# Lipo Batteries - Max & Safe Min Voltages?

Guys,

I have a 3.7V 2400mAh LiPo cell (https://core-electronics.com.au/search/?q=polymer+lithium+ion+battery+(lipo)+3.7v+2400mah) connected to a LoPy4 & Expansion Board.

Using the median battery voltage code provided by a Core tutorial, my max median voltage appears to be about 3.8V when charged via USB (I havent tried charging from a wall socket (iphone charger for example).

My main question though relates to when the LoPy is running on battery power…

Currently, when the median battery voltage is <=3.05V it sends me a Pushbullet notification as a reminder to charge the battery.

I dont want to kill any part of the project through running a too-flat battery so thought Id seek an experienced view point.

Any thoughts on what voltage I could expect to start seeing odd / unreliable behaviour from the LoPy and at what voltage should I not let the battery fall below? I have read anywhere from 3.2V down to 2.7V - Given that fully charged is 3.7-3.8V, thats a massive “flat range”.

Thanks
Jon

Hi @Jon13024,

For 3.7V LiPo’s, you should be able to expect:

• 4.2V = fully charged
• 3.7V = nominal voltage
• 3.0V = near flat

There are some caveats depending on the age of the battery etc, especially given the swing is very fast toward the end.

Fyi, the time it takes to go from 4.2V to 3.7V, and from 3.7V to 3.0V are not equal. The voltage discharge curve will look something like this:

Thanks Graham, thats most useful.

My battery was at 2.99V this morning at 06:30, its been plugged into USB since then and is now at 3.85V, the lopy4 is in machine.deepsleep() for most of the time, only waking up every hour at the moment.

The drop-off curves are interesting and steep!.. Over the weekend I watched as the voltage dropped sub-3.2V. At one point it simply stopped running the program and presumably re-started but seeminly did not execute main.py. I plugged it in, re-booted and its been fine since (wierdly it was even running fine overnight on battery at 2.99V).

One other question… Is it OK to leave the battery connected to the expansion board if its also plugged into the USB? I have read some reports that say yes, others say no! Im guessing it depends on how the charging circuitry works…

Thx
J

Hi Jon,

The Expansion board has built in Lipo charging, and when connected via USB it will charge the Lipo Battery.

From the Expansion board documentation:
"The Expansion Board features a single cell Li-Ion/Li-Po charger. When the board is being powered via the micro USB connector, the Expansion Board will charge the battery (if connected). When the `CHG` jumper is present the battery will be charged at `450mA` . If this value is too high for your application, removing the jumper lowers the charge current to `100mA` .

To use the battery, pull `P8/G15` high (connect to `3v3` ). If you want to use the SD card as well, use a 10k pull-up."

Thanks Stephen,
I did read that part of the docs. i am currently charging at 450mA.

The battery works as soon as the USB is pulled out and there doesn’t appear to be any break in service.

I suppose my question about having the battery connected whilst powered from the USB was more related to damaging the LiPo. I am expecting my battery & LoPy4 to be on my bench for a month or so whilst I continue to play and tweak, after-which it will be deployed into the paddock.

Hi Jon,

There is no risk from leaving it connected. The Expansion board has a charge controller so the battery will only be charged until full then it will be electrically disconnected. The expansion board will choose whatever power source is providing higher voltage.

Just to clarify the figures here,
as @Graham pointed out:
4.2 = fully charged, do not exceed
3.7 = nominal voltage, usually at the rated C of the battery but this rarely happens these days
3.0 = Flat, there is no more useful current at this voltage
2.5 = damage. Most chargers won’t recover a LiPo at or below this voltage.
Below 2.5V you will start to damage the LiPo, and below 3.0V you will shorten it’s useful cycles.

Also, not all LiPo’s are the same, they vary GREATLY in quality, which also relates to how well they stand up to low voltages.

3.8V is normally considered the storage charge for LiPo’s and if you want the best use between charges, bringing it up to 4 to 4.2V would be better.

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Thanks Michael - great, clear advice.

One question though - My 3.7V 2400mAh LiPo cell (https://core-electronics.com.au/search/?q=polymer+lithium+ion+battery+(lipo)+3.7v+2400mah) has been plugged into the USB port (via the Pycom Expansion Board (v3.1)) since 06:30 yesterday (charging at 450mAh) and it is still only recording a median voltage of 3.88V - Not sure why its not getting to the 4.1V - 4.2V area??

The BQ24040 on the Pycom Expansion Board (v3.1) is a LiIon charger, not a LiPo charger.
But the charge voltage should still reach 4.2V
Considering you’re charging a 2400mAh battery at 450mA then a ‘flat’ battery should take from 5 to 6 hours to charge.
If it does not get above 3.8v then there are other issues at work here.

Thanks Michael,
Does that mean I have the wrong battery for the expansion board, or should it still be fine?

The voltage is definitely not getting over 3.88V - 3.9V. Have you any thoughts about what ‘other issues at work’ could be?

From what I read on the BQ24040 spec sheet it should be doing a charge correctly.
That is its function.
First off I would check the voltage on the battery pins with no battery connected.
It should be 4.2v or higher.
Then check the voltage with the battery connected, it should be the same.
It’s only when it gets saturation charge that the voltage come down to 4.2v while the current decreases to keep it there.
This is standard LiPo charging.

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Thanks Michael, I’ll give that a go and see what I get.

Lipo batteries are unforgiving. I never run them below 3.5V. If you aren’t seeing it recharge properly to 4.2V or it’s not able to supply the mA hours it is supposed to, you may have already damaged it. Is it puffy or bloated at all?

Thanks @Lindsay36840, I discovered it wasnt getting up to the full voltage because I was only powering the Lopy4 / Expansion Board & LiPo from my USB port on my Mac. As soon as I plugged it into a 5V 2A supply the voltage / charge went straight up to 4.1xV. The board also became far more stable / reliable.

Its a 2400mAh unit so should be more than capable of supplying the required current for a suitable period of time.

Battery is physically just as it was when it arrived from Core.

Thx
Jon

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In the RC industry, modellers never let their Lipo’s get below 3.80 volts, as their life (the battery that is…) is shortened. Going down to 3 is death for the battery. I didn’t see if it was one or more cells, but from experience going down to 3.1 or below makes it almost impossible to balance multiple cells ever again. Most model chargers try to balance and if they can’t due to a faulty battery cell, it will never fully charge or switch off. Storage charge is 3.85, but you’d never use it in your application. 4.2 is fully charged.

… just re-read the start. Only one cell, so ignore my balancing waffle…

3.8v is the standard storage voltage of the pack and is not even down to the nominal voltage.
You are severely limiting your flight duration for no good reason.
3.5v is the standard cautionary voltage that may preserve your pack longevity.

At 3v a LiPo has no usable current left but can recharged OK, but you are degrading it.
2.5v is an unrecoverable voltage that most chargers will not attempt to charge, but they can be brought back, you should expect damage to that cell.

In between 3.5v and 3.2v is a very grey area where you may experienced very shortened life for some LiPo packs and not others.