Charge/recharge battery backup system.
Marky-Mark
Posts: 26
Can anyone point me at a recharge setup for batteries?·
I have an electronic key pad locked box and it uses 8 AA batteries.· The thing is supposed to last a year on a set, but it never does.· I would like to change the setup to run off a wall transformer, and have a set of rechargeble batteries in the system.· The setup needs to do all of the following:
I am just learning about electronics and micros, and could use a pointer in the right direction.
Thank you.
I have an electronic key pad locked box and it uses 8 AA batteries.· The thing is supposed to last a year on a set, but it never does.· I would like to change the setup to run off a wall transformer, and have a set of rechargeble batteries in the system.· The setup needs to do all of the following:
- run off the wall transformer (DC power)
- monitor the rechargeble batteries and charge when needed (should be infrequent)
- when wall power fails, switch to internal batteries
- do so without disrupting internal electronic combo
I am just learning about electronics and micros, and could use a pointer in the right direction.
Thank you.
Comments
Regards
Chet
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I have done what· Chet is talking about and it·work pretty good and it is easy to do
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Sam
After reading up on charging circuits, I am thinking of changing direction.I think the right approach is to have the wall power run the whole contraption and have non-rechargable batteries as back ups.· A battery status indicator/monitor will be added and will test the system periodically (probably once a week or so)·and sound an alarm if battery replacement is needed.· This reduces the problem to finding the right power supply, probably post regulating it to supply a real 12V, adding a relay, and caps for the switch over period.· I have a feeling the keypad already has a 5V regulator, so I might not have to even worry about post regulation.· I have to figure out also if the device allows for battery change without loosing memory.· (I know it looses it's code if the batteries run dry for extended periods, but I don't know if changing the batteries quickly will erase the memory)
Next step will be disassembly and figuring out the existing circuit.
I am still very open to sugestions, this will be my first electronics hack
· You do not need to make this hard to do ,you take the 8 AA batteries
and you take a LM317 Reg......
Set it up·in the current mode for 20 mil amps this will
keep the batterys charged and ready for when ·you loss your·power
with out over charging them
The only draw back is in this setup is the batterys charge very very·slowly a this rate
If you loss the line power for a long time
·Then it will take a·long time·to charge back to full power if you can live with
this i do not see an reason that this will not work
One· thing when you chose a wallwart get one that let say 300 milamps
I have seen some that are 100 milamps this would even be better one to use
I hope this help you in what you want to do
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Sam
Post Edited (sam_sam_sam) : 9/13/2007 3:38:19 AM GMT
That might be the easiest way to go about it, thank you for that sugestion!
I will probably add the current required by the device to keep on to the supply current, such that the batteries will see a total of 20mA. What battery technolgy, NiCad?
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Here where i saw a 100 milamp tranformer at the one i would use if i was doing this project
tranformer
http://www.goldmine-elec-products.com/prodinfo.asp?number=G8996
LM 317 Reg
http://www.goldmine-elec-products.com/prodinfo.asp?number=A10360
The only thing is that goldmine-elec has a $15.00 MIN order
but you may·find other thing·there for your project i have· order many time in the past they are very good about shipping order·and you will get it in·a few day
I hope this help you in what you want to do
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Sam
Post Edited (sam_sam_sam) : 9/13/2007 8:59:37 PM GMT
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- Rick
Also, it would provide you with the kind of battery that is really made for long-term trickle charging between interim uses if that is what you really want.
Battery chemistry creates some undeniable facts - for longevity, batteries prefer to be completely charged, a constant charge always creates heat and some gases that made build up to excess, and newer battery technologies are intended more for smallness more than ruggedness. The manufactures have enjoyed creating huge amounts of confusion about the proper charging of batteries so that consumers no longer make DIY chargers. They want to sell you a charger at a huge profit rather than build one for pennies. But, the Lead Acid gel cell is still an easy DIY charger project and one of the best documented.
Anyway, if you have a near constant 120VAC source, then it can allow you to rely on a much smaller Lead Acid gel cell battery as the AC will provide power at all times unless there is an outage. I have such a 6 volt that is smaller than the 8AA batteries which is rated at 1.2AH. At the same time the Ac is providing power to the device, it is also trickle charging the standby battery. As you can see, the standby battery need only provide power for a day or so until the 120VAC is restored.
Anything in between being fully battery dependent and being fully AC dependent except for outages is likely to be unnecessary. Toss a coin and decide one or the other.
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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········
Post Edited (Kramer) : 9/16/2007 5:13:48 PM GMT
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David
There are 10 types of people in this world,...
Those that understand binary numbers, and those that don't!!!
Before we go building a band-aid for something, however, it is curious that a product advertising a battery life of 1+ year is failing after only 3-6 months. Do you use this electronic lock a lot? Is it exposed to unusually high or low temperatures? Have you tried different brands of batteries? Could the bolt in the lock be sticking or jamming, causing excessive battery drain? Just want to make sure we cure the problem, instead of covering it up.
