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Overcharging NiMh batteries?

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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Reply to
Bill Bowden
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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

Reply to
ehsjr

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

Reply to
David Eather

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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Reply to
Bill Bowden

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