220 amp alternator

Chebbyfan · April 2, 2025

I have an order in for a 25 trail boss, and on the order, I opted for the 220 amp alternator. Researching this, I came across something wierd, so I’m just trying to get some info on it. I found a truck at a dealer similar to my options, but it shows a 220 amp with a 170 amp backup? Does that mean it will have two alternators?

Another JR · April 2, 2025

There are two alternator option that can be ordered. The base configuration is one 170 amp alternator. It was sized with some extra capacity for added accessory loads, but I have never seen a limit or GM recommendation for maximum added loads. Maybe newdude will chime in if he knows the excess capacity.

One option is to replace that with a 220 amp alternator. This will allow you to support extra loads of up to 50 amps greater than the base alternator could support. As stated above, I don’t know what the excess capacity of the base alternator is, but it is surely at least 30 amps. So I think you can safely add up to an 80 amp load with the larger single alternator. If you were going to go above 60 amps of extra load, I would suggest doing more research to verify the above assumed excess capacity.

Another option is to have that larger 220 amp alternator plus a second 170 amp alternator. The second alternator is not set up to function as a redundant backup alternator. It is instead for supplying very high accessory power, such as for charging a large trailer battery bank or powering a winch or hydraulic pump that is heavily used. I believe you are forced to get the aux battery if you get the second alternator.

My suggestion is (as you appear to have done) get the 220 amp alternator regardless of what you think your future plans are. It’s not much money, and you’ll kick yourself if you find you need it later and didn’t get it. (Same with adding the second battery, and the high idle switch.) I would also suggest you not add the second alternator unless you are sure you need it. It adds complexity, makes the engine bay more crowded, adds accessory drag and slightly lowers fuel efficiency, and possibly adds maintenance cost if it fails at some point.

These are just my opinions, and those who have dual alternators may identify advantages I’m not aware of. I ordered the single 220 amp alternator and aux battery for my truck, plus the high idle option. I have a slide in camper with a 200 ah lithium battery, and only pull boat and utility trailers. I charge the camper battery primarily from solar, but have a 30 amp Victron DC to DC charger driven by the truck system as a backup to charge while I drive if needed. It draws about 35 amps maximum from the truck. Victron’s new 50 amp charger, which draws about 55 amps from the truck, would be easily powered by the 220 amp alternator even at normal idle.

Edited April 2, 2025 by Another JR
Typo

64BAwagon · April 2, 2025

I ordered my truck with the dual alternators for future resale as much as any other reason. Along with the high idle switch, upfitter switches and the heavier front torsion bars.

I thought that the dual alternator was an option on the Duramax only but I was wrong.

My 2024 gasser that I just sold was built with the 220a alternator and the dual battery option. All things being equal the guy that bought it was glad it was equipped that way since he tows an RV.

I believe the L5P Duramax comes with a 220a single and dual batteries (not wired like the L8T gas with dual batteries) the charging option is the dual alternators 220/170. My experience with heavy trucks was that the secondary (slaved) alternator was an on demand and triggered by the regulator on the master alternator. Not sure if GM follows that same operation though.

Like Tim Taylor says "More power"

Edited April 2, 2025 by 64BAwagon

Chuck FB · April 3, 2025

23 hours ago, 64BAwagon said:

I ordered my truck with the dual alternators for future resale as much as any other reason. Along with the high idle switch, upfitter switches and the heavier front torsion bars.

I thought that the dual alternator was an option on the Duramax only but I was wrong.

My 2024 gasser that I just sold was built with the 220a alternator and the dual battery option. All things being equal the guy that bought it was glad it was equipped that way since he tows an RV.

I believe the L5P Duramax comes with a 220a single and dual batteries (not wired like the L8T gas with dual batteries) the charging option is the dual alternators 220/170. My experience with heavy trucks was that the secondary (slaved) alternator was an on demand and triggered by the regulator on the master alternator. Not sure if GM follows that same operation though.

Like Tim Taylor says "More power"

My order was very similar to 64BAwagon's in these ways and at this point really don't know what the future holds so thought I may as well spend a bit more now rather than kick myself for not choosing a few items that added up to something but not that much in the grand scheme of the insane prices of pickups in Canada. I chose the dual alternators, they do then force one to get the second battery since the second alternator is only charging the second battery. Otherwise with the single 220 alternator there is the choice of getting the second battery if one desires. I did get the Hi Idle although it will only operate when warming up the engine as its non functional once the engine is warmed up and the electronic park brake has to be applied for it to function through the cruse control on the steering wheel and it idles up to 1200 rpm with no choice of engine speeds between normal idle speed and 1200 rpm. Upfitter switches I got which they don't wire in the under hood and into the interior harness but all the components are there and the switch is already in the dash, that would be very expensive to order as a kit through the dealer. I chose the fifth wheel prep for the box, I wasn't going to go through all the trouble of ordering an HD 3500 and not get that. I also got the four pack of tpms sensors that can be mounted into trailer tires, its very cheap to purchase when ordering the truck. I made sure to not forget to order the block heater option, I live in Canada after all "eh".

