Rechargeable Batt

System · May 2006

Hi,

I have a Yuasa NP2.3-12 VRLA battery http://www.yuasa.com.tw/english/vrlafile/NP2.3-12.htm . As noted in the spec, the capacity is 2.3AH(20hr rate of 0.115A TO 10.5V). Assuming that the max no. of charge/discharge cycles is 250 times, does is means that about 6kWh(20 x 0.115 x 10.5 x 250) of energy can be stored and utilised?

If the operating voltage of the device to be powered by the battery is less than the specified 12V, do I need to step down the supplied voltage and if so how can I do that? Finally, is a charge controller required if a 5W solar panel is used?

Appreciate any advice. Thanks.

Regards

BB. · May 2006

Re: Rechargeable Batt

kWatt*Hrs = 250 cycles * 2.3AH * 12v / 1,000 = 6.9 kWhrs

However, usually it is not a good idea to discharge a lead acid storage battery below 50% capacity--so the 6.9 kWhrs becomes 3.45 kWhrs over the ideal life of the battery (you can pick 12 vdc or 10.5 vdc--they are probably both roughly bracket the answer).

If the voltage of the battery device is less than 12 volts, you will need to drop it down to the voltage required... There are many different ways to do this--however, the choices are dependent on battery voltage, device voltage, device power, runtime required, electrical noise accepted, ability of the user to build the required circuit, etc...

It can be done, but need more information to make a step down circuit recommendation (or off the shelf circuit suggestion).

If the charging current to the battery is more than C/10, you will probably need a charge controller.

C/10 = 2.3 amps / 10 = 0.23 amps or:
P=VI=12 vdc * 0.23 = 2.76 watts

So, the solar panel is about 2x the size recommended for charging the battery without a controller. However, if the application can replace the battery (cost does not matter), the amount of sun to the panel is limited, the amount of power your draw is a couple of watts, or you don't care if the battery lasts more than a year, then a charge controller can probably be ignored (there are simple IC's that you can probably wire up as charge limiting device for only a few bucks--if desired).

More information/questions?

-Bill

System2 · May 2006

Re: Rechargeable Batt

Hi BB,

thanks for reply.

Like to know what is C?

As noted in http://www.solar-electric.com/charge_controls/solar_charge_controllers.htm, the purpose is to prevent "overcharging" the batt with higher than intended voltage. Would charging the batt over a long period of time cause any damage?

Also, the "MAX charged current" for the Yuasa batt is 1.725A, I assumed that's the max charging current, would charging with current higher than that cause problem too?

Regards

niel · May 2006

Re: Rechargeable Batt

c is the total ampere/hours rating of the battery, so 1/10th c is 10% of that said capacity.
follow the recommendations naws has listed as following bb's recommendation of being under 10% of c would result in overcharged batteries. i did this with a 1.8% c charge rate in the past and it boiled out the electrolyte some and exposed the plates leading to severe sulphation. it becomes a fine line between an overcharge and not enough capacity to reach full charge. it is better to get enough capacity to charge from a pv and just make sure through regulation that it does not overcharge the battery. controllers are types of regulators, but not all regulators can or should be controllers. in conclussion and to answer your question, yes, you need a regulator or controller on a 5w pv to charge that battery.

BB. · May 2006

Re: Rechargeable Batt

Anytime you can protect the battery with a charge controller/regulator/low voltage cut-off (for the load), that is a good thing. If you were going quick and dirty 10% charge rate or lower without a controller would be a quick and dirty, short lifetime use (~months). Yes, you can boil the water out of any lead acid battery without some sort of charging controller. Not arguing with Niel on this point--I was just not sure what the battery application is...

However, selecting a battery really depends a lot on the application. Lead Acid batteries are OK (cheap and heavy), but some others types may actually perform better for your application (like the good old standby NiCad or Metal Hydride types). You can better match your voltage requirements (NiCad and MHD have flatter discharge curves, can be charged/discharged at higher current levels, don't normally bubble off gasses, no acid spills, will better withstand poor charging/discharging practices that will kill a lead-acid, can be easy to replace just by placing D, C or AA cells in a battery holder).

An R/C (radio controlled) model site might be a good place to get some information for small/high performance battery packs and charging information.

-Bill

niel · May 2006

Re: Rechargeable Batt

bb,
i bought some nimh batteries at wall mart recently. they are 2.5amphrs(2500mahrs) for about $18 or $19 and a quantity of 8. to get to 12v you would need 10 batteries though and you should not mix and match different batteries. they had 4 packs i believe as well of this same battery so that would leave 2 extra batteries leftover that you could use in something else. buying chargers that accomodate that many high powered nimh batteries are out there, but not as commonplace as the ones that charge 4 or 2 at a time. those that can do it from a dc source are even fewer and far between, but again they are out there.
here's a typical link you can use to see some of the chargers available in this example: http://www.batteriesamerica.com/
thomas distributing may have some as well as mahaenergy.

