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I am trying to do this prototype where I require a smartphone to be charged for a substantial amount of time (probably as long as possible, like months or if possible years). Hence I was thinking of charging it with a deep cycle battery (like a lead acid marine battery) with a USB converter like this.

But I want to calculate how long will one marine battery last me to charge the phone. I am quite new to electrical engineering and wonder how can I check the power discharge rate of a smartphone, and what is the battery capacity of a marine battery?

Null
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user47617
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  • I require a smartphone to be charged for a substantial amount of time The charging stops as soon as the phone's battery is full so what you mean is "I require a smartphone to be powered from USB for a substantial amount of time". What we need to know is how much power the phone needs when it is connected like that. Which depends on what the phone is doing. Without this information it is impossible to give a reasonable answer. – Bimpelrekkie Jun 07 '18 at 11:34
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    @Bimpelrekkie it is possible to teach user47617 a method to determine the answer for his setup himself, which will be useful for everyone. This is what Colin__s is doing. – Arsenal Jun 07 '18 at 11:37

2 Answers2

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Assume your phone has a 3.5 Ah battery with a nominal voltage of 3.7 V, that gives you 12.95 Wh. Your lead acid battery may have a nominal voltage of 12 V and a 100 Ah capacity, that gives you 1200 Wh.

Watt Hours (Wh) = nominal voltage (V) * capacity (Ah)

Assume the process of charging your phone is 100% efficient (it isn't), you get 92 charges from your lead acid battery. If one charge of your phone lasts 2 days that gives you 184 days from the car battery.

More likely is you'll get ~70-80% efficiency from the adapter, and a bit more inefficiency from the phone's internal charging circuitry, also, even deep cycle batteries won't be able to give you all their energy so you lose a little more.

The numbers are all made up, but hopefully gives enough information to work out what you need.

Colin
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First you need to calculate the power demand of your smartphone. This depends heavily on the used smartphone and what it should do. I would suggest the following:

  • Take a look at the battery of the smartphone and calculate the Wh of the Battery: \$Wh = Ah \cdot Volt\$. You should be able to find the Ah and Volt ratings written on your battery.
  • Use the smartphone and measure how long the battery holds.
  • Calculate the power demand: \$P_{smartphone} = \frac{Wh\space of\space Battery}{Operating \space time \space in\space \mathrm{h}}\$
  • Calculate Wh of your marine battery: \$Wh_{marine battery} = Ah \cdot V\$
  • Operating time of your device with the Battery in hours: \$ \frac{Wh_{marine\space battery}}{P_{smartphone}}\$

That should give you an estimate on how long the marine battery will last.

sdalhues
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