2008 4Runner: can I draw 30 amps without damaging the alternator? - Fiberglass RV
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Old 02-14-2024, 12:20 AM   #1
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2008 4Runner: can I draw 30 amps without damaging the alternator?

My question has to do with how much current I can safely and sustainably draw from my 4Runner alternator? My alternator would already be charging the vehicle battery right near it, as well as an ordinary group 27 lead acid RV battery. These are both newish interstate batteries and I keep them in good shape, i. e., 12.5 V or higher, so their power draw is not excessive. (I wish I had a more precise idea what they draw in amps; thoughts on that are welcome and are possibly relevant to my main question.)
Anyway, I’m wondering if, I can draw another 30 to 50 A to charge a LiFePO4 battery; if my alternator will be able to handle that? i’m assuming a DC to DC Converter would be involved, and my understanding is that that works by keeping the voltage above a certain threshold to protect the alternator? But my question is, how much current can I reasonably expect to be able to draw on the output side of that DC to DC converter? As I write this, I’m thinking it might be unreasonable for me to expect to be able to add another 30 to 50 A of continuous draw to an alternator that is already charging or maintaining charge for two lead-acid batteries.
I understand that there are several other ways to charge, including solar and shore power through a Converter. My question is specifically about charging using a tow vehicle. Let’s assume I can get it together in terms of wire gauge, wire length, and getting an appropriate DC to DC converter. What can my alternator in a 2008 forerunner actually handle?
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Old 02-14-2024, 07:43 AM   #2
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Are you planning to replace your group 27 with the Li? I think your alternator is 130A and I think it takes about 100A max for your vehicle. You might check a 4runner forum. It's been pointed out many times that the scenario of a Li battery sucking an alternator to death exists only on YouTube.
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Old 02-14-2024, 10:43 PM   #3
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Are you planning to replace your group 27 with the Li? I think your alternator is 130A and I think it takes about 100A max for your vehicle. You might check a 4runner forum. It's been pointed out many times that the scenario of a Li battery sucking an alternator to death exists only on YouTube.
Actually, I was thinking of adding an additional 100 or 200 Ah. But before I commit to that, I am trying to get an idea how much I can charge in an hour of driving. If I can charge 50 Ah in an hour or driving (using a Victron Orion Tr Smart), that would probably make it worthwhile for me. It is still unclear to me if that is really a prudent thing to do with the alternator already charging the TV and RV lead acid batteries.
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Old 02-15-2024, 07:25 AM   #4
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Understood. I've pondered similar, seems a shame to let that potential "juice" go unrealized. There is a good bit of electrical expertise on the Casita forum, you might ask over there.
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Old 02-21-2024, 11:57 AM   #5
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talk to an expert. you may need a DC to DC charge controller.
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Old 02-21-2024, 12:58 PM   #6
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Originally Posted by zack sc View Post
My question has to do with how much current I can safely and sustainably draw from my 4Runner alternator? My alternator would already be charging the vehicle battery right near it, as well as an ordinary group 27 lead acid RV battery. These are both newish interstate batteries and I keep them in good shape, i. e., 12.5 V or higher, so their power draw is not excessive. (I wish I had a more precise idea what they draw in amps; thoughts on that are welcome and are possibly relevant to my main question.)
Anyway, Iím wondering if, I can draw another 30 to 50 A to charge a LiFePO4 battery; if my alternator will be able to handle that? iím assuming a DC to DC Converter would be involved, and my understanding is that that works by keeping the voltage above a certain threshold to protect the alternator? But my question is, how much current can I reasonably expect to be able to draw on the output side of that DC to DC converter? As I write this, Iím thinking it might be unreasonable for me to expect to be able to add another 30 to 50 A of continuous draw to an alternator that is already charging or maintaining charge for two lead-acid batteries.
I understand that there are several other ways to charge, including solar and shore power through a Converter. My question is specifically about charging using a tow vehicle. Letís assume I can get it together in terms of wire gauge, wire length, and getting an appropriate DC to DC converter. What can my alternator in a 2008 forerunner actually handle?
I don't think you can hurt your alternator by drawing all it is rated to produce. But its output will drop as it warms up. This is normal. What size alternator do you have? Charging the lead acid bats is not a "continuous" load on the alternator. There is a surge in power the the truck battery when the truck is started, then shortly, the power reduces to an absorption phase (in modern charging systems) and finally to a current that matches what is being used in the vehicle, such as running the engine loads, headlights, heater fan, etc.

