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I'm having a problem determining resistor specs (especially wattage) for my voltage divider circuit [ 27 V to 17.5 \$+-\$ 0.5 V ] on 10 ampere DC supply.

I have an AC (220V 50Hz) to DC (27V 10A) power supply which I want to use to charge a (14.8V nominal) Li-ion battery. Any voltage above 16.8V (full charged) will do, as long as it's not too high. (Should I even consider using 27V directly?)

I have 5 each of [1 Ohm,10 Watt] and [2 Ohm,20 Watt] resistors lying around, can they be used in some combination?

Any suggestions on how can it be done, any inexpensive alternatives?

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closed as unclear what you're asking by Chris Stratton, Finbarr, JimmyB, PeterJ, Nick Alexeev May 5 at 3:32

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    \$\begingroup\$ I,m not sure if I understand you, but if you want to use a voltage divider to charge the Li-ion battery, you'd better quit this whole project. Li-ion will explode when not carefully charged. Buy a Li-ion charger instead. \$\endgroup\$ – Huisman Apr 26 at 17:31
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    \$\begingroup\$ A very unwise experiment but a potential nomination for the Darwin Award. \$\endgroup\$ – Sunnyskyguy EE75 Apr 26 at 17:47
  • \$\begingroup\$ @SunnyskyguyEE75, Li-Ion are not that bad, they maybe would cost only a burn-down house. I think the first nominee for Darwin Award should go to the guy who tested 8-kV charged caps with his fingers... \$\endgroup\$ – Ale..chenski Apr 26 at 18:19
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    \$\begingroup\$ As mentioned above, don't do this. Li-Ion batteries are dangerous and voltage dividers are always to wrong way to generate a voltage for anything that draws current. Buy an off-the-shelf charger and familiarize yourself with electronics. \$\endgroup\$ – MapleTronix Apr 26 at 18:20
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A voltage divider is not the right tool for this job.

You need a voltage regulator.

Considering that you are trying to charge a lithium ion battery, you probably ought to use a charge controller rather than a simple regulator.

You need to stop and reconsider your project.


A voltage regulator will reduce the voltage. The problem is that you need more current to flow through the resistors than your load consumes.

If you need 100mA through your load, then your divider needs to have about 1A through it. This will keep the voltage somewhat close to the voltage you used in designing the divider.

Voltage dividers are really only useful when your "load" draws nearly no current. Say, providing a voltage to an opamp in order to add a DC offset to an AC signal.

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I want to use to charge a (14.8V nominal) Li-ion battery. Any voltage above 16.8V (full charged) will do, as long as it's not too high. (Should I even consider using 27V directly?)

This is absolutely wrong and dangerous premise. Li-Ion batteries must be charged wich CC (constant current) and then CV (constant voltage) algorithm, and the CV level must never exceed 4.2V per cell, never, which means 16.8 V is the ABSOLUTE MAXIMUM for your case. [there are newer cells with 4.35 V nominal charge, but I assume a 4.2V cell]. Please take some time to digest relevant materials from BATTERY UNIVERSITY.

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