Do-it-yourself uninterrupted power supply for a router and a GPON terminal

GPON Internet connection technology is one of the most successful innovations of recent years. It allows you to completely get rid of the power of the “last mile” devices, which is especially important during long power outages. Almost all other technologies make it possible to provide the Internet only temporarily – for an hour or two, and this time does not depend on the end user – only on the goodwill of the provider, who provides his equipment with uninterruptible power supplies “in the attic”.

In the case of GPON, the only thing that the end user should take care of is providing power to the terminal on his side. Some terminals are immediately equipped with a built-in WiFi router, but most often this does not happen and the provider provides a separate device. The most common configuration is a GPON terminal, consuming 0.5A at 12 volts, and a WiFi router, consuming 0.3A at 9 volts.

Of course, you can buy an expensive charging station with high-capacity batteries and fast charging, connect device power supplies to the 230v station inverter, but this is wildly inefficient. In my case, the consumption of this configuration was 16 watts.

The next option is to buy USB-DC converters for 9 and 12 volts, and plug them into a powerful power bank. Consumption immediately becomes more economical. In my case, it dropped to two watts, but I used quite expensive devices that support QC2 / QC3 technologies. Although, simple laces without these bells and whistles and boosters inside the cord allow you to achieve approximately the same parameters. We’ll be back to boosters…

The disadvantage of this approach is that almost all power banks do not support simultaneous charging and discharging, and those that do are marked that this mode is “not recommended”. For the “not supported” case, you will have to manually change the device, reconnect it, put it on charge, etc. And for the case “not recommended” – it is already clear there that the degradation of batteries will occur faster than usual. This is due to the fact that lithium batteries require compliance with a certain charge algorithm, the so-called “constant current -> constant voltage” mode, and so – this current should be measured on the battery without load. If the load is hanging on the battery, charging will not occur correctly, and it will never switch from the “constant current” mode to the “constant voltage” mode. This is true for the case of a sufficiently large load compared to the charging current. For example, for a 10000mAh battery, the recommended charge current is 0.5C (half capacity) = 5A and a miserable 1-2 amperes on the load should not affect, but in reality, power banks are rarely charged with the recommended current, since they take power from USB, which is current limited . There are nuances with QC2 / QC3 and PowerDelivery, but these technologies are almost never used in cheap power banks, but we are moving towards the most budget option.

The next candidate for solving the problem is specialized devices. I found 3 candidates for free sale, but the “coolest” of them provides a capacity of only 10800 mAh, which, according to reviews, is only enough for 4-5 hours for the two devices mentioned above. For those who do not turn off the light for such a long time – quite a suitable option. For me – no. I have to provide myself with Internet for 8-10 hours “for anyone”, and during the presence of light I must have time to charge to a sufficient level so that the next day I again have an Internet of 8-10 hours. You can, of course, buy two such devices, put one on a router, the second on a GPON terminal, but this is “not our method”;) I have not tested such devices, so I have no information about the real charge time of them, but believe the manufacturer, or rather – his marketing – do not respect yourself. I want to emphasize once again that for many users such devices are a completely acceptable solution.

We turn to the main thing – “with our own hands.” What do we need for this? The first and most important thing is the ability to handle a soldering iron! For those who do not know how, it is better to use ready-made solutions or the help of specialists.

The second is the batteries themselves. You can choose any, but in terms of “price-quality” I personally won this “race” with Chinese “nonames” for 2000mAh.

18650 3.6V 7.2Wh
18650 3.6V 7.2Wh

Capacity measurements showed that they correspond to those declared by the seller with a not very large spread. The choice of capacity coincided very well with the technical characteristics of the cheapest charge controller boards based on TP4056 / DW01A chips. Their maximum charge current is 1A, which is the recommended half of the capacity of these batteries.

