Charging 12 v a lead acid battery using 24 v solar panel

Thread Starter

The_Grim_Reaper

Joined Feb 11, 2018
5
Greetings !
I'm working on a project in which i have to charge a 12 v sealed lead acid battery using a solar panel which produces approximately 24 volts.
I am wondering if i should use a charge controller along with it.
If i decide to simply charge the battery using panel, is there any risk of explosion?
 

ebp

Joined Feb 8, 2018
2,332
You should definitely use a charge controller to prevent overcharging the battery. Explosion is unlikely be gross overcharge will cause the battery to vent, spraying out hydrogen, oxygen & probably some some electrolyte.

If the maximum power point of the panel is 24 volts (a bit of an oddball, if it is), an MPP tracker converter will deliver a very substantial benefit over a simple on-off type controller.
 

Thread Starter

The_Grim_Reaper

Joined Feb 11, 2018
5
What is the solar panel rated output.
What is the battery capacity in ampere-hours?
Rated output as mentioned at the back of the panel
Maximum power 10 Wp
Vmp 17.4 volts
Imp 0.58 Amps
Open circuit voltage 21.2 V
The battery capacity is 12 v 7.5 Ah

But when i simply checked the voltage using multi-meter under no load, i got up to 24 volts.
Is the rated output details wrong?
also i don't know how to measure the current in the panel.
 

Thread Starter

The_Grim_Reaper

Joined Feb 11, 2018
5
If the maximum power point of the panel is 24 volts (a bit of an oddball, if it is), an MPP tracker converter will deliver a very substantial benefit over a simple on-off type controller.
Maximum power 10 Wp
Vmp 17.4 volts
Imp 0.58 Amps
Open circuit voltage 21.2 V
But when i simply checked the voltage using multi-meter under no load, i got up to 24 volts.
Very odd isn't it?
 

crutschow

Joined Mar 14, 2008
23,362
Rated output as mentioned at the back of the panel
Maximum power 10 Wp
Vmp 17.4 volts
Imp 0.58 Amps
Open circuit voltage 21.2 V
The battery capacity is 12 v 7.5 Ah
Yes, you will need a charge controller to prevent overcharging the battery (something like this).
Typically you charge until the battery reaches about 14.4V and then drop to a trickle-charge voltage of 13.6V.
But when i simply checked the voltage using multi-meter under no load, i got up to 24 volts.
Is the rated output details wrong?
No.
The open circuit voltage is higher than the maximum power output voltage, which is were the voltage drop across the internal panel resistance equals the voltage across the load (according to the maximum power transfer theorem).
That's what an MPP circuit does -- adjusts the current until the maximum power is extracted from the panel.
 

ebp

Joined Feb 8, 2018
2,332
Those are very typical specs for a panel intended for charging a 12 volt lead-acid battery. Without an MPP charger you would get something around 7.8 watts from the panel at 25°C panel temperature with full sun and the battery at 13 volts.

Specifications are generally standardized at 1000 watts per square metre "insolation" and 25°C. The voltage from the panel has a significant temperature coefficient of about -2 mV/°C per cell (probably 36 cell in series for your panel) and the current a much smaller positive coefficient. Current at MPP varies directly and quite linearly with illumination. The MPP voltage doesn't vary much with illumination. (all of this for silicon cells; other types have different characteristics)
 

Hymie

Joined Mar 30, 2018
708
Yes, you will need a charge controller to prevent overcharging the battery (something like this).
Typically you charge until the battery reaches about 14.4V and then drop to a trickle-charge voltage of 13.6V.
No.
The open circuit voltage is higher than the maximum power output voltage, which is were the voltage drop across the internal panel resistance equals the voltage across the load (according to the maximum power transfer theorem).
That's what an MPP circuit does -- adjusts the current until the maximum power is extracted from the panel.
Wow – Crutschow, that’s one hell of a circuit just to charge a battery.

I offer my own solution (drawn on the back of a cigarette packet), which I reckon will suffice.

Given the maximum available current from the solar panel, transistor Q1 may not require any additional heat-sinking.

Diode D1 is required to avoid the battery voltage being applied to the solar cells (at night), with the Darlington transistor having an internal diode (collector to emitter).

