LiPo (Lithium Polymere) batteries are interesting batteries for IoT as they are nt so much expensive, easy to get with multiple form factors and multiple capacities. As they are used for radioguided planes and such toys, you have a lot of really small batteries available for a couple of euros.
The Lipo battery is typically providing 3.7V. This sounds good for 3.3V circuit. The charging circuit is quite easy and low expensive compare to some others battery technology.
But there is no miracle and a couple of problem I’m actually discovering and this post will introduce.
To quick start, I’m not a battery expert so I may write a lot of incorrect things, use wrong word and it is also possible that you battery burn because you try to do something bad after reading this post. By-the-way I’ve tried to read some battery expert post on different sites and franckly speeking the content was not understandable. So the following will be quite wrong but more easy ! Now, you can read carefully 😉
The problem with LiPo battery is that they are not stable when reaching more than 4,x V and going down under 2,xV. This means that you have to use it, basically between 3V and 4,2V.
Regarding a typical discharge curve, once you reach 3.3V you have to be really careful as the discharge is really fast. When a LiPo is discharged under 3V you have a high risk to destroy it. As an example, since I missed to charge my fitbit when it asks me to do it and let it discharged to the maximum, my autonomy decreased from 10d to 5days.
It means that in your LiPo circuit you have to ensure you will stop to drive energy as soon as the battery level reach 3V. If any have a simple circuit to do this with a 0,1V precision, I’ll appreciate. By the way I’ll give you an integrated solution above. The last thing you can do is monitoring the power voltage and shutdown your circuit before reaching the limit. But you have to ensure that no load will completely discharge the battery even if your system is down. You can choose to buy protected LiPo, these battery have a circuit for over and under voltage protection integrated.
The graphic above is a LiPo discharge over 20mAh. On X the available battery capacity, on Y the voltage (read 420 = 4,20V).
We can notice:
- 75% = 3,92V
- 50% = 3,79V
- 25% = 3,57V
The discharge rate depends on the current, as an example, I discharged the same (officially) 500mAh at 3 different rate. The following graph indicates the power level regarding the battery capacity available :
The total battery capacity is different depending on the requested current ; in this example for a given 500mAh I had :
- 320mAh @ 8mAh
- 320mAh @ 20mAh
- 275mAh @ 30mAh
Regarding the charging process, it is basically easy to manage but you have to ensure to now overcharge the LiPo or it will burn … To avoid this problem, you have a large number of components ; one of the simplest is the Microchip MCP73831T it costs not more than 0,5 euros. This chip will charge and protect the LiPo just using a 5V source. This component is a 5 pin you can solder at home on a SO8-Dip8 adapter eventually.
More over you need to regulate the Voltage output from 4V to 3.3V to fit your application. The voltage drop is too little for using a LDO regulator in this role. A solution I used was a diode in series (eventually two) to reduce the circuit voltage. But this solution does not prevent battery over-usage.
I was surprise to find only one device managing these two problem in once, ans I assume some other exist, so please, let me know. The solution is a TI component TPS61200. This component provides a regulated output able to be under or higher the LiPo voltage from 1.8V to 5.5V. This component also includes a programmable undervoltage lockout to protect the LiPo. Unfortunately this component have to issues : it’s price (2 euros) and its package really small and as a consequence not easy to hack at home – I experiment and it is hard to put exactly the needed solder ; it can be done only with reflow and if you have no mask I recommend to put solder on the pad instead of the PCB. Once-again, any one that have something better to propose is welcome to comment.
With these two components you can build a LiPo charger / regulator for you arduino or sigfox modules when needed 🙂
Update : After reading your comments, I made a test with a LM3670 ; this component definitely not protecting the lipo against deep discharge. But it have two big advantages : price and form factor. So if you are able to control the discharge and switch your circuit off or really low consumption until user refill it, it can be a good choice.
LiPo and High Temperature
LiPo are given for working up to 60°C and not more. On large LiPo pack you should have see youtube video where the LiPo burn in fire and explosure. For small LiPo I was looking for what is happening when you let them at temperature over 60°C. I do not recommend to execute the same test to anyone, if you use battery at such temperature and do the same kind of test, you do it at your own risk !
By-the-way, for this test I use my heater with a thermometer able to give me a precise temperature at the battery level. In my test I first cook the LiPo at 70-80°C for 5:30. As the battery was not impacted by this test, then I push the temperature up to 85-95°C for 2:30 more.
As you can see, after 5:30 at a temperature of 70-80°C the battery was not really different than at start :
There was 2 LiPo in this test :
- a 350mAh one starting at 35x20x5 mm and finishing at 35x20x6 mm
- a 500mAh one starting at 50x35x3 mm and finishing at 50x35x13 mm
Even if none of these LiPo have burn into a large fire due to over temperature, they have been really impacted, especially for the 500mAh one. It is important to care about the temperature and leave enough space in the casing to allow the battery growth and preserve the packaging.