Replaced NiMh battery in small solar light

[fixit7]

I replaced a 600 Mah NiMh battery in small solar light with a 2300 Mah
since I had no 600 Mah hr batteries in my stock.

It worked great for about a week, but then the light started blinking when it should be on steadily.

Is it because the solar panel can not charge a battery with 4 times the capacity?

 

[betwixt]

It is possible, the original manufacturer would select the biggest capacity battery the PV was capable of charging so it almost certainly wouldn't fully charge a bigger one.
We would have to do an analysis to find the cause but my best guess without seeing it is that the battery is only just reaching "start up" voltage. What happens when a good battery is almost discharged is the light goes out, the drop in load on the battery lets it recover a little and that turns the light back on again. You get a cyclic action which you see as flashing. If the bigger battery you fitted is sitting on the borderline of operation because of lack of charging current it could be causing the same effect.

Brian.

 

[barry]

Won't the PV just keep charging the battery until it's fully charged? Why would the PV have a limit?

 

[betwixt]

In theory the PV will keep charging but assuming the manufacturer decided on battery capacity so it charged in say 6 hours, to go from 600mAH to 2300mAH would take approximately four times longer so you would run out of daylight hours in most places. The lower impedance presented by the higher capacity battery might also reduce the PV efficiency. "small solar light" PV panels typically produce less than 50mA short circuit current and much less into a partially charged cell, I think most of them are made using fragments or off-cuts of larger cells.

Brian.

 

[fixit7]

https://imgur.com/XiruKUT

https://imgur.com/MaM7Ejo

 

[Externet]

There is a good chance the initially-charged higher capability new cell was able to power the led for more days without being recharged by the assembly.
So check the charging circuitry is performing, not only its panel.
Measure the (connected - in place) cell at dark and at full sunlight. If no difference in voltage, is not getting charged.

 

[fixit7]

There is a good chance the initially-charged higher capability new cell was able to power the led for more days without being recharged by the assembly.
So check the charging circuitry is performing, not only its panel.
Measure the (connected - in place) cell at dark and at full sunlight. If no difference in voltage, is not getting charged.

Thanks.

0 volts at dark, 2.02 volts with full sunlight.

Should I measure the current?

 

[Audioguru]

It is impossible that your 2300mAh Ni-MH cell charges as high as 2.02V. A good one charges to 1.4V or 1.5V. Maybe you measured the voltage from the solar panel without a battery?
I replaced all my Chinese 600mAh batteries that are probably almost full of sand and rust away in a couple of months, with Energizer 2300mAh cells and they work fine for years.

 

[fixit7]

I think the chip in the pic is a solar led driver.

http://www.datasheetcafe.com/yx805-datasheet-pdf/

So, is it a "small battery charger" than uses electricity from the solar cell to charge the battery?

 

[c_mitra]

0 volts at dark, 2.02 volts with full sunlight...

Is this the open circuit (without the battery connected) voltage measured with a regular digital multimeter?

The voltage is independent of the size of the panel (except for series-parallel combinations) whereas the total current (short circuit) depends on the active area of the panel.

Remember that the open circuit voltage is the max possible whereas the short circuit current is the max current possible.

Maximum power transfer is possible when the input and output (source and load) resistances are equal. Then you are losing half the power in the panel itself.

You should measure the voltage with the NiMH cell as load. Also measure the current.

Change the cell externally (with a regular charger) and measure the voltage and current when connected to the solar panel.

If your panel is too small, it may not be able to fully charge the cell but there is the charge controller in between. That can make the measurements messy.

 

[fixit7]

It was without the battery.

I think, as was mentioned, is that the charge circuit is not designed for a battery 4X of the original.

Maybe there is a large self discharge?

I can get (2) 600 Mah nimh for $2.75 from China.

 

[Audioguru]

A cheap solar garden light has no charge controller. The solar panel is small enough to limit the charging current. The schematic of most ICs shows a Schottky diode in series between the 2V solar panel and the Ni-MH cell to prevent the battery from discharging into the solar panel during darkness.

You cannot accurately measure charging current because the meter's shunt resistor reduces the voltage and current, and the sun is in different positions giving different intensities during daylight hours.

 

[Audioguru]

I do not trust Chinese batteries. I got some marked 300mAh and 600mAh and both perform the same and are actually 200mAh and might be old Ni-Cad instead of newer Ni-MH.
They rust away in a couple of months (rain or condensation) while Energizer cells do not rust. They might light the LED for only a couple of hours while the Energizer cell lights the LED brightly all night long.

 

[fixit7]

Ok, so no Chinese batteries.

 

[betwixt]

The IC isn't a charger either. It is an oscillator and switch in one package, it works in the same way as the venerable "Joule Thief". All it does is switch the current through the inductor to allow it to produce a higher than battery voltage to operate the LED. Bear in mind a rechargeable cell has a nominal voltage of about 1.2V but it takes 2v or more to operate a white LED. The fourth pin on the IC is an inhibit input, it senses when the PV is producing voltage (so it must be daytime) and disables the switch so the LED doesn't come on until dark.

Brian.

 

[c_mitra]

They rust away in a couple of months (rain or condensation) while Energizer cells do not rust.

But my personal experience is exactly opposite; But if I pick up randomly from the store self, both are likely to last the same.

Recently I bought a few chinese cells from Aliexpress and they are very good (running for more than 1 year).

not really relevant, but a few years ago I attended a conference on electrochemistry in China and I was impressed by the work they have done on batteries. It must have been a national priority research area for them.

If you put the question differently: do you get your monies worth with chinese batteries, then I shall say yes. The most expensive ones- that I can find on the supermarket shelves- Duracell - is also made in CHina. They have never leaked but they do not last either.

 

[Audioguru]

The most expensive ones- that I can find on the supermarket shelves- Duracell - is also made in CHina. They have never leaked but they do not last either.

The solar garden lights are made very cheaply using the cheapest Chinese Ni-MH batteries they can find. The batteries do not produce the mAh marked on them and they might be old Ni-Cad instead of newer Ni-MH.
In Canada the Duracell and the Energizer Ni-MH cells are the same expensive price and both are made in Japan, not in China. Their mAh ratings are almost the same and are truthful as shown on published tests. Both do not rust like the Chinese ones.
Tests show that Ikea (store chain from Sweden) batteries made for them are as good as the name brands but cost much less.

 

[fixit7]

It is impossible that your 2300mAh Ni-MH cell charges as high as 2.02V. A good one charges to 1.4V or 1.5V. Maybe you measured the voltage from the solar panel without a battery?
I replaced all my Chinese 600mAh batteries that are probably almost full of sand and rust away in a couple of months, with Energizer 2300mAh cells and they work fine for years.

The 2.02 volts is without the battery.

 

[Audioguru]

The solar panel for a garden light usually has 4 solar cells, each producing about 0.5V with no load. They charge the battery all day at a low current to about 1.4V when fully charged.

 

[D.A.(Tony)Stewart]

My experience with Alkaline bulk purchases from Walmart in Canada are that cheap 100 pack AA cells last 1 to 3 years with low load ( e.g. CO alarm) or no load The Energizers are designed for 10 year shelf life with a date stamp on them.

The reason I am learning is due to the quality of the seal and impurity level of the carbon electrodes and electrolyte that results in high ionic salts to self-discharge and also leak from the seal. Graphite can be produced in various grades from 95% to 99% to 99.9% to 99.95 to 99.9995 percent with processing costs (e.g. $1200/kg for nuclear grade) ) and material source quality as well. We are considering these for Graphene and Fullerene carbon for high volume production in a custom machine.