Your opinions on alternator failure

[curtis73]

Ok, here's the setup. 73 Impala station wagon. I have two batteries; 700 CCA in the front and a marine in the rear. They are isolated with a solenoid that allows them to be linked with the ignition switch, hot all the time, or isolated completely. The alternator is a 12si standard GM case that has been used for years. It was purchased from Summit and rated at 140 amps with an extra 4-ga wire directly from the alternator to the front battery.

The purpose was to use this car as our RV during our cross country trip. It worked great. During the night we ran our 12v cooler and a fan on the rear battery and charged it during the day while we drove. It worked famously, but by the time we got to Cincinatti, the alternator died. I replaced it with a parts store 63-amp. Since then we've not drained that rear battery and the new alternator has worked perfectly for a few days now.

I have some theories on why it died, but I want some of your expert opinions.

[DRW1000]

Expert opinions eh? hmmmmm. I doubt that includes me but here goes.

If I understand what you have done here it would seem that when you start the car using the front battery the solenoid closes effectively connecting the rear (somewhat drained battery) into the charginging circuit in a parallel set up with the front. The fact that both batteries probably have a slight voltage differential due to the amount of charge is in my opinion negligeable. Basically you have, as far as the alternator is concerned, a battery almost twice the size in capacity (but still the nominal 12 or so volts). You also now have the internal resistance of the battery as seen by the alternator reduced (to something close to half) due to the parallel hookup. This could be hard on the alternator but I would suspect that you have more than compensated for this via the 140 amp alternator. '

This is somewhat of a longwided way of stating "I don't know" In theroy I think what you have done should not have been a problem. In the practical world though other factors enter the equation such as interconnecting wire resistances etc. but nothing stands out as obvious.

I am very interested in hearing some of your theories though.

[MagicRat]

IMHO if modifying the alternator for 140 amps was such a good and durable idea, GM would have already done so. However, they redesigned their more recent alternators for higher (100++) amps rather than use the 12 SI design.

I suspect a sub standard aftermarket part was used in the rebuild.

I would look at the internal voltage regulator first. Replacement ones are about $15 and easy to install, but you must take the housing apart to do to. Just be careful to when reassembling the unit to use 2 toothpicks to hold the brushes away from the armiture when reassembling the unit.

Likely that alternator has to recharge the deep cycle battery extensively in the morning, and at 140 amp max capacity produces more heat than the regulator was intended to handle.

When you have it apart, just test the copper coils to make sure they are not grounding on the armiture and also have continuity (ie, the copper wire is not broken) The heat might have melted the insulating coating on the copper windings

GM does make a 12 SI alternator in 78 amp, 85 and and the rare 93 amp configuration. These units may last longer under heavy loads because the heat build up is less, due to the lower amp rating.

If you are going to do this a lot, consider two stock alternators instead of one mondo 140 amp unit.

BTW my stock 93 amp 12 SI produces enough heat to fry the stock voltage regulator, but it takes many months of abuse to do so.

[curtis73]

Well, I was afraid of that. I would start the car in the morning and kick the rear battery on at the solenoid and the idle would draw down like you kicked on the AC. At one point I even noticed a slight whine from the belt and a little rubber smell.

My original thought when I designed the system was that modern ambulances, and diesel pickups use a 200-amp alternator and at least two batteries. What I failed to take into consideration was that the alternator technology I was using to get my 140 amps was 30 years old.

I think the 63 will do fine. I don't run continuous high loads, I'm just trickling a battery to death. I don't need the high amperage rating to charge, I would need it to sustain power with high-load draws. I'll see if the stock 63-amp will keep up. If it doesn't, maybe I'll run two alts. That engine came with the brackets for an AIR pump, maybe I can modify that bracket for another alternator.

On the other hand, maybe this is the sign I need to drop a Duramax in there

[pind]

Curtis,

The reason your alternator smoked is not necessarily the result of faulty aftermarket parts. The 12si will handle loads of grief.
One thing it will not handle, is a near dead battery, over and over, at idle. This is one of the reasons that highway truck alternators go bad so often. They insist on idling with the lights on a lot. This places a constant draw on the battery, and the alternator has to compensate.

Remember that most alternators are rated at somewhere between 5 and 6000 Alternator RPM. ( not engine RPM) At idle, the alternator is not turning anywhere near that. Therefore although you are placing a load on it, it does not get proper airflow to cool it either.

These are just casual observations over the years, hope it clears some stuff up.

[curtis73]

Good stuff. I didn't know that, thanks. *absorbing electrical information*