For maximum charge efficiency, such a handkerchief needs as much as batteries. How many batteries to take? In the course of experiments, I determined that 8 pieces confidently hold the load I need for 15 hours, but after that a full charge after that requires a whole 6 hours of waiting, which is a bit much, so I increased my battery to 12 cans (total capacity is about = 85Wh). A full charge-discharge experiment has not yet been carried out, but according to calculations, it turns out that this is quite enough (even redundant).

What to charge? Any 5 volt power supply with a good current will do here. You can adapt an old computer power supply, or buy a PSU suitable for your load (the current limit of 5 volts must be more than the number of batteries * 1A). Since I was experimenting, I chose a PSU with overestimated characteristics – 5V 40A – a cheap Chinese semi-open block for powering LED strips, but for “serial” use it is clearly redundant.

There are a lot of options for holders, but I liked a single battery box with wires already stamped. You can do without them altogether by making your own installation option, but I was just too lazy to mess with it.

It is clear that such boxes also need as many as batteries.

And how do we “get” 9 or 12 volts from the battery? There are several options, but I chose the simplest one – the voltage boost board. There are a lot of such boards now, with different characteristics, but the cheapest and most popular are XL6009E1 and MT3608.

Each of these budget solutions has its advantages and disadvantages, but I will not dwell on this in detail, since there are a lot of videos on YouTube comparing these devices. I personally rejected the XL6009E1 (for myself), since the lower operating voltage threshold for it is 3 volts, and the TP4056 board provides battery discharge control up to 2.8 volts, plus, the voltage drop across the protective diode … to which I will return. That is, the MT3608 board was chosen, which starts from two volts. More precisely, two boards. I set one to 9 and the other to 12 volts.

Now back to the key drawback of such boosters – impulse noise. In my case, the GPON terminal completely “tolerated” it, but the WiFi router began to work extremely unstable – the connection could disappear for 10-20 seconds every five to ten minutes. This problem is solved by installing two capacitors at the output of the MT3608 – an electrolyte at 470-1000uF, which will eat the main pulsation frequency (envelope) and a film or ceramic one at 0.1-0.2uF, which will crush the high frequency of the main spectrum.

Do not be afraid of hanging installation – this is a temporary building (but there is nothing more permanent than temporary).

Back to the cheap USB-DC converters mentioned in the fourth paragraph of this article – they have the same problem! If with them your router began to “fail” – think about how to crush the power interference.

Now about diodes. All this can be connected and “on the way.” That is, when the power supply is working, the load remains connected to the batteries, but this mode is not very good, and those who want to make everything “beautiful” can make a bypass from the power supply directly to the load. This is where a diode (at least one) is needed to isolate the power plus, otherwise 5 volts will “come” to the charging circuit from the other side and the batteries will not be charged. It is better to use a Schottky diode, since this type of diode has the smallest voltage drop. And now … the second diode! When the load is powered by batteries, and the PSU is not powered from the mains, it turns out to be powered by batteries, or rather, its output circuits. Usually there is nothing dangerous there – a pair of capacitors, a choke and then a diode bridge, which will “not let” the voltage go further. But this is also an extra load, albeit a small one, therefore it is also desirable to insert a diode into the bypass circuit, which will prohibit the power supply circuits from this parasitic activity.

It remains to solder the shoelaces-plugs to 5.5mm and that’s it! Do not reverse polarity. For this type of plugs, the plus is always inside, the minus is always outside.

For convenience, you can add cheap Chinese “voltmeters” to the outputs of boosters, a switch, a fuse and other “beauty”. But this is already “excess”.

PS An important note for those who bought the MT3608 and cannot adjust the output voltage – turn the resistor counterclockwise 30-40 turns. These boards have a “dead zone” where the resistor does not work. This is due to Chinese savings on one resistor, which should be there and set the offset, but it is not;)

PPS If you are interested in any details – please in the comments. And if there are a lot of people who are interested, I will publish an article with a continuation and analysis of incomprehensible moments in more detail.

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