# how does this solar lamp work?

#### zenite

Joined Dec 21, 2009
15
http://www.ikea.com/us/en/catalog/products/90154371

The product above from Ikea is a solar powered lamp. I am doing a project similar to this, using solar power to charge a battery. Can anyone guide me to what I need to know to build the circuit to charge NiMH batteries like the lamp from Ikea. The batteries are being charged and the lamps can still operate. From what I know, NiMH must be disconnected and placed into the charger to charge. So how can the batteries in the lamp get charged w/o removing it? I need a push in the right direction, totally clueless.

#### zenite

Joined Dec 21, 2009
15
Another question, I am looking into this company solar panels...
http://www.jethobby.com.sg/cgi-bin/ezsite/prod/manager.cgi?action=list&cid=114&gid=25

I need 8-12V and 2A for my device. Which solar panels should I use? I read from another post about using current regulator to increase the current (maybe I misunderstand it), but is it possible to increase current? The panels only give 100mA, which is typical of most panels. Those that come close to my need are too heavy (3kg) for use (in my proj).

#### rjenkins

Joined Nov 6, 2005
1,015
Charging a battery while still in circuit is simple - it just means that the charger can supply more current than the battery is supplying to the load, so more power is going to the battery than is being taken out.

The little solar lamps can cheat, as the solar cells can not give enough power to ever overcharge and damage the battery.

They also sense the output from the solar cell and turn off the lamp once the solar cell starts to produce power, so most of the power goes in to the battery.

Higher power circuits need more complexity to regulate the solar panel output and protect the battery from overcharge.

You need solar panels that give rather more power than your device uses, if you also want to charge a battery so it can run at night.

Also, don't be taken in by 'ideal case' figures for solar panels; these are when the cells are at the perfect angle in full sunlight - in real world conditions, with changing angles and clouds, the typical output will likely be rather lower.

There is no regulator that will give more power out than in. You can increase current but at reduced voltage, or increase voltage at the cost of current, but the power will always be less due to efficiency losses in the conversion process.

#### Bernard

Joined Aug 7, 2008
5,228
How many hours per day does your device operate? A 20W load seems trivial when operating from the mains , from batteries it's a big deal. A 20V open ckt, 5W panel at peak power puts out 370 mA, so need about 5 hr for one hr of 20W load, and that is 12:30 PM Tucson AZ, clear sky.

#### zenite

Joined Dec 21, 2009
15
my device will operate for (hopefully) 2-4 hours on battery before it goes flat.

How do I increase the current while decreasing the voltage? It might be useful for my device.

also, as long as current is flowing into my battery, the battery will be charged no matter how small the current is? So all I have to do is connect the solar panel output to the terminals of the battery, and the circuit board of my device to the terminals of the battery too to achieve what I mentioned in the earlier post? (charging the battery while the battery is being used to power up the device)

#### Bernard

Joined Aug 7, 2008
5,228
Using 8 size D NiMH batteries you might operate for about 3 h @ 2 A load. It would take about two days to recharge with 5 W panel with no external load. A 20 W panel might be about right. Might consider SLA battery.

#### rjenkins

Joined Nov 6, 2005
1,015
my device will operate for (hopefully) 2-4 hours on battery before it goes flat.

How do I increase the current while decreasing the voltage? It might be useful for my device.

also, as long as current is flowing into my battery, the battery will be charged no matter how small the current is? So all I have to do is connect the solar panel output to the terminals of the battery, and the circuit board of my device to the terminals of the battery too to achieve what I mentioned in the earlier post? (charging the battery while the battery is being used to power up the device)
Yes, as long as the solar panel is providing more power than the device uses, the battery will be charging to some extent.

Use a Switched mode voltage regulator such as a National Semiconductor 'Simple Switcher' IC to step the voltage down. If you go from eg. 12V input to 5V output, the current at the 12V side will be roughly half that at the 5V output.

Don't forget you need a diode or whatever between the solar cell and the battery so it can charge, but not drain power back from the battery when there is no light. (The solar panel may already include this).

#### Darren Holdstock

Joined Feb 10, 2009
262
NiMH are trickier to charge than the old NiCads, which I guess is why battery chargers aren't as cheap as they used to be. And they're never quite the same again if they're discharged too low (I've been working on chargers recently, and today I finished building a precision battery flattener that discharges down to 1 V and no further. An odd but necessary item if one is validating chargers).

BatteryUniversity is the best resource I currently know of.

Last edited:

#### rjenkins

Joined Nov 6, 2005
1,015
The 1V discharge limit is the safe limit if discharging a battery pack; this is just to avoid the chance of a weak cell becoming reverse charged whilst other cells are still discharging.

Single cells can be safely discharged to zero - this is in fact a good way of rejuvinating both NiCD and NiMH cells that appear to have low capacity, just connect a 10 Ohm resistor across each cell and leave for a few days.

#### Bernard

Joined Aug 7, 2008
5,228

#### Darren Holdstock

Joined Feb 10, 2009
262
rjenkins said:
The 1V discharge limit is the safe limit if discharging a battery pack; this is just to avoid the chance of a weak cell becoming reverse charged whilst other cells are still discharging.

Single cells can be safely discharged to zero - this is in fact a good way of rejuvinating both NiCD and NiMH cells that appear to have low capacity, just connect a 10 Ohm resistor across each cell and leave for a few days.
rjenkins is quite correct about the cell reversal phenomenon, and this is the best way to cause acute damage to a NiMH battery pack. But I'd caution against regular deep discharging of NiMH cells as though they won't be immediately ruined they will suffer cumulative chronic damage that steadily reduces the cycling life. Perversely, occasional deep discharge of NiMH cells is a recommended technique for extending working life. These things are awkward to deal with, to say the least.

The same isn't true of NiCads, which (as rj says) benefit from regular deep discharge.

Last edited:

#### Darren Holdstock

Joined Feb 10, 2009
262
As luck would have it, I've been doing some more work on NiMH chargers today, and came across this handy document from Duracell (Scribd). They recommend a 3-step charge procedure for NiMH cells:

1. Charge at the 1C rate, terminated by using dT/dt = 1 deg.C/minute.
2. Apply a C/10 top-up charge, terminated by a timer after 1/2 hour of charge.
3. Apply a maintenance rate of indefinite duration at C/300.
[Here C is the charge capacity of the cell, i.e. the mAh rating but without the h, just the mA, and dT/dt is the rate of increase in temperature when charging, as measured by a temperature sensor on the battery charger].

There's a rare note on discharge and lifetime too:

"Cycle life is also affected by the depth of discharge. Depending upon the charge termination method, up to 500 cycles can be obtained with the battery being fully discharged on each cycle (100% depth of discharge or "DOD"). Considerably higher cycle life can be obtained if the battery is cycled at shallower charge/discharges."