# Charging 12v car battery with 16v, but low current okay??



## black_z (Jan 23, 2015)

Its only drawing 190mA, so I dont see it doing much damage. Voltage at the battery is only 12.5 when wired up to the 16v source.


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## black_z (Jan 23, 2015)

And to make this more interesting, what I have done is dissected 3 USB phone charging cables, plugged them into three USB wall adapters(put out 5.xV each), then wired them up in series for right around 16V. 


The battery voltage is SLOWLY creeping up. It's definitely charging it, just not sure about the safety of it.

I guess to take this further... is the watts being transferred the dangerous part? So long as the Volts aren't too insanely high?

(my only electrical knowledge is what I have learned reading this forum!)


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## mattheww50 (Jan 23, 2015)

Actually as long as the current is kept low, the voltage doesn't make much difference. Most battery chargers operate at substantially more than the nominal 12 volts, and rely on the resistance built into the charging cables to llimit the current (on some chargers it is a really bad idea to shorten the charging leads). On a cold winter day, the voltage regulator might crank the charging voltage up near those levels to charge the battery.


The internal resistance of an automotive storage battery is a tiny fraction of an ohm. It has to be to deliver 200+ ampere to the starter to crank the engine. So no matter what the open circuit voltage at the charging leads is, losses somewhere are going to reduce it to not much more than open circuit battery voltage. The only hazards in what you are doing is that you might burn up the wall wart USB power supply if it it lacks overcurrent protection.

As far as storage batteries go, you have a choice when you design them. You can design to either tolerate deep discharger very well, or design them to tolerate overcharging very well. Deep cycle batteries don't tolerate overcharging very well, but are just fine being discharged to almost nothing. Automotive batteries tolerate overcharging very well, but deep cycle discharge is really bad news for them.

Bottom line is you can probably charge the battery at 190 ma forever, and you won't damage it.


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## black_z (Jan 23, 2015)

So, at a low current like that, where the voltage start getting dangerous for the battery?


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## SilverFox (Jan 23, 2015)

Hello Black z,

An automotive regulator keeps the voltage of the charging system below 15 volts. 14.2 is typical.

The problem with high voltage is that the electrolyte can boil and you loose fluids from the battery. However to maintain a boil you need some current and your set up may not supply enough current to do that.

I don't think you should have an immediate problem, but if you left your charging going for a year or so you may end up with less electrolyte in the battery than when you started. 

Tom


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## mattheww50 (Jan 23, 2015)

black_z said:


> So, at a low current like that, where the voltage start getting dangerous for the battery?


The open circuit voltage of the charger is irrelevant believe it or not. The voltage drop across the battery is current x intenal resistance + battery voltage. Since the internal resistance of a automobile storage battery is a tiny number, I x R is also a very small number relative to 12 volts. So regardless of the open circuit voltage on the charger, the charger, or the leads to the charger are going to have to have losses that bring the voltage across the battery down to about 12 volts. In this case it is likely that the internal resistance of the USB power supply is where the voltage losses will occur. So even if you have 120V DC at 190ma, Ohm's law say that the total resistance in the circuit must be about 600 ohms, almost none of which is in the battery. At 16 volts, the power supplies together would have an resistance of about 20 ohms to delivery 12V at 190 ma (4 volts at 190ma) R=E/I


You really need to run signficant amperage into the battery to make the voltage to rise much. And yes if you charge the battery at 100 amps, it will indeed get quite warm, and you probably will boil off electrolyte. The heating losses in the battery are I^2 x R. At 190 ma, I^2 is .037, and the resistance is on the order of .03 ohms, so you are dumping a whopping 1 milliwatt into heating the battery! By contrast lif you charge at 100 amps, the I^2 x R loss is 10,000 x .03 or 300 watts, and that can and will heat up the battery.


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## black_z (Jan 24, 2015)

Wow! Very informative..thanks! So, within reason, it really doesn't matter how much voltage and current you apply to the battery, as long as you keep an eye on the battery voltage?


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## mcnair55 (Jan 24, 2015)

Common sense tells me no to trying unless you are 100% competent in electrical issues.Bogey man stories occur everyday on this forum with stupidity and going against instruction manuals,they are written for a reason.

You cause a fire in your home and the fire people will report the seat of the fire and no loss adjuster would pay out imo.


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## StandardBattery (Jan 24, 2015)

black_z said:


> Wow! Very informative..thanks! So, within reason, it really doesn't matter how much voltage and current you apply to the battery, as long as you keep an eye on the battery voltage?


As stated above you need to keep an eye on the current draw and voltage... and going excessive with the voltage is not a good idea.


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## black_z (Jan 24, 2015)

So, the reason the voltage goes up in a car so fast(from resting to running) is because of the amperage the alternator is putting into it? NOT just because its 14.4V, or whatever...?


