USB charging via the Auto battery

[Mark]

If I were to charge my iPad and iPhone via USB from my auto battery, it would pretty much take forever for those two items to drain the battery, wouldn't it?

Even if, let's say, I was boondocking for two weeks?

I'll have other USB ports that run directly from my coach batteries (charged via solar) but I'm just wondering.

Thanks in advance!

 

[AuricTech]

Ignoring for the moment the potential for damage to a regular starting battery (they're not designed to supply power for extended periods of time without recharging), let's do the math:

Assuming a nominal capacity of 50 Amp-hours @12 Volts, that translates into 600 Watt-hours. Obviously, you want to be able to start your vehicle, so I'll start with a conservative assumption that you can use 150 Watt-hours (25% depth of discharge) without risking being stranded.

Most tablets can draw about 2 Amps @5 Volts while charging. That works out to 10 Watts, which means that your starting battery could conceivably charge a tablet for 15 hours without going below 25% depth of discharge.

I don't think it's something I would want to risk, unless you're driving enough to keep your starting battery at close to a full state of charge.

ETA: Your house batteries can take more abuse, being designed to be drawn down to 50% depth of discharge without damage.

 

[Mark]

Thanks,

So, given that I have easily accessible USB powered from the solar charged batteries, it just makes sense to use those connections when I'm boondocking.

I was just thinking if I accidentally left an item plugged in while I was gone from the rig.

Thanks again.

 

[Mark]

Once an item like that is charged, does the battery keep sending power? You (I) would think that once the item is charged it is no longer sucking power.

 

[LeeRevell]

If I leave my little 12v USB charger plugged into the ciggy socket without the phone/GPS attached, it still remains warm - heat is power drain! Likely not much, but it is there.

 

[VanLifeCrisis]

once they charge, smart phones stop pulling power, they basically smart charge themselves, the draw will be tiny unless its sitting there running gps or something, youd have to leave it in for days to kill battery with a simple charger (some glow or light etc...).

 

[One Awesome Inch]

Assuming a nominal capacity of 50 Amp-hours @12 Volts, that translates into 600 Watt-hours.

What is the formula that was used to get 600w?

 

[AuricTech]

Another variable, which I did not discuss in my original response, is that not all USB chargers are capable of supplying that full 2 Amps / 10 Watts of power. I've seen quite a few cigarette-lighter-outlet USB chargers that are limited to 1 Amp / 5 Watts. For one of those to pull 150 Watt-hours from your starting battery, as I stipulated in my initial back-of-the-envelope calculations, would take 30 hours.

As to how much power your devices would continue to demand once fully charged, I'm going to try an experiment tomorrow. I have a Goal Zero Yeti 400, which features both a display of the Watts its load is drawing and also a cumulative Watt-hour counter (digital, of course). It has two 2-Amp USB ports, so I'll partially drain the batteries of a couple of tablets I own, then plug them into the Yeti to recharge. That should answer how much power they draw after reaching a fully-charged state.

ETA: To answer a question that came up while I was typing the above:

Volts times Amps = Watts
Volts times Amp-hours = Watt-hours

ETA2: Note that "Watts" and "Watt-hours" are entirely different beasts.

 

[MikeRuth]

What is the formula that was used to get 600w?

Ohms law ---P over IE or E over IR Commonly refered as PIE or England over the Irish Republic, LOL .

.p.
----
I*E

.E.
----
I*R

Where E=Volts I=Amps(Current) R=Resistance

50(I)X12(E)=600(P)

 

[One Awesome Inch]

Thanks bro!

 

[ccbreder]

Those power plugs use milli-amps when they are plugged in. They use more when charging, but always use some.
It all depends on how long between charging. If you leave for vacation, when you return the battery will be dead.

 

[Spirituallifetime]

I've charged cell phones and ipod using vehicle battery many times without problems, and don't think anyone would have a problem with it as long as you are starting and driving the vehicle every couple of days. These devices use so little power to charge it never effected the starting power of the battery. I haven't charged a tablet or ipad this way yet, I don't think even these devices will use much more.

---

You could have a problem charging devices if the battery is old.

 

[Spaceman Spiff]

My USB port (Blue Sea) has a .01 amp parasitic load, meaning it uses 1/4 amphour a day with nothing plugged into it. My iPhone also draws about the same current when fully charged and plugged in. Not much, but things add up. Your vehicle will have other parasitic loads also (clock, anti-theft, auto door locks, etc.).

Age and condition of the battery is an issue. I've had vehicles that would not start after 48 hours with an old battery and just the vehicles parasitic loads (and self discharge).

-- Spiff

 

[Bdog1]

I do have good dual battery's, but I charge two phones each day and leave my LED dome lamps on for many hours a days without any issues.

The goal zero experiment would be interesting. Those are a nice piece of kit I wish I had!

 

[Spirituallifetime]

The ECM (computer) that runs today's vehicles draws power off the battery when the vehicle is off, a battery that is in good condition will last 3 months if the vehicle is not started and run. If you park it for any length of time it is best to put a trickle charger on it, this keeps the battery charged and keeps the computer from loosing its memory.

 

[AuricTech]

I have a Goal Zero Yeti 400, which features both a display of the Watts its load is drawing and also a cumulative Watt-hour counter (digital, of course). It has two 2-Amp USB ports, so I'll partially drain the batteries of a couple of tablets I own, then plug them into the Yeti to recharge. That should answer how much power they draw after reaching a fully-charged state.

Well, I ran my experiment the other day, and here are the results so far:

With two partially-discharged tablets (an Emerson 7" Android and a Polaroid 9" Android, both made by Southern Telecom) plugged into the two USB ports on my Yeti 400, the Watts display indicated that they were drawing a total of 13 Watts. That indicates to me that the Yeti 400's two USB ports are rated for a total of around 2 Amps, rather than 2 Amps each.

Once they both indicated that they were fully charged, the Watts display on my Yeti 400 dropped to around 3 Watts. I had not turned either tablet completely off, so the experiment is only partly complete: while it demonstrated that two fully-charged tablets will draw less power than they do when still needing a charge, it did not address the power draw of two fully-charged tablets when both are turned off. Since I used the integral USB ports on my Yeti 400, this experiment also failed to account for tare losses from a 12V-to-USB adapter. Nevertheless, it's clear that at least some tablets draw much less power upon reaching full battery charges.

The next step is to see how much power they draw when they are both fully-charged and turned off....

 

[Mark]

This is great information, thanks!