TheSamba.com :: View topic - How many amp hours of aux battery to charge a phone?

Sat Jun 05, 2021 2:59 pm

If my iPhone 11 battery runs very low, how many amp hours of my aux battery capacity are needed to full charge it?

Sat Jun 05, 2021 4:07 pm

Too many variables. What amperage is your USB charging port? Or the charger itself? Are you disabling wifi and cellular while charging?

This question is useless without details.

kourt

Sat Jun 05, 2021 4:21 pm

I think you're in the 2 amp hour range +/-1

Sat Jun 05, 2021 4:31 pm

iPhone 11 batteries are ~3100mAH. so... figure a charge conversion efficiency of a conservative 60% therefore:

XmAH x .60 = 3100mAH
XmAH = 5100mAH or 5.1AH

that's for a fully discharged phone batt, which few let get that low but this is all armageddon scenario figuring a dead batt and poor charge efficiency.

Sat Jun 05, 2021 5:01 pm

I didn't ask how long it would take, just how much energy would be needed from the aux battery. The phone battery only holds a certain amount of energy and that is roughly how much must come from the aux battery. I realize there losses and some small variables.

Mark

kourt wrote: Too many variables. What amperage is your USB charging port? Or the charger itself? Are you disabling wifi and cellular while charging?

This question is useless without details.

kourt

Sat Jun 05, 2021 6:43 pm

You missed a key aspect which is the relative voltages of the 2 batteries in question. That puts your 5.1ah number way off, too high.
I'd say your efficiency assumptions are off too, but don't know that for certain.

Mark

DanHoug wrote: iPhone 11 batteries are ~3100mAH. so... figure a charge conversion efficiency of a conservative 60% therefore:

XmAH x .60 = 3100mAH
XmAH = 5100mAH or 5.1AH

that's for a fully discharged phone batt, which few let get that low but this is all armageddon scenario figuring a dead batt and poor charge efficiency.

Sat Jun 05, 2021 6:49 pm

I don't have an iPhone, but I do have a USB amp meter in my van. I think my Android is something like 1.3 amps... I'll double-check when I get home next week.

Sat Jun 05, 2021 7:58 pm

For $13 you can read how many mAh passed thru this device.
a "USB Meter" on Amazon

The USB outlet adds some losses but it's probably not much different than the power that departed your aux battery.

Sun Jun 06, 2021 4:49 am

It took more energy to makes these posts than the energy in a phone.

:lol:

Just looking at my battery meter, it charged from 2100-2300 to full cap last night ona 1 amp brick at 5 volts, that’s 10 watt hours

So fir your 12 volt battery, that’s less than 1 amp hour

It’s marginally on par with the self discharge rate of a flooded lead scid battery!

Sun Jun 06, 2021 6:51 am

crazyvwvanman wrote: You missed a key aspect which is the relative voltages of the 2 batteries in question. That puts your 5.1ah number way off, too high.
I'd say your efficiency assumptions are off too, but don't know that for certain.

Mark

DanHoug wrote: iPhone 11 batteries are ~3100mAH. so... figure a charge conversion efficiency of a conservative 60% therefore:

XmAH x .60 = 3100mAH
XmAH = 5100mAH or 5.1AH

that's for a fully discharged phone batt, which few let get that low but this is all armageddon scenario figuring a dead batt and poor charge efficiency.

ack!! you're right!! need to equivalent everything to watt-hours, which accounts for the voltage diff. oops!

Sun Jun 06, 2021 7:09 am

crazyvwvanman wrote: You missed a key aspect which is the relative voltages of the 2 batteries in question. That puts your 5.1ah number way off, too high.
I'd say your efficiency assumptions are off too, but don't know that for certain.

Mark

DanHoug wrote: iPhone 11 batteries are ~3100mAH. so... figure a charge conversion efficiency of a conservative 60% therefore:

XmAH x .60 = 3100mAH
XmAH = 5100mAH or 5.1AH

that's for a fully discharged phone batt, which few let get that low but this is all armageddon scenario figuring a dead batt and poor charge efficiency.

As best as I can tell, iPhone 11 batteries are 3.7 volts and 3100 mAH so charging one from zero will consume 11.5 watt hours or .9 amps from your aux battery assuming it's @ 12.7 volts.

Sun Jun 06, 2021 3:03 pm

I think you meant to say .9 amphours so I'll agree, plus a little more for efficiency losses.
What meager info I have found for car charger efficiency indicates a 5% to 10% loss, far less than many home wall chargers waste.

Mark

jimf909 wrote:
As best as I can tell, iPhone 11 batteries are 3.7 volts and 3100 mAH so charging one from zero will consume 11.5 watt hours or .9 amps from your aux battery assuming it's @ 12.7 volts.

Sun Jun 06, 2021 4:30 pm

so, Mark.... what's the application for determining the rilly small demand? curiousity? running an array on the edge? off grid budget balance? inquiring minds and all that.