18650 battery monitoring issues

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Antoine1
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18650 battery monitoring issues

#1 Post by Antoine1 » 25 Jul 2021, 09:39

Hello guys,

I am working on a project which I want to power via 2 18650 batteries.

I'm using this board to power it: https://fr.aliexpress.com/item/40012828 ... 6c37zPQr7D

Powering is working properly, no issue on that but I am now trying to monitor the battery level and this is where I am loosing track.

I have read several posts and explanations on how to proceed and I do not manage to reach the expected result.

First of all, base on this page, I have added a 100k resistor between 3v3 and A0: https://arduinodiy.wordpress.com/2016/1 ... hingspeak/

When reading the value within ESP I have 845 with fully loaded batteries but if, using a volmeter I measure between GND and A0 I get a value of 2.3v (where I should expect something below 1v no?)

To be honnest, I am unsure on where I should put the voltmeter, maybe I am wrong?

If I apply the following formula %value%*4.2/1024 the value it returns is 3.45 with fully loaded batteries, I was expecting to get something around 4.2v no?

Thanks in advance for your help,

Regards

TD-er
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Re: 18650 battery monitoring issues

#2 Post by TD-er » 25 Jul 2021, 11:15

The ADC plugin supports a 2-point calibration, so you can calibrate it without the need for finetuning a formula.

If you're seeing a value < 1023 when the battery is fully charged, you know you have the correct resistor.
Most boards like the NodeMCU or Wemos D1 mini already extended the range to 3.3V using resistors.
So if you now see 2.3V and the reported ADC value is < 1023 you're good to go.

Antoine1
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Joined: 01 Apr 2018, 09:20

Re: 18650 battery monitoring issues

#3 Post by Antoine1 » 25 Jul 2021, 11:50

Hi,

Thanks for the feedback.

I just saw that I forgot to mention that I am using a Wemos D1 Mini.

So if I understand properly what you mean, I should just assume that: 845 = 4.2v

Then:
- Let the battery discharge for a while
- Measure again the voltage on my battery
- new value = new voltage measure
- Use this new reference as a second value for the calibration

Am I right?

Thanks a lot,

Regards

TD-er
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Re: 18650 battery monitoring issues

#4 Post by TD-er » 25 Jul 2021, 13:07

You can also power it using an external power supply, which makes it easier to measure.
If you do it with the batteries at a more discharged level, make sure to measure the voltage while the ESP is connected to get some load on the cells.

The discharge curve is rather difficult to measure as it may fluctuate due to variations in load and the discharge curve is rather flat for a long time.
It also depends on the temperature.

See for some example charts: https://www.richtek.com/Design%20Suppor ... ment/AN024

Also if your board uses a switching power supply, the discharge current may increase when the cell voltage decreases.
So if I understand properly what you mean, I should just assume that: 845 = 4.2v
Yep


Edit:
You can also let the 2nd point be at 0 => 0V
I don't know why it would be different.

AlexisMori
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Joined: 27 Jul 2021, 18:10

Re: 18650 battery monitoring issues

#5 Post by AlexisMori » 27 Jul 2021, 19:06

Hi...Has anybody built a battery pack out of 18650 cell batteries to supplant their house batteries? I know the voltage for the lithium particle batteries is higher than lead corrosive battery. The lithium batteries I have are 3.6 ostensible 4.2vdc max. So 14.4 to 16.8vdc when 4 are utilized in arrangement. I'm either planning to replace the house batteries with these or I'm planning to construct a sun based generator with them with a 2000 watt inverter and fair plug the total rv into it. We have a 30 amp benefit. I know I won't be able to run the ac but I can everything else. I will be utilizing 4p with 100 cells each at that point interface them together in arrangement to induce the 14.4 ostensible voltage. There would be four packs like this with 100 cells in each for 260 ok capacity.

TD-er
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Re: 18650 battery monitoring issues

#6 Post by TD-er » 27 Jul 2021, 20:14

AlexisMori wrote: 27 Jul 2021, 19:06 Hi...Has anybody built a battery pack out of 18650 cell batteries to supplant their house batteries? I know the voltage for the lithium particle batteries is higher than lead corrosive battery. The lithium batteries I have are 3.6 ostensible 4.2vdc max. So 14.4 to 16.8vdc when 4 are utilized in arrangement. I'm either planning to replace the house batteries with these or I'm planning to construct a sun based generator with them with a 2000 watt inverter and fair plug the total rv into it. We have a 30 amp benefit. I know I won't be able to run the ac but I can everything else. I will be utilizing 4p with 100 cells each at that point interface them together in arrangement to induce the 14.4 ostensible voltage. There would be four packs like this with 100 cells in each for 260 ok capacity.
One simple reply possible here: DON'T DO IT!!!

A slightly more elaborate reply:
You can't be sure that cells in series will be charged well, so you will almost certainly damage a few and thus expose the good cells to a higher charging voltage which may cause other serious issues and even a fire hazard.
Apart from that working with 2000 Watt from low voltage requires quite high currents => very serious fire hazard.

So to summarize, DON'T DO IT!!!

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