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12v battery charging current limit?

gregd

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Dec 31, 2014
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Living in the foothills of the Sierra Nevada of California has its advantages and disadvantages. One disadvantage right now is that strong winds combined with very dry conditions have prompted the state's power utility (PG&E) to cut power to areas that are fed with high voltage lines in wind-prone areas. I seem to be in a small zone where power outages have mostly been skipped, I suspect because I'm near a hospital and medical center so may be on the same "grid" as they, but we still had a 16 hour outage a few weeks ago.

I have a 92 AH deep-cycle marine battery and 2kw inverter that can run our kitchen refrigerator, but it only has enough storage capacity for about a day's worth of use. The challenge then is how to recharge the battery. I can use my wife's SUV and some jumper cables, but sitting right next to the SUV is my Roadster with some 53kwh of power sitting inside. A few years ago when I had the 12v battery replaced I had the SC put in a short pigtail to the battery to make it accessible. The idea was to use it to jump the car if necessary, but, as with the SUV, I wonder if I could use it to (slowly) charge my deep cycle battery?

So, the question: What is the current capacity of the car's 12v battery charging? I don't want to fry the car by putting a large load on the circuit. Maybe need to put a small resistor in series,to provide a current limit. I know the charging will be slow, but it might be better than running the gas car if I'm not in a hurry. At least, get it to a low float level.
 

X.l.r.8

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Feb 18, 2018
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Toronto/Miami
APS is 12v or what ever the output is. I remember when I was looking to put more power for the class D amps I was poking around. If you are not running the 12v systems it should be fine, but a replacement DC/DC converter scares me. I have a volt one that has a high output for the race car but they are super cheap.
 

ewoodrick

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Apr 13, 2018
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Buford, GA
Folks, we are getting all sorts of things confused here.
First, I have no idea what the recharge current for the roadster looks like.

But, 15A on the plug isn't that much when using an AC inverter. 12V*15A = 180 watts

At 120V that's only 1.5A of service. That's not enough to power a refrigerator, let alone the AC.

To power a house, you've got to pull directly off the big battery and spend some money on serious inverters. Not a trivial or even supported solution.
 
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gregd

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Dec 31, 2014
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So, let me clarify...

The car will not be powering anything except recharging a 92AH deep cycle battery. After it's charged, I'd take the battery off the car, hook it to an inverter, and run the fridge, etc. from there. I'd never want to hang an inverter off any car system, as there have been reports that even a small inverter (under 100w) plugged into the accessory port can damage the APS.

I understand that the total 12v output from the APS is over 100 amps. But, how much of that is reasonably available to charge the 12v motorcycle battery that runs the car's systems when asleep? Since the battery is very small, I'm guessing that there is not much current available for recharging. When totally flat, what's the in-rush current for the small battery? I'd probably want to limit the current to my larger battery to that value.

Perhaps another way to look at it... Which fuse drives the 12v battery? I don't see one specifically marked as such in the manual.
 

X.l.r.8

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Feb 18, 2018
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I would take the power from the APS fuse. Not the 12v accessory plug. With the car sans 12v systems (even the brake pump is 2.5A) you could pull a lot more. I will look for the relevant pages if work is quieter than last night.
 

gregd

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Dec 31, 2014
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I would take the power from the APS fuse. Not the 12v accessory plug. With the car sans 12v systems (even the brake pump is 2.5A) you could pull a lot more. I will look for the relevant pages if work is quieter than last night.
Ok, interesting idea. Hadn't considered digging in there.

The easiest thing for me, however, is to pull the power from the Power Pole connection I already have that connects directly to the 12v battery. The question was, how much can I safely pull, without draining the 12v battery or damaging the power supply feeding it.

I know the accessory plug maxes out at around 8 amps. That's trivial to access as well, but I think it's a diode drop or so lower than the 12v battery's float voltage, which is what I need.
 

ML Auto

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Mar 8, 2014
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SW Florida
The 12V battery is charged thru the switchpack when the car is awake. Hooking a load to it is asking for an expensive switchpack replacement. The APS on the later cars output is 120 amps at 13.5v. The 1.5 is only 12.5v. But you would have to keep the car from going to sleep if you wanted to charge something from the APS output.
 
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X.l.r.8

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aps.png
View attachment 471316
 
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gregd

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Dec 31, 2014
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Thanks X.l.r.8 for the details!

Basically, the current available for the 12v battery should be a bit more than what I can get from the Accessory outlet (it's on a 15A fuse), but not hugely so. As you say, maybe 15A, but that would be pushing things. Back of the envelope... 92AH / 15A = 6 hrs. Hmmpf. And that's only to the float voltage, and only if it maintained that current for the whole duration, and only then if I had a current regulator to protect things. Thinking now this won't get me what I need (regardless of the risk). And, yes, MLAUTO, I realize that pushing things this far is not a good idea. Given all of the above, it's become an academic exercise...

So, back to Plan A, using my wife's gas car to recharge the deep cycle battery. Our neighbor used to be a car mechanic, and figures I can get a good charge in just an hour or two. Fortunately, the last two outages (there's one in progress right now) have avoided my street, though I can hear multiple generators running in the not-so distance. Fingers crossed.

Thanks again for the research.
 

X.l.r.8

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Feb 18, 2018
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I think you are better off with a one wired car alternator of 150A output and finding a way to drive it, even an ebike would power that for an hour with no additional load. Most generators are 120/240v but to drive a 12v alternator is pretty simple. A crashed moped motor would run for hours on a drop of gas.
 
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gregd

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Dec 31, 2014
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I think you are better off with a one wired car alternator of 150A output and finding a way to drive it, even an ebike would power that for an hour with no additional load. Most generators are 120/240v but to drive a 12v alternator is pretty simple. A crashed moped motor would run for hours on a drop of gas.
Not sure about your ebike example (seems to violate some law of physics), but yes, there are lots of ways to generate power. The trick is generating enough. One suggestion was to use an old lawn mower to spin the alternator, which would be good except that my lawn mower is electric... I also have a creek in the back yard (but not enough elevation drop), and solar panels would be great except for the local tree cover.

If I were to get a real generator, it'd be one of the dual-fuel models, and would be run from the BBQ's propane tank. Most have a direct battery charging outlet on them. Pollution-wise, I'm not sure which is better - using the car with its emission control stuff, or a small generator running on propane without it.
 

X.l.r.8

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Feb 18, 2018
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Toronto/Miami
The ebike was only because you can use stored battery like a 48v up to 96v lithium that is cheap and easy to charge and store energy. A motor without a rite running a belt to a alternator would spin it without breaking a sweat. At that point you could pretty much build storage system (which is where my race car batteries will head after I’m done with them) but where do you stop. The argument for a good 240v generator means you always have a back up to charge the roadster. I would let my model S go flat long before the roadster. That being said there is already an inverter modification for the volt to let the car power the house. But then you have to hide a volt somewhere lol.
A propane or LPG generator would be a super clean way to have back up power.
 
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