Supercaps used with battery ??

KnockaT

Hi, I have a home-made battery lighting system built using a 12v lead/acid battery, 40w solar panel, which is driving about 90 leds in various rooms. The circuits are adjusted via dimmers and used as general lighting during the hours from sunset until bedtime, then dimmed as night lights until sunrise. The system has been running now for 5 years without fail but recently as we go into winter, with the low angle of the sun, the amount of stored capacity  has decreased somewhat. I want to install another battery in parallel but am worried about any unbalance which may occur. I want to wait until the original battery requires changing, then buy 2 batteries of the same type. In the mean time, a friend has given me a bank of 6 x 2.7V 120F supercaps and I wondered if I put them in parallel with the battery, would I get any increase in capacity to keep me going temporarily? The 6 caps are mounted on a balanced board.
Any help would be appreciated for this old newbie.

vtmaps

Welcome to the forum,

no the caps are not a solution.  The energy in them is proportional to their voltage... there's not that much energy stored in the narrow range of voltages that the battery works at... you don't want to let the battery get below 50% discharge (about 12.0 volts resting, a bit less if you are drawing current).  Thus, you are only using the energy in the caps from about 14.4 volts to 12 volts.

and you are correct that you should not put new batteries in parallel with old batteries.  sadly, when batteries cannot carry the load, it is time to replace them.

--vtMaps

littleharbor2

Five years is a pretty good run on your battery. It's probably time to replace it. Have you ever given it an equalizing charge? 40 watts of solar panel is likely not putting out enough amperage to correctly charge your lead acid battery, let alone two batteries. You might consider getting a larger panel when you replace your battery. Two series wired 6 volt deep cycle golf cart batteries would be much happier than two 12 volt batteries in parallel. Costco or Sams club have golf cart batteries at very good prices ie; under 90 dollars apiece.

KnockaT

Subsequent to my earlier post, maybe the term increased capacity was not a GOOD SELECTION. Thinking on, would it be possible to have a switching arrangement where on one side you had 12v battery and on the other the 6 x 120Farad bank. As the voltage in the supercaps died away to a certain value then the switch operated and the caps would charge up again? This would not store energy but I am guessing that the battery might live a longer and happier life? Maybe a current limiting resistance to stop any initial dead short from happening? I am a little out of my comfort zone here, just trying to keep the old grey matter stirred up.
Thanks guys for you help already, much appreciated.
Cheers KnockaT

 

BB.

The only common usage of a super cap in parallel with a lead acid battery bank is for people with high power automotive audio systems.

http://www.crutchfield.com/S-c0BOZz20nJH/learn/learningcenter/car/capacitors/faq.html

Note that these are supplying surge current for on the order of 1/60 second (30 Hz low end audio) or a few times longer (low frequency audio).

A super cap in parallel with a lead acid battery is not going to be much use when, for example, starting a motor--Which can take upwards of sever seconds to come up to speed. Batteries maintain their output voltage at a fairly constant voltage because of chemical action--And does pretty well on longer time frames. Capacitors store energy based on voltage differential (charged to discharge voltage change). So--A capacitor can store/deliver much less total energy vs a battery of similar size.

In some cases, engineers have designed special circuits that allow a wide voltage range on the capacitor (say 2:1 voltage range) and deliver this power to the load. One was a subway car that needed short term storage for powering lighting/electronics when power is interrupted during track switch over. The plan was a capacitor bank would have less maintenance vs a true battery--Problem is that supper caps (the very large ones) typically have a limited life too--In that application it was a about 2 years. (not much different than a battery bank, and probably much more expensive).

So--What is your energy need? If you need very fast/high surge current, perhaps a super cap is a possible solution. However, if the application also needs to supply a lot of energy over seconds/minutes/hours too--A large battery bank, is needed to supply power over these longer time periods may already supply any needed short term energy spike (fractions of a second). Such as for starting a well pump motor.

High energy capacitors are dangerous because they can supply their entire energy storage in a fraction of a second--Need to ensure you have fusing/breakers and shields to prevent an unintentional short or tool dropped across the capacitor terminals from causing an Arc Flash (high energy arc+vaporize metal causing a vary dangerous explosion).

-Bill

KnockaT

Thanks Bill...,