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## magellan (Jan 24, 2015)

black_z said:


> So, the reason the voltage goes up in a car so fast(from resting to running) is because of the amperage the alternator is putting into it? NOT just because its 14.4V, or whatever...?



That was my question too. A car alternator can put out as much as 18V, which is normally reduced by the voltage regulator to 14.4V, which can vary somewhat. Presumably there's a reason for that, but I'm no expert. I would wonder, though, that if you hooked the battery up to 50V if it would just punch thru the battery and short it out.


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## StandardBattery (Jan 24, 2015)

black_z said:


> So, the reason the voltage goes up in a car so fast(from resting to running) is because of the amperage the alternator is putting into it? NOT just because its 14.4V, or whatever...?



No the voltage is determined by the regulator, the condition of the battery determines what amperage it will draw at what voltage. It's better to think about a load (in this case the battery) taking current rather than being fed current it's a bit perspective but when someone is not familair with electric current I think it's the better view to get straiight first. As the battery charges the conditions change. So time is a factor as is the current state of the battery with any input.


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## black_z (Jan 24, 2015)

I guess what I'm asking is why when I apply 16v with my little setup, does the voltage on the battery only rise to 12.25(or whatever), but an alternator INSTANTLY makes it read 14.x?


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## SilverFox (Jan 24, 2015)

Hello Black z,

The alternator is capable of putting out 50 - 60 amps. At that charging rate the voltage quickly climbs higher.

If your device could put out 50 amps you would see higher voltages.

Tom


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## black_z (Jan 24, 2015)

That's what I'm figuring. Seem slike it sorta boils down to watts...

Next question...is it possible at all to charge with LOWER voltage. In my head, it doesn't make sense, but I've hooked one of these phone chargers up to a car battery and saw the voltage increase a little. Despite it only being 5V.

I've seen videos on youtube of people doing it through the cigarette lighter and eventually starting their car...from a dead battery


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## mattheww50 (Jan 24, 2015)

Think of voltage as water pressure. It always flows from higher to lower pressure, so there is no possibility of charging a 12 volt battery with 10 volts unless 1 or more cells are dead. If the battery voltage is higher than your source, the current simply flows in the other direction and discharges the battery you are trying to charge. I suspect that if you were to check the open circuit voltage on the USB adapter, you are likely to discover it is considerably more than the nominal 5 volts. This is a 'feature' on linear power supplies, the voltage will come down to the nominal voltage once any reasonable load is applied to the supply.

Automobile storage batteries store energy, which is accumulated as the battery charges. If you charge the battery for 2 minutes at 10 amps, you can expect to be able to draw 20 amps for about a minute, or 40 amps for about 30 seconds from the battery. Consequently as long as the voltage applied to the cigarette lighter is more than the battery voltage, you can 'push' energy into the battery. Sounds simple, but keep in mind that invariably the cigarette lighter is fused, so it is going to take minutes,not seconds to deliver enough energy to the battery to be able to start the car.

10 amps is about the limit before the fuse will blow.


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## black_z (Jan 24, 2015)

Fake videos then!

These are cheap chargers, but they do read 5V. 5.2, or so. That's why three in series were right at 16V.

So, not matter how many amps, you can not charge a higher voltage with a lower one. Right?


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## magellan (Jan 24, 2015)

black_z said:


> Fake videos then!
> 
> These are cheap chargers, but they do read 5V. 5.2, or so. That's why three in series were right at 16V.
> 
> So, not matter how many amps, you can not charge a higher voltage with a lower one. Right?



Yes. Here's where the pressure analogy to voltage comes in that Matt spoke of.


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## IonicBond (Jan 25, 2015)

black_z said:


> Its only drawing 190mA, so I dont see it doing much damage. Voltage at the battery is only 12.5 when wired up to the 16v source.



That's because your 16v source does not have enough current to do anything but a superficial surface charge, which would bleed off instantly as soon as you put any load on it. The minimum for a flooded battery to be effective and get deep into the pores is C/12, where C equals the 20hour ah rating. Thus you'll never get up to a somewhat sustainable 14.4v despite having 16v source. Too little current. You also have to have enough to beat the batteries own daily self discharge.

On the other hand, if you charged your battery with another higher current source so that it is fully charged, and then left your 16v wallwart on it as some sort of maintainer - bad news - even though very little current is flowing.

1) 16v at very low current is typically what is used for an "EQ" charge on a flooded battery that has already been fully charged as best as possible. You don't want this to go on forever, and a wall wart left here forever is bad news, besides making a poor EQ charger.

2) Also, even with very little current flowing, but high voltage like 16v forever, you have unwanted parasitic reactions now occuring, namely positive plate corrosion - and bus bars too usually. Now as soon as you put a heavy load like starting a car on it, the corroded internal terminal connections / bus bars just open up like a small fuse.

