Solution needed for 15VDC 350mA Output

[Gerry]

I have an armoured street payphone that works off a 15VDC 350mA transformer Output, 240V Input.

I am trying to find a solution to solar power it.

Need to know if a suitable panel is available, what type of rechargeable battery can I use, and do I need any type of regulator?

Have not got a clue what to do.

Many thanks for any suggestions.

[Sunking]

Gerry can the phone work with 12 volts? 15 volts is really an odd number for a solar battery system.

[Gerry]

I think it can. We had it working of a 12v 7Ah rechargeable Yuasa battery with a charger at one time The mains came on at night and recharged the battery

[Sunking]

OK then this is doable and won't cost an arm and leg. I assume the phone needs power 24 hours a day right? Consumes a constant power of 15 volts x .35 amps = 5.25 watts? It is important to nail these numbers. So your phone consumes 5.25 watts x 24 hours = 180 watt hours per day.

Ok to account for battery charge efficiency multiply by 1.5 so 1.5 x 180 wh = 270 watt hours. So you have to size the solar panel to generate 270 watt hours for the shortest days of the year. I have no clue where this will be to determine the amount of Sun Hours you receive in December/January. Location means everything. To determine the solar panel wattge take the 270 watt hours and divide it by the amount of Sun Hours you receive on the shortest day of the year. For example if you live in Tuscon you receive 5.1 Sun Hours, so 270 wh / 5.1 h = 53 watts. If you live in Seattle you receive 1.2 Sun Hours 270 wh / 1.2 h = 225 watts So as you can see location is everything.

As for the battery size in Amp Hours is easy take the 270 wh and multiply by 5, then divide that by the battery voltage. So [270 wh x 5] / 12 volts = 112.5 AH battery ar 12 volts.

For the charge controller take the solar panel wattage you have to use, and divide by the battery voltage. So in Tuscon 55 watts / 12 volts = 4.6 amps. Gloomy ole Seattle 225 w / 12 v = 19 amps. i

[Gerry]

Thanks, Studying this as hard as I can!

The phone definitely used a 15v 350ma transformer powered from a 240v mains.

It really only uses power when it is offhook in use, which won't be much these days.

It will report once per day automatically to a centralised payphone management system with an 'ok' report or any faults.

I live in Belfast Northen Ireland. Average sunlight hours in Belfast range between 1.1 hours per day in December and 6.3 hours per day in May.

Many Thanks again for your help.

Gerry

[Mike90250]

So, how would you guess usage pattern? 60 minutes a day? 3 hours a day ? If it only needs power at those times, that saves you quite a bit of power, but the first test, is to see if it will still work off a 12v battery.

[Sunking]

The phone definitely used a 15v 350ma transformer powered from a 240v mains.

It really only uses power when it is offhook in use, which won't be much these days.

This is what I was after, so you are going to have to take an educated guess how many hours per day off-hook condition exist. At any rate this is going to make the system a lot smaller and less expensive.

Just for example I will assume 3 hours per day. I guess this for the phone to be used 2 hours, plus one hour for being on-hook for 22 hours. That gives us 5.25 watts x 3 hours = 15.75 wh. Add adjustment factor of 1.5 x 15.75 wh = 24 wh per day. Piece of cake.

Just use a 20 watt solar panel, 5 amp charge controller (smallest they make), and a 10 AH 12 volt battery. That will be more than enough. With A 10 ah BATTERY WILL GIVE YOU 3 DAYS RUN TIME FOR CLOUDY DAYS. You should be able to do this for less than $200 USD

First thing you need to do is test the phone and make sure it can work with a voltage range of 11 to 16 volts. If it does you are good to go. If not back to the drawing board using a special Lithium batter pack and custom charger.

[Gerry]

Thanks for this Derek.

Will test this out over the next week and see how it goes.

Will let you know the results.

Kind Regards

Gerry

[Sunking]

You are welcome. Let us know the results and keep us posted. Where do I send the bill?