What I decided not to order was the heavier front torsion bars and this is where things get funky with the GM. If the standard torsion bars are selected the truck will more than likely come with the grill shutter system, at least my trim level truck did. I don't know what they do if the camper bars are selected but I have been told that if the snow plow prep bars are ordered then the truck does not come with the shutter system in front of the rads. Also when getting the standard torsion bars GM does NOT rate the truck to carry a camper which is odd given that its exactly the same truck but the front bars are a bit lighter. In the glove box it makes that note on a sticker. However even if one has the heavier torsion bars the rating is quite a bit less for the max camper weight vs the allowable weight the truck can carry on the B pillar.

Edited April 3, 2025 by Chuck FB

Another JR · April 4, 2025

Hi Chuck. You said the second alternator is charging the aux battery only. Does that mean your truck does not have the battery isolator that connects the two batteries on the single alternator trucks when ignition is switched on? I thought the two-alternator gas trucks still have the isolator, but have the big positive output wire from the second alternator wired to the aux battery fuse block. This was just my assumption though.

Edited April 4, 2025 by Another JR

Chuck FB · April 4, 2025

1 hour ago, Another JR said:

Hi Chuck. You said the second alternator is charging the aux battery only. Does that mean your truck does not have the battery isolator that connects the two batteries on the single alternator trucks when ignition is switched on? I thought the two-alternator gas trucks still have the isolator, but have the big positive output wire from the second alternator wired to the aux battery fuse block. This was just my assumption though.

Its been a while since I had read anything on the system and its not easy to find correct info on it. I won't claim to be right but I was under the impression that the two alternator system is more separate than what the single alternator charging both batteries is. It would seem to make sense and also that second alternator is no ordinary alternator from what I had read either, it was claimed to have sort of a freewheel mode if no demand was asked of it. I don't want to create information out of thin air and pretend its correct but that was my understanding that it is wired differently and also that the second alternator isn't wired to feed power to the main battery so its not like a redundant system that the secondary alternator would take over if the main one had an internal issue.

I do know that lets say I open the door to the truck and it gets all hot and bothered that someones going to press its button ( there is a case to be made to name ones truck a female name LOL ) the voltage drops rather dramatically on the starting battery as I check it and the aux battery voltage and while the main battery voltage is down, the aux battery remains exactly the same as it did prior to opening the door. Does that mean there is an isolator between the batteries or just no connection at all. I also don't know if there is an isolator or more like a low voltage shut off if lets say that aux battery is hooked up to a load as per a trailer or camper and the engine is turned off, what if anything would prevent a discharge of the aux battery. Sadly I have far more questions than answers myself.

I had seen a youtuber with a single 220 and single battery install a second battery and I believe he bought the battery connection system that the duramax uses so the could tie the two batteries together for more amperage to run the snow plow electric lift motor and did a bit of fancy wiring to get that connected the way he wanted.

I've also seen another youtuber that has a high amperage system in his slide in camper he designed and is truck has the two alternator system and he has a dc to dc charger that that is drawing off that aux battery. I think his theory for specing the truck that way was to not push the main alternator as alternators like these are not large industrial alternators that can put out their rated amperage and not have a melt down.

Do you have the second battery option with your single 220 amp alternator ?

Pryme · April 4, 2025

My truck has the 220a with a single battery.

Another JR · April 4, 2025

1 hour ago, Chuck FB said:

…

Do you have the second battery option with your single 220 amp alternator ?

I have the single 220 amp alternator and the factory installed optional auxiliary battery. There is an isolator relay that connects the batteries when the ignition is on. There was another thread recently where some of us were discussing exactly how it worked. It’s a dumb (meaning not smart) system, and the batteries are simply connected when ignition is on. No time delay. The aux battery therefore contributes some current to the starter, limited by the resistance of its charging wire, which has a 100 amp fuse on it. I measured the aux battery voltage while cranking and it dropped.

The attached picture shows the aux battery fuse block with the isolator relay.

Edited April 4, 2025 by Another JR
Added picture

Chuck FB · April 4, 2025

35 minutes ago, Another JR said:

I have the single 220 amp alternator and the factory installed optional auxiliary battery. There is an isolator relay that connects the batteries when the ignition is on.

That would make sense for that setup, the only way the second battery could be charged is by connecting the two batteries together via the isolator and then when the engine is off the second battery can be discharged by the load from a camper etc but can't draw from the main starting battery due to the isolator. I have an older Chev that has the two battery system with isolator but I never did end up using it for its intended purpose. I had thought of connecting them together for increased cranking amperage but never did bother as I normally had access to a plug to plug the block heater "s" and battery warmer blanket in and that whole combo does amazing things along with at the time the more rare 0W-30 oil.

