There are three scenarios:

  1. 4 AA batteries connected in series would produce 6V. I'm planning to supply the output to an LM7805 regulator and produce 5V. The output from the regulator then will be connected to the charger cable (torn apart from the original phone charger) which will be inserted in the phone (3.7V 1300 mAh Li ion battery).

  2. I'm guessing although the batteries will produce 6V, the current supplied will be too low to charge the phone. So adding 6 batteries instead and following the same procedure as mentioned in (1) could work.

  3. 4 AA batteries connected in series and the output (6V) is delivered to a XL6009E1 boost converter (5V to 35 V) tuned to produce 15V. The output is then supplied to an LM7812 regulator to stabilize the voltage output and the output from the regulator is directly supplied to a 12V lead-acid battery.


  1. I need to know if all these systems will work. If not, then please mention the corrections required.
  2. The current supply will be quite low to charge the batteries in all these cases. Is there any combination of series-parallel connection for the AA batteries to obtain the optimum voltage and current ratings?
  • \$\begingroup\$ Your AA battery bank has insufficiency storage power to exceed a typical smart phone so higher current and mWh rather than higher voltage is best matched to ideal output load voltage. \$\endgroup\$ Commented Jun 21, 2019 at 15:04

1 Answer 1


your approaches

  1. won't work – the LM7805 is so ancient, that it's voltage drop at reasonable charging currents will be > 1V (and that would be bad already if your alkaline batteries are totally fresh and not half discharged. Also, using a linear regulator in a battery-powered device is a bad idea; you'd be wasting one sixth of your energy. Use a switch-mode regulator instead.

  2. no, AA batteries can easily supply several amperes of current. So, while twice the amount of cells doubles the capacity of your energy storage, it's not necessary for current reasons.

  3. Now you're crossing into totally senseless land: use a switch-mode supply to regulate up, and a linear one to regulate down. That makes no sense. Simply directly use a switch-mode supply architecture for AA voltage->USB voltage. Switching up to 15 V and then using a linear regulator to go down to 5 V wastes 10 W of power if your charging current is 1 A. That's twice as much as the power actually used for charging. This is so inefficient that I'm willing to say, no, this won't work.


  1. your first system won't work, your second is not necessary, your third is a bit... stupid, sorry.
  2. no, the current sourcing ability of AA batteries is quite good, actually.

So, simply use four AA batteries to get 6V, and then use a Buck converter to go down to 5V.

However, there's a reason why power banks use lithium batteries: these have a higher energy density than alkaline AA batteries, so you might be pretty disappointed how many batteries you use up just charging your phone once.

  • \$\begingroup\$ Thank you. The idea of using a buck converter to get 5V seems perfect. And I am sorry if I sound stupid. I don't have enough expertise in this field and I don't have enough time to study everything before the deadline of this project. In the 3rd case....forget about the LM7812. What about simply using that boost converter to charge the lead-acid battery with 4 AA's? \$\endgroup\$
    – Rahul
    Commented Jun 22, 2019 at 4:24
  • \$\begingroup\$ I don't see what charging a lead acid battery (which also has way, way, way more capacity than your poor AAs) has to do with charging your phone. \$\endgroup\$ Commented Jun 22, 2019 at 12:45
  • \$\begingroup\$ Alright. Let me break it down. I was just trying to build a demo system. In the actual project, I'll have to use not more than 4 TEG modules to charge a phone battery and then if possible, modify the set up to charge a lead-acid battery. The TEGs supply around 1.5-1.8V (as per the operational condition) each and that's why I was trying to use AAs instead in the demo project ( which may sound silly). The demo project is supposed to help me determine the actual set up. \$\endgroup\$
    – Rahul
    Commented Jun 23, 2019 at 4:13
  • \$\begingroup\$ yeah, TEGs and AAs aren't overly similar. What's the maximum output power of your TEGs? \$\endgroup\$ Commented Jun 23, 2019 at 10:20
  • \$\begingroup\$ 0.75W to 1W per TEG \$\endgroup\$
    – Rahul
    Commented Jun 23, 2019 at 15:26

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