regulator questions

[sonictimm]

Hey all I'm working on my first GCP and I was wondering about a few regulators. What I want to do is use Ni-MH battery packs because I already have 14 Ni-MH cells and I don't have extra money for Li-ions. If I put them in series, I get 14.4 V but then there are only 3.3Ah and the battery only lasts 1.5 hours (I tested it many times!). So, to save time and money as opposed to making my own custom regulator board, what I want to do is:

put my NiMH's into three 4-cell packs and put them in parallel to get 4.8 V and 9.9Ah and use a couple PTN04050C's to get 12V and enough amps to power my screen, spkrs, and the GC using the original reg. board. That way I would get more mah's without going thru all the work of making my own regulator board. would I get a better battery life than the 1.5 hrs. that I otherwise get?

also, I noticed that the PTN78000A outputs in -V and I am wondering: wth? I'd appreciate any explanations/answers thanks!

 

[superben51]

It outputs negative voltage. So you can use it as ground and use the difference as the normal voltage. So 12v and -12v would be the same as 24v

 

[sonictimm]

OK Cool thanks again SuperBen. So would my setup work OK with the two step-up regs in parallel with each other? and would it still get me more than 1.5 hours battery life?

 

[superben51]

I don't know a lot about step-up regs and their affect on current draw (YAY! my screen works now!)But if you were to build your own regs, to run at 4.7v and use two different step-up circuits, one to step it up to 5v for the 5v line and another one to step up to 12v for the screen, then it should increase battery life. After all there is very little chance of that using three times the amps use. However you would need a very beefy step-up reg to use the original GC regs and I'm pretty sure that it would be brutally inefficient.

 

[vskid3]

You'll probably get pretty similar play time.

No matter how you configure your batteries, you'll always have 47.5Wh of power (volts*Ah). The only gains you would see would be from efficiency from using better regulators. Using your current runtime of 1.5hrs, I'm guessing 1hr 45min to 2hrs would be as much as you could squeeze out of your batteries.

 

[sonictimm]

Sorry but I don't get it. I have plenty of volts with 14.4 V and 3.3 Ah, but not enough amps for more than 1.5 hours. If I do my setup, and have 9.9 Ah, then shouldn't it last for at least 4 hours when you factor in the 10% efficiency loss? (and yes I checked the datasheet it is ~89% effficient) I just don't see how it could die in less than three considering it will have enough voltage and amps for about four.

@superben: also, I'm not putting the two regs one after another, they will both be put in parallel with each other. They will both have the same input and output voltages, I just need more than one of them to have enough amp output.

 

[vskid3]

The amp draw changes with the volts. The watts will always be the same. So going from your numbers, the draw is about 2.2A at 14.4v, which is 31.7W. Assuming there is no change in efficiency, it will draw 6.6A at 4.8v, which is still 31.7W, still giving you 1.5 hours of play time. Your only gains would be from using more efficient regulators, which I'm guessing wouldn't give you more than 15-30 minutes more.

It also sounds like your batteries are a little worn. I'm pretty sure the GC draws less power than I'm getting from looking at your runtime vs Ah.

Trust me, you're not the first person who's thought of doing this, and it still doesn't work. Dang physics.

 

[sonictimm]

Dang Physics.

Yea I agree. Thanks a lot for clearing that up for me. I didn't realize how much amps and volts go hand in hand.

Physics, we will meet again!