6V battery to power 5V device

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annaek
 
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6V battery to power 5V device

Post by annaek »

Context: I have a temporary need to power a project that usually runs from a 5V power supply without access to AC power. (I have a sleep disorder, and it is ridiculously hard to wake me up. I've built an alarm clock that does a pretty good job. There is a planned power outage in my apartment building due to maintenance. I'm not willing to go without my alarm clock if I can avoid it, because my circadian rhythms are also very sensitive to disruption. I would rather cover my nightstand with batteries than fail to wake up!) The most sensitive piece of equipment is probably a Raspberry Pi 3, though I also am running a strip of LEDs (https://www.adafruit.com/product/306?length=1) which apparently are quite sensitive. I do have a UPS, which does a good job of powering the system through brief interruptions, but it does not have anywhere near enough capacity to handle the multi-hour outage I am anticipating.

Based on easy availability, low-cost, high-capacity, and ability to handle relatively high currents, I have settled on (alkaline) 6 V lantern batteries. I have found a data sheet for a Rayovac heavy-duty lantern battery and have done the calculations to verify that it meets my needs in terms of both total capacity and ability to deliver the current that I need. (I took some measurements, and my raspberry pi does not draw anywhere near the 2 A that my power supply can provide under actual operating conditions. It's a headless configuration with no networking. The current I measured peaked at 400 mA during boot and then settled to around 200 mA during typical operation.)

The documentation that I could find on the pi says it can safely accept a power supply up to 5.5 V, which means my 6 V battery is just slightly too much. (And the LED strip is a similar story) It's now clear to me that I need a voltage regulator, but I know basically nothing about voltage regulators.

In total, I need to be able to supply 1.4 A (peak) at 5 V. For most of the operating time, current draw will be closer to 200-300 mA.

I did find this linear voltage regulator (https://www.adafruit.com/product/2164) in the Adafruit shop, but the documentation there states that I would need at least 7 V input in order to have a consistent 5 V output. I can do that by putting 2 lantern batteries in series, but I assume I would then have to deal with some serious heat dissipation. If I use this product, what sort of heatsink would you recommend?

Are there other (perhaps more efficient) options for voltage regulators that I should consider?

Thanks!

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zener
 
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Re: 6V battery to power 5V device

Post by zener »

There are a lot of ways you could do it. First of all it might be helpful to confirm the exact voltage that battery puts out when it is new. It might be higher than 6V.

The most efficient way is with a switching regulator. The other main type is linear, which is not as efficient but sometime is simpler.

If you can live with 1A max then this is your solution:

https://www.adafruit.com/product/2190

But it can't do 1.4A AFAIK.

Here is one from Pololu that can do 1.5A

https://www.pololu.com/product/2870

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annaek
 
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Re: 6V battery to power 5V device

Post by annaek »

Thanks for the links! The first one has a link (in the product description) to a 'UBEC DC/DC Step-Down (Buck) Converter' which can output up to 3 A. I think that's pretty much exactly what I need. Here's the link:
https://www.adafruit.com/product/1385

I'm not sure if it will run well off of a single 6V lantern battery or if I'll need 2 in series (I've seen the pi get finicky about a 4.9 V input supply), but, hey, that's why I have a multimeter!

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adafruit_support_bill
 
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Re: 6V battery to power 5V device

Post by adafruit_support_bill »

One problem with alkaline batteries in general is that the voltage output declines steadily over the usable part of the discharge cycle. A 6v battery may be closer to 7v when fresh, but closer to 4v when nearly drained. The voltage drop will be more severe with higher current loads. NiMH rechargeable cells have a flatter discharge curve and generally perform better into high-current loads.

The UBEC can handle up to 16v input , so 2 of your lantern batteries in series should be OK.

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