Auxiliary batteries and isolators - blowing up! 2

[LaunchD]

Has anyone seen similar experiences or have any ideas to the situation below?...

On a fleet of 2006 GMC trucks (modified into shuttle coaches), we have experienced a problem with battery isolators and auxiliary batteries that "blow up".

The stock GM truck has two main batteries and two 105A alternators. The modified bus has the addition of an auxiliary battery and a 200A isolator. The aux battery is used for some systems that need to run while the bus is turned off. It prevents the need for jump-starts because of drained main batteries. Both gel-cell and deep cycle batteries have been tried (and failed) as the aux battery.

At one point or another, there have been occassions where the aux battery has boiled over and the isolator has blown (diode is shorted out on the main battery side). Data loggers have been installed to monitor the condition of the aux battery.

On most tests, the electrical system appeared normal. However, on one bus, we were able to record and observe that voltages up to 20V were measured across the terminals of the aux battery for a period of about 20minutes. Shortly after, the battery was replaced. Refer to the attachment for voltage readings. The graph does not show the entire period that was logged, only the time immediately before the event.

In the few days before the high voltage spikes and the battery boiled, the data logger showed that the charging voltage on the battery was about 13.5V (it's usually about 14V normally). After the battery and isolator were replaced, the voltage returned to a more stable reading. The graph looks a lot more stable, at least.

What could cause this? Can an automotive alternator output 20V? I am considering the voltage regulators as suspect. We will try to reproduce the events to see what conditions could cause this. But for the meantime, I am really stumped.

Any help/ideas would be appreciated. You may have more questions for me... fire away. Other facts: the main batteries are never damaged. Main electrical systems like headlights, ECU, etc are not damaged (fault codes from the diagnostics port were not found, though, the ECU history is cleared after 40 ignition cycles with no events - which can happen within 2 days). The aux battery and isolator are mounted in their own compartment away from heat. Temperature loggers have never recorded high temperatures (but have only been installed on buses that have not had batteries boil yet).

Thanks,
Dave

 

[GregLocock]

are you sure that the alternators, or main batteries, never work in series (eg when starting?). If they do, how do you know that the auxiliary battery is isolated from them?

That data looks very odd, never mind the spikes, the regulation appears to be working in a strange fashion - the chargeing voltage when fully charged is lower than when chargeing the discharged battery.

Could be as easy as a dodgy connection



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Thecardoc]

The most common cause for high alternator output voltage, "When the regulator is NOT to blame" is a loss of case ground for the alternator.

Be very specific here. How is the secondary alternator excite circuit wired? The excite circuit normally flows throuh a charge lamp. Does this van have a charge lamp or is the primary alternator PCM controlled? The excite wire is typically brown in color. If the PCM controls the alternator I believe you will find a white wire in the voltage regulator connector.

By chance are the two alternator circuits sharing the same voltage sense circuit? The voltage sense circuit should be the red wire that goes to the voltage regulator. If both alternators are using the same voltage sense input (the one for the first alternator) then the actual voltage output of the second alternator could be quite random.

The answer to this one will very likely be an oversite of the installer in reguards to how the primary alternator is controlled, and then the fact that the second alternator is "miss-married" to the first one. Your solution may well be add a switch to control alternator #2 voltage sense, and altrnator excite circuits.

 

[LaunchD]

I've gathered some info from GM. The charging system schematic is linked, below.

The isolator and aux battery are installed by a third party (no schematics or info available), so I will have to verify how it is actually wired up.

Thanks for your input, I'll post more info once we 'scope' it out and play around a bit.

Dave

Description of operation from GM (refer to schematic):

When the engine is running, the generator turn-on signal is sent to the generator from the ECM/PCM,
turning on the regulator. The generator's voltage regulator controls current to the rotor, thereby controlling
the output voltage. The rotor current is proportional to the electrical pulse width supplied by the regulator.
When the engine is started, the regulator senses generator rotation by detecting AC voltage at the stator
through an internal wire. Once the engine is running, the regulator varies the field current by controlling
the pulse width. This regulates the generator output voltage for proper battery charging and electrical
system operation. The generator F terminal is connected internally to the voltage regulator and externally
to the ECM/PCM. When the voltage regulator detects a charging system problem, it grounds this circuit to
signal the ECM/PCM that a problem exists. The ECM/PCM monitors the generator field duty cycle signal
circuit, and adjusts the system voltage depending electrical load. On Vehicles that have a auxillary
generator the ECM/PCM uses the GEN 2 turn on signal to control the auxillary generator, the auxillary
generator regulator does not use a F terminal. The two generators work together to maintain system
voltage.

 

[Thecardoc]

OK, I'm happy with my memory of the wiring schematic. (VBG)

This would be the GM factory schematic you provided, note the red wire shared by the two alternators pins "D". That is the voltage sense circuit, and is the one that the alternator regulators use to control charge output. The grey wire (which I called white) terminal "F" is the one controlled by the PCM for voltage output. If the PCM is happy with vehicle voltage level, it will regulate that alternator down. Meanwhile alternator #2 is running on only it's internal regulator and if it's watching #1's sense voltage output and not it's own that could easily cause the output voltage to climb. Note how pins D, and the alternator outputs are all connected through fusible links. Your problem may well be that your isolation of the two circuits has also isolated the voltage sense (regulation) of alternator #2.

 

[LaunchD]

Update

Dear Friends,
We've discovered the cause of the "weird" voltage spikes on the aux battery. In short...

The "main output" of the isolator would blow/open circuit (reason still not known - however, possibly to due under-sized isolator for the application).

With the main batteries disconnected, only the aux battery was being charged by the alternators. However, the sense wire for the alternators were still hooked up to the main batteries. Since the batteries are not being charged, the alternator thinks that it's only outputting 12V... the alternator will continue to increase the output - causing the boiling of the aux battery.

Cheers,
Dave

 

[MacGyverS2000]

Thanks for the update... always good to know a logical reason was found for a problem.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Thecardoc]

Nice job solving the problem. Looks like I would probably have gone right at it if i had seen it in my shop. Funny thing is, in our industry we actually struggle to be paid correctly for being able to diagnose a circuit issue like that, but see others make big bucks slamming alternator, after alternator.....