Lithium battery recommendations for custom LED light design

[swschultz77]

As a mechanical engineer I'm in the process of redesigning a portable LED-based lighting device that originally ran four 5mm LEDs off of 4-AAA batteries. The new device will have both motion and light-sensing built into it and as suggested will run off of a rechargeable battery or batteries. The light will be only used under limited lighting situations when motion is detected. The light will most likely again have four or five super bright white 5mm LEDs. My knowledge of Lithium batteries is quite limited. I'm trying to determine what type of Lithium battery will work best to drive the LEDs. I want the light to be as small as possible. The circuit will more than likely contain something such as a charge pump or boost converter. I considered using something such as 2-18650 batteries but at the same time due to the overall size of the batteries they would require the device to be substantially larger than desired. The plan is that the batteries are not meant to be replaceable if they wore out but instead the entire device would be replaced. The goal is to make the light/batteries last a minimum of 3 years.

So my main limited knowledge questions are these:

- Are there any what I'll call non-standard lithium power packs (possibly thin and rectangular in shape, maybe like a cell phone battery) that are reasonably priced that can provide 6 to 7 or so volts to drive these LEDs? Is there any chance of finding a small package lithium battery package such as this that is fairly comparable in cost to similarly powered off the shelf pair of lithium batteries?

- Is there any chance of using a single 3.7V battery and the appropriate charge or boost circuitry that will drive the LEDs a decent length of time without constantly needing to recharge the battery?

- Can I expect to find any semi-standard pair of off the shelf Lithium batteries (10440/14650/16340/18650?) that will last the desired 3 years and drive the LEDs properly? With the limited research I've done I was considering a pair of 16340s, but don't really know if they're going to do what I need. As mentioned 18650s sound great other than their massive size. I'm probably missing some information for folks to fully answer my questions but any and all input is deeply appreciated.

Thanks so much!

 

[swschultz77]

Does Anyone have any insight that they can provide on my post?

Thanks for your help!

 

[swschultz77]

I'm STILL looking for input on my original question above. I know there are folks here that have knowledge of such things. What might it take to get some input on this? In conjunction with the original question, I'm now considering using SMD LEDs instead of the originally planned standard 5mm LEDS. Battery-wise 17500's are also under consideration. PLEASE someone help point me in the right direction on this. THANK YOU!

 

[xxo]

I would think that using 3 AA Energizer L91 ultimate lithium (which are primary/non rechargeable batteries) would be better. Why do you want to use rechargeable Li-ion cells for your application?

Energizer ultimate lithiums should have comparable energy density and specific energy to Li-ion cells and 3 L91's will provide the same or a little higher Watt hrs as a high capacity 18650.

 

[Lynx_Arc]

The problem is without a lot of specs I really don't know if you need a whopper of a battery setup to drive LEDs for 3 years or what.
18650s have the highest power density of rechargeable batteries at this time but 21700s are close to that density but larger by a bit.
Also need to know beam pattern flood or throw and lumen output desired. One huge issue on any type of light is the standby current as motion detecting lights have to draw power constantly even if you have it set to not turn on when it is daytime there still can be a substantial power draw that could deplete batteries alone by itself. The highest efficiency LEDs have around 200 lumens/watt while SMD chips are 80-100 lumens/watt and 5mm can be in that range or a bit higher but unsure if they are anywhere closer to 200 lumens/watt.
I strongly recommend not using a boost circuit in the design even a very good one will still lose power in converting. Another thing is what voltage does your sensor circuitry require.

 

[swschultz77]

The problem is without a lot of specs I really don't know if you need a whopper of a battery setup to drive LEDs for 3 years or what.
18650s have the highest power density of rechargeable batteries at this time but 21700s are close to that density but larger by a bit.
Also need to know beam pattern flood or throw and lumen output desired. One huge issue on any type of light is the standby current as motion detecting lights have to draw power constantly even if you have it set to not turn on when it is daytime there still can be a substantial power draw that could deplete batteries alone by itself. The highest efficiency LEDs have around 200 lumens/watt while SMD chips are 80-100 lumens/watt and 5mm can be in that range or a bit higher but unsure if they are anywhere closer to 200 lumens/watt.
I strongly recommend not using a boost circuit in the design even a very good one will still lose power in converting. Another thing is what voltage does your sensor circuitry require.

