dead battery and alternator charging question

[LCA078]

I've searched the forum and TM's but couldn't find a good answer. If a M939 has completely dead batts, I know we can slave start it, remove the slave cable, and the alternator will supply the voltage to keep the truck running while also charging the batteries. Back in my younger days when we had dead batts in the motorpool from vehicles sitting for long periods, the motor sergeants would slave start the vehicles but then drive them to the shop to put on a charger. I vaguely remember the advice from same motor sergeants was to disconnect the batteries for extended driving back to the shop as to not put strain on the alternator and associated systems. Of course we could charge the dead batteries just by just driving the truck around but that wasn't the preferred method.

What say ya'll? Is charging completely dead batteries via the alternator too hard on the system?

 

[M35A2-AZ]

You are right, it is not a good idea to try to charge dead batteries with the truck. It is way to hard on the Alternator, you could damage it.

 

[Elijah95]

General rule of thumb “Alternators are for maintaining and topping off SLIGHTLY discharged batteries but never to recharge a dead battery unless in a dire emergency” for any vehicle

 

[Anvilface]

I concur, putting that much strain on an alternator isn't good. If you've got the time to put them on a charger it's much easier on things. Besides, a charger plugged in to a socket in the shop isn't burning diesel just to charge batteries...

 

[Scrounger]

General rule of thumb “Alternators are for maintaining and topping off SLIGHTLY discharged batteries but never to recharge a dead battery unless in a dire emergency” for any vehicle

The above is the best way to approach this issue.

A few things to keep in mind. If you have the standard wet 6TL batteries and they are discharged more than a little, it will take an alternator a long time to fully charge them. As in maybe a week of on road driving. That is one of the reasons to use a battery charger.

As far as the alternator goes. Car and lite truck vehicle alternators are usually rated for a duty cycle of 20-30%. The stock alternator in these trucks, just like commercial OTR trucks, has a 100% duty cycle. What that means is that it can deliver its full rating continuously without harm. However, given the age of the equipment, where it has been, and that Uncle Sugar isn’t paying for the replacement, I would strongly recommend using a shop charger.

Probably one of the best reasons to use a shop charger is being able to charge each battery separately. There is always one that is a little different and when they are charged as a group that can lead to some being over charged and some under charged.

 

[Anvilface]

The above is the best way to approach this issue.

A few things to keep in mind. If you have the standard wet 6TL batteries and they are discharged more than a little, it will take an alternator a long time to fully charge them. As in maybe a week of on road driving. That is one of the reasons to use a battery charger.

As far as the alternator goes. Car and lite truck vehicle alternators are usually rated for a duty cycle of 20-30%. The stock alternator in these trucks, just like commercial OTR trucks, has a 100% duty cycle. What that means is that it can deliver its full rating continuously without harm. However, given the age of the equipment, where it has been, and that Uncle Sugar isn’t paying for the replacement, I would strongly recommend using a shop charger.

Probably one of the best reasons to use a shop charger is being able to charge each battery separately. There is always one that is a little different and when they are charged as a group that can lead to some being over charged and some under charged.

That's one aspect that happened to me (sort of). Batteries were acting all squirrelly, staying charged for a day then being dead the next after driving. Pulled the batteries and had 1 out of 4 that was shot. Harder to tell when they are all hooked up.

 

[74M35A2]

If you have updated your alternator to a modern unit, then by all rights let the alternator do its job and charge at an above-idle condition. A few points on this subject:

1. The stock alternator usually dies trying to charge dead batteries because of low cooling airflow at idle, combined with low temp rated electronics in the end, as well as a 1900’s telephone system type connector block. Yuck.

2. If your batteries don’t test out at labeled CCA level, they will always be putting an un-necessary load on the alternator.

3. Don’t run your vehicle with the batteries disconnected. They act to stabilize the alternator system reference voltage.

4. At idle, if loaded, the voltage regulator FET (transistor) is full on and does not get a break. If alternator output can exceed load, then duty cycle falls below 100% and some additional electronics cooling time can occur during running. This correlates strongly to #2.

