Arduino boards can use the serial USB to provide the power source when connected to a computer either directly, like the UNO and the MEGA, or through the FTDI adapter like the Mini Pro. But if we want to use them when disconnected from the computer and/or in standalone mode we need another form of providing power to them.
When using my Arduino boards in standalone mode, I’m powering them up using rechargeable Ni-MH AA batteries, which works fine, but has the issue that when the batteries are exausted, recharging them can take a long time. In fact to fully recharge some of the 2600mAh batteries that I have can take as much as 12 hours… A common solution for this issue is to have several groups/packs of batteries, but I wanted something better.
In a first approach I started to look at LiPO batteries commonly used in powering up RC toys, drones, and so on. These batteries pack a lot of energy, come at several power ratings and sizes, and need specialized chargers for charging them. They also have the issue that can be quite dangerous and can bring houses down in uncontrolled fires if not properly used. They should not be overcharged and over discharged, otherwise they get destroyed, which means proper chargers and proper electronics to shut them down when bellow a certain voltage level is a necessity.
These batteries have normally per cell/battery 3.7V and are packed in parallel or serial with notations like 7.2V 2S that mean two connected Serial 3.7 cells. (2P means two packed in parallel). Also there is a discharge and charge rate, like for example 20C that means that the battery can discharge at 20 times it’s nominal capacity, for example 2600mAh 20C would mean that a peak current of 46A for 10s can be achieved. Normally charging them is at 1C rate, which means 2.6A for a 2600mAh battery, which should take less than an hour to fully charge, a far cry away from the safer Ni-Mh 12 hours charging time….
The fact is that for using LiPO batteries for powering up my Arduino boards mean that I needed specialized chargers and specialized control electronics to avoid over discharge. These chargers and monitoring chips/boards are readily available and are easy to find. Also if I wanted to use the 1S 3.7V batteries on my 5V Arduinos I also needed to have a step up DC converter from 3.7V to 5V, or in the case of a 2S (or above) a step down to 5V converter.
The fact is that while it might be cool to add a LiPO battery, charging board, protection and step up/down converter to power up an Arduino, it might be overkill at least for my endeavours… If my bet was on battery physical dimensions, than size/vs power might be the only available solution.
USB power banks normally use a form of LiPO batteries known as 18650. And these power banks have an all in one board that allows charging through USB, keep the batteries safe above their ratting discharge voltage, and a step-up or step down converter to 5V provided on another USB port: it’s an all in one. In fact we can buy them just for the single board solution for charging and monitoring.
These power banks can be bought with a single 18650 battery (starting at 2600mAh) and multiple 18650 batteries providing much larger power output, while keeping the charging times decent.
Most of the capacities of these batteries that are sold on eBay do not have truly their announced capacity, but hey, if needed we can swap the provided battery for a real Panasonic/Sanyo ones.
So I’ve just bough aluminium encased single cell power banks and an USB to DC male jack cable and let’s see how it works out…