You will need a dc-dc charger to charge lithiums from your vehicle. The dc-dc charger you select will determine the power it draws. And the dc-dc charger, at least the Victron ones, have setable parameters. You might select, for example, the 30 amp model that will output 30 amps and draw a bit more to make up for inefficiency.

Bottom line, a surge load to the lead acid batteries and then a longer 30 amp load to the lithium, seems like no big deal at all. But again, what is your alternator rated output?

And to state it for the umpteenth time, the Youtube video of a smoking alternator is pure hype. Not a real world thing. Not related to what you are doing. Alternators don't just burn out because you ask them to do what they are designed to do. Alternators in vehicles run at sufficient RPM and with sufficient cooling to be able to run at their rated output continuously. Your loads are not continuous.
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Old 02-21-2024, 01:07 PM   #7
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talk to an expert. you may need a DC to DC charge controller.
He is already planning a dc-dc charger. And he does need one for lithiums. It''s not a matter of "may" need one. Proper lithium charging from a vehicle charging system requires a dc-dc charger because lithium charging voltage is different, they don't need a float cycle, they should not see more than 14.4 to 14.6 volts and they need to be disconnected from the vehicle when not charging. The disconnect stops the lithium from seeing the truck as a load and re-connects when the truck is running. The dc-dc charger also allows us to determine the max current going to the lithium and allows a low voltage disconnect. It also can eliminate the need for a switching wire to turn the charger on and can eliminate the problem of drawing down the truck battery when parked.
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Old 02-28-2024, 10:11 AM   #8
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I don't think you can hurt your alternator by drawing all it is rated to produce. But its output will drop as it warms up. This is normal. What size alternator do you have? Charging the lead acid bats is not a "continuous" load on the alternator. There is a surge in power the the truck battery when the truck is started, then shortly, the power reduces to an absorption phase (in modern charging systems) and finally to a current that matches what is being used in the vehicle, such as running the engine loads, headlights, heater fan, etc.

You will need a dc-dc charger to charge lithiums from your vehicle. The dc-dc charger you select will determine the power it draws. And the dc-dc charger, at least the Victron ones, have setable parameters. You might select, for example, the 30 amp model that will output 30 amps and draw a bit more to make up for inefficiency.

Bottom line, a surge load to the lead acid batteries and then a longer 30 amp load to the lithium, seems like no big deal at all. But again, what is your alternator rated output?

And to state it for the umpteenth time, the Youtube video of a smoking alternator is pure hype. Not a real world thing. Not related to what you are doing. Alternators don't just burn out because you ask them to do what they are designed to do. Alternators in vehicles run at sufficient RPM and with sufficient cooling to be able to run at their rated output continuously. Your loads are not continuous.
Thanks very much Raspy. I haven't been able to find out yet what the technical rating of my alternator is, but I believe it ia 100 amps or more. I hear what you are saying. The two lead acid batteries do not, either of them, establish a continuous load. Only the new LiFe battery will be a continuous load, and that will be managed by the DC to DC converter, which I think keeps the alternator output voltage above a certain threshold value? I believe that the current draws associated with the lead-acid batteries tend to be highest in the first five minutes or so after one first starts the engine. (That's what I have typically seen when monitoring power draw when charging lead-acids from a converter.) I am imagining that a smart dc-dc charger takes that into account, although maybe I would be wise to manually turn off the dc-dc charger for the first 5 minutes after starting. I am thinking about red-arc brand as well as Victron for the dc-dc converter.

One place where I think there might be some vulnerability, and I wonder if you may disagree, is idling. If I am idling at around 650, the cooling in the alternator would tend to be much less than driving at 2000 rpm.

One reason I am thinking I would prefer having the LiFe battery with the TV rather than the RV, is so that I can charge it when I do an errand or an adventure that involves an hour or two of driving without the RV. I have been thinking about battery location. Maybe behind the passengers seat on the floor? I am thinking that a lithium battery probably should not be anywhere in front of the fire-wall/under the hood, even if there is room, but I am really not sure. Wish I had a better idea about where to put a LIFe battery in a 2008 4runner.
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