I'm puzzled at what happened to two Hampton Bay solar path lights I installed at my mom's house. They replaced older Hampton Bay lights that had failed. Well, in less than a year, these have failed too...but differently: They now light during the day, and go off at night...they are supposed to work the other way around!

There is a solar cell and a small CdS photocell glued into in the top of the light housing. This is wired to a small circuit board that powers the single 5mm white LED. The light is powered by two 1.2v NiCd cells.

I've had some of these lights fail where the photocell would keep the light on constantly, draining the batteries. But this is strange...if I put my hand over the photocell, the LED goes out, and comes on when exposed to light. All I can think of is somehow polarity reversed itself somewhere in the circuit...but how?

Sounds like the Photocell has started Dayburning (Knackared) and thinks its Dark when it's light. So instead of the panel charging the Batteries the power coming in from the sun is going straight to the LED instead of charging the batteries . Try a new CDS Cell of the samne type (if you have one) and that should be your probolem sorted. I have had 100s of solarlights that i  have fixed for other people that do this.

Danny

Maybe battery failure ? That would explain it :

In the day, the dead battery is shorted so it pulls max current from the cell. As the battery is charging through a diode, the output from the cell is probably clamped to about the Vf of this diode - 0.7V. For the sense circuit, this may be low enough to decide that this is night. And it may be sufficient to run the inverter as well (those lights run the LED on an inverter to step up the voltage from 1.2V Ni-Cd to about 3.4V for the LED) - so the LED lights up

In the night, the battery is dead so nothing works

Try to figure out by swapping parts between a working and not working light, or inserting a charged AA battery (you can use alkaline as well, just dont leave it there permanently) in the light and see what happens

Beware : Some solar panels have really nasty connection points on the back which are near impossible to solder to. Try to not tear off the wires. Or try to supply 1.5V from alkaline battey instead of from the solar cell

I would rather guess broken connection (on the PCB) of the converter control input:
Except come one light about 8 years old, all these use the main solar cell as the light sensor to control the LED converter (voltage present => light => LED OFF and vice versa). The detection circuit depend, if the converter uses single transistor selfoscillating circuit (the voltage from the solar panel block via an auxiliary diode the transistor base from turning it ON, so stop oscillation when the solar panel voltage exceed the actual battery voltage) or it is separate comparator at about 0.2..0.4V (when using specialized IC - this is in most lightf from last two years).
In both cases, if the path have some non-conductive "joint" (open auxiliary diode, open detection input pin of the IC), the inverter stay all the time ON. This discharge the battery and prevent it from being charged, so it does not light at night. During the day, the energy is provided from the solar cell.

Dead battery won't cause this, as when the solar cell is illuminated, it have always higher voltage than the battery (what would stop the single transistor converter) and/or higher than the detection threshold (and that wouldstop the IC version).

The inverter could run only, if there is enough voltage on the battery (>0.7..0.9V for start, >0.4V for sustaining for the selfoscillating circut, >1.1V for start and >0.7V for sustaining for the IC's) and at the same time no voltage on the solar cell.

Danny was correct. I tried new batteries. The lights dayburned and also then burned at night. With no batteries in the units, they lit during the day only.

I've tried to repair these lights in the past, but some parts are glued in and they are hard to repair without breaking the plastic. In one that I did successfully fix, it was soon dead again due to water getting in. Another one I tried to caulk back together but it looked like hell afterwards so I trashed it.

Avoid water damage : seal it well on the top, leave a hole in the bottom so whatever flled it up would escape

Seal it with silicone and hot glue they work well

Seal it (all the ugly glue work) on the inside and it'll look excellent on the outside