So yep - even a wallwart of the wrong voltage can cause damage eventually. In bulk charge, you just won't make it or will be fooled by a superficial surface charge. But if the battery is already full or nearly full, look out for positive plate corrosion.

On a related note about how damaging a high voltage / low current setup can actually be:
https://www.youtube.com/watch?v=z5RtZe9AW2E

Similar situation with solar panels with no controller. Eventually, it will kill your expensive battery without it.


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## magellan (Jan 25, 2015)

Great info, ionicbond, thanks. I've copied it into my battery notes.


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## black_z (Jan 25, 2015)

IonicBond said:


> That's because your 16v source does not have enough current to do anything but a superficial surface charge, which would bleed off instantly as soon as you put any load on it. The minimum for a flooded battery to be effective and get deep into the pores is C/12, where C equals the 20hour ah rating. Thus you'll never get up to a somewhat sustainable 14.4v despite having 16v source. Too little current. You also have to have enough to beat the batteries own daily self discharge.





It brought it up to 12.35V at resting. And started my car. A lot better than 11.9V


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## IonicBond (Jan 25, 2015)

That is better. If you want to do the job right, there are better and safer ways to do it. Consider something like a Tecmate-Optimate 6 lead acid charger.

SAFETY FIRST

A lead-acid battery under charge produces hydrogen, especially so at high voltages nearing the end of charge, and even worse at "EQ" voltages like 15-16v. That is what the vents are for. If your vents are clogged, even with a 16v wallwart, you run the risk of cracking the case. At worst, if the positive plate grid corrosion is so bad, and the hydrogen buildup large enough due to clogged vents, AND those corrosive connections INSIDE the battery fuse open with a small spark, you have an underhood disaster.

It doesn't happen often, but it does happen it just doesn't make the news much even if it blinds the operator, and people just shrug it off like it is a natural thing.

I know I sound like a high-school auto-shop teacher with horror stories, but seriously, don't take the chance. That is why in the video earlier, Compass Marine said that this was a "dangerous condition" with the tiny unregulated panel. And it is. At the very least, if you are playing around with even jump-starter packs, look away before making the switch. You never know.


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## black_z (Jan 25, 2015)

I wasn't leaving it on there forever. Just seeing if I could charge it with some stuff I have laying around the house. It worked! 

As for as charging goes, which would be quicker?

14V @ 2A 

or 15V @ 1.87A

They both equal 28 watts

From what I've read so far, the higher voltage one, right? Or would they be equal, since it's the same wattage?


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## black_z (Jan 25, 2015)

IonicBond said:


> 1) 16v at very low current is typically what is used for an "EQ" charge on a flooded battery that has already been fully charged as best as possible. You don't want this to go on forever, and a wall wart left here forever is bad news, besides making a poor EQ charger.
> 
> 2) Also, even with very little current flowing, but high voltage like 16v forever, you have unwanted parasitic reactions now occuring, namely positive plate corrosion - and bus bars too usually. Now as soon as you put a heavy load like starting a car on it, the corroded internal terminal connections / bus bars just open up like a small fuse.



It's not really charging at 16V if it doesn't read that on the meter, though, right?!


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## black_z (Jan 26, 2015)

Yay? Nay?

What about driving all day long. 14.4v for hours and hours. ..


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## IonicBond (Jan 26, 2015)

Knowing (P/I*E) is going to serve you well, but in this case we are just throwing darts at the board with too many variables at this distance. Your battery's pedigree for one. New? Used? Has it ever had a full charge? Is it hard-sulfated already to the point that internally it would only test out as a 45ah battery?

What we do know is that unless you can deliver C/12 (or about 8.33A voltage limited to 14.4v), charge will be ineffective and take waaay longer than it should. Until you do, at best all you are going to experience is superficial surface charges to make your voltmeter happy, but as soon as you put a load on it - other than a cellphone - that voltage is going to crash. Or it will start the car, but you are doing so from a discharged standpoint.

Measuring voltages and being able to start the car does not mean your battery is happy until you load test it.

Note that most modern low-current maintainers include warnings these days about doing precisely what you are doing. That is, voltage-limited low-current is fine for maintenance on nearly or already fully charged batteries, but charging them in bulk is too inefficient. Actually, instead of providing enough current to get the chemical reaction going properly, all you are doing is heating the battery internally, although you wouldn't actually feel it.

Start by finding a source that can provide voltage limiting no larger than 14.5v, anywhere from 7-10A minimum, and try again. Do NOT go larger than C/8 for flooded, in this case that would be 12.5A maximum for these long-term chargers.


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## black_z (Jan 27, 2015)

I really appreciate the long responses! Those numbers look like you are assuming I have a 100ah battery, right? It's more asking the lines of 40-50.


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## black_z (Jan 31, 2015)

All this was just to see "what if", you know... what if I was in a bind and had no help, or no battery charger. Nothing. Just seeing what all I could come up with to get back on the road.

Thanks for all the replies!


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