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I have a LiPo 4S battery which the output should be 14.8v but it outputs, fully charged, 15.6v. The issue is that I have a voltage limit of 15v on a non-drone device that gathers data from sensors. So, my first thought would be to discharge it a bit until it drops below 15v. Does that make sense?
Also, I would like to know if that "over voltage" can be avoided?
As others pointed out, 16.8 volt is what that battery should be reading (unless there is a specific chemical composition that makes that different).
Discharging a battery to use with your application is an anti-pattern, please avoid. Put a voltage regulator in there with the correct amp rating (draw) and you will be all set.
Pololu has some 1-2a 12v step-downs that cost $3-10. You can also go adjustable, here is a 9-30v 1 amp step up/down.
I know a few good ones on Amazon. Bottom line, this is a common part, easy to use, and I have had high reliability for nearly identical situations.
A LiPo reaching a higher voltage than stated on the LiPo is quite normal and is not actually "over voltage." Generally, when talking about a LiPo battery cell, 3.7v is mentioned because that is the nominal voltage of the cells. However, when fully charged, a LiPo cell should reach 4.2v. So for a 4s LiPo, 14.8v would be considered the nominal voltage but when fully charged, a 4s LiPo can reach 16.8v.
Seeing as you have a 15v limit, there are a few options. Firstly, as you mentioned, you could discharge the 4S LiPo to 15v and this should work just fine, although you would be losing a significant part of the battery's capacity. Alternatively you could get a 3s LiPo (if that is sufficient voltage) because a 3s LiPo will not go above 15v. And lastly, if this is an option for your project, you could use a 15v power supply to deliver steady, consistent power.
It is probably easiest to charge the battery as normal, but reduce the voltage to your equipment.
You could just add a diode in line with the battery; this would introduce a voltage drop of about 0.7V, plus give you reverse-polarity protection. If you can't modify the sensor device, you could add it in the power cable. Just make sure the current rating is high enough for your device, and that you can remove it for charging!
Another small, simple approach would be to add a 15V LDO (low drop out linear regulator) to your circuit. During the first part of operation, it would drop your voltage down to 15V at reduced efficiency, on the order of 88% to start out. Once your battery is discharged to 15V, it will drop the voltage only by its dropout voltage, so unlike a diode it won't be reducing your efficiency much during most of the discharge cycle. These are a few samples, but you can run your own search based on the exact voltage and current you want to use.
