I am trying to make my own 9 volt battery from 18650 cells. I have 3 cells (Samsung INR18650 35-E) in series and have a voltage divider (7k and 15k resistor) at the end. When I measure the voltage I measure 8.5 volts, almost the exact same as a normal 9v battery.

My problem is that I almost have no current. I can power a simple LED without any resistors. When I try that with my 9v battery it blows up immediately. I also can not power a simple 9v (Lego) motor with it.

What can be my problem? Any help is really appreciated!


closed as off-topic by Chris Stratton, StainlessSteelRat, Edgar Brown, Bimpelrekkie, Voltage Spike Mar 21 at 21:06

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  • 2
    \$\begingroup\$ Before going any further, I suggest you stop what you're doing and look up how to safely use lithium-ion batteries. Your voltage divider is severely limiting how much current it can output, which is why you're unable to power anything from it. \$\endgroup\$ – Hearth Mar 14 at 18:27
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    \$\begingroup\$ In any case, what you have here is not a 9V battery, but a 12V battery with a 7k resistor in series with it. (and a 15k resistive load) \$\endgroup\$ – Hearth Mar 14 at 18:36
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    \$\begingroup\$ You should change your project to one that is safe while you still have time to. No sane teacher is going to force you to stick with something once safety issues are pointed out. \$\endgroup\$ – Chris Stratton Mar 14 at 18:36
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    \$\begingroup\$ Then get rechargeable AA's. You are using the wrong parts for the job without the expertise to understand why they are wrong, how to use them safely, or how to bridge the difference. Thus you should be using the correct parts as intended by the manufacturer - AA or AAA cells. And it is your teacher who should be taking the lead on safety. \$\endgroup\$ – Chris Stratton Mar 14 at 18:54
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    \$\begingroup\$ @LuukWuijster 18650s are lithium-ion cells, infamous for bursting into flame if you so much as look at them the wrong way. It would not be hard for you to accidentally burn down your house working with them. That said, some 18650s have built in protection circuitry, and those are safer, but still not something I'd recommend for someone as inexperienced as you seem to be. Using multiple cells together is especially full of pitfalls, and if you are dead-set on using li-ion cells, I would have to recommend you use only a single cell, boosted to your needed voltage. \$\endgroup\$ – Hearth Mar 14 at 19:12

You can't use a voltage divider that way.

The current to your load has to pass through one of the resistors. I your case, it has to pass through the 7k resistor.

Consider Ohm's law:


E is voltage in volts

I is current in amperes

R is resistance in ohms

Rearrange it to give current:

I= E/R

You have about 11 volts.

Divide that by 7000 ohms, and you have diddly/squat for current. About 1.5 milliamperes through the 7k resistor.

It won't help to lower the values of the resistors, either. You have current flowing through your voltage divider all the time. Right now less than 1mA.

If you lower the resistor values so that you get more current for the load, you will also increase the wasted energy. Your battery won't last very long.

What you need is a regulator.

A plain old linear regulator would be a start. They are easy to build using common parts. The common 7809 wouldn't work because your battery voltage is too close to the needed output voltage. There are many models of linear regulators available though.

Linear regulators also waste a lot of energy. This will shorten the run time of your battery.

A more efficient option would be to use a buck regulator. These are more difficult to build, but waste far less energy than linear regulators do.

As a simpler alternative, consider using 6 AA or AAA cells in series.

That gets you very nearly 9V, and is in fact the way 9V batteries are made. If you carefully saw open a 9V battery, you will find 6 smaller batteries inside. Each is a small 1.5V alkaline cell.

If you need to be able to recharge your battery, use NiMh cells. 6 in series will get you around 8V, which should work well enough for most things.


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