How many amps can I put through a 12v power socket? "Poor Man's Goal Zero"

[dwh]

Next day, start the vehicle, plug the fridge into the vehicle, and plug the power box into the 12v socket. If I can get 6ah out of the socket, and if I drive 8 hours, it should be fully charged by the time I get to my destination (6ah x 8 hrs of driving = 48ah.)
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So is there anything wrong with this plan? Anything I haven't taken into consideration? Let me have it, I'm looking to learn here. :ylsmoke:

Nothing specifically wrong with the plan, but the numbers are optimistic. First of all lead-acid batteries are only around 80% efficient, so if you draw out 48ah, you'll have to supply around 48 x 1.2 = 57.6ah to get it back to 100%. Call it 60ah.

Also the assumption of a 6a continuous charge rate from <battery at 50% charged> to <battery at 100% charged> is unrealistic. You could very well see more than 6a at the beginning*, but once the battery reaches 90%, you might only see 2a-3a. So you might get it 100% charged in 8 hours of driving, but personally, I wouldn't count on it.


*Without something to limit the current, you could easily find yourself popping a fuse every time you plug in the 50% depleted battery.



For what you're talking about, I would probably opt for installing an inverter in the vehicle and using it to power a decent battery charger.

 

[Martinjmpr]

Ah, I was hoping you'd chime in! :ylsmoke:
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For what you're talking about, I would probably opt for installing an inverter in the vehicle and using it to power a decent battery charger.

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Dumb question, I guess, but do they not make 12v-to-12v chargers? Something that would have some kind of regulator to prevent overcharging?

 

[dwh]

Ah, I was hoping you'd chime in! :ylsmoke:
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Dumb question, I guess, but do they not make 12v-to-12v chargers?

Cig plug to cig plug jumpstarters, yea. Only one I remember right off the top of my head had like a 10 minute timer built-in. You'd have to unplug/replug it every 10 minutes.

You might find something suitable if you look around enough.

How many hours it would last before it failed...dunno, but I wouldn't expect something intended to work a few minutes once a year to last long running all day for days on end.

Something that would have some kind of regulator to prevent overcharging?

That's not the problem, besides your truck already has a regulator to prevent overcharging.

The problem is a good sized, half drained battery sucking enough amps to blow the fuse.

So you don't need something to limit the volts, you need something to limit the amps (current).

The resistance of the wiring itself, combined with the resistance of the battery, will limit the current. But as always, "it depends".

If you used #12 to feed the cig plug, and #12 to feed from the cig plug to the aux battery, would it limit the current enough so that say a 20a or 30a fuse wouldn't blow? Dunno. Depends on how long the wire is, what the internal resistance of the battery is, and what the supply voltage is.

You might find that the fuse doesn't actually blow until you start the truck and the alternator kicks up the supply voltage. Or maybe a few minutes after.

You'd have to try it to find out.


But the flip side of the coin is that if the wiring does have enough resistance to limit the current, it might also reduce the charge rate and make it take longer to get the battery back to 100%.


No matter what you do, you aren't going to get much of a charge from a cig plug. Fine for little batteries, but mighty slim when trying to recharge a 100ah battery.

Oh, you can make it work. As long as you've got enough drive time to get the job done.

 

[Martinjmpr]

No matter what you do, you aren't going to get much of a charge from a cig plug. Fine for little batteries, but mighty slim when trying to recharge a 100ah battery.

Oh, you can make it work. As long as you've got enough drive time to get the job done.

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Which brings me full circle back to my original question: How many amps can I pull through a factory 12v power outlet safely? IIRC the 12v outlet on my 4runner had a 15a fuse and the one I put on my Suburban I also put a 15a fuse on. This is where my inexperience with things electrical fails me. What determines the amperage of the current drawn by the depleted battery? Assuming the 100ah battery is at 50% (worst case), is it going to try and draw all 50 amps at once when connected to a power source?
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Would a resistor on the power box (between the battery and the 12v power coming in) draw the current down to a level that would keep from popping the fuse? Even 10a would be sufficient for what I'm trying to do. 10a constant draw ought not to blow my 15a fuse would it?
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Or is the better alternative, as you say, to put an inverter on the vehicle side and use a 120vAC charger? In that case, what wattage of inverter would I need? Maybe something like this?
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http://products.batterytender.com/A...R-5A-Power-Tender-Series-High-Efficiency.html
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Advertised as a 5a charger, my only question is, what wattage of an inverter would I need? 5a x 12v = 60w which would mean I could connect it to a basic 150w inverter like this one:
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https://www.walmart.com/ip/12V-DC-t...00002364&wl14=12v power inverter 150w&veh=sem
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But that can't be right, can it? 60w is a light bulb, surely it would take a bigger inverter to run this wouldn't it?

