

This issue causes no end to system failures. General Motors and many Japanese manufacturers deemed partial load dump to be severe enough to warrant a major design change. The inclusion of AVALANCHE RECTIFIERS in alternators. The value of the devices is 28-32 VDC. Excessive voltage dumps back into the stator because it is an AC source it can absorb this damaging voltage. Hundreds of millions of dollars are spent avoiding PARTIAL load dump. The same type of load dump that the opening of a switch to hard charging house batteries would cause.

Keep in mind. The word CUMULATIVE in this explanation means something. $$$$$$$$$

My load dump protection for fire engines and ambulances included a 20,000 micro-farad 100 volt capacitor, and three 50-amp rated avalanche rectifiers strategically located throughout the electrical system.

Does it work?

The city of Los Angeles wrote a letter to me in 1991 stating the suppression package reduces ancillary failures of electronic components by 80% over a six-year period. Because of KKK-1822-AA regulations, ambulances with the Sign of Life qualification must use Schottky isolators for the cargo bay. The failure rate for the finned isolator went from 10+ in 1986 to 0 in the following FIVE YEARS. This stuff isn't hocus-pocus. It really does work.

BTW The Kelly II has eight capacitors 220K uf caps, and a dozen avalanche rectifiers.

Excuse me while I go powder my nose...

RJsfishin wrote:
Quote:
, but I'd wait for Mexwanderer to confirm this.
----------------------------------------------
Oh yeah, like we need the battery Godess to figure
out a simple thing like this.

Ooooooooo Godess........

Well I guess I can add "Gender Confusion" to the list. I'll smack ya if you try and kiss me. Well, in most places anyway.

BTW try Goddess. Or the spell check.

A few minutes after starting my engine, my starting battery is back down to sub 1 amp acceptance levels at 14.4v+

I imagine if one has a large depleted bank of AGM batteries taking everything the alternator can produce say 80 amps, then flips the switch to off. I could imagine the rectifier bridge in the alternator not being happy with the instantly dramatically reduced load on the alternator.

Enough to blow the diodes? I don't know

jplante4 wrote:

Ozlander wrote:
Actually, no battery is required on a running alternator, as long as it has a load on it.

How does the field get energized??? :h

It stays energized by the power generated by the alternator. It is often necessary to have some external excitation when starting the alternator from cold, but not to keep it generating once started. (It might also happen, at least sometimes, that there's sufficient residual magnetism to start generating without a battery, but that's not at all a certainty with modern alternators.)

I used to have an old car with a loose battery terminal clamp that would occasionally bounce out of connection while driving. The alternator would keep everything running more or less properly until the engine was shut off. When idling, the voltage in the system would drop noticeably, though--the wipers would slow down, the lights dim, the heater fan die out somewhat...but it would keep running and working until the engine was shut off, at which point everything went eerily dead, at least until the battery connection was reseated. (The bolt on the terminal clamp had stripped out. I was a poor college student and didn't care to spend the money for a replacement connector.)

You should have some form of battery control center that has a charge relay. As I said before, the switch should be in the control circuit and not the charge line.
Have you looked for the battery control center?

Ozlander wrote:
Actually, no battery is required on a running alternator, as long as it has a load on it.

How does the field get energized??? :h

If you have a battery isolator installed OEM, you already have a switch, as Roy B stated in your other thread on this topic. Your Ford Alternator is not at risk but if you want manual control over the charge circuit, a solenoid switch would be the most elegant choice as it can be controlled by a small switch on the dash.

wolfe10 wrote:
He said: ", I'm thinking of putting a switch on the positive chargeline running from the alternator back to the batteries..."

That is only a partial quote. What he actually said was:

Gerald55 wrote:
I would like to be able to sometimes turn off the engine alternator charging of the house batteries on my rig. To do that, I'm thinking of putting a switch on the positive chargeline running from the alternator back to the batteries.

Note that he stipulated "house batteries."

Actually, no battery is required on a running alternator, as long as it has a load on it.

Quote:
, but I'd wait for Mexwanderer to confirm this.
----------------------------------------------
Oh yeah, like we need the battery Godess to figure
out a simple thing like this.