Will This work?

System · September 2010

Hello ,

I picked up some solar lawn lights to use as battery rechargers. they came with Nicd 600mah but I have tried 2000mah Nimh's. How can I check if they are fully charged? How long should I expect the cells to make that much energy to fill them?

I have multimeter but have no idea how to use it.

THanks
(running Trisquel 3.5)

BB. · September 2010

Re: Will This work?

You are kind of left with whatever the light/solar panels are designed for.

NiCad batteries are used because they have fairly good life when discharged 100% (typical inexpensive landscape lighting with no real charge controller) and can tolerate the poor recharging done by the solar cells in landscape fixtures.

The AA batteries (or whatever size you have) will recharge to whatever the amount of sunlight that day will support. And discharge 100% the next night.

I am not sure that your fixtures will make use of the ~3x larger capacity NiMH batteries you have replaced them with (maybe during long summer days?).

More or less, try the new NiMH batteries and see how they work over time for you. If they perform satisfactory for 1 year or more--Why not continue to use them. You may even wish to try a couple different brands and see how they do over time and seasons.

I personally would pick NiMH AA batteries with 2,000 mAH or less capacity (the 3,000 mAH batteries don't seem to last very long). Also, the Sanyo Eneloop type are some of the best quality NiMH batteries out there right now--But they are not cheap.

If they don't work well, you probably will have to go back to NiCad (remember to recycle them--Cadmium is a pretty deadly heavy metal and is banned/highly regulated in most countries outside the US) or change to some other landscape lighting technology (hardwired to home's power system).

For your original question:

A fully-charged cell measures 1.4–1.45 V (unloaded), and supplies a nominal average 1.25 V/cell during discharge, down to about 1.0–1.1 V/cell (further discharge may cause permanent damage, and the risk is increased with multi-cell packs). This voltage varies depending on the discharge rate of the cell (lower discharge loads result in an increased voltage output for longer periods, approaching the 1.4 V unloaded cell voltage).

Under a light load (0.5 ampere), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts; some measure almost 1.5 volts. This voltage falls rapidly to about 1.25 volts at 10% depth of discharge (DOD) and then remains almost constant until the cell is over 80% discharged. The voltage then falls rapidly from about 1.2 volts down to 0.8–1.0 volts at which the cell is considered "flat" in most devices. Mid-discharge at a load of 1 ampere, the output is about 1.2 volts; at 2 amperes, about 1.15 volts; the total effective differential internal resistance is about 0.05 ohms. Nickel metal hydride batteries provide a relatively constant voltage for most of the discharge cycle, unlike a standard alkaline where the voltage falls steadily during discharge. [17]

-Bill

niel · September 2010

Re: Will This work?

there could be a problem as the amount of charge available for the battery is usually quite low to begin with. this is determined by the current available from the solar cells and charging circuit and primarily the amount of sun they are able to receive. i have trouble keeping 600ma nicd batteries charged in mine during the winter months as there is far less sun available to keep them up to snuff at that point. you are placing a battery with more than 3x the capacity there and if a 600ma nicd won't reach full charge then a higher capacity certainly won't. keeping the led load off and just recharging daily until fully charged is possible to do though.
your experience with circuits and meters will be a problem though in making this work. the circuitry isn't the best inside of these little solar lights either. if you wish to charge them reliably from dc then i suggest getting a battery charger that has the option of 12v or even 18v dc input. many have wall adapters that go from your 120vac wall socket and have a small plug into the charger (noting that some just run the wires permanently into the charger and i don't advise you doing that if you can't use a meter to determine + or - leads). from there you only need to worry about getting a small pv panel and cheap controller to feed the charger power with. again you need to know enough to determine polarities and how to wire it up correctly.
with the solar lights you can place them outside during the day to charge and bring them inside under a nightlight or any light that is bright enough to keep the leds from coming on and discharging the batteries. this will need roughly 3.5 days to reach full charge and maybe more in the shorter winter months. you have no way of knowing easily if they are reaching their full charge doing this and is unreliable if not burdensome to do.

