Overcharging NiMh batteries?

[Bill Bowden]

My neighbor has a solar powered outdoor decoration light that cycles through
several colors but only runs for maybe an hour at night. It has a single
1200 mAh 1.2 volt rechargeable battery and a couple small solar cells to
charge the battery. I measured the load current at 15 milliamps and the
charge current at only 1.3 milliamps so it appears the solar array is not
charging the battery sufficiently. She has another one of the same type that
runs all night so this one must have a problem. I have a few small solar
cell pieces where 4 in series charges the battery at around 70 milliamps but
I'm thinking that may overcharge the battery? I have some 3 inch square
cells that produce about 1 amp, but they are very hard to cut down to
smaller sizes.without breaking intp pieces. I tried an exacto knife and
managed to get 4 small pieces to do the job but I'm thinking they may still
be too large at 70 millamps. The circuit is all in one chip with the cells
and battery common to the positive side, and the negative side of the
battery and cells goes to 2 connections on the chip. The voltage drop from
cell to battery is about 200 millivolts while charging so it appears to work
like a blocking diode so the battery doesn't discharge into the solar cells.
There may be some sort of regulator that limits the battery charge but I'm
not sure. I guess I could just add a load resistor across the cells if
needed?


.



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[ehsjr]

On 8/20/2015 5:50 PM, Bill Bowden wrote:

My neighbor has a solar powered outdoor decoration light that cycles through
several colors but only runs for maybe an hour at night. It has a single
1200 mAh 1.2 volt rechargeable battery and a couple small solar cells to
charge the battery. I measured the load current at 15 milliamps and the
charge current at only 1.3 milliamps so it appears the solar array is not
charging the battery sufficiently. She has another one of the same type that
runs all night so this one must have a problem. I have a few small solar
cell pieces where 4 in series charges the battery at around 70 milliamps but
I'm thinking that may overcharge the battery? I have some 3 inch square
cells that produce about 1 amp, but they are very hard to cut down to
smaller sizes.without breaking intp pieces. I tried an exacto knife and
managed to get 4 small pieces to do the job but I'm thinking they may still
be too large at 70 millamps. The circuit is all in one chip with the cells
and battery common to the positive side, and the negative side of the
battery and cells goes to 2 connections on the chip. The voltage drop from
cell to battery is about 200 millivolts while charging so it appears to work
like a blocking diode so the battery doesn't discharge into the solar cells.
There may be some sort of regulator that limits the battery charge but I'm
not sure. I guess I could just add a load resistor across the cells if
needed?

70 mA won't overcharge a 1200 mAh battery. However, it might be
more advantageous to fix the problem than to jury rig by adding
more solar cells. Always suspect the rechargeable battery
first. You could swap the batteries between the good one and the
bad one and test for several days, to see if the problem stays
with the light or follows the battery.

The 1.3 mA charging current is too low, so either the battery
isn't drawing enough, the cell isn't producing enough or the
circuit is not passing enough charging current.

For a typical one-chip "garden light" schematic, google YX8018

Ed

 

[David Eather]

On Fri, 21 Aug 2015 15:22:27 +1000, ehsjr <ehsjr@mverizon.net> wrote:

On 8/20/2015 5:50 PM, Bill Bowden wrote:
My neighbor has a solar powered outdoor decoration light that cycles
through
several colors but only runs for maybe an hour at night. It has a single
1200 mAh 1.2 volt rechargeable battery and a couple small solar cells to
charge the battery. I measured the load current at 15 milliamps and the
charge current at only 1.3 milliamps so it appears the solar array is
not
charging the battery sufficiently. She has another one of the same type
that
runs all night so this one must have a problem. I have a few small solar
cell pieces where 4 in series charges the battery at around 70
milliamps but
I'm thinking that may overcharge the battery? I have some 3 inch square
cells that produce about 1 amp, but they are very hard to cut down to
smaller sizes.without breaking intp pieces. I tried an exacto knife and
managed to get 4 small pieces to do the job but I'm thinking they may
still
be too large at 70 millamps. The circuit is all in one chip with the
cells
and battery common to the positive side, and the negative side of the
battery and cells goes to 2 connections on the chip. The voltage drop
from
cell to battery is about 200 millivolts while charging so it appears to
work
like a blocking diode so the battery doesn't discharge into the solar
cells.
There may be some sort of regulator that limits the battery charge but
I'm
not sure. I guess I could just add a load resistor across the cells if
needed?