===
Thanks folks for your responses. As suggested above I don't the user to have to deal with replacing the batteries, or taking them out to charge and putting them back. For this reason the light will obviously require charging circuitry. I do realize that there will be a constant drain on the battery(s) with the motion sensing requirement. I obviously also want this drain to be minimized as much as possible. Beam pattern-wise I don't really need much in the way of throw. It's much more important that the light floods the area close to the user.

Regarding the LEDs themselves, I see that a number of the SMD-type LEDs have built-in lenses. If I don't require much in the way of throw, how important is the reflector configuration on such a light? Might the lenses suffice to the point where reflectors aren't required at all if just flooding the surrounding area?

I'm talking from a point of very limited expertise on lights, but if a boost circuit isn't recommended, what might you suggest efficiency-wise taking into consideration the other requirements above that is able to provide enough current to drive four or five LEDs off of whatever rechargeable battery configuration ends up being used? Obviously something such as a single 18650 isn't going to to the trick without something to bump up the power in some fashion.

Thanks up front for any and all additional input!

 

[Lynx_Arc]

If you don't require throw then any LED will do, you just need to decide how many lumens is required from them.
If this is an emergency backup type light that won't have someone checking it and manually recharging it then likely lithium ion and nimh are out you will likely need to resort to using nicad or lead acid and have it constantly trickle charging. Depending on the runtime and lumens needed before recharging (manually) 18650s or multiple cells in parallel can suffice. I have single LED headlamps that can do about 100 lumens for 15 hours before needing a charge on a single 3000mah cell and 100 lumens can light up a decent area well enough to see with maybe not a huge area or be able to read an pocket bible or something though at a distance.

 

[Dave_H]

As a mechanical engineer I'm in the process of redesigning a portable LED-based lighting device that originally ran four 5mm LEDs off of 4-AAA batteries. The new device will have both motion and light-sensing built into it and as suggested will run off of a rechargeable battery or batteries. The light will be only used under limited lighting situations when motion is detected. The light will most likely again have four or five super bright white 5mm LEDs. My knowledge of Lithium batteries is quite limited. I'm trying to determine what type of Lithium battery will work best to drive the LEDs. I want the light to be as small as possible. The circuit will more than likely contain something such as a charge pump or boost converter. I considered using something such as 2-18650 batteries but at the same time due to the overall size of the batteries they would require the device to be substantially larger than desired. The plan is that the batteries are not meant to be replaceable if they wore out but instead the entire device would be replaced. The goal is to make the light/batteries last a minimum of 3 years.

So my main limited knowledge questions are these:

- Are there any what I'll call non-standard lithium power packs (possibly thin and rectangular in shape, maybe like a cell phone battery) that are reasonably priced that can provide 6 to 7 or so volts to drive these LEDs? Is there any chance of finding a small package lithium battery package such as this that is fairly comparable in cost to similarly powered off the shelf pair of lithium batteries?

- Is there any chance of using a single 3.7V battery and the appropriate charge or boost circuitry that will drive the LEDs a decent length of time without constantly needing to recharge the battery?

- Can I expect to find any semi-standard pair of off the shelf Lithium batteries (10440/14650/16340/18650?) that will last the desired 3 years and drive the LEDs properly? With the limited research I've done I was considering a pair of 16340s, but don't really know if they're going to do what I need. As mentioned 18650s sound great other than their massive size. I'm probably missing some information for folks to fully answer my questions but any and all input is deeply appreciated.

Thanks so much!

With a single Lithium-ion cell and the right LEDs you might get away with direct-drive which does not require a boost circuit. There is not much voltage overhead to regulate LED current: cell down to about 3.4v when it's nearly discharged, and LED forward voltages typically up to 3.0v or a bit higher. This would have all LEDs in parallel. If LEDs are properly matched you may not need any current-
balancing, which would add overhead; or multiple drivers. Could be tricky but is do-able.

You need to decide on some LEDs with correct brightness, tint, beam angle, and forward voltage at the current you need to run at. 5mm leaded LEDs are still around but for a new design, consider SMT (chip) or perhaps DIP-style. Solar garden lighting has been switching to SMT.

