Dead starting batteries and a high output alternator

[jamesmriding]

Well, I did a bit more research beyond this forum on my OP while everyone here was chiming in.

It is hard on an alternator to charge dead batteries? Yes. That's clear to me now.

Generates a lot of heat which is tough on every part of the alternator (like the 240 I have that does bench tested 270Amps at 1800 RPM), from bearings to the rectifier. That's magnified by the extra heat generated by a high output alternator. Maybe it fails, maybe it just has a shorted life. In any case it is hard on the equipment.

Backed up by Mechman's installation docs, and other HO alternator manufacturers documentation.

Looks like the battery manufacturers don't think it is a good idea either.

Fact: Alternators are not designed to charge dead batteries

www.optimabatteries.com

But it's also unavoidable. Which was why I wanted to know if there was a way to mitigate the impact. Looks like when you gotta do it, you gotta do it. And just charge up the battery properly as soon as you can.

 

[dwh]

If you're afraid to work the alternator hard, then WTH is the point of buying a high output alternator? It's either rated to handle the load or it isn't.

Better stay away from aftermarket off-road lights and winches as well - they'll surely work the alternator harder than charging a couple lead-acid batteries.

 

[dwh]

From the Mechman home page (at the bottom)...


"We specialize in bolt-in high output charging solutions for your vehicle, with applications to fit almost any application."


(And that should say "solutions to fit almost any application", not "applications to fit almost any application".)


https://www.mechman.com/

 

[dwh]

And no, automotive "charging systems" are not designed to charge batteries. That's not because the alternator can't handle it - most can - it's because the system is designed to maintain a steady operating voltage, which makes it take bloody forever to fully recharge a flat battery to 100%. Since most won't drive the many hours needed, the battery never does get back to 100% (that situation is described in the linked article from Optima).

That is called "chronic undercharge" and is probably the #1 reason for premature battery failure.

And, since it also puts the alternator under a somewhat inceased load whenever the engine is running, could (theoretically) wear out an alternator prematurely.

So after you run the battery flat, put it on a proper shore powered charger ASAP. That's S.O.P. anyway.

 

[dwh]

The internal wiring in most vehicles alternators aren’t rated to handle high current for prolonged periods, they’re meant to top off a nearly full battery and keep the electronics running, so eventually you’ll burn it up. I would assume two dead batteries would make it even worse.

Yes and no. First you have to define "high current". A modern fully loaded vehicle running radio, lights, a/c compressor clutch, a/c blower, computers, etc. could easily draw 70a or more just to drive the truck down the road.

Then, how much does battery charging add to that? A dead battery has almost as high a resistance as a full battery. Doesn't draw much. At around 40%-50% charged the resistance is lowest, so that's where it draws the most. BUT, by then it's built up a surface charge, so that again limits how much it can draw. Let's just say...50a for a pair of cranking batteries for a diesel.

So 70a+50a=120a.

Then there are alternator ratings. All have a max rating, continuous rating, cold rating and hot rating.

So say you have a 100a alternator (on a diesel? Unlikely.) That'll be the max cold rating to make the marketing sound good. The hot continuous rating is probably 80a. It just won't produce more than that.

Which means there are 70a to run the truck and only 10a left over to charge the batteries. Gonna take a long time to charge.

So the question is...will running that 100a alternator at its hot max of 80a gonna burn it out? Not likely, unless it's a cheapo rebuild.



The OP has a Mechman alt rated 240a (270a@1800rpm). Dunno what the hot rating is. Let's just say 80%, which would be 192a (216a@1800rpm).

Is a 120a load going to burn that alternator out?

No.

 

[67cj5]

Well to avoid any more alternator damage why not add a DC to DC charger so one battery it being fed from the alternator and the other is being fed from the DC to DC Charger, That might work ??

 

[Alloy]

I've had a 7.3 for 19 years. The batteries have gone dead 3-4 times. I've replaced the batteries 4 times and I've had the alternator rebuilt twice which cost $40-$50 each time.

 

[john61ct]

It is an absolute crock of ******** to say asking for some moderate percentage of rated current output from an alternator is abusive.

Just turning up a decent stereo will demand higher amps than a discharged lead Starter battery.

Sure an LFP bank, or say 800Ah of AGM, put in a current limiting VR or a B2B

but a flat Starter?

ludicrous!

