Electronic – Correct charging current for lithium-ion batteries

When charging lithium batteries in series, you need what's known as a balance charger to keep the cells at similar voltage. One way that some chargers do this is by stopping the charge of both when one is full, discharging the full one until they are even, and then charge them both up again. There are fully featured chargers meant for the remote control hobby, but they require setting the charge options on the screen before every charge. I cannot seem to find any "plug and play" balance charger boards similar to the TP4056. I am fairly sure that ICs exist that will handle the buck/boost power supply and the balancing, but I can't seem to find one and you would need to manufacture your own circuit board, and the chip is almost certainly surface mount.

That being said, there are a few workarounds. One is to have the batteries in series for operation and parallel for charging, and use either one or two single cell chargers. This can be accomplished through a 3PDT switch, and a good explanation is here. Note that you don't need to use the same charger, your TP4056 should be able to be substituted.

A simpler and more elegant way to do it would be to use a boost mode power supply, and don't put the cells in series. You can attach multiple of your current cells in parallel, so that if you have two cells and are drawing 5W, it would be 2.5W per cell or ~0.67A. There are many chargers that are designed for this application, handling both the CC-CV charging and the boost regulator for the output, such as the Lipo Rider Pro from Seeed Studio which will do 1A output. This is what I would recommend.

Alternatively, you can buy more powerful batteries that will put out more than 1A instead of putting multiple in parallel.

Or if you want to take all the fun out of this project and just buy something that sort of works, there are USB battery banks available from China that will take care of all the charging and discharging of 18650 LiPo batteries. I have seen banks that will take from one to six 18650 cells in parallel, and some of the larger ones can do as much as 3A output. Here are a few examples, and a video review. Their build qualities and user interfaces look pretty bad, but apparently they work.

Lots of questions! Instead of just pointing you to a certain IC or battery, I will try to answer your questions so that you can understand the concepts and pick or build your own battery.

1. What precautions do I need to take so that there are no explosions? Certainly something that is possible with lithium ion batteries!

   A) Do not overheat. You can find charging systems with thermistors that monitor the temperature of your battery.

   B) Do not over discharge. Lithium ion should not be discharged below 3 volts, but 3.3 volts is a better goal to aim for. You Over discharging is more likely to cause the battery to lose capacity than explode, but best to keep over 3 volts regardless.

   C) Do not over charge. Very important not to charge over 4.2 volts.

   D) Do not short circuit. Allowing the positive of the battery to come in contact with the negative for longer than a few milliseconds will likely cause a fire. Usually the short circuits are accidental and there are circuits that will protect the battery if short circuit occurs

   E) Do not discharge the battery at a rate faster than the battery is rated for. Again there exist protection cicuirts that will protect against this.

   F) Cells in series must be balanced. Simply applying a current to the battery pack might result in some of the cells charging at slightly different rates leading to imbalance. As a result it is a very good idea to monitor each cell as it is being charged. There exist IC that will do this.

2. How does the use of boost and buck converters affect battery life? Boost and buck converters are quite useful, but you will lose some battery life due inefficiencies. Most of the boost and buck converters that I have seen have about an 85% efficiency although occasionally I have seen 95% efficiency claimed. Thus you lose 5-15% battery life due to inefficient conversion.

Don't forget that a fully charged 3 cell lithium ion battery is 12.6 volts - is this too high?

Good luck.