One would think in the wrong conditions with an extended crank or lets say a weak starter battery but a stronger aux battery, too much amperage demand would try to get through the charge wire and blow the fuse ( and perhaps that is what would happen in an off balanced worse case scenario )

In looking at your photo now and I looked at my aux battery, that setup appears to look the same visually. The question is what are those wires hooked to, on the other end on mine. It would make sense with mine that the 100 amp wire is connected to the second alternator and if so effectively would separate the two batteries. What the 30 amp fuse feeds, is that the output feeding your camper ?. And as to the 100 amp fuse, the mystery of how a 170 amp alternator is not going to blow that fuse unless there was a dead short, that high a resistance in that cable so it can't really support the output of a 170 amp alternator ?.

I would have to try that if I have some alligator clip adapters for a multi meter to fasten onto the battery and long enough leads. And maybe it would tell nothing but I could put the truck into full on but not running mode that I assume shops use for all systems on and need a battery charger to keep up that rate of draw for any length of time.

Edited April 4, 2025 by Chuck FB

Another JR · April 4, 2025

The 100 amp fuse is on the battery interconnect wire. If you have the second alternator, it would be connected to the aux battery via a larger fuse that is not on my fuse block.

The 30 amp fuse and wire go to the trailer 7 pin connector hot pin.

Chuck FB · April 4, 2025

7 hours ago, Another JR said:

The 100 amp fuse is on the battery interconnect wire. If you have the second alternator, it would be connected to the aux battery via a larger fuse that is not on my fuse block.

The 30 amp fuse and wire go to the trailer 7 pin connector hot pin.

Do you have a wiring diagram or a photo of the aux battery with this other fuse as my truck definitely has the second alternator and yet looks the same under the cover of the aux battery connection system. I wonder if your main battery top fuse system looks different as in if the cable joins both battery top systems or if they have it set up so each battery runs its charging cable from the alternator. Whatever I had come across before with the explanation of that second alternator I haven't found so far.

That 30 amp fuse, yes that would make sense that the second battery would feed the 7 pin connector to charge the RV battery and with ignition off it then may maintain that connection ( I am assuming ). So with a single battery system I would guess there would have to be an isolator relay to avoid sucking the starting battery down.

64BAwagon · April 4, 2025

Just for reference. When the main battery on my 2024 gasser with 220a alternator and factory dual batteries went bad the truck would not start even though the second battery was fully charged. I am assuming that whatever the second battery contributes to the system when the ignition is on is limited by the wiring size between the batteries

When placing a charger on either battery while the truck was parked the battery not on the charger received no charging.

This setup would be great for someone who needed a power source in order to run a warning light (construction or roadway worker) without running the engine constantly. I could see where a simple LED flasher could be powered from that battery for quite a while.

Chuck FB · April 4, 2025

14 minutes ago, 64BAwagon said:

Just for reference. When the main battery on my 2024 gasser with 220a alternator and factory dual batteries went bad the truck would not start even though the second battery was fully charged. I am assuming that whatever the second battery contributes to the system when the ignition is on is limited by the wiring size between the batteries

When placing a charger on either battery while the truck was parked the battery not on the charger received no charging.

This setup would be great for someone who needed a power source in order to run a warning light (construction or roadway worker) without running the engine constantly. I could see where a simple LED flasher could be powered from that battery for quite a while.

I tried the charging theme myself with a charger and tested the voltage of the apposing battery and just as I suspected there was no charge able to find its way through and given however this two alternator system is set up I wasn't surprised. And you found the same scenario with the single alternator as that relay is off and ensures no tie between the two.

Do you have any idea why the main battery died, was there a drain on the battery that discharged and ruined the battery ?. I don't know what gremlins the GM's are having but certainly I have been hearing about some Fords having struggles that Ford has not figured out what's going on in some cases and is a never ending nightmare for the vehicle owner.

64BAwagon · April 4, 2025

We tested the truck and could find no parasitic drain, all the modules were sleeping when commanded. Even with a 1a NOCO maintainer on it the battery was still losing charge. It was likely an internal fault/short. Rumor had it that GM had gotten some bad runs of batteries when the truck was built.

Chuck FB · April 4, 2025

5 minutes ago, 64BAwagon said:

We tested the truck and could find no parasitic drain, all the modules were sleeping when commanded. Even with a 1a NOCO maintainer on it the battery was still losing charge. It was likely an internal fault/short. Rumor had it that GM had gotten some bad runs of batteries when the truck was built.

I gather the replacement battery was the fix, no more issues after that ?.

As to the Ford issue, I heard about it on a youtubers channel as it started from day one as per a battery issue even before he bought the truck new. Of course a dealership isn't going to stand up and admit to issues so I imagine they replaced the battery at the dealership before it was sold and off he goes and at first he was using the truck more, but then as he parked it for some days the issues began to show up as its draining and either won't start or functions on the truck are not working because the system is trying to save the battery by shutting down features. He put a Noco 2 amp maintainer on it but it won't keep it charged up to full as the drain is just too high for the maintainer to keep up. Others commenting on his issue were claiming having a similar problem, its like the more features a vehicle has the more prone to these sorts of gremlins. That truck has been back to the dealer a few times as they replace this, test that and on and on and so far have not figure out what is causing it.

Remember the GM commercial with the catch phrase "Like A Rock", well they were not wrong, your truck was stone dead ... like a rock