Solar LED Flood light .. need some help..

Thread Starter


Joined Feb 9, 2017
Hello AAC Community !

I am new here, it's actually my 1st post. and the purpose of this message is to get some help ( classic..)
First, please note I am not LED expert, The story is like this :
I have made a small solar PV offgrid system to power a LED flood light at home.
Here the details:


-I use 2x 3.2V 20Ah Lithium 0.33C LiFePo4 batteries togeher in serie it gives me 6.4V 20Ah
-I use a solar controller 5A 6V
The LED flood light I use is rated 6V 6W 1A. ( but I have some doubts)
So I connected everything together to light my garden at night. The controller is set for 6 hours. It has been a while since the 1st trial..
about 7 months running everyday..

Some facts :
- The LED is still very bright at the beginning of the night.
- 2 or 3 hours later it starts dimming. when we reach almost 6 hours of use, it's hardly's very very weak


what is wrong with my system ? why the LED cannot be bright 6 hours ?

in terms of safety of discharge, i feel we are good :
- Battery 0.33 x 20Ah = 6.6A and I use only 1Ax6hours = 6A in the night.

in terms of watts, I feel we are good too..6Wx6hours = 36 Watts per night . Battery has 6.4V x 20Ah = 128AIf I draw 50% of the battery i can run 2 nights ( 2x 6hours) without charging but everyday it does ( may be not fully, but I assumeI get 3-4 hours of sunshine at least, which gives me 3 hours x 5A = 15A everyday. and LED only uses 6A terms of solar panel:I use 3x 6V 10W solar panels in parallel . so total is 30W 6V ( and 5A)Today , at noon, it was cloudy and my charging voltage was 6.17V ( solar panel side) the charging voltage for batteries was 5.84V I measure directly on the solar controller's bolts.

At the beginning of the night, when the LED is bright, the measured voltage is 5.77V. When it dims and is very weak, the measured voltage is 5.14VHow is it possible ?

My solar controller is supposed to have a discharge cut off voltage of 5.4V. So, how come the LED is still on with a voltage of 5.14V ?

I try to understand what is wrong but I cannot see throught ..
is my battery dead ? I doubt it..and it's Lithium LiFePo4..
is it not enough solar power to charge ?

What would you change to get 6 hours of brightness everyday ?

Thanks : )


Joined Jun 19, 2012
DIY solar setups are often disappointing.

1) Battery charging is an inefficient process- losses in the battery electro-chemical process and the charging circuits waste significant power.

2) Solar cell ratings are enthusiastic, assuming full sun, straight-on a dead-clean cell, hardly realistic for a real install.

3) Charge controllers are often not matched correctly to the specific battery.
Every battery type has a personality, the charger must be optimized to charge the battery correctly.

Take some measurements of the true power levels when the system is operating, that's a start to understanding what's going on.
Beware of the "listed specs" of any solar panel or battery, remember it's marketing, not truth.


Joined Apr 7, 2016
I was on Wikipedia for some data on Lithium iron phosphate battery:

Cell voltage:
  • Minimum discharge voltage = 2.5 V
  • Working voltage = 3.0 ~ 3.3 V
  • Maximum charge voltage = 3.65 V
The voltages of daily operation, indicates that the battery is never fully charged.

For what battery chemistry is the charging controller?

For seasonal variations, where in the world do you live? (Just country / state / region)


Joined Aug 7, 2008
You need more information on solar panels, open circuit V, & short circuit current. To charge 2 LiFePo4 batteries in series I use a 18 V OC SP @ 160 mA,
just a little on the high V side , but the physical size was just right. Charge V needs to reach 7.2 to 7.3 V for about 90 % charge. Two of your SPs in series would be about right if the 6 V is loaded V. Just for kicks I'll throw in my solar -battery controller.LH Battery Manage. 00000.jpg


Joined Sep 9, 2010
Can you charge your batteries properly in some other device, and then test them in your system?

What you are describing is very similar to what happens with a cheap solar landscape light. Brand new with a fresh battery, they'll often run all night until the sun comes up. A year later, in the winter, they may run only a few minutes in the evening. Removing and recharging the battery can often help them a lot, but of course they quickly run down again. But if your batteries give you the expected life after a good and thorough charging, that tells you the problem is a chronic lack of charge. If they do not, then you know the cells have lost capacity.

It is a problem that your controller is not performing the low-voltage cutoff. It's very damaging to some battery chemistries to deplete them past the critical voltage level.


Joined Feb 5, 2010
7 months would be getting close to 250 charge discharge cycles.
IF the charging was also insufficient the batteries may be damaged at this point and no longer be capable of delivering 20 Ah.
(Some battery specs use a very misleading 100 hour discharge curve to calculate ampacity. This always make the batteries seem twice as good or better than they are in real service)

Thread Starter


Joined Feb 9, 2017
Thanks everybody for your replies!!

To Kjeldgaard : It's in Australia. I do not know the battery chemistry for the solar controller. it's not mentionned in the documentation. I have been told it was for lithium.. i do not know more..

To wayneh Yes I would like to charge them. just need to find the time to do it ahah . I agree with your comments. You also mention "It is a problem that your controller is not performing the low-voltage cutoff." The measures show the controller still work with 5.14V which is below the cut off voltage.. I do not understand that.. would it be a malfunction ? a lie from manufacturer ?

I am going to fully charge them then try again. I will keep you posted.

Thanks again !

pumpkin king

Joined Jan 22, 2016
As Bernard said, you need 7.3v to charge your batteries, the 6v solar panel isn't enough to fully charge your batteries.
To get round the charging problem, you can use a DC-DC step up power supply with built in current limit between the panel and battery. Set the output voltage of the DC-DC supply to 7.3v so not to over charge the batteries, and set the current limit to around 4 amps. But if you say you only get about 4 hours sunshine then the panel needs moving, if its already in the best spot then you need a larger panel to fully charge the batteries.

Thread Starter


Joined Feb 9, 2017
one new information a little odd..

the 6V solar panel reach 14V if i measure its voltage ( unplugged) under bright sunshine..
which is odd is that under bright sunshine, the controller ( 5A 6V ) seems not able to manage the voltage and send 12.65V to the batteries..
so it seems my batteries are charged under 12.65V when it's bright, 6.84V when it's cloudy..
I measured the values at the controller and also on batteries to be sure..

the controller's cut off voltage function seems not working either..

should I understand this 6V 5A solar controller is total crap ?

besides that I found a lithium battery charger and charge each battery separately. so far i reached 3.2V for each

What do you think ?