70 mA won't overcharge a 1200 mAh battery. However, it might be
more advantageous to fix the problem than to jury rig by adding
more solar cells. Always suspect the rechargeable battery
first. You could swap the batteries between the good one and the
bad one and test for several days, to see if the problem stays
with the light or follows the battery.

The 1.3 mA charging current is too low, so either the battery
isn't drawing enough, the cell isn't producing enough or the
circuit is not passing enough charging current.

For a typical one-chip "garden light" schematic, google YX8018

Ed

YX8018 is possibly the worst chip ever made (at least the Chinese ones)

I bought 100 from AliExpress for about $5 (IIRC). 40% were DOA with
various defects. Of the rest, using all variants of the circuits on the
data sheets, with the recommended and also with superior components, the
best efficiency was 30% (might have been low thirties). Yuk.

I don't remember all my test results but he might have a defective YX8018

 

[Bill Bowden]

"ehsjr" <ehsjr@mverizon.net> wrote in message
news:mr6cf6$j0u$1@news.eternal-september.org...

On 8/20/2015 5:50 PM, Bill Bowden wrote:
My neighbor has a solar powered outdoor decoration light that cycles
through
several colors but only runs for maybe an hour at night. It has a single
1200 mAh 1.2 volt rechargeable battery and a couple small solar cells to
charge the battery. I measured the load current at 15 milliamps and the
charge current at only 1.3 milliamps so it appears the solar array is not
charging the battery sufficiently. She has another one of the same type
that
runs all night so this one must have a problem. I have a few small solar
cell pieces where 4 in series charges the battery at around 70 milliamps
but
I'm thinking that may overcharge the battery? I have some 3 inch square
cells that produce about 1 amp, but they are very hard to cut down to
smaller sizes.without breaking intp pieces. I tried an exacto knife and
managed to get 4 small pieces to do the job but I'm thinking they may
still
be too large at 70 millamps. The circuit is all in one chip with the
cells
and battery common to the positive side, and the negative side of the
battery and cells goes to 2 connections on the chip. The voltage drop
from
cell to battery is about 200 millivolts while charging so it appears to
work
like a blocking diode so the battery doesn't discharge into the solar
cells.
There may be some sort of regulator that limits the battery charge but
I'm
not sure. I guess I could just add a load resistor across the cells if
needed?

70 mA won't overcharge a 1200 mAh battery. However, it might be
more advantageous to fix the problem than to jury rig by adding
more solar cells. Always suspect the rechargeable battery
first. You could swap the batteries between the good one and the
bad one and test for several days, to see if the problem stays
with the light or follows the battery.

The 1.3 mA charging current is too low, so either the battery
isn't drawing enough, the cell isn't producing enough or the
circuit is not passing enough charging current.

For a typical one-chip "garden light" schematic, google YX8018

Ed

The battery is good and when fully charged powers the circuit for over 24
hours. With a larger solar cell they wouldn't need to switch off the LEDs in
the daytime. The charge current reads 70mA through the circuit, so it must
also be good. That only leaves the cells themselves and I haven't any idea
what's wrong with them. They would be difficult to remove and test. I'm
going to enclose the additional cells in a small plastic box and run wires
through a small hole in the fixture and attach the additional cells in
parallel with the original. Doesn't look too professional so the neighbor
will figure out a way to hide the box so it gets light and doesn't attract
attention. She has about 50 of these things of different sizes and a bunch
of statues in her yard, and a red Ferrari in the garage.




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