Calculating the Maximum and Minimum Capacity for a LiPO battery with varying power requirements

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jenks123

Joined Nov 10, 2019
1
I am working on a project where I need to find the power consumption of a few sensors that will run off of a LiPO battery—not too difficult. The problem that I am running into is the conceptual side of how everything I see says that battery capacity is only dependent on the current consumption of the sensors, but not the voltage that they need. To me this doesn't make any sense as surely a device that needs 10mA at 3.3V will need less power than a device with the same current but runs at 5V, correct? If anyone can elaborate on the details behind battery life of a LiPO battery I would appreciate it.

While my question is more hypothetical/theoretical, the details of what I'm using are as follows:
• 2S 2800 mAh 40C
• 4 sensors each with a different active and idle current
• The project aims to be low-power consuming, so the best and worst case for battery life is desired.

Thanks in advance!
 

oz93666

Joined Sep 7, 2010
668
OK ... so you have 2s that means max voltage will be 8.4 V and lowest will be about 6.6V (it will need recharging at about 6.6V) Lithium are great but their voltage does drop off much more than other chemistries which can be awkward ...

Your 2s battery can supply 2.8A for hour as the voltage slowly drops from 8.4 to 6.6 ... to get that constant current the load will have to adjust (get greater) to maintain that constant current , but that's how they test them it would be called "1C discharge"

Your battery is 40C that means it can incredibly deliver over 100A !! It would be flat in less than 2 mins (realistically 1 min)

Another way of looking at it is the power in the battery , average voltage is 7.6..... x 2.8A = about 20 Whrs
 

MrAl

Joined Jun 17, 2014
6,469
I am working on a project where I need to find the power consumption of a few sensors that will run off of a LiPO battery—not too difficult. The problem that I am running into is the conceptual side of how everything I see says that battery capacity is only dependent on the current consumption of the sensors, but not the voltage that they need. To me this doesn't make any sense as surely a device that needs 10mA at 3.3V will need less power than a device with the same current but runs at 5V, correct? If anyone can elaborate on the details behind battery life of a LiPO battery I would appreciate it.

While my question is more hypothetical/theoretical, the details of what I'm using are as follows:
• 2S 2800 mAh 40C
• 4 sensors each with a different active and idle current
• The project aims to be low-power consuming, so the best and worst case for battery life is desired.

Thanks in advance!
Hello,

The power consumption is not that important unless you are using a converter of some type like a buck converter otherwise the current is the more important factor.

To know the run time you need to know the current of each sensor and how long it runs at active current and how long at idle current, and ideally the P factor of the battery.
If you dont know the P factor then a certain amount of testing is required. After the testing you need to evaluate the aging of the battery too so you know what to expect a year from now. Battery capacity goes down with age so you should take that into account also. At some point which you determine the battery needs replacing. One way to test for this is to use a microcontroller to monitor the battery drain time and when it gets too short you replace the battery with a new one.

The voltage level may be used to determine when to charge. When it gets too low charge the battery. If you can not charge at random times then you probably have to charge as much as possible.
 

Alec_t

Joined Sep 17, 2013
10,355
There is also the matter of self-discharge of the battery, even when no intentional load current is drawn. Depending on your application the self-discharge current may or may not be significant.
 

noweare

Joined Jun 30, 2017
100
I dont know this is what your looking for but....
Amp Hours are a measure of the charge delivered by the battery and Kilowatt Hours are a measure of energy delivered.
from http://all-about-lead-acid-batteries.capnfatz.com/all-about-lead-acid-batteries/lead-acid-battery-fundamentals/amp-hours-vs-kilowatt-hours/

A battery has a certain life which is usually given in a number of charge/discharge cycles. There are a lot of variables though that can effect the useful life of a battery like partial discharge compared to full discharge, temperature and so on. Lot of reading to do.
 
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