I welcome comments – good or bad on my proposed circuit.
Solar charging circuit.jpg
 

dendad

Joined Feb 20, 2016
3,023
Wow – Crutschow, that’s one hell of a circuit just to charge a battery.

I offer my own solution (drawn on the back of a cigarette packet), which I reckon will suffice.

Given the maximum available current from the solar panel, transistor Q1 may not require any additional heat-sinking.

Diode D1 is required to avoid the battery voltage being applied to the solar cells (at night), with the Darlington transistor having an internal diode (collector to emitter).

I welcome comments – good or bad on my proposed circuit.
View attachment 150023
Yes, something like that will work. On a small unit I build, a power zenner across the battery is all I use.
BUT, a MPPT controller will extract significantly more charge from the panel to the battery so it depends on what you want.
A cheap "will work ok " solution, or to get best efficiency and maximum charge for your battery.
Oh, I think R1 needs to be on the other side of D1 :)
 

crutschow

Joined Mar 14, 2008
23,362
I welcome comments – good or bad on my proposed circuit.
That will not prevent long-term overcharging of the battery and shortening its life.
That's why you use a charger that will reduce the charge voltage (and consequently the charge current) when the battery is fully charged.
But if the battery is being used when the sun isn't out and then charged otherwise, that circuit should be sufficient.
 

crutschow

Joined Mar 14, 2008
23,362
Another simple solution using a LM317
That regulates current, not voltage.
It's better to regulate voltage when charging a lead-acid battery to reduce overcharging.

In this case the maximum output current of the solar panel is small enough that you don't need to add a current-limit when it's charging the TS's battery.
 

sghioto

Joined Dec 31, 2017
1,039
Disagree, been using six of the above charger circuits off one solar panel for 9 years. If you set the current correctly it will work just fine.
SG
 

crutschow

Joined Mar 14, 2008
23,362
Disagree, been using six of the above charger circuits off one solar panel for 9 years. If you set the current correctly it will work just fine.
That's very odd because all recommendations for charging lead-acid batteries recommend constant-voltage charging, (which is how your automobile does it), not constant-current. :confused:

What is the "set the current correctly" value?
 

dendad

Joined Feb 20, 2016
3,023
If sghioto is using "12V" solar panels with 12V lead acid batteries it will probably work as the "charge controller" will drop enough volts to limit the max volts, and its constant current will actually be almost irreverent. Also, if the batteries are big enough, the max current available from the LM317 circuit will not hurt the battery.
Still, a proper charge controller, designed for the battery chemistry, will work so much better. As I mentioned in #9, you can do short cuts that will work, but doing it the right way will get more charge from the panel and protect the battery.
 

ebp

Joined Feb 8, 2018
2,332
With industrial solar power applications it is common to have batteries so large that the PV array can't even come close to supplying too much current. This is because batteries are sized to be able to sustain system operation with multiple days of poor insolation. It is also hard to pound really high current into lead acid batteries without raising the terminal voltage above float voltage, which is something that is not always easy to manage when the load is permanently connected. Float voltage needs to be be accurately regulated and should be temperature compensated for best results - avoiding "boiling" the battery in hot weather while avoiding the penalty of inherent low capacity in cold weather. You can get away with treating the battery badly if it is small and inexpensive, but when you've got many hundreds to thousands of dollars worth of batteries you really want to maximize their useful life.

Many of the consumer charge controllers are simple hysteritic on-off "bang-bang" types - there is a direct connection between the PV array and the battery that is either on or off. With panel(s) such as that described here, the results aren't great, but they aren't too terrible. MPP converters are much more complex. To truly operate at MPP they must be set up to match the specific PV cell type or must actually seek the MPP. Because MPP is a peak, you can't get to it with a monotonic process. You have to go up one side of the hill and you won't known you've gone past the peak until you find yourself going down the other side. If you operate your array over a wide temperature range, such as -40°C to +45°C the MPP moves a long way. When you have to operate at -40°C you can save a lot of money on the array and battery by using a good MPP tracking converter.
 

sghioto

Joined Dec 31, 2017
1,039
What is the "set the current correctly" value
Just means the charge current is determined by the size of the battery and the duty cycle. In my particular case the batteries are removed from the charger and only used once a month. Trickle charged at .4 amps for several hours each sunny day.
SG
 
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