 

[Martinjmpr]

DOH! Guess I need to read through replies when I ask a question! Someone already suggested a DC-to-DC charger and here it is:
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https://www.amazon.com/OptiMATE-TM-...sr=8-1-spons&keywords=optimate+dc+to+dc&psc=1
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Says it charges at 2a. Any reason to prefer something like this over the AC charger above + a reasonable sized power inverter? I'd rather charge at 5a than at 2a, other things being equal. Cost is pretty much the same.

 

[dwh]

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Which brings me full circle back to my original question: How many amps can I pull through a factory 12v power outlet safely?

Depends on who made the parts and what the various parts are rated at. You might have a receptacle rated 15a with a plug plugged into it rated at 10a, feeding #12 wire rated at 20a.

The weakest part is the limit. So in that example, 10a.

IIRC the 12v outlet on my 4runner had a 15a fuse and the one I put on my Suburban I also put a 15a fuse on.

I just added a twin power receptacle. It came with #12 wire. I spliced that to #10 (had some laying around) and ran it back to the fuse block and a 20a fuse. Is each individual socket good for 20a? I doubt it. But I won't be plugging 20a of load into either socket. 2 10a loads is more likely.

But theoretically I could overload the individual sockets, which are probably rated for 15a each because the fuse that protects the whole thing is 20a.

What determines the amperage of the current drawn by the depleted battery?

The total resistance of the wiring and battery vs. the supply voltage potential.

Higher wiring/battery resistance, or lower source potential - less amps flow.

Lower wiring/battery resistance, or higher source potential - more amps flow.

Assuming the 100ah battery is at 50% (worst case), is it going to try and draw all 50 amps at once when connected to a power source?

The resistance of the wire and the internal resistance of the battery will determine how much of the alternator's potential is actually allowed to flow through the battery.

So, with BigAss wire, it could easily be possible to see 50a or more flowing through the battery - initially. But as the voltage of the battery rises, so does its resistance, and the amount of current (amps) slowly gets more and more choked off.

Would a resistor on the power box (between the battery and the 12v power coming in) draw the current down to a level that would keep from popping the fuse?

Yes, but it could also lower the voltage, which could make the battery end up taking a lot longer to get full.

Besides, the battery itself is one giant resistor.

Even 10a would be sufficient for what I'm trying to do. 10a constant draw ought not to blow my 15a fuse would it?

It won't be constant. No matter what, with a lead-acid battery, as its voltage rises (assuming you start at 50% and not 0%) its resistance rises and amps flowing goes down.

And when it's at 50%, its resistance is just about at its lowest point - allowing the most current to flow.


But if you find a way to limit the current to 10a, then no, you won't blow a 15a fuse.

And it's possible that whatever battery you buy, together with whatever wire you use, might achieve that naturally.

Or is the better alternative, as you say, to put an inverter on the vehicle side and use a 120vAC charger? In that case, what wattage of inverter would I need? Maybe something like this?
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http://products.batterytender.com/A...R-5A-Power-Tender-Series-High-Efficiency.html
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Advertised as a 5a charger, my only question is, what wattage of an inverter would I need? 5a x 12v = 60w which would mean I could connect it to a basic 150w inverter like this one:
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https://www.walmart.com/ip/12V-DC-t...00002364&wl14=12v power inverter 150w&veh=sem
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But that can't be right, can it? 60w is a light bulb, surely it would take a bigger inverter to run this wouldn't it?

Dunno. At work and gotta run. I'll come back to inverter + battery charger later.

 

[IAm_Not_Lost]

DOH! Guess I need to read through replies when I ask a question! Someone already suggested a DC-to-DC charger and here it is:
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https://www.amazon.com/OptiMATE-TM-...sr=8-1-spons&keywords=optimate+dc+to+dc&psc=1
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Says it charges at 2a. Any reason to prefer something like this over the AC charger above + a reasonable sized power inverter? I'd rather charge at 5a than at 2a, other things being equal. Cost is pretty much the same.

https://www.amazon.com/Wagan-EL9796...rd_wg=r4p87&psc=1&refRID=QNWZE8J865E8N0KG7HAM

This may be another option, it's basically a cigarette lighter battery to battery trickle charger that regulates how many amps get pulled and increases the voltage, pretty much what you were originally looking for. You wouldn't be using it exactly as designed, but it may work for your purposes. Online reviews state that it gives the battery that needs to be charged 5amps. It has some negative reviews but mostly because folks don't understand that it isn't a jump starter.

