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Thread: Rechargeable lantern

  1. #1
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    Default Rechargeable lantern

    I've been wanting to make a type of rechargeable lamps for some time now, but I'm uncertain about the charging part.

    What I want is basically to drive a couple of LEDs from an 18650 battery, which should be easily charged by connecting the lamp to a power source. The lamp should also continue to operate while the battery is charged and be able to handle being connected indefinitely.

    My current plan is to use a protected 18650 battery with resistors as current limiters for the LEDs and have the battery charged at a low current (again limited by a resistor) by a 5 V power supply.



    I want to keep it as simple as possible, hence the use of a protected battery. I don't know how the protection circuit works though. Does it completely cut off the current to the battery once its voltage exceeds a threshold (4.2 V)? Are they reliable? We're talking cheap chinese (trustfire) batteries here.

    Any feedback is very welcome.

  2. #2
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    I think you'll need more than that. Googling shows a single Li ion cell, nominal 3.7V, and the CR123A's I have and a charger for same work by putting current in during a second or so, then sampling the voltage for a few milliseconds, repeating till threshold is passed. As the discharge curve is rapid to fall to around nominal voltage, you might get by with a zener circuit to keep the input voltage strictly at 3.8V max. That way it should never exceed safe limits, and still give you 90% of full charge at best. An LM317 regulator would be better, too, but a zener circuit might be adequate if you test carefully to see what voltage results for the current you find there. Try it on a spare cell, and do it in a place that won't mind if one explodes.

    Edit: I'm assuming it is wise to provide your own protection. Best not to rely on existing stuff, for these batteries.
    Last edited by The_Doctor; 12-19-2008 at 18:10.

  3. #3
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    If they are NiCd or NiMh batteries, they wouldn't mind being charged from a resistor, but if its a lithium battery, then you need to think about the charging a little more. I'm not sure of the protection circuit in thee battery, but I wouldn't really want to trust it, going by how hot the lithium fires are.

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    Ok, so if I replaced the lithium battery by 3 NiMH cells such a simple circuit would be safe?

    I kind of like these lithium batteries though. Only a single one needed to get a suitable voltage for driving LEDs and very good capacity. How about if I use a 3.6V supply instead? Is it OK to maintain such a voltage over a lithium battery for a long time?

    I just looked at my CR2 (3 V) battery charger. It seems it is simply a switched 3.6V 200 mA power supply. No real control circuitry as far as I could see. That should mean I could just drop down the supply voltage to 4.2-4.3 V with a normal diode.

    Granted, cheap chinese lithium chargers might not be what I should be basing my designs on. Especially considering my 18650 charger recently blew up (yes, the charger, not the battery). It was only the power supply though, it still works fine if I supply it with 12 V.

  5. #5
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    Yeah, if you replaced them with NiCd's or NiMh's it would be fine.

    Then again, you mentioned you only want them to slowly charge. I guess if you chose the resistor, so it only trickle charged the lithium battery, it should be fine.

    Lithium batteries dont like being fully discharged, so usually there is a little circuitry that cuts off the output if the voltage gets below a certain point.

    Lithium batteries are fine if you treat them properly. Like you said, if you are just wanting to slowly trickle charge them, it should be fine.

    I believe the protection circuitry in the battery should have a low voltage cut-off, but i'd try to confirm it before you waste batteries.

  6. #6
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    You could look at some of the A123 LiFePO4 batteries, or the NanoSafe batteries. They have a more conventional charging profile, come with smart controllers, and don't explode if you overcharge them. Capacity is about the same or slightly better than the conventional Li-Mn batteries.

  7. #7
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    Quote Originally Posted by The_Doctor View Post
    I think you'll need more than that. Googling shows a single Li ion cell, nominal 3.7V, and the CR123A's I have and a charger for same work by putting current in during a second or so, then sampling the voltage for a few milliseconds, repeating till threshold is passed. As the discharge curve is rapid to fall to around nominal voltage, you might get by with a zener circuit to keep the input voltage strictly at 3.8V max. That way it should never exceed safe limits, and still give you 90% of full charge at best. An LM317 regulator would be better, too, but a zener circuit might be adequate if you test carefully to see what voltage results for the current you find there. Try it on a spare cell, and do it in a place that won't mind if one explodes.

    Edit: I'm assuming it is wise to provide your own protection. Best not to rely on existing stuff, for these batteries.
    I'm going to rephrase this. I'm not implying it might explode. This isn't a careless suggestion, you should always test Li ion systems with full caution if you set up a new one. Saying this doesn't mean I think it will go wrong, you just have to be rigorous.

