We have designed an led lamp that is run by a constant current source. The constant current source requires a minimum of 6.2 volts. With a 9 volt input to the constant current source the current draw is 400 ma. As the voltage input drops (battery voltage) the current will go up (constant current = constant power). We need a rechargeable battery and charger for this application. We would like the battery life to over 1/2 half hour and in a small package. What is a good choice for this application?
Battery Reguirement led load
- Thread starter Bronx68
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LEDninja
Flashlight Enthusiast
9V*400 mA=6.2V*580mA =3.6V*1000mA minus the loss of your power source at the LED.
2*RCR123A (also known as 16340) should work. You will have roughly 1 hour runtime as the batteries are ~700 mAH.
There are smaller batteries but finding replacements when the original set dies and finding chargers would be difficult.
6.2V minimum voltage means 2*3V CR123A primaries can not be used in an emergency. Maybe you should look for a power source that has a lower minimum voltage.
For example a 3*AMC7135 based buck regulator will work down to 4V input. Don't know if the output waveform is good enough for your application.
http://www.candlepowerforums.com/vb/showthread.php?t=192925
2*RCR123A (also known as 16340) should work. You will have roughly 1 hour runtime as the batteries are ~700 mAH.
There are smaller batteries but finding replacements when the original set dies and finding chargers would be difficult.
6.2V minimum voltage means 2*3V CR123A primaries can not be used in an emergency. Maybe you should look for a power source that has a lower minimum voltage.
For example a 3*AMC7135 based buck regulator will work down to 4V input. Don't know if the output waveform is good enough for your application.
http://www.candlepowerforums.com/vb/showthread.php?t=192925
Ledninja,
This is an existing design that needs to be modified to also have battery operation. We looked at the RCR123A batteries but I believe they are only 3 volts each so we would only get 6 volts with 2 of them. Is there a higher voltage version of this battery? Can two of them be charged at a time from and external charger?
Also we would need to leave the batteries in the unit and connect the charger externally to charge them. So if we put 3 of them in series to get 9 volts, I'm not sure how we would be able to charge them externally (i.e. connect charger through connector to batteries in unit).
I am considering a 7.4 volt LiIon battery pack. We don't have much experience with LiIon packs and just learned about excessive shipping charges for LiIon batteries.
Thanks,
Bronx68
This is an existing design that needs to be modified to also have battery operation. We looked at the RCR123A batteries but I believe they are only 3 volts each so we would only get 6 volts with 2 of them. Is there a higher voltage version of this battery? Can two of them be charged at a time from and external charger?
Also we would need to leave the batteries in the unit and connect the charger externally to charge them. So if we put 3 of them in series to get 9 volts, I'm not sure how we would be able to charge them externally (i.e. connect charger through connector to batteries in unit).
I am considering a 7.4 volt LiIon battery pack. We don't have much experience with LiIon packs and just learned about excessive shipping charges for LiIon batteries.
Thanks,
Bronx68
Lighthouse one
Flashlight Enthusiast
Bronx: Primary 123 batteries are 3 volts. The chemistry used in RCR123 batteries gives 4.2 volts fully charged...which drops off to about 3.5 volts when depleted. You can use (2) RCR123 or any other li-ion batteries. You may even consider the 18650 size (2) if the larger size is not a problem. They will provide much more runtime than requested.
Two batteries are about $10.00- and a charger $10.
Two batteries are about $10.00- and a charger $10.
Last edited:
In-place charging without removal of the battery pack adds all sorts of "interesting" issues on top..
A 7.4 or 11.1V Li-Ion *pack* usually has balance connectors, and low cost balancing LiPo pack chargers are readily available. .
If it actually stops hard at 6.2V, it might be fine.. If it doesn't start at <6.2V, but a lit device continues to operate with diminished output below 6.2, there's a risk of overdischarging.
A pair of RCR123 would give you overdischarge protection and overcharge protection, but charging them in series will push them out of balance over time, reducing runtime..
A 7.4 or 11.1V Li-Ion *pack* usually has balance connectors, and low cost balancing LiPo pack chargers are readily available. .
If it actually stops hard at 6.2V, it might be fine.. If it doesn't start at <6.2V, but a lit device continues to operate with diminished output below 6.2, there's a risk of overdischarging.
A pair of RCR123 would give you overdischarge protection and overcharge protection, but charging them in series will push them out of balance over time, reducing runtime..
Shadowjk,
Here is what I'm thinking: I'll build in a switch that removes the load when the 7.4 volt Li Ion pack is charging so there will be no load on the charger while the batteries are charging.
You mentioned "A pair of RCR123 would give us overdischarge protection and overcharge protection, but charging them in series will push them out of balance over time, reducing runtime.." I don't understand why a pair of these batteries will give overdischarge and overcharge protection. Please explain.
I am looking at 7.4 volt Li Ion pack with built in protection IC that prevents overcharging and discharging. I can send you a link if you are interested in it. Do you see any problems with this approach?
Here is what I'm thinking: I'll build in a switch that removes the load when the 7.4 volt Li Ion pack is charging so there will be no load on the charger while the batteries are charging.
You mentioned "A pair of RCR123 would give us overdischarge protection and overcharge protection, but charging them in series will push them out of balance over time, reducing runtime.." I don't understand why a pair of these batteries will give overdischarge and overcharge protection. Please explain.
I am looking at 7.4 volt Li Ion pack with built in protection IC that prevents overcharging and discharging. I can send you a link if you are interested in it. Do you see any problems with this approach?