I may actually pick one up to use in friends vehicles that don't have dual battery systems.

 

[john61ct]

A good quality AGM designed for deep cycling costs a lot more than that.

I use Firefly Oasis at $500 since it uniquely withstands PSOC abuse, which for what you're talking about is hard to avoid away from mains power.

The only other good makers in the US market are Odyssey, Lifeline and Northstar.

Again, ciggie ports are a terrible design, flat out dangerous even at the 6-8A max I would recommend for Load devices.

For CHARGING, where minimum healthy rate is 30-40A per batt, trying to hook that up with a ciggie port would be **very** dangerous.

Anderson plugs are the only way to go for that IMO.

Even low voltage stuff can easily start fires or explode. Please learn the basics before messing around.

 

[john61ct]

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Which brings me full circle back to my original question: How many amps can I pull through a factory 12v power outlet safely? IIRC the 12v outlet on my 4runner had a 15a fuse and the one I put on my Suburban I also put a 15a fuse on.

Just because they put a 15A fuse doesn't mean that much current is safe through the ciggie port.

The main problem is the design was never intended for electrical devices other than a resistance lighter.

The spring on the plug gets weak, there is no positive force holding it in place, you end up with intermittent connections that arc (spark) which can cause melting and fires, usually in surrounding plastic.

To answer your question: 5-6 Amps.

Blue Sea's twist lock design is 10A I believe far far superior to automotive cr^p.

.What determines the amperage of the current drawn by the depleted battery? Assuming the 100ah battery is at 50% (worst case), is it going to try and draw all 50 amps at once when connected to a power source?

Amps are not amp hours! Former is a rate, latter a quantity.

A quality 100AH AGM battery can easily "pull" 500A from an unregulated charge source for a short period.

Even with puny little start batts, you've seen sparks from jumper cables, heard of people melting tools, bush welding rigs right?

You need something explicitly designed to limit currents, usually for this use case a DC-to-DC charger, aka batt-to-batt, like Sterling or ProMariner make, also some good Aussie mfg and CTEK.

But you need to get the battery to 100% full if you want to get a decent lifetime out of it. Except for Firefly.

And that takes 4-7 hours no matter your charge source.

At only 5-10A you won't get good lifetime even if everything else is done right, and take maybe an extra 2-3 hours to get to full.

And still to dangerous IMO for continuous use with a ciggie port.

The inverter + mains charger idea is silly.

 

[john61ct]

I'm not advocating using these, but check them out in consultation with a pro that works with 12V.

&#8203;Samlex BBM-12100

Sterling Power Pro Connect CVSR (the C is critical)

IMO rather than limiting current, build the interconnect infrastructure to handle the big amps.

Also realize that many car alternators can get damaged trying to pull rated amps, or even half the rating for long periods.

A B2B charger IMO is really the way to go, in conjunction with 100A Anderson connectors.

 

[john61ct]

Low amp trickle chargers are for maintaining full status for starter batteries that are already full or nearly so.

They are not designed to charge a depleted deep cycle batt, I think even at 48 hours still won't get right up to 100% unless featuring a user programmable Absorb cycle. Would need to log with an accurate DMM to find out.

And even if you don't care about that, again, AGM lifetime will really suffer at such low amp rates.

 

[john61ct]

This is the right sort of unit:

https://baymarinesupply.com/sterling-60-amp-battery-to-battery-charger-bb1230.html

This is way too underpowered:

&#8203;Tecmate OptiMATE TM-500

 

[Martinjmpr]