System2 · September 2010

Re: Will This work?

First off, I am using this as an intro to using solar energy project. I am not interested in the lights ( I live in the desert climate and have no traditional lawn) but am treating my 8 panels as a huge wall charger only sustainable. I will be using the batteries elsewhere in devices. My units have an on/off switch so no chance of the LED's coming on unless I choose. do you know how long the batteries will contain their charge level when removed from charger and just sit?

so to charge say a 200-2300 mah battery 3.5 days in west texas is a fair estimate...

that means I should learn more about solar soon.

Thank you all very much.

Slappy · September 2010

Re: Will This work?

"I am using this as an intro to using solar energy project. I am not interested in the lights"

they make good night lights, made four holders out of 1 inch pvc pipe, two for in the house and two for the out side mounted to a pole, set them out during the day to charge and bring them in at night. Have one in the bath room and one in the kitchen, so who ever wakes up in the middle of the night to do what ever in either room can see where they are going with out turning on the lights, and disturbing other people. it works very well. and they dont blind ya like house lights do

Just a thought

System2 · September 2010

Re: Will This work?

Thanks but I am using it more like a solar panel already wired.

BB. · September 2010

Re: Will This work?

Jane,

Rechargeable batteries are actually a very sticky problem in terms of design and performance.

There are many different chemistry and many different designs in a chemistry (by brand, model, application).

If you are interested in rechargeable NiMH cells--A few things to know.

Standard NiMH can recharge without a special charger if the charge rate is C/10 or less (2,000 mAH would be 200 mHA or 0.2 amp charge rate). That will take around 10-15 hours to recharge.

You can charge them faster (C/5, C/2, C/1 and some faster) -- But the charger has to be pretty sophisticated. Basically, as the cell nears fully charged the cell voltage actually drops a little bit. And at that high rate, the cell temperature has to be monitored too. Actually a very challenging engineering design problem.

Next, "standard" NiMH have a pretty fast self discharge rate--Depending on the battery brand and age, they can self discharge 100% in a month or even less.

And, NiMH AA size batteries over 2,500 mAH capacity or higher--Tend not to last very long before they either fail to hold a charge or have so high a self discharge rate that they are useless a day or few after recharging.

Also, there have been in recent history lots of counterfeit NiMH batteries.

The newer NiMH like the Eneloop or "Pre-Charged" NiMH (Kodak and others), can keep upwards of 80% of their energy over a 1 year period. Otherwise, they work/charge just like normal NiMH batteries. They are some of the best NiMH batteries out there for general purpose use.

"Fast Charging" NiMH batteries (faster that C/10) should be done with a real charger... It is possible that you can connect a commercial battery charger directly to a solar panel -- But because these usually have microprocessors in them--the variable output of a solar panel may confuse the processor... So, it is usually easier (and better) to have the solar panels charge a 12 volt battery bank--then plug the AA charger into the 12 volt battery bank.

-Bill

System2 · September 2010

Re: Will This work?

What about AAA Nimh's? Will they charge?

BB. · September 2010

Re: Will This work?

There are a lot of missing information (how big of panels, what sort of charger you have or are looking at, what your expectations are, etc.)...

A lot of this can be done with knowledge from an Electronics 101 class...

Say you choose to charge the cells with a C/10 rate (10% of the capacity of the battery). So, for a 2000 mAH battery... They charge at 1.7-2.0 volts or so:

2.0 AH * 1.7 volts * 0.10 rate of charge = 0.34 watts

So, a 1 watt 6volt solar panel with a ballast resistor/circuit could open loop charge a pair of standard fully discharged AA 2,000 mAH cell (like the Eneloop) over a period of 2-3 summer days.

Anything else would require an "active charger" plus a (for example) small 12 volt lead acid battery to perform a "quick charge" (under 2 hours) safely and effectively.

-Bill

Will This work?

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