BB. · May 2006

Re: Rechargeable Batt

Niel,

Yea, it kind of blows me away at what they can get from 10 AA nimh batteries (roughly equal capacity as the heavy lead-acid battery at the beginning of this thread).

They can also be more tolerent of overcharging and deep cycle discharging too and can probably live with a very simple voltage cut-out for over/under charge protection. Usually, mimh batteries survive just fine if you don't "reverse bias" one of the cells in a string. With small, low voltage strings (like 2-3 batteries in series) it is pretty easy to detect one battery going to zero. It is harder to detect (using bulk voltage) to detect reverse biased cells in a series string of 10 (don't know if one cell at zero, the rest at higher voltage or all 10 are just drooping under load).

Just to get an idea, the NiCad and NiMH batteries are those typically used on those cheap solar powered LED yard lights. They will last a year (365) of charge/discharge cycles with very little electronics or weather protection. And, depending on your (windsunner's) requirements, if it is low voltage (like 2-4 cells), you might just gut a few old solar powered yard lights from friends/family and put something together quick and dirty from the parts.

-Bill

PS: By the way, the R/C folks will match battery capacities to make sure that they don't have one or more "weak" cells that gets reversed biased during heavy loads. -BB

Patman3 · May 2006

Re: Rechargeable Batt

windsunner wrote:

If the operating voltage of the device to be powered by the battery is less than the specified 12V, do I need to step down the supplied voltage and if so how can I do that?
Regards

For 6 and 9 volts (and in between like 4.5V) I use cigarette lighter plugs from a certain Electronic store in the mall. They have low power (800mA or so) and a high power (2.5A) with those little adaptiplugs. I run my portable phone and a 6V flourescent lite off those adapters.

System2 · May 2006

Re: Rechargeable Batt

Thanks Patman3, I had never used any cigarette lighter plug before, I assumed they work as a DC to DC step-down?

BB and Niel, the batt would most probably used to power the Linksys WRT54G wireless router http://www.linksys.com/servlet/Satellite?childpagename=US%2FLayout&packedargs=c%3DL_Product_C2%26cid%3D1124916802645&pagename=Linksys%2FCommon%2FVisitorWrapper or the Motorola SB5100 Cable Modem http://broadband.motorola.com/noflash/sb5100.html. However, I might use it for other device. Would it be feasible to deploy it on a multi-purpose setup(router, modem, radio, phone etc) with the appropiate step up/down for each device, either powering multiple or single device at any time?

Hope someone would answer my earlier questions regarding overcharging and charging current

Thanks again to All.

BB. · May 2006

Re: Rechargeable Batt

Windsunner,

There are many types of electrical conversion electronics. Some are not energy efficient (they waste heat/energy as they step down the voltage), and others that can step down the voltage, or step up the voltage, or step up/down around the input voltage pretty efficiently (80%-90% or better).

Knowing the source (battery) and the sink (router or wall wort) does help--sometimes, if you are really into saving power, you would look at the router which, according to the specs., is a nominal 12 DC input. However, a 12 vdc lead acid battery will supply power from 10.5 volts to 14.5 volts... And, typically 12 volt computer gear is +/- 5% or 10% tolerance (10.5 to 13.2 vdc if 10%). You may damage the device when the battery is charging and you would have to use some sort of inverter/converter or other device to manage the voltage to the router...

Looking at the devices, the Routher is 12 vdc at 1 amp, and the cable modem is 9 watts (did not see its voltage--other than 120/240 VAC 50/60 Hz). Together, that is about 21 watts of power, or about 2 amps. It would flatten the battery to 100% in a little over an hour. Just to power the devices during the day, you are talking about 25+ watt panel. And if you wanted to power them for 24 hours in a typical sunny area during the summer using a lead acid storage battery:

Preq = 21 watts * 24 hours/day * 1/80% battery charging losses * 1.17% (solar panel "real numbers) = 737 watt*hours per day from a solar panel.

Pbat = 737 watt*hr * 3 days * 1/50% (discharge level) = 4,422 Whrs or 4.422 kWhrs (that is about the physical size of 4-5 car batteries)

Ppanel-load = 737 watts / day + 4,422 (3day storage) / 3 days =1,474 watt*hrs per day (fully charge battery after 3 days of clouds + load in 3 days of sun)

Ppanel-size = 1,474 watts*hrs / 5 hours of full sun = 294.8 watts of solar panels or two 160 watt panels for summer. Could be 3-4 panels for winter (depending on location and weather).

There are still many issues to look at (chargers, inverters, panels, location, what to do during winter/bad weather, is is 24hrs per day, or only during the day, etc.). But this kind of gives you the scope of the problem and the solution +/- 20-30% or so.

