Re-using an old 18v printer power supply to charge a 12v battery

I recently recycled my first printer, an HP DeskJet 710c, but just before leaving it in the recycle bin I looked at the printer power supply and wondered what I could do with it; How else could I use it? It can operate with an input voltage from 110 volts to 220 volts while at the output it gives a little over 18 volts and a current of 1.1 amps.

The first thought that came to my mind was an old cordless screwdriver that worked with 18-volt rechargeable batteries. The batteries that came with it, since it was many years old, were now obsolete. I powered the screwdriver with the correct polarity with the 18 volts of the old power supply and … it worked! It could stay that way, that is, turn it into a cordless screwdriver. However, I did not like this idea very much as it would not be very useful to me since I have a good battery screwdriver while a wired screwdriver would not have much power.

One night I needed a battery for testing. The projects I develop with microcontrollers are almost always powered by 12-volt batteries. The reason is simple: the ease of charging them from a small photovoltaic system since the recording devices I make (beekeeping scales, weather station, etc.) are placed away from a high voltage power source. They send the data through the mobile network and so they can be placed anywhere that receives a mobile phone signal, ie 99% worldwide.

Having tested the output of the solar panels with the voltmeter in the past, I noticed that their output voltage is about 19 volts during the day. In the same way, I tested the output voltage of the old printer power supply and found that, as its technical characteristics said, its output voltage was 18.7 volts. The first thing that came to my mind was … just stop, this looks like a solar panel. It was night and of course, there was no sun. The small photovoltaic system I have to charge the batteries certainly could not help overnight. I also do not have a charger for 12-volt batteries that runs on mains power (although many times I needed it). Could I replace the solar panel in my system with the old power supply? What do you think about this?

Solar charge controller. Notice that polarity is important.

I did try it, I plugged the old power supply in the solar panel with the right polarity and the lights on the charge regulator lit up as they would when the sun came up. It seems logical to me. Let me clarify at this point that it is not a solution that I suggest as good, I just tried it and present the results. The battery was also connected (I actually tried a lot of both old and deep discharge car batteries as well). The first test was done with a deep discharge battery at 12 volts and 9 amps which were almost empty. Within a short time, it had reached levels close to 13 volts. I must note that I did not like the way the battery charged. The tendency and behavior of the system were very unstable. I think the reason was the small size of the battery but also that the voltage difference was large as the battery was almost empty. After disconnection from the power supply, however, the battery kept the voltage normal and behaved satisfactorily.

A 12v old car battery before charging


During charging. A voltage siilar to that of a car dynamo is noticed here


I also tried 2 car batteries at 12 volts (45 and 100hp) with which I can say that things went very well. The behavior of the system was stable, the voltage at the battery poles was stable and the end result was very good. Keep in mind that these are old 12-volt car batteries that I use to power microcontrollers and not to start cars. As you can read here the last time I used the 12 volt car battery with a capacity of 45 ah without charging it in any way in the meantime (solar panel), it kept my beekeeping scale alive for over a month. I know for some friends this is a bit much, but keep in mind that my beekeeping scale does not send one or a few SMS every day but is basically permanently connected to the internet via the mobile network and creates live graphs on the live connection. This requires a little more energy. I must also admit that I can probably improve the energy consumption of the beekeeping scales microcontroller.

The same experiment on a bigger battery


Battery voltage during charge


My friends, if you notice a lot of power supplies, either from printers like in my case or from laptops, they convert the 100 – 220 volts of the network to 18-19 volts. A trend that if handled after these thoughts I shared with you maybe you too see the old power supplies with a different eye. I reused them to charge batteries this time. I’m sure there will be even better ideas out there.

Author: tommykar

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