Battery Pack Woes

My battery pack and I are having relationship problems.  I think it should work every day, but it thinks it deserves a day off once in a while.  So I have fired it from its job.  Let me back up for a moment.

The solar hot water system needs a small amount of electricity to run the pump, valves, and controller.  You can get this electricity from the wall outlet, but I decided to get it from a separate 40 watt photovoltaic solar panel.  But if the sun ever ducks behind a cloud, even for a moment, that power is lost, valves start closing... and repeated cloud passing can make the system cycle on and off repeatedly.  So to stabilize the system I am using a charge controller and a battery.

Charge controllers mix together electricity from the solar panel and the battery to optimize the power output.  If excess power is available, the system charges the battery.  If solar output drops, the power can be supplied from the battery, at least for a short while, and prevent valves from closing.

Charge controllers are built assuming that you are going to use either one 12 volt lead acid battery or two together (24 volts).  I have no interest in using lead acid batteries because they only last about 3 years.  So I decided to make my own 24 volt battery from AA Sanyo Eneloop NiMH batteries.

The first challenge was how many batteries did I need to mimic the 24 volts that the charge controller is expecting.  Each NiMH cell is about 1.25 volts so twenty batteries would produce a theoretical 25 volts.  This seems close enough.

Next challenge is to find a battery holder for 20 AA cells.  Since I could not find one,  I decided to put together three holders, two 8 cell (2×4) and one 4 cell (2×2).  The photo below shows what I came up with.

At the top of the battery holder is three 9V style connectors that I had to connect together to get the total 24V output.  To hold this assembly together, I bent some sheet metal and glued it to the outside.  Of course I needed to be careful that the metal did not short out any of the electrical contacts.

The Problem

Shortly after installing the system I started noticing problems with the battery.  Sometimes when I touched the battery holder, the voltage would drop to zero.  After some fiddling, the voltage would be back to normal with no clear reason way.  Other times, I would find the battery pack completely dead with the cells reading 0V each.  Upon removing the batteries from the pack they would slowly recover to about 1V suggesting they were being loaded in the pack.  I am not sure why these problems were happening, but decided a new, cleaner, battery holder might help.

I found batteries holders on the web that could hold 10 cells, so I only needed two of them rather than three holder used in the last system.  By placing them back to back, I would make one nice twenty cell holder.  I just needed to solder two terminals at the top, solder some leads, and tape the outer edges with electrical tape.

The new battery holder is installed and working well. 

I have had a discussion with my new battery pack and it has agreed to work every day and not just occasionally.  Hope springs eternal and I believe it. (for now).