Pauli says:

"It’s been five months since I joined AirJaldi. When looking back, I think these must have been the most challenging, interesting and educating months of my life so far in many ways. Needless to say that my language skills have improved quite much while working with people who don’t speak my mother tongue (Finnish) at all. Also my knowledge in electronics has improved a lot when repairing, servicing and assembling many kind of electronic and electrical equipments (UPS’s, SMPS’s, battery chargers, WiFi routers, laptops, consumer electronics). One of the main goals for my five months volunteering period was to improve the battery chargers used in AirJaldi WiFi relay and hotspot installations. Following entry describes the progress in general terms.

Needs behind Jaldi Charger

I was assigned to design and implement a battery charger for 12 volt lead acid batteries used to backup WiFi- routers in rural areas of developing countries. Doesn’t sound very complicated a task in the first place, does it? At least that’s what I thought before finding out the facts (most of them using trial and error- method) that makes this task challenging. First of all, in some places here in Dharamsala the mains power can be anything between 80-380Vac + 1000V surges. This big variation puts any kind of power supply under heavy stress. Secondly, I couldn’t have any idea how hard it can be to find proper tools, equipments and components here in Himachal, not to mention the quality of some Indian- (hand) made components…

So, the main goals were to design a unit with following features:

• Extremely large input voltage range
• High reliability
• Long battery life
  
• Affordable price
  
• Locally made using commonly available tools and equipments
• Modular (possibility to select only relevant functions for different installations)

Specifications

The given specifications for our new locally made charger unit turned to be following:

Battery charge control

• Charge voltage regulation
• Charge current limiting
• Charge voltage negative temperature compensation

LVD

• Low battery voltage limit (adjustable)
• Delay on connecting and disconnecting load
• Voltage hysteresis between load disconnection and reconnection.

POE injector

• Screw terminal for feeding battery voltage over ethernet cable

Surge protection

• Protection against surges coming through ethernet cable

Implementation

Jaldi Charger V.1.1 (and newer) is implemented using custom design, one sided printed circuit board that can be quite easily produced using Press’n’Peel technique. All parts are easy to solder (big size, non-smd) and mostly easily available in India (except some of the surge protection diodes). The currently used power supply solution is heavy duty, robust linear transformer with rectifier bridge, big filter capacitor and varistors for surge protection. Charger unit is based on highly reliable, well protected L200 linear regulator, that can efficiently work in this implementation with input voltages between 17 and 26Vdc. LVD unit is based on low cost voltage dual comparator LM393 connected to provide hysteresis and delay on connection and disconnection of the load. Ethernet surge protection is implemented using ultra fast Transient Voltage Suppressor Diodes. All cable connectors used are high quality screw terminals.

Final words

The final product is still missing an enclosure and I will have to make some modifications to the circuit in Finland but, generally-speaking, the project has been successful and very educating in many ways. I hope that when ready, this product can help people facing problems with bad mains- voltage to prolong their backup-batteries lifetime and improve stability of the devices running on batteries."