On a long rafting trip I will be charging a cell phone (charger output = 5.1V/1A) and a head lamp (charger output = 4.2V/1A) using a 12V/8A dry cell battery as the power source. The cell phone will be charged once every two days. The lamp will be charged once every four days. How many charges can I get from the 12V/10A dry cell? What is the formula? I assume there is some ratio for the 12V step-down to the lower voltages.

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    \$\begingroup\$ While not a duplicate, please see this question and its answer to get a sense of the data required for any meaningful calculation, and some relevant calculations as well. \$\endgroup\$ Feb 13, 2013 at 20:28
  • \$\begingroup\$ Batteries store chemical energy that will convert to electrical energy when given a path for electricity to flow between their terminals. Knowing the voltage and current (Amps) capability of the battery gives us a way to estimate the power (rate of energy flow) capability of the battery, but not how much energy it can store, so there is no way to answer your question with the information given. \$\endgroup\$
    – Phil Frost
    Feb 13, 2013 at 20:57
  • \$\begingroup\$ You need to know target battery mAh capacity as well as voltage. Energy to charge = Vbattery x mAh x k where k is an efficiency factor. Use k=1.5 to start. So say cellphone = 1500 mAh= 1.5 Ah (moderate smart phone) and say lamp = 2000 mAh = 2Ah . Wh Phone = 1.5 Ah x 5V say x 1.5 = 11.25 Wh Say 11 Why. Lamp = 2Ah x 4.2V x 1.5 = 12.6 Wh. Available = 12V x 8Ah = 96 Wh. Phone = 11/2 = 5.5 Wh/day. Lamp = 12.6/4 = 3.15 Wh/day. Total = 8.65 Wh/day. Days = 96/8.65 = 11 days. Reality = somewhat more to substantially less with these values. Plug in your actual mAh. Best - measure 12V current for each. \$\endgroup\$
    – Russell McMahon
    Jan 17, 2015 at 14:51
  • \$\begingroup\$ Note lamp of 4V x 2Ah = 8 Wh = 2 Wh/night. That's 0.5W x 4 hours or about 50 lumens with good modern LEDs = plenty for most tasks. Rigging a portable water powered charger at each stop will work wonders!. Propon shaft with alternator at end. This can run all night. If you have say 10 hours at 1 Watt you can run light and charge phone on water power alone. 1 Watt is "easy enough". \$\endgroup\$
    – Russell McMahon
    Jan 17, 2015 at 14:55

1 Answer 1


You need to measure how much current each of the chargers draws from the 12 battery, and multiply this by how long each charger needs to operate in order to charge the corresponding device. This will tell you how many amp-hours each "charge" of each device requires from the 12V battery.

Divide this number into the capacity of the 12V battery (is it 8 A-h or 10 A-h?) to determine how many charges you can expect.

For example, suppose each charger draws 500 mA while operating, and requires 1 hour to charge its device. That means each charge costs you 0.5 A-h, and an 8 A-h battery can supply 8 / 0.5 = 16 charges. Since you need three charges every 4 days, this will last you 16*4/3 = 21 days. YMMV.

  • \$\begingroup\$ Thanks for the answer. I thought there might be a way to figure it out without actually using up some of the battery (empirical). I've got my cell phone time-to-charge estimated, but my 750 lumen head lamp takes many hours - at least 12 - unfortunately, I don't know the current. I'll try to put a multimeter on both of them and get the current. then estimate the hours. Again, thank you. \$\endgroup\$
    – Ken
    Feb 15, 2013 at 3:35
  • \$\begingroup\$ You can measure the current using a different 12V supply, including an AC-powered one, or even your car battery (engine off). \$\endgroup\$
    – Dave Tweed
    Feb 15, 2013 at 4:34

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