    So you can't just use a lower current, with a higher resistance. And using 3.6V isn't going to put any significant charge to a 3.7V cell. I suggested 3.8V in my post, because that will. The voltage will closely approach the 3.8V value and never reach the normal 4.2V upper limit used in most fast chargers (note, those switch off the charge at that point). That means you can leave it connected indefinitely. It might be best to use a current limit resistor to limit initial charging current to 400 mA, but with the small difference between cell voltage and 3.8V you might not need one. It won't be a fast charger but it will be a safe one. But that upper voltage is critical. You might improve performance with careful adjustment to 3.9V, but it must be strictly controlled, and never reach the full 4.2V unless your charger has an auto-switchoff if it gets there (either that or strict current limiter). I think using an LM317 regulator is best, because it's far easier to set a safe value than to try to 'economise' with a simple zener circuit. And if you wire it with an adjustable pot, make sure the Adj pin goes LOW, if the pot wiper fails, not high or open circuit, otherwise the regulator output voltage will go high.


    EDIT:
    http://www.batteryuniversity.com/partone-12.htm
    That confirms a strict limit of 4.0V (4.05), stating that trickle charging causes risk to safety by plating metallic lithium if the voltage exceeds this. I suggest 3.8V to leave a margin, as temperature affects this. That page says 3.8V lets you get around 50% charge, but that's good, it's close to the 40% for nominal best storage life. Anyway, this is about the only way I'd consider leaving a Li-ion battery on a constant maintenance charge.
    Last edited by The_Doctor; 12-20-2008 at 19:48.

  8. #8
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    Now I've finally built the thing (and given it away, unfortunately forgetting to take pictures). I ended up using a voltage regulator anyway to get the right voltage for "safe" charging of the battery. Here's the schematic.



    The voltage regulator gives 3.86 V, which means a maximum charging current of 1.1 A (since the low voltage protection of the battery kicks in at 2.75 V).

    This mixture of LEDs gave a nice yellow white, albeit with a color temperature far higher than a candle. I ended up covering the LEDs with cotton to diffuse the light. The battery (Trustfure "2.5" Ah) lasts at least a whole night. I think this kind of light source is quite useful as a night light and in case of a power outage.

    Thanks everyone for your help. That last link from doc was especially helpful and convinced me to change the design.

  9. #9
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    Neat report. Nice to see that the optimal voltage is so easily had with preferred values too. I love it when a plan comes together like that. Nominal output is actually 3.8257 but your value is within 1% deviation from that so using 1% metal film resistors should make it safe repeatedly if you make more. Might still need to watch what temperature does though. I suggested 3.80V as a safe margin, and only watching real performance for a while will show whether that margin can be tightened safely because temperature and extremes of value tolerance range might conspire to do something nasty. If it does, then either cherry-picking resistors, or adding a series Schottky diode should help. (Diode should drop just 100-200 mV).

    Once I feel like doing some actual work again I'll put together something similar and see how it works with a few Li-ion's I have here (detailed in another thread in this sub-forum). They're PDA batteries for a device I haven't got any more so they're in good shape, and also entirely dispensible for a good cause.

    EDIT: Related to that circuit, I'm going to build a modification of the linear current regulated driver I put in my LED lamp. It's dimmable with a 10K pot with a 1K2 resistor to change the pot law so fading is nicer for light. Standard practise is to use SMPU's for efficiency, but linear is good so long as the LED chain's Vf plus that of the regulator are below the supply voltage by a small margin. You won't see 92% or better but you might still see over 80%, which ranks it with many SMPU methods, but it's simpler, and there's no flicker. If you're using LED's to light machine tools, this is crucial. My modification will be designed to almost eliminate the extra 1.25V drop across the current sense resistor by displacing 90% of that voltage drop into the control network. I think the therory is good but I'll not post details till I confirm it. Anyway, if it works, it raises the efficiency of a fadable linear current controlled LED driver to SMPU levels of performance, if you also use a low dropout regulator and match the device with the supply voltage. If optimising for Li-ion or 12V lead acid, this should be easy because LED Vf and the voltages of those battery types are very predictable.
    /ramble
    Last edited by The_Doctor; 12-27-2008 at 07:26.

  10. #10
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    Quote Originally Posted by The_Doctor View Post
    Nominal output is actually 3.8257 but your value is within 1% deviation from that so using 1% metal film resistors should make it safe repeatedly if you make more.
    There's an extra 34 mV from the R2*I[ref] error term. I use 5% resistors, but I checked the output voltage of the regulator and it was 3.85V or so.

    I'm probably going to make more of these kind of lanterns, so I procured a few TI BQ2057 "Low Dropout Linear 1-cell Li-Ion Charge Controller with AutoCompTM, 4.1V". They seem fairly easy to use and provide all the control needed to safely and efficiently charge Li batteries. Will need to etch my own PCB though, as they're SOIC. Never done that before.

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