solar charger for 12V battery - Solar panel is 2x 5V 1.25w 250mA

Thread Starter

mubarakk

Joined Dec 21, 2010
7
Hi,
I am not too good with circuitry and electronics. I want to Build a Solar Charger for my 12V Lead Acid battery (8Amp). i have two solar panel of 5 Volt 1.25W 250mA. I also have a 40Amp Charge controller. Controller doesn't seam to charge the battery. The Battery just kept draining by the controller even without load. i tried adding capacitors to the Solar panel terminals to boost the panel volt inorder to charge the batteries but to no success. i connected the panel to a boost converter, the boost converter did not power on. the peak panel volt without load is 13V. and the the boost converter only requires 3V to power up and boost up to 35 volt. my question is, how can i make the boost converter power on with the solar panel volt? would appreciate a circuitry explanation or low cost solution. thanks.
 

gramps

Joined Dec 8, 2014
72
You just don't have enough power from the solar panels to charge the battery. 1.25 watts isn't going to cut it. The charge controller and the boost converter that you have will themselves use way more than the power that the panels produce, leaving nothing to charge the battery..
 

Thread Starter

mubarakk

Joined Dec 21, 2010
7
You just don't have enough power from the solar panels to charge the battery. 1.25 watts isn't going to cut it. The charge controller and the boost converter that you have will themselves use way more than the power that the panels produce, leaving nothing to charge the battery..

So the the Panel is useless? Can Ultra Capacitor boost the panel power?
 

spinnaker

Joined Oct 29, 2009
7,837
So the the Panel is useless? Can Ultra Capacitor boost the panel power?

Buy the appropriate panel. If you boost voltage then you reduce current. Ain't no such thing as a free lunch.

And why would you want to build a charge controller when they can be purchased so cheaply?

https://www.ebay.com/itm/PWM-30A-Solar-Charge-Controller-12V-24V-LCD-Display-Dual-USB-Solar-Panel-Charger/162626774113?hash=item25dd4fa461:rk:1:pf:0

A charge controller circuit is not a project for someone "not too good with circuitry and electronics". Charging a lead acid battery is a science. You just don't apply a voltage and expect it to charge efficiently.

And how is this digital design? This is an anlog issue unless you want to consider the PWM part of the circuit as digital.
 

oz93666

Joined Sep 7, 2010
702
. Charging a lead acid battery is a science. You just don't apply a voltage and expect it to charge efficiently.

And how is this digital design? This is an anlog issue unless you want to consider the PWM part of the circuit as digital.
I disagree slightly with that .... when panels first came out the standard open circuit voltage was close to 20V , and this was so it could be connected directly onto a 12V lead acid , and it would be fairly efficient at charging . Lead acid is very tolerant of overcharging , it just electrolyses the water , so as long as you keep it topped up everything is fine.

Then electronics appeared which made things more efficient , power point tracking , and this made sense with high panel price... But now panel price has dramatically dropped , and electronics is very vulnerable to damp and other damaged.

For our TS who is just playing around on a small scale , I would recommend forgetting about all electronics . Since he already has two 5V panels , get two more identical panels, in series these will give 20V open circuit , fine for charging a 12V battery direct ... Of course you can only get out what you put in to the battery , so for heavy use you will need more panels.
 

spinnaker

Joined Oct 29, 2009
7,837
I disagree slightly with that .... when panels first came out the standard open circuit voltage was close to 20V , and this was so it could be connected directly onto a 12V lead acid , and it would be fairly efficient at charging . Lead acid is very tolerant of overcharging , it just electrolyses the water , so as long as you keep it topped up everything is fine.

Then electronics appeared which made things more efficient , power point tracking , and this made sense with high panel price... But now panel price has dramatically dropped , and electronics is very vulnerable to damp and other damaged.

For our TS who is just playing around on a small scale , I would recommend forgetting about all electronics . Since he already has two 5V panels , get two more , in series these will give 20V open circuit , fine for charging a 12V battery direct ... Of course you can only get out what you put in to the battery , so for heavy use you will need more panels.

You are wrong.. No they can't be connected directly. And never could. At least not if you wanted to have your battery last for very long. There is a very specific protocol to charge lead acid batteries. Battery charge controllers have been around pretty much since solar panels have been made available to the public.


https://batteryuniversity.com/index.php/learn/article/charging_the_lead_acid_battery
 

BobTPH

Joined Jun 5, 2013
2,138
If you could charge it with 100% efficiency, at 250mA, an 8 Ah battery would take 32 hours. On a good sunny day you will get 6 hours of charging. So if you are planning on charging the battery in a week, with luck, you might almost get there.

Bob
 
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