You can change any of the requirements that I have assumed, and see how that would affect the components and operations of your setup.

-Bill

niel · May 2006

Re: Rechargeable Batt

"However, a 12 vdc lead acid battery will supply power from 10.5 volts to 14.5 volts... And, typically 12 volt computer gear is +/- 5% or 10% tolerance (10.5 to 13.2 vdc if 10%)."

do know that you will run into this same problem with the nimh batteries as voltages will be around 14.5v at full charge to a set of 10 nimhs.
even your original battery would not have the capacity to last very long with all of the loads you proposed to put to it. maybe consider a much bigger lead acid battery with appropriate ability to charge it as the 5w pv may now be severely underated to charge a bigger battery. the circuitry going to the other items may need another regulator to keep the voltages inline with the specs needed for those items while allowing more for the battery to become properly charged. you could regulate this with diodes from radio shack as each diode will drop the voltage roughly 1/2v. 3 to 4 of them in series would be about right and 4 being better because you aren't close to the edge of other items tolerance range. diodes are rated for typical currents to pass and you should stay within that amount of current. if you need 3amps to pass then use 3amp diodes. if you need 4amps move up to maybe 6amp diodes. note that putting one or more in backwards will stop the current from flowing.

BB. · May 2006

Re: Rechargeable Batt

And, as always, conservation is your friend. The router you listed is a pretty full service kind of guy. Perhaps you only need an access point that had been designed for low power consumption. Or perhaps one of those lowpower *nix boxes with a PCMCI wireless card may use less power...

Again, many ways to slice and dice the problem--depending on what the problem is.

-Bill

Brock · May 2006

Re: Rechargeable Batt

I have the older version of that lynksys router and it is powered directly from my 12v battery bank. I have never had a problem with voltage, but my max was 14.8 and minimum was maybe 11.5v. The router pulls max about 250mA or .25 amps. It does vary in loading depending on what it is doing as well. Maybe the G uses more power, but I wouldn't think that much more.

BB. · May 2006

Re: Rechargeable Batt

Also, you need to measure the average power of any equipment... It is possible that the 1 amp is just a short term peak, and not a long term average.

-Bill

System2 · May 2006

Re: Rechargeable Batt

As BB had stressed, there are lots of factor to consider regarding using or not using volt regulator and charge controller. I did some research on the prices of both, and reckon that it should be more cost-efficient to build my own. Would be grateful if anyone can provide some reference circuit diagrams

Also, if the cost of these devices are much higher than those of the items(batt, phone, router, modem etc) being protected, while the risk of damage w/o them is low, I might consider not using them at all.

Finally, thanks All for your valuable suggestions and comments. I will take time to carefully consider them.

BB. · May 2006

Re: Rechargeable Batt

I am not a power supply engineer, but I have designed and put into volume production various "buck" and 3 terminal (voltage and current) regulators. And I have spec. for external design many larger supplies too...

My recommendation for the "output" regulator side would be to go into a few different manufacturer's catalogs of components. Most of them will include application/design guides, and sometimes even have reference design circuit boards available to bread board them.

If you want to design/build your own, I would suggest a 24 VDC nominal battery voltage (~20-30 volt range), and a "buck" type circuit which will give you anything from roughly 18v down to a couple of volts with the minimum amount of circuitry. They are easy to build, relatively forgiving and stable, have high efficiencies, and have many options available (current limits, voltage cut-offs, soft start and others). Also, going much above 24 vdc, the number of devices that can handle the higher voltages (Controller IC's, FETs, Transistors, etc.) is much reduced, giving you fewer choices. Doing it for a 12 volt to 12 volt regulated conversion will require a more complex buck/boost type regulator.

However, your success will depend on your experience bread-boarding a circuit as any switch mode inverters requires a good ground plane and low resistance current paths. And laying out your own board, etching, etc. can be fun--but hardly time efficient for one device.

If you have not done this before, I would suggest that you stick with a pre-made circuit board. Other designs can give you other options (boost--output above input voltage; buck/boost, voltage above or below input voltage--i.e., 10.5-15 volts in, 12.0 volts out).

If you are only building a hand full of these systems, you can probably look around for the web for a 24 to 12 vdc converter for $0.50 to $2.00 a watt retail pricing.

Cost effectiveness is a difficult thing to quantify sometimes. If you can get a 25 watt converter for less than $50, and you don't blow up a $100 router, and flatten a $100 battery, plus lose the wifi services you were trying to provide in the first place--then saving $30 is not a big deal.

If my battery/panel calculations are approximately correct--purchasing a good solar charge controller is going to save you hundreds of dollars in battery replacement costs over a few years by preventing over and under charging of the batteries (a well maintained battery set should last you 7-15 years) plus can include automatic generator starting, and charge management information (current/voltage/capacity meters, or even data logging).

-Bill

Rechargeable Batt

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