Bit of Battery Advice

[Hippogriff]

Hi all,

It's late here... my eyes are drooping, time for a potentially dumb question.

I'm running some LEDs off a PIC 16F628A which is being powered by a single 9V PP3 battery pack, so 9V going via an ADP667 and a little 10µF Capacitor, spitting out 5V for the PIC... it's all hunkey-dory.

I was just! wondering about simplifying the overall circuit even more by having a 5V battery and eliminating the voltage regulation bit... just having the battery output feed the power pin of the PIC.

Seems they're a bit harder to find and are often designed for cameras and suchlike (or really expensive when Google tells you about 5V USB battery packs for about $50). But, of course (I thought!), you can put the batteries in series... so I could connect, like, 4 x AAs in series and get 6V... or 3 x AA and you'd get 4.5V.

Simple question, really... is there a way of getting 5V? Do I even need the full 5V for my PIC and LEDs..? I think the PIC Datasheet infers it would actually work at less than 5V. How do I know what I don't know?

I see Duracell 4.5V batteries for sale, I am assuming these are just a repackaging of 3 x AAs?

 

[cr0sh]

The reason for the regulator has to due with voltage sagging due to current needs of the rest of the circuit; if the voltage sags too low, you'll cause a reset to the microcontroller. The regulator helps to prevent this from occurring. With that said, if all you are running are some LEDs, it might not be a big deal (until the battery runs low). Some microcontrollers are tolerant at being run slightly under 5 volts, so try it and see with a 4.5/4.8 volt 3-cell pack; couldn't hurt...

 

[Reloadron]

Per the data sheet the chip itself has a voltage range of 3.0 to 5.5 volts. However, per cr0sh you don't want your other loads dropping the battery voltage. I think things should work fine with a good 4.5 volt battery combination.

Ron

 

[ben7]

A full 1.5volt battery will read over 1.5 volts. It should be ok to use 3 1.5v batteries to get 4.5 volts.

The PIC should run ok on 4.5volts and the voltage will drop over a loonng time because of the low current being drawn from the batteries.

 

[#12]

I just ran a test on NiMH batteries last month and found that they spend most of their life at 1.25 volts each (4 x 1.25 = 5).
I had a problem with a cheap flash camera that was eating alkaline batteries like popcorn...10 to 12 flash pix per pair of AA's. I switched to 2450 maHr NiMH batteries and fired 94 pictures in one day without the battery low indicator showing up because the NiMH batteries have very low internal resistance, like .05 ohms, thus they don't trip the "low battery" sensor when the flash pulls a surge of current. After 94 pix, the voltage on each battery was 1.25. That sounds like a pretty good source for a 5 volt PIC. ps, the self discharge is 1/2% to 1% per day, if that matters to you

 

[Hippogriff]

Thanks for this - bits of advice like this are invaluable as I continue building up my collection of 'bits'. As I don't have everything I need already, I am loathe to go out and spend £2 on a AA battery holder from Maplin if there's a chance it won't do what I need. Of course, saving up my orders and using an Internet shop would be better but sometimes you want to try it right now. I think that I might give this a 4.5V angle a shot if it's going to simplify my pretty simple circuit even more.

 

[Hippogriff]

So........ if I was thinking of using my PIC 16F628A circuit to power a 12V LED strip (at the moment I'm using normal LEDs) then how might I best go about creating a 12V power source from batteries - that could be used to power the 12V LED strip via a ULN2003AN.

At first I thought two 9V PP3s linked in series would do the trick, creating 18V... but that would not be within the limitations of my ADP667, which tells me it can take a source up to 16.5V only.

I've actually connected a short run of the LED strip directly to my bench PSU and it took 18V quite happily (inbuilt resistors)... but drew 0.17A (as opposed to 0.06A at 12V) so I guess this is not such a good idea.

So, assuming there isn't a simple 12V battery available, I think I need something that's between 12V and 16.5V that's, hopefully, in a compact package.

I guess I could create 12V from 6 x 1.5V AA batteries, but that seems like a lot of batteries. This is dead easy from my bench PSU and from wall-warts, but less-so from batteries... it seems... and it does need to be a battery, as it will be portable.

EDIT: Actually, it's working OK (maybe not as bright as I would like) with a single PP3.

 

[audioguru]

A PP3 9V alkaline or Ni-MH battery uses AAAA cells which are tiny and have a very low capacity that is about 1/13th the capacity of AA cells.

 

[blueroomelectronics]

Just connect 4xAA NiMH to the device. Many newer 18F PICs have a built in battery level circuit.

 

[PatM]

I stopped using normal NiMH cells a couple of years ago.
Just sitting for a month or so with no use, they run down quite fast
I switched to Sanyo Eneloop cells which still hold 85% of their charge after a year.
NiMH cells come with higher capacities - so it depends what your usage is.
For infrequent use in a digital camera, the Eneloop cells are much better.

 

[audioguru]

Energizer "new" Ni-mH cells also hold their charge for a long time. They are made in Japan (by Sanyo?).

 

[PatM]

It appears the Energizer and Eneloop cells supply about the same usage.
The only thing I can find is that the Energizer cells may be recharged about 500 times, while the Eneloop Cells are good for 1000 recharges.

AA rechargeable battery shootout: Energizer, PowerGenix and Sanyo Eneloop -- Engadget