Dead starting batteries and a high output alternator

jamesmriding

New member
I recently installed a Mechman 240A alternator on a Ford 7.3 truck. I managed to discharge both start batteries to zero by leaving the headlights on.

In the situation I was in i was able to charge the batteries before running the truck, as I understand that using the alternator to recharge them from dead can damage or destroy both the alternator and the batteries (both Optimas in this case).

But got me to thinking - if I was in Death Valley and this happened, and I could get a jump start but there was no access to a charger prior to running the truck, what is the strategy to get the batteries recharged without damaging the truck, and possibly disabling the truck even further?

And yeah, I get that not leaving the lights on is the first line of defense, but occasionally I make mistakes.

Thoughts? Thanks.
 

dreadlocks

Well-known member
you dont need two batteries for starting, an isolator would mean at least one of em could not be drained if engine is not running.. there is no law saying isolators are for house applications.
 

jamesmriding

New member
@DaveInDenver thanks. I'm not looking for a line of defense for discharge. The question is this - is there a safe way to charge two dead batteries with a high output alternator that does not put the alternator or batteries at risk? I though I was clear on the OP but apparently not. Sorry for that.
 

jamesmriding

New member
@dreadlocks You may not be familiar with staring a 7.3 Ford diesel, particularly in cold weather. On a sunny day in LA you are right - on batter would be fine. On a cold morning in the mountains you are wrong. That is why the truck shipped with TWO batteries as standard from the factory on every 7.3 truck from 99 to 03. Because you DO need both. Now, any ideas on the question I actually asked?
 

dreadlocks

Well-known member
if the truck shipped with two batteries standard I'm sure your overthinking this.. two batteries flat lead batteries are highly unlikely to kill a 240A alternator..

I'm familiar with getting diesels going in cold weather, but thats not really the death valley scenario you proposed is it? If you only had one dead battery, you wouldn't need a jump start.. and it would not be such a big frigging hit to your alternator to bulk up just one battery and do this unlikely damage your concerned with.. if you drained em both flat on a cold morning in the mountains, good luck finding a vehicle capable of jump starting you, but if you had one of em good.. you probably could get going w/a 4cyl honda.

IMHO, an isolator is still your best bet.. both to reduce load on alternator after you leave your lights on, and so you dont need to get a jump start. Its both cheap and effective. (Admin edited)
 
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john61ct

Adventurer
I have no idea where that damage idea comes from.

Not a problem, so no solution required.

Jump start, go driving, get the batteries back to full as soon as possible
 

luthj

Engineer In Residence
Your not going to damage anything by jumping and letting it run. This assumes your wiring is rated for the ~150A of charge current that alternator will supply to the batteries in the first 15 minutes or so. The current will rapidly taper down.

If desired, you could wire an isolator for the second battery. It will get charged when the engine is running. Wire a combine/override switch, so you can use both batteries in cold weather, or if one is dead.
 

Mike W.

Active member
Run it your not going to hurt the batteries. What I have found with my 7.3 is when I isolated the battery life decreased noticably. Then changing one battery does in the older battery. I always change both batteries when it's time.I have owned my F350 since new 2001. It's got 450,000 on it and it still runs fine. 7 sets of batteries and 3 alternators in that time. One tremendous motor..
 

roving1

Active member
Thousands of commercial vehicles and equipment with way more and bigger batteries and not particularly different alternators are jumped from flat dead and sent on their merry way every single day. Overthinking the whole thing assuming the batteries are not wired with speaker wire.
 

dreadlocks

Well-known member
What I have found with my 7.3 is when I isolated the battery life decreased noticably.
Did you use a smart ACR or just a big dumb starter solenoid to isolate them? for starting you'd want the later as it connects batteries before/when starting instead of after engine is running.. if they are isolated when not charging having varying ages and states of battery life is pretty much moot since one wont drag the other down.. if they are always connected, they need to be same age and damaging one damages both.
 

Mike W.

Active member
Did you use a smart ACR or just a big dumb starter solenoid to isolate them? for starting you'd want the later as it connects batteries before/when starting instead of after engine is running.. if they are isolated when not charging having varying ages and states of battery life is pretty much moot since one wont drag the other down.. if they are always connected, they need to be same age and damaging one damages both.
I don't know if the smart ARC in 2004..it's a working truck or was it's pulls my favorite overland vehicles now..Horses...i made the one battery switch change once..
 

jamesmriding

New member
Your not going to damage anything by jumping and letting it run. This assumes your wiring is rated for the ~150A of charge current that alternator will supply to the batteries in the first 15 minutes or so. The current will rapidly taper down.

If desired, you could wire an isolator for the second battery. It will get charged when the engine is running. Wire a combine/override switch, so you can use both batteries in cold weather, or if one is dead.
 
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