 

[jamesmriding]

Thanks gents. Got what I needed here. You all have fun talking amongst yourselves.

 

[ducktapeguy]

Well to avoid any more alternator damage why not add a DC to DC charger so one battery it being fed from the alternator and the other is being fed from the DC to DC Charger, That might work ??

Where is the energy for the battery supplying the DC to DC charger coming from? In the end, all the power is coming from the alternator.

Personally, I will avoid using the alternator to charge a dead battery if I can avoid it. I've had to replace many different types of alternators before, ever since I started using a battery charger, I haven't needed to replace one in a long time. Maybe alternators gotten better, or maybe there's some truth in these old wive's tales and manufacturers warning labels that say it'll damage the alternator. Either way, I see no reason why you'd want to do it except in an emergency, in which case I'd just pay close attention to the heat. Electrical loads have definitely gone way UP in modern cars, and engine bays are becoming more packed, so either one can't be good for heat. If you've seen the winding on the rotor, I just can't imagine that handling sustained high currents

When alternators were $40 and could be replaced in 5 minutes, it wasn't such a big deal and I didn't care too much about damage. However the last one I had to replace cost $150 and took about 4 hours, it required removing the radiator, power steering pump, serpentine belt, ECU, and a bunch of other stuff, just to access it. An OEM alternator on my GX cost about $800 for the alternator alone, and I can't even see it in the engine bay, so it's not something that can be fixed on the side of the road. I'm not willing to risk damage to it when a $30 battery charger can do the same thing

 

[67cj5]

Where is the energy for the battery supplying the DC to DC charger coming from? In the end, all the power is coming from the alternator.

Personally, I will avoid using the alternator to charge a dead battery if I can avoid it. I've had to replace many different types of alternators before, ever since I started using a battery charger, I haven't needed to replace one in a long time. Maybe alternators gotten better, or maybe there's some truth in these old wive's tales and manufacturers warning labels that say it'll damage the alternator. Either way, I see no reason why you'd want to do it except in an emergency, in which case I'd just pay close attention to the heat. Electrical loads have definitely gone way UP in modern cars, and engine bays are becoming more packed, so either one can't be good for heat. If you've seen the winding on the rotor, I just can't imagine that handling sustained high currents

When alternators were $40 and could be replaced in 5 minutes, it wasn't such a big deal and I didn't care too much about damage. However the last one I had to replace cost $150 and took about 4 hours, it required removing the radiator, power steering pump, serpentine belt, ECU, and a bunch of other stuff, just to access it. An OEM alternator on my GX cost about $800 for the alternator alone, and I can't even see it in the engine bay, so it's not something that can be fixed on the side of the road. I'm not willing to risk damage to it when a $30 battery charger can do the same thing

Yeah, Like you I also use Battery Chargers, My big wheeled charger cranks out 35 Amps on High and it also has a Boost Start function that puts out over 300 Amps, I also have 3 NOCO Smart Chargers and I also have a Small 4 Amp Charger, Either one of them I can run off a small Genny, But Smart chargers are a pain when using a Genny because they have their pre set programs to run through and when charging FLA's that can take up to 3 X longer than when using the 35 Amp Charger even on it's lowest setting,

I agree with what your saying, You can't beat a good Battery Charger.

 

[john61ct]

Where is the energy for the battery supplying the DC to DC charger coming from?

Yes but the whole point of using a DCDC in this context

**if** you had a big bank of a chemistry with enough CAR to actually stress the stock alt setup

is to limit the current pulled from the alt.

Or, to protect an LFP bank from too high a C-rate.

Yes of course if you have a high-amp mains charger and shore power available, that can be more convenient.

If an alt is rated for 240A, especially a Mechman, then even in Death Valley 150-180A for twenty minutes is a walk in the park. That's only 2.5kW, I've owned car stereos that pull more than that.

With even the highest CAR AGM, it would take a 300Ah battery to draw that much power, and within that 20 min climbing SoC and resistance would start reducing the draw, 120A, 100A soon under 80A.

Even if it cost $3000 to replace the alt, in this scenario - dinky little Starter batt at much lower CAR, there is no chance of damage compared to its normal duty cycle.

And of course, if you did want to continuously charge at 200+A, e.g. an 800Ah LFP bank,

you can just mod the alt to fit a Balmar MC-614 VR to do the temperature-based de-rating for you automatically.