A solar battery charger and battery Ah query

[gregmcc]

I've got a microcontroller project that I would like to power off a battery (lead acid) and keep the battery changed via a solar cell.

A few questions.
1) The circuit draws 100mA at 5V - I'm looking to get a 1.3Ah 6V battery. First off, how long will this battery keep my project running if its not being charged. Its 1300mA per hour? So it will be 13 hours? Is that then 13 hours until the battery is dead? How will I be able to work out when the 6V will drop below say 5V at which time my microcontroller won't have enough juice to work.

Or do I just buy the battery - connect up my circuit and start a stop watch

2) I assume to charge the battery via solar cell I will need around 3 x 2V 100mA solar cells? Or maybe 4.
One cell at 2V 100mA won't work will it?

 

[ronsimpson]

1) Assume you are using a low drop out linear regulator. Then the regulator will work down to 5.1 volts. (depends on regulator) The battery is rated at 1300mAhr but to what discharge voltage? OR We need to know how many mAhrs there is between full charge and 5 volts. Please send the data sheet for the battery or the part number.

2) Your load is 100mA and the solar cells are 100mA so the battery will never charge! Read the data on the solar cell. That 100mA is at 12:00 on a clear day, at high altitude, near the equator, every thing working just right. At 4:00 you current is more like 25mA if you are lucky. Just off the top of my heat I think you need 1000mA solar cell.

3) In some places it can rain for three days. (I live in a almost desert environment and it can be cloudy for 3 days). I think you need a week of battery, depending on your weather.

 

[gregmcc]

Thanks for the info. I was looking at using a LM2937-5 which is a 5V low dropout regulator. The battery I was looking can be found here https://www.electro-tech-online.com/custompdfs/2012/02/RT645.pdf

Guess I'll have to look for a much bigger solar cell. The cost of these things skyrocket with increased output current.

 

[ronsimpson]

I looked at your battery. They show a Discharge characteristic Curve with 0.93C or 120mA. It looks like they don't want you to use the battery below 5.4 volts, but you could use it to 5 volts but the time from 5.4 to 5.0 is very short. (120mA for 12 hours)

On the charge side they don't want you to go past 1.35A.

Is there a way to reduce the discharge current?

 

[gregmcc]

How do you go about reading the graphs from the datasheet and getting 12 hours? Do you think a higher voltage battery would be better. Maybe this one - 12V 4.5Ah https://www.electro-tech-online.com/custompdfs/2012/02/RT1245.pdf

I'll look into trying to reduce the current.

 

[ronsimpson]

A higher voltage battery hurts you if you stay with a linear regulator. You doubled the voltage, cut the current in half and have the same energy.
If you use a switching power supply it will help.

Back to the 6 volt battery. The energy between 6v and 5v is lost in heat in the low drop out regulator. about 1/6 of your total power.

If you used the 12 volt battery; it will discharge in 5 hours (using linear) because the current rating is 1/2. The voltage from 13 volts and 5 volts will be wasted in the regulator. Using a switching regulator, it will output 5V @ 100mA and take from the battery about 45mA. The switcher will move power form one voltage to another at maybe 90%. There will be less power loss.

One reason to go 12V is car accessories. I have two solar car chargers. OK, they will not charge a battery but they keep it from going down in the winter. (100mA)

Because there is no power left in the 12V battery below 10.75V, and because batteries will not charge well if totally discharged, it is not a good idea to take the battery down to 0 volts. Stop at 10.7 (and 5.4).

 

[Reloadron]

I am missing something here? The first mentioned battery was a 1300 mAH battery. Then this battery is mentioned which is a 4.5 AH (4500 mAH) battery? The latter would maintain a 100 mA load about 45 hours.

A few questions.
1) The circuit draws 100mA at 5V - I'm looking to get a 1.3Ah 6V battery. First off, how long will this battery keep my project running if its not being charged. Its 1300mA per hour? So it will be 13 hours? Is that then 13 hours until the battery is dead? How will I be able to work out when the 6V will drop below say 5V at which time my microcontroller won't have enough juice to work.

What am I missing here?

Ron

 

[ronsimpson]

Yes Ron I think the battery moved from 1.3ah to 4.5ah between post #1 and #3.
Moving the battery from 1/2 day to 2 days is a big help.
Now we need to 10X the solar cell.

 

[gregmcc]

I am missing something here? The first mentioned battery was a 1300 mAH battery. Then this battery is mentioned which is a 4.5 AH (4500 mAH) battery? The latter would maintain a 100 mA load about 45 hours.



What am I missing here?

Ron

I thought I could use the first battery I found - which was 1.3Ah. I found another one now rated at 4.5Ah

@ronsimpson - You recommend I use a switch mode ps and a 12V battery.

Let me give you some more info - the project is a arduino weather station which uploads data to the internet. I've checked the current on the ethernet module and its drawing 90mA. I don't think there is a way to reduce the current.

I would like to put the completed unit on my roof with a solar charger and battery. At this stage maybe I should just run a 12V wire up there

The project will be on 24x7 taking reading so I dont want the battery to go flat during the night.

 

[gregmcc]

Now we need to 10X the solar cell.

Oh dear - I"m going to have one almighty solar panel on my roof just to charge this

 

[ronsimpson]

I would get a 5 or 6 volts wall wort. (a new one that uses a switching power supply not a old one that uses a transformer) either one will work. My phone charger is 5V 200mA "USB".

Next question: what Ethernet module are you using? I am using SitePlayer.

 

[gregmcc]

Thanks - I'll look into it.

Its a enc28j60 module

https://dereenigne.org/wp-content/uploads/ENC28J60.jpg

 

[gregmcc]

Thanks for all the replies - After rereading all the above and thinking about my project I think I'm going to give up on the solar idea. While it would be nice to implement and experiment with, its going to be expensive. I have to run a UTP cable to the box anyway, I might was well add in a 12V cable (No - I'm not going to use POE)