NiMH in solar lights - A fire risk?

I was getting tired of my solar lighting dying at night and looking stupid in the morning because only one of the two would be on. Turns out the NiCDs that were in them were junk. I thought I would be smart and toss in a set of Energizer 2300mAh NiMHs. Wow, they worked great for a couple of days! A couple of days because I have since removed them. Heres the deal:

During a bright day, around midday, I thought it would be a good idea to put a finger on the cells in the lights to see if they were getting warm or anything. I think my finger print is still on the wrapper of one of the cells. Holy crap were they hot!

So, here I am, thinking that using NiMH cells in these solar lights is going to cause a fire!

Is there a fire risk with these?

Should I play it safe and use Eneloop plus' in these lights? I don't want to go back to NiCD because they are just horrible.

A couple of interesting things about these lights:
LED on power usage: 23mA
LED off and charging in sunlight: 67mA
Open circuit voltage in sunlight: 2.56 volts (WOW!)

Should that open circuit voltage be as alarming as it seems to me?

Hello Erik1213,

There isn't a fire hazzard, so don't worry about that.

The voltage you are measuring is it with the battery installed or open circuit? You mentioned that it was open circuit but I just wanted to make sure. When you install the batteries what is the voltage then?

I am not sure why your batteries got so hot, but if that continues it will kill NiMh batteries. You would be better off with NiCd. With that said I have used NiMh in solar lights without problems for years.

Tom

That voltage is with the battery removed. With the battery in, depending on charge level, the voltage doesn't go much over 1.35 volts with a NiMH.

What I have read is that the cells will heat up, especially under a slow overcharge, because there is a catalyst to recombine water broken down by electrolysis and it will get hot while it is working.

I did a refresh on the NiCDs I had to bring them back up to rated capacity. I'm going to try those for a bit and see how they do.

Hello Erik1213,

The heating of NiMh batteries occurs at voltages in the 1.5 volt range. At 1.35 volts they are simply charging and shouldn't be getting hot.

Tom

A couple of things to note. First of all, as far as the high open circuit voltage from the solar cell, keep in mind that photovoltaic cells are, to a limit, current sources rather than voltage sources. Without a load connected, the output voltage will rise to levels far in excess of the nominal voltage. As an example, a nominal 12V panel will typically have an open circuit voltage of around 22V. However, connecting a battery will bring the voltage down to the charging voltage of that battery. As far as the battery getting hot, this may not be due to overcharging. Remember that the battery is in a light that is out in the sun. Simply being in the sun will get the battery hot. As long as voltage isn't rising to insane levels (say 1.45-1.5+V), you probably aren't overcharging. However, the heat could reduce battery life significantly. That's why solar lights generally use hardier chemistries (than NiMH) like Nicad and LiFePO4.

No. As said above you're good.

Lithiums however ( left on a dashboard 210F ) would be another issue ( I think ).

Quality solar chargers like the WakaWaka use hi temp lithium batteries that can withstand high temperatures, they don't however advise to charge on a dashboard. 210F is freaking hot.