 

[LCA078]

That's one aspect that happened to me (sort of). Batteries were acting all squirrelly, staying charged for a day then being dead the next after driving. Pulled the batteries and had 1 out of 4 that was shot. Harder to tell when they are all hooked up.

That's even happened on my 1-ton Ford Powerstroke with two just batteries. One started to fail and was causing odd starting issues. My pickup is a 12v system and the batteries are wired in parallel in order to provide regular 12v's but double the amps for starting. My understanding is the parallel system also causes the stronger battery to constantly 'charge' the weaker battery for the system provide a single voltage. When one battery starts to go south, you replace both.

 

[LCA078]

If you have updated your alternator to a modern unit, then by all rights let the alternator do its job and charge at an above-idle condition. A few points on this subject:
1. The stock alternator usually dies trying to charge dead batteries because of low cooling airflow at idle, combined with low temp rated electronics in the end, as well as a 1900’s telephone system type connector block. Yuck.
2. If your batteries don’t test out at labeled CCA level, they will always be putting an un-necessary load on the alternator.
3. Don’t run your vehicle with the batteries disconnected. They act to stabilize the alternator system reference voltage.
4. At idle, if loaded, the voltage regulator FET (transistor) is full on and does not get a break. If alternator output can exceed load, then duty cycle falls below 100% and some additional electronics cooling time can occur during running. This correlates strongly to #2.

Solid advice and from your signature it sounds like you're a subject matter expert in this field. #3 is the catch-22 issue. In a situation without ability to use a battery charger, it's either overload the alternator with the dead batteries still connected or deal with non-buffered alternator output with the batteries disconnected. And you can't use the slave vehicle for charging because you're just heating up their alternator instead. Maybe I'll ride the fence on this one and say it's a trade off then to disconnect two batteries (if still using the 4 battery setup) and cut the time in the half for the alternator to be pegged at max output. Or keep the slave truck hooked up after getting the dead one started to cut the charging time half again but then airflow sucks for the alternators...dunno. I'm overthinking it but that's what I do best.

Scrounger- I figured our alternators where high-duty cycle since the output was fairly low (60amps per the -10) compared to my pickup (~110amps IIRC) which is a smaller frame alternator. Granted our alternators are putting out 26-30v which means similar power output but it's the amps that cause wiring and components to heat up and fail.

 

[74M35A2]

It is not just the amps, it is the total power that will make heat, so voltage included. Watts total.

My signature is a side business, I do automotive and heavy duty starters and alternators for a living, but I don’t know everything. We currently have 82% of the new truck market for heavy duty, and working on same direction for medium duty. I don’t list the company I work for, but there are a few here that know it, and respect my desire to keep that separate from here.

I’m not saying people are wrong, but a lot of the old ways and thoughts don’t hold true anymore with modern stuff. There have been a lot of improvements. It is a hard case here, because in this hobby, most people want to run their batteries for as long as they possibly can. Best thing you can do is keep your batteries fresh at 5 years old or newer. This takes the load off everything. I just slam 2 new group 31’s in every 5 years ($100ea), have a modern alternator, and have never needed a jump or even a battery charger. She can sit for 6 months, and crank right up. Old batteries will just suck the life out of your alternator, and your time, trying to charge them, when they can never reach full up, and just get worse from there.

New batteries, new civilian alternator, and new new suction side fuel lines are the best things you can immediately do to keep these trucks enjoyably reliable. Very low cost to do all. First thing I did was give my stock 6TL batteries away for free because I did not know how old they were. Never looked back.

Today’s automotive alternators are rated way higher than a typical heavy duty truck. Every since cars went to electric power steering, alternator size jumped by 100amps. Typical F-150 alternator today is a 240 amp unit (12v). A semi truck has a run a long time between shut downs, so it does not need such a high output alternator to recover the lost crank energy from the batteries. Plus it does not have electric power steering, 3 rows of heated seats, etc...