Now by looking at the spec sheet the typical current consumed in wait mode·is .45mA at 5v, and .25mA at 3.3V.· That is an average power, and if mine consumes above average, it would explain the frequent power outs.· I am not sure what voltage the micro runs at.· According to Wikipedia an Alkaline could have 3000mAh of life if the current is small.· .45mA translates roughly into nine months.· But if the device runs the motor to open the latch, the current draw has to be much larger and that would eat into the life expectancy.·
The manufacturer warns batteries should be changed once a year regardless, and that batteries should last 9 months to a year.·
Incidentally, the second generation of the device can be powered of an AC adapter, but still requires regular batteries.· This leads me to believe that they use a cap and a relay, and no recharge circuits.
Next I will need to get it powered up and figure out the schematic, and if the latch motor and v-reg could tolerate 9.6V (8 rechargable batteries) or if I need to add a couple to boost the power.· If it can't deal with lower voltage, I am thinking of going with the relay and cap approach.
I will look into sealed lead acid batteries, but I need the battery to not emit gases.
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Considering what the manufacturer did, using a wall wart and a changeover relay would be a very simple and yet elegan solution. One relay and a steering diode, and that's it. I'm sure someone could come up with an all solid state circuit based on a couple FETs as well. And, if the wall wart puts out a higher voltage than the 8 AAs, you might also be able to simply use two steering diodes (just make sure you use a diode with a very very low reverse leakage current, so you don't wind up charging your AA stack!) Another option would be to use Lithium AAs, if the circuit can tolerate 13.6V (Lithium AAs while marked "1.5V" start out at about 1.7V). The only advantage the lithium AAs give you though is extended storage life in temperature extremes - if the batteries are located inside, then I'd just go with the alkalines.
As for your current consumption, 450uA at 5V and 250uA at 3.3V sounds like the current consumed by the microcontroller itself. What about a voltage regulator? That could quickly add up - and personally I think they could have designed an electronic lock circuit that had a standby current of less than 450uA, but that's just me.
I am still wonder if this really isn't a 6volt DC set up with a low drop out regulator. In that case the 8AA are providing 6AH of capacity.· The following assumes a 12volt system is required, but it gets easier with a 6volt system.· Seems like you should measure some in-place voltages.
Those 7805 regulators commonly waste 30% of a battery's power in the heat generated during regulation. From the above text, it seems like the circuit needs the 12 volt for interface purposes though.
Your biggest dilemma is getting a small enough battery to fit your housing conviently. As mentioned before, the smallest I have is a 6volt 1.2AH. And that is a bit larger than 8AA batteries. There may be a 12volt .6AH of similar size.
The beauty of the Sealed Lead Acid is that you can easily sustain a very small trickle charge.
NiCad and NiMh don't like a small trickle [noparse][[/noparse]they prefer about 1/12th the AH rating], they age quickly on slow trickle. Lithium are very high density and quick charging, but were never intended for trickle charge and need very specialized charger- maybe explosive if handled wrong.
You might find a 12v rechargible photo battery. Or, you might just use a 12v primary photo battery as a reserve and forget the trickle charging. It certainly would allow you to have a compact solution. You could even leave the 8AA cells as a·non-charging reserve by providing one blocking diode. Very clean and simple.
Furthermore, you can actually use a smaller AmpHour capacity than you are now using because the usual power source is the 120VAC. So instead of AA cells for months that are rated at maybe 3AH for alkaline, you could easily get by with as little·as 0.6AH, maybe less. After all you only need battery power for a couple of days [noparse][[/noparse]if you power goes out for a long weekend]. Providing a low battery light would help in those situations.
The trickle charge is usually described as a fraction of the AH rating. You may check in The Art of Electronics, but 1/30 to 1/50th is okay for Lead Acid. So you supply need only provide 1/30 of an amp plus the current that the device requires. I suspect that 100 ma wall wart at 15VDC would be more than enough, but you will likely find that there is nothing less than 500ma available.
The real question is if the device will tolerate a constant 14.4 volts [noparse][[/noparse]required for the trickle charge] or if you have create a dual supply of 14.4 to the battery and 12.0 directly to the device. Much can be done with diodes alone. You also should consider having a current limiting device to the battery so that it is a true trickle charge. A single 2n2222 approriately configured can work as a current limiting device and will drop the 15v to 14.4 because of its causes an internal .6 diode voltage drop. Thus the battery won't overheat and have an early death.
So the real trick is to get from·15 to 12vdc. Lets see that is 3.0volts difference. At roughly .6 volts per diode, a string of·5 rectifier diodes in series [noparse][[/noparse]over-rated at 1amp or more] will deliver roughly 12volts to the device.
Similarly a string of 4 rectifier diodes acts one blocking diode will make sure the battery isn't getting a second charging source, but will deliver a 12volt maximum. If you have any doubts about it being enough, this string might have less diodes and the battery will deliver a higher intial voltage. Much depends on your device being able to tolerate higher than 12 volts. Since Alkaline batteries are often 1.7volts when fresh, I don't think you will have too much trouble. But double check for safety's sake as I am making an assumption.
At this point, I would try to verify if I had 6 volts or 12 volts. The 12 volts doesn't make any sense to me.·And 6 volts seems a bit tight with a regulator.··The device might be using two diodes to drop to·4.8 volts which is reasonable.
Then I would strongly consider forgeting the trickle charge as too much of a modification.· I would just have a battery standby with a low battery warning light to keep you aware of status. You would likely have to still change them once a year.
But of course, the choice is up to you.
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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········
Post Edited (Kramer) : 9/16/2007 6:30:59 PM GMT