# Regulating DC Voltage

I have constructed a circuit that has an Arduino Pro Mini (3.3v) along with an LCD screen (3v) that is being powered off of a single 18650 battery ( 4.2v – 3.6v). The purpose of the Arduino is to take a analog reading of the voltage of the battery while the 18650 is also powering a higher ampere secondary circuit. The problem I’m running into is when I’m running the secondary circuit, the backlight and the contrast in dimming on the LCD readout due to a drop of voltage (brings it to 2.5v) along with effecting the Arduino as well seeing that I’m running the 18650 to RAW and the VCC is out to the LCD.

Now, is there anything I can use to eliminate that voltage drop while the secondary circuit is in use? I'm not able to use a voltage regulator due to the minimal amount of provided voltage so a LM317 is out of the question... If you have any ideas, I would love to hear what you have to offer. Thank you in advanced for your help!

Need the current for the higher power secondary circuit.

Go buy a boost regulator from eBay (they are listed by the hundreds - or at least dozens of the variety you want) and run a 5V LCD screen from it.

The 3V LCD module uses a “charge pump” to double the voltage to the HD44780 equivalent chip, so it is already operating at 3V logic levels with the same chip as the 5V one. The extra voltage is only required to generate the contrast. If the voltage drops to 2.5V, the charge pump is delivering only 5V and you have set your contrast potentiometer in the expectation of 6V, so Vo will be set to around 1.3V giving a contrast voltage of 3.7 instead of 4.7V.

Actually, if you could identify the 5V from the charge pump (it is probably jumper J2 or J3 adjacent to the charge pump U3) of your 3V LCD, you could probably wire a 4.7V Zener from that to Vo and a 2k2 resistor from Vo to ground instead of the potentiometer, to deliver a fixed voltage to the contrast resistor chain. The 4.7V Zener may however be too low (at this low current), perhaps a 5.1V Zener with a 2k2 to ground and a resistor to Vo of the order of 1 to 2k.

The backlight may be more difficult to manage - if you use the option of the boost regulator, you might just have to use it to power the backlight as well.

I think there are some buck-boost DC converters out there, which would be ideal. Just remember to monitor the battery voltage and not let it fall below the safe minimum for your lithium battery type. Any kind of boost regulator may suck charge out of a flat battery too well...

Thank you for the info, I'll give the boost regulator a shot! Hopefully that will workout. As for the battery I'm using one rated for 35A so I don't think it's going to lack in that perspective.

You need to be more specific about your battery specs. There are two ratings regarding current 1 mAh rating (mAmps continuous for one hour) 2 Discharge rate (called "C" rating for Lipo batteries) is usually specified with two numbers; A. Continuous discharge rate (self explanatory) B. Max burst discharge rate (discharge rate for 10 second burst)

The same voltage battery may be available in many different mAh ratings and discharge rates 2500mAh/35A = 14C (14 times the discharge rate of the mAh rating , in this case 14 times 2.5A/per hour) Stating that your battery is rated for 35A in no way means in could not have the shape or size of the one shown in the link I posted since the battery shown in this link is the same size but as you can see if read the description , is rated for 35A

You need to be more specific about your battery specs. There are two ratings regarding current 1 mAh rating (mAmps continuous for one hour) 2 Discharge rate (called "C" rating for Lipo batteries) is usually specified with two numbers; A. Continuous discharge rate (self explanatory) B. Max burst discharge rate (discharge rate for 10 second burst)

The same voltage battery may be available in many different mAh ratings and discharge rates 2500mAh/35A = 14C (14 times the discharge rate of the mAh rating , in this case 14 times 2.5A/per hour) Stating that your battery is rated for 35A in no way means in could not have the shape or size of the one shown in the link I posted since the battery shown in this link is the same size but as you can see if read the description , is rated for 35A

The link you posted is in fact the batteries I have.

Are they not the same size ? There's no dimensions on that page.

I'm sorry, when you say dimensions you're talking about the physical size of the battery, correct? The dimensions would be 18650, ~18mm (w) x 650mm (h).

The dimensions would be 18650, ~18mm (w) x 650mm (h).

650mm long ? Are you sure ? That can't be right. that's 25 inches , more than 2 feet long. A meter is 1000mm

raschemmel: 650mm long ? Are you sure ? That can't be right. that's 25 inches , more than 2 feet long.

[Deep South Negro Voice] Man, that sure is a long one! [/Deep South Negro Voice]

raschemmel: A meter is 1000mm

A meter is something you use to measure electricity. Not a unit of measurement.

LONG ONE
TAKE-2

650mm long ? Are you sure ?
That can’t be right. that’s 25 inches , more than 2 feet long.

That’s almost a meter. The battery can’t be that long if it is only 2500mAh.

Dimensions: Length: 65 mm (2.56") Diameter: 18 mm (0.71") Weight: 46.5 g (1.64 Oz)

From first spec link.

Tom..... :)

raschemmel: The dimensions would be 18650, ~18mm (w) x 650mm (h).

650mm long ? Are you sure ? That can't be right. that's 25 inches , more than 2 feet long. A meter is 1000mm

Woops! I just skipped right over the decimal, lol! 65.0mm... Sorry! Hahaha!

Now that we have the correct size, you are correct in that the battery I originally linked is noticibly smaller than this one which is almost 3" long.

Well I picked up a DC buck boost regulator from eBay, just waiting on it to come in the mail.