Battery powering NeoPixels

[kadamr]

Hi,

I'm trying to power a strip of 60 NeoPixel LEDs using as lightweight and compact batteries as possible. I've looked at all the relevant tutorials on this site but I haven't found a solution that quite works perfectly. The following are my criteria:

- The NeoPixels require 5 V and up to 3.5 A (although 2 A should cover it for continuous usage)
- I need only power the LEDs for about 40 minutes (for consideration of capacity - mAh)
- Batteries must be no wider than about 40 mm but aren't limited in length
- Batteries must be rechargeable
- Recharging must be done within the product using either a USB or DC input *so this rules out using AA batteries*
- Whilst recharging the LEDs must still be able to function (so no 'run-or-charge' solutions!)
- Rocker switches only, no toggles!

So I could use one 3.7 V cell and convert/step-up to 5 V but then I couldn't output 2 A and the batteries would run out fast but charging would be easy and I could run the LEDs at the same time.

Or I could use two 3.7 V cells in series to get 7.4 V and convert that to 5 V which I think should give me the current and capacity required, but recharging would require balancing so I couldn't run the LEDs while charging, and a 3-pole toggle switch would be required as shown here. However the last two points go against my criteria! That is no toggles and no run-or-charge solutions. Is there an alternative/better solution?

Something I was wondering was could I not place the two cells in series with the load whilst simultaneously being wired in parallel with the charger? This would mean I could charge them in a balanced manner whilst running them at 7.4 V simultaneously if required. The load would have the 7.4 V to 5 V convert and a simple rocker switch to turn the LEDs off. This seems simple but I image it's against all the safety rules for using lithium ion batteries...?

Any ideas? Thanks for your help!

[adafruit_support_bill]

could I not place the two cells in series with the load whilst simultaneously being wired in parallel with the charger?

No. They are either in series or in parallel. Trying to do both just creates a short circuit. You might want to consider using NiMH cells. They would be a little bulkier for the same capacity, but you really only need 2000mAh, so a 6xAA (7.2v) pack should be sufficient. http://www.pololu.com/catalog/product/2225

NiMH cells can be charged in series and you can find NiMH chargers at RadioShack or any on-line battery or RC hobby vendor.

Our BEC will take that back down to a safe 5v for your neopixels: http://www.adafruit.com/products/1385

[kadamr]

Thanks for your help, however, the NiMH cells might be a bit too heavy/large. I was trying to get batteries that weigh around 60-80 grams but the NiMH weigh over 120 grams for 4 or 5 of them. The main problem, however, is recharging them inside the device; regular rechargers require the batteries to be removed and put into the charger, which isn't possible. There are chargers like this one but they aren't really appropriate given their size and cost - the rest of the device is cheaper than just the charger! I was hoping there would be something like your LiPo chargers using 12 V DC or a USB input - i.e. a small pre-built circuit, but I haven't found anything like that yet after having looked for quite a while. Do you know of any way of recharging NiMH batteries in series using a small circuit that allows the batteries to remain in place? ...or a solution to the lithium poly battery problem described in my last post? Cheers.

[adafruit_support_bill]

The Pololu charger is a pretty sophisticated charger. You can get by with something much simpler and less expensive for NiMH - and they can be safely charged in series. These chargers are designed for charging cells in-place. They do not need to be removed from the device.

[kadamr]

That is a great solution, thanks a lot! As I understand it that means I could leave the batteries in the device and even charge it while it is powering the LEDs without having to use any complicated toggle switch circuits etc.. I just have one last query - having looked into the brand you suggested I found this rechargeable LiPo battery and this matching charger and wondered whether this wouldn't be a similar solution but about 50 grams lighter. I imagine this is two 3.7 V cells although it doesn't say so and doesn't say it needs to be charger in parallel or anything. So would this work the same as with the NiMH solution you described but be a little lighter? That is to say: could those two products be used to charge the batteries and power the LEDs simultaneously or are is it the same parallel vs. series problem?

[adafruit_support_bill]

That doesn't appear to be a 'balance' charger, since it only has a 2-pin connector. I suspect that the 2 cells in the pack have been carefully matched to minimize the potential for cell imbalance. Many people do use these cells/chargers for RC car/boat/plane purposes. If it suits your needs it may well be a good choice and should allow you to charge the cells in-place while powering the LEDs.

Many of our customers are working on wearable projects where the consequences of a catastrophic LiPo failure are more serious. Out of an abundance of caution, we recommend balanced charging for those applications.

[kadamr]

I see, I will look into which option is best for my purpose but thanks for your guidance, it certainly helped me with my project.