The last time we broached the subject, we found a USB battery from WaveShare that could power experiments based on Yoctopuce modules with quite respectable performance: 19 days of continuous use. But we're not giving up just yet. This week, we're turning our attention to products from Voltaic Systems: if the specifications published on their website are anything to go by, there's a good chance that their USB batteries work exactly the way we want them to.
Summary of previous episodes
Today's USB batteries, which can be found just about everywhere on the market, were primarily designed as mobile chargers. This is why these batteries automatically switch off as soon as they detect that little or no current is being drawn from their output. This prevents the battery's internal electronics from draining the accumulator when not in use. On the other hand, this feature prevents these batteries from being used to power YoctoHub-Wireless or YoctoHub-GSM based experiments, because they switch off as soon as the hub goes into sleep mode, preventing it from waking up. In recent weeks, we have determined that variants C and D of Waveshare'sSolar Power Manager batteries are perfectly acceptable solutions.
Voltaic Systems V75
Selling for around USD 60, Voltaic Systems' V75 battery claims a capacity of 71Wh and comes in the form of an aluminum parallelepiped measuring around 150x80x25mm and weighing around 380g. It features two USB-A outputs, a micro-USB output and a USB-C connector that can be used as an output or input to recharge the battery. It can be recharged using a solar panel, and it so happens that Voltaic Systems sells solar panels and adapters for building complete solar systems.
The user interface is reduced to the bare minimum: a single push-button and four small red LEDs that remain off most of the time. To check the battery status, press the button once; to switch off the output, press twice; to switch the battery back on, press once more. It's not necessarily very intuitive, but we can live with it. There are no visible screws, so it will probably be difficult to disassemble the battery.
The V75 battery from Voltaic Systems
The Voltaic Systems website insists that this battery remains "Always On", and if you dig a little deeper, you'll realize that their batteries were clearly designed with a scenario very similar to the one we have in mind, namely powering a small installation indefinitely using a solar panel to recharge the battery.
So we launched our experiment, which consists of a Yocto-Meteo-V2 connected to a YoctoHub-Wireless-n that wakes up every ten minutes to send the measures from the Yocto-Meteo to a VirtualHub for Web server. The results are spectacular.
The experiment with the V75 battery lasted nearly 72 days
The experiment ran for a remarkable 71 days and 23 hours before the battery decided to shut down. At equivalent capacity, this battery lasts almost twice as long as Waveshare batteries.
At a rate of 1.4mWh per callback and 6 callbacks per hour, the experiment consumed around 1.4mWh*6*24h*71.9d = ~14.5Wh, or roughly 20% of the battery's capacity. From this we can deduce that the battery consumption is approximately (71Wh-14.5Wh)/71.9d/24h/5V = ~6.5mA.
Once the battery is empty, we also checked that a small 18x22cm solar panel providing a maximum of 6W at 6V enables the battery to automatically reactivate itself, thus restarting the experiment after a few hours of sunshine.
A small solar panel and a little sunlight can recharge the battery
Voltaic Systems V88
The Voltaic Systems V88 battery takes the form of a large aluminum brick, measuring around 220x135x25mm and weighing 700g. The mounting screws are visible, and it will probably be possible to open it to apply conformal coating, for example. The advertised capacity is 89Wh. Given the difference in weight and size, we would have expected a capacity roughly double that of the V75 version, but in fact the gain is only 25%. This battery will be yours for around USD 150.
The V88 battery from Voltaic Systems
It has a USB-A output and a USB-C socket that can be used as an output or charging port compatible with "Power delivery" technology. It also has two round sockets: one that serves as an input for connecting a solar panel, and a second for connecting a load. You can choose the voltage delivered by the latter: 12, 16, 19 or 24V. The battery also features a handy little LCD display showing charge, output voltage, and charging current. Like the 75 version, the battery has only one push-button, but as you can see the effect of each push on the display, it's less of a problem.
By default, the V88 battery switches itself off as soon as no charge is drawn, but you can activate the "Always On" function by holding the button down for around 4 seconds.
Despite its size, the V88 battery lasted only 16 days
We launched our experiment and it ran for 16 and a half days. That's respectable, but a little disappointing when you consider that the 75 model, far more modest in every respect, lasted over ten weeks. This confirms what we already suspected: to power a low-consumption system, the most important factor is not the battery's capacity, but its internal consumption.
Our solar panel works with the V88 model, but it's a bit small
At a rate of 1.4mWh per callback and 6 callbacks per hour, the experiment consumed around 1.4mWh*6*24h*16.5d = ~3.3Wh, i.e. less than 4% of the battery's capacity. We can deduce that the battery consumption is about (88Wh-3.3Wh)/16.5d/24h/5V = ~42mA. which may be a bit much. It's possible that the DC/DC for the 12..24V output has something to do with it.
We checked that our little 6V solar panel could resuscitate the battery by plugging it into the USB-C socket, but this is clearly not the most efficient charging method: it took a good day in full sunlight for the battery to reactivate its USB-A output. According to the documentation, you're normally supposed to use an 18-24V panel connected to the appropriate round socket.
We contacted Voltaic Systems support to find out why there was such a difference in efficiency between the V75 and V88 batteries. They quickly replied that the V88 battery is based on older, more energy-hungry electronics, and that in any case, the V88 model is about to be discontinued. Obviously, this kind of kills the point of this review, but if you find a V88 battery on sale, you'll know where you stand.
Conclusion
Voltaic Systems products are a great find, and have obviously been designed for applications very similar to ours: running a small system indefinitely in the middle of nowhere, powered by a battery recharged by a solar panel. In fact, Voltaic Systems sells solar panels of various powers and all the accessories needed to connect them to their batteries.
The only downside is that Voltaic Systems online store and all the brand's official distributors apparently refuse to deliver these batteries to Switzerland. To get ours, we had to have them delivered to France and then re-exported to Switzerland. When asked about this, Voltaic Systems' highly responsive support team answered by suggesting that shipping to Switzerland was negotiable, provided we were prepared to pay the price.