Alternative is to use a 1-cell boost converter typically used as backlight driver, with LEDs in series. Current balance is not an issue. A number of vendors offer these, you'd need to check: TI, Analog Devices, ON Semi, Microchip, Intersil etc. Many can easily drive up to 5-6 series LEDs.

Two Li-ion cells in series need to be managed, charging and discharging, which is added complexity. With this setup you could run LEDs two in series, two parallel sets with direct-drive and some form of current limiting, even resistors.

Small devices often use flat pouch-style Li-ions which are thinner than cylindrical cells e.g. 18650. I've seen small ones with 350mAh or so; ones the size a matchbook with 800mAh or so.

Also, instead of individual LEDs have you considered COB? It is a single assembly, usually thin metal plate, with multiple LEDs premounted/wired. The cost can be quite low as they are common in cheap flashlights, even dollar-store items.

Dave

 

[Dave_H]

By the way, one of the biggest design flaws I have found in battery-powered LED devices
having remote control (IR or RF) or PIR detection, is excessively-high standby current.
Good designs are in the tens of micro-amps, or at most the low hundred's. I have measured some
as high as 1mA or higher, which will run a set of AAA's down in a matter of weeks, even if the device
is never activated; even less time if it is.


Dave

 

[Lynx_Arc]

By the way, one of the biggest design flaws I have found in battery-powered LED devices
having remote control (IR or RF) or PIR detection, is excessively-high standby current.
Good designs are in the tens of micro-amps, or at most the low hundred's. I have measured some
as high as 1mA or higher, which will run a set of AAA's down in a matter of weeks, even if the device
is never activated; even less time if it is.


Dave

+1 to this. I've had several tap lights that ended up in the trash because one had a 1ma standby current 3 weeks later the batteries were dead and another set that were radio remote controlled lasted not even 3 days off the heavy duty batteries supplied with it making them completely useless because you would have to install/remove batteries for each use making for little need of a remote. I've had remote controlled Fluorescent lanterns that told you to turn them OFF instead of standby because of the parasitic stand by drain.
I bought a used CFL Rayovac 8D lantern that worked fine when I got it but 3 days later the new batteries in it were dead I had to ship it back to get it replaced. I since opt for clicky switches on stuff by default I have to read up on some devices to make sure they run for a long enough time before needing recharged.

 

[Dave_H]

===

Regarding the LEDs themselves, I see that a number of the SMD-type LEDs have built-in lenses. If I don't require much in the way of throw, how important is the reflector configuration on such a light? Might the lenses suffice to the point where reflectors aren't required at all if just flooding the surrounding area?

Regarding beamwidth of SMT LEDs you can find them with and without "domes". Common beamwidths are 60 degrees and 120-130 degrees, which is defined as angle where the optical power drops to one-half, so there is still light outside of that. You may not need any reflector but might have some behind or to the sides if it helps.

Check out LED companies such as Kingbright.

Dave

 

[Dave_H]

Direct LED drive (note that is not directly from the battery to the LEDs) with some form of current limiting is probably
the simplest and likely OK for efficiency (around 80%). It could use a constant-current circuit, or resistors. With the latter,
you would notice some drop in brightness as cell voltage goes down but might be acceptable. Best to set up a simple
circuit and check with LEDs you intend to use.

If using a boost converter, the standby (quiescent) current needs to be low to avoid excessive cell rundown.

I still recommend avoiding series cells, it would be my last choice. Cases for series cells are devices such as two-way
radios and larger flashlights, which have high current drain and need or benefit from higher voltage.

For Lithium-ion charging I can't recommend specific circuit or chips, but it's so widespread in products that the solution
should not be complex or expensive. Not rocket science but you need to know what you're doing, with Li-ion. A small
device I opened up uses a tiny 5-pin IC with a few supporting components.Every USB phone charger pack uses a step-down
charge circuit combined with a boost circuit to get output voltage back up to 5v. That may be overkill for the setup being
considered. Of course required circuit depends on cell capacity, charge rate, and charging time.

That's why I recommend OP comes up with a spec sheet (with actual numbers) if you don't have one already, for this
(re-)design. It could prevent a lot of spinning of wheels.

Dave