Just because they put a 15A fuse doesn't mean that much current is safe through the ciggie port.
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The main problem is the design was never intended for electrical devices other than a resistance lighter.
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The spring on the plug gets weak, there is no positive force holding it in place, you end up with intermittent connections that arc (spark) which can cause melting and fires, usually in surrounding plastic.
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To answer your question: 5-6 Amps.
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Blue Sea's twist lock design is 10A I believe far far superior to automotive cr^p.
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John: Thank you for your input, I do appreciate it. Not sure if you read through the whole thread or not but using a different sort of connector (Anderson, etc) would defeat the whole purpose of the exercise.
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If you're talking about putting in new, heavy gauge wiring and using non-standard connecters, well, I don't need that: I have a dual battery setup in my Suburban that is perfectly adequate for running the fridge. Heck, last weekend we were out camping and I shut the truck off from Friday afternoon until Sunday afternoon - nearly 48 hours of not running the engine, and the 68ah "house" battery did a perfectly fine job of keeping the fridge running.
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What I'm trying to build here is, as I said, a kind of "poor man's ArkPak." - That is, something that I can pick up and plug into virtually ANY vehicle with ZERO preparation done to said vehicle. Something that is portable and switchable, that will run the fridge and that can be recharged as I drive.
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Unless I'm terribly mistaken, what I described above is EXACTLY what an ArkPak is: A portable power source that runs off of a Group 31 battery and that can be recharged by a multiple of ways: 120vAC, 12vDC, Solar, etc.
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And it may be that the difference between what I'm trying to do and an ArkPak is that the ArkPak has some kind of sophisticated computerized charge controller that can regulate the current going to the battery to allow for safe charging no matter what the source. If that's the way it is, then it may be that the $350+ price tag of the ArkPak (plus the battery, which the user supplies) is well worth the cost. I'm absolutely willing to consider that this might be the case.
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Amps are not amp hours! Former is a rate, latter a quantity.
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A quality 100AH AGM battery can easily "pull" 500A from an unregulated charge source for a short period.
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Even with puny little start batts, you've seen sparks from jumper cables, heard of people melting tools, bush welding rigs right?
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You need something explicitly designed to limit currents, usually for this use case a DC-to-DC charger, aka batt-to-batt, like Sterling or ProMariner make, also some good Aussie mfg and CTEK.
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But you need to get the battery to 100% full if you want to get a decent lifetime out of it. Except for Firefly.
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And that takes 4-7 hours no matter your charge source.
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At only 5-10A you won't get good lifetime even if everything else is done right, and take maybe an extra 2-3 hours to get to full.
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And still to dangerous IMO for continuous use with a ciggie port.
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Well, understand that first of all, the 12v recharging idea is obviously not the preferred form of charging. If I made such a box, I would charge the battery fully before leaving home. Furthermore, if I was ever at a location with a 120vAC power source I would use that as well. So don't think that the 12v power port would the the "only" method of charging this up, I just want to have that option if I have no other.
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The inverter + mains charger idea is silly.

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I agree that it sounds kind of "redneck" but assuming that the inverter was of sufficient wattage, why would it not work? Serious question.

 

[Martinjmpr]

https://www.amazon.com/Wagan-EL9796...rd_wg=r4p87&psc=1&refRID=QNWZE8J865E8N0KG7HAM

This may be another option, it's basically a cigarette lighter battery to battery trickle charger that regulates how many amps get pulled and increases the voltage, pretty much what you were originally looking for. You wouldn't be using it exactly as designed, but it may work for your purposes. Online reviews state that it gives the battery that needs to be charged 5amps. It has some negative reviews but mostly because folks don't understand that it isn't a jump starter.
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I may actually pick one up to use in friends vehicles that don't have dual battery systems.

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Thank you! Yes, this is the kind of thing I'm looking for. Now others have said that some of these have a time-out feature, though? At that price point it might be worth trying.

 

[dwh]

The inverter plus mains charger is not a silly idea. It's also not a new idea; it's been done many times. It's an old trick to bulk up off grid battery banks.

It would be pointless however, unless you used a mains charger of more amps potential than what you'd get from a cig lighter plug.

It would also not be portable between vehicles, unless you used a tiny inverter, tiny charger, and a cig plug - which would make it pointless.

So let's shelve that.



So the goal is to get as many amps as can be safely drawn from a cig plug to a battery, without blowing the fuse.

Well, one option would be a Xantrex Echo Charge:

https://www.amazon.com/Xantrex-82-0123-01-Echo-Charge-Systems/dp/B0016G8RT8

But that's a 15a unit, and could be just a bit much (could blow fuse or overheat the socket). But it is designed to do battery to battery charging.


Another option would be a Morningstar Sunguard:

https://www.solar-electric.com/sg-4.html

It's a PWM solar charge controller, but would also work to charge one battery from another battery/alternator. Current limited to 4.5a though, so a bit on the small side.


For a few bucks more, you could go with a 6a or 10a SunSaver PWM:

https://www.solar-electric.com/ss-6.html

https://www.solar-electric.com/ss-10.html


If you don't care about cost, a 10a MPPT might be the way to get the most into the aux battery while not quite overloading the cig plug:

https://www.solar-electric.com/12vo10ampmpc.html

But I wouldn't bother. The voltage differential from the engine battery to the house battery isn't large enough to make MPPT worth the cost. PWM is fine for this sort of thing.



I'd say go with a 10a SunSaver. 10a is enough to make a (slight) difference in charge time (drive time), but small enough it probably won't blow a fuse or melt a socket.

The solar charge controller is powered by the aux battery, so you'll have to mount it in the portable battery box so it stays connected to the aux battery. Plugging/unplugging from the cig socket will be like the sun going up/down.