Low voltage question

[kirbyfanner]

Hey modretro,

I just registered today after half a year of reading the site, seeing the awesome projects you guys finish, and learning about how to make my own. Now I feel ready to tackle my first portable, a handheld sega genesis, but I have a question about the battery i'm using.

I found this low battery circuit http://kyorune.com/modding/article.php?id=55 that seems simple enough, but I don't know what the voltage is for when the low battery light goes on. I'm using a 7.2v NiMH battery pack, and seeing how it's headed for a 7805 i'm assuming if it drops low it'll auto-turn off, but what voltage should I set the indicator for? 6.8v or something? Or really low, like 6?

Thanks so much, looking foward to posting my project when I get started!

 

[Prog]

I think what actually happens is the battery's voltage drops slightly as it depletes, until it finally simply lacks the amp output needed to run anything. Some batteries get damaged if they output too little voltage, so they're rigged not to.
Also, don't use Ni-Mh. You're going to want lithium-polymer or lithium-ion batteries.

 

[vskid3]

Also, don't use Ni-Mh. You're going to want lithium-polymer or lithium-ion batteries.

Why not? If cash is tight and space and weight are flexible, NiMH is a great choice.

 

[hailrazer]

Oh let's see. They are heavy, they have less power per cubic inch. They have a memory and lose their ability to charge very quickly.

And on and on and on......

Why use an inferior product to save $10 or so. Makes no sense. If you have them and want to use them till you can put in some Li-ions or polymers that's fine. If you are buying save up a few dollars and buy the better product.

 

[vskid3]

Why use an inferior product to save $10 or so. Makes no sense. If you have them and want to use them till you can put in some Li-ions or polymers that's fine. If you are buying save up a few dollars and buy the better product.

Of course, I'm not saying to choose NiMH over Li-ion, but if someone is on a strict budget, NiMH can do the job. There is also the greater fire risk with lithium that some people don't want to chance it even though it should be safe with protection circuits. And at least NiMH aren't as heavy/bulky as lead acid.

 

[hailrazer]

And at least NiMH aren't as heavy/bulky as lead acid.

Oh come on. Who doesn't want a N64 portable with 2000 cold crankin amps ?

 

[kirbyfanner]

Thanks for all the advice, yeah I know li-ions are way better but I happen to have the NiMH battery here now, so i'm just gonna use it for this portable. Also, because it's my first I think it's best to use a battery that's just so-so instead of going for a really good one, but i'll definitely use li-ions for now on.

My concern is still what will happen when the battery gets really low. I'm assuming if it drops below 7v, the system will shut down because the 7805 won't work, but because NiMHs have memory effect and other things I wanted to build a low battery indicator so I know when to stop playing and charge. Unless you guys think that it's okay to just let it run till it dies? Thanks again everyone!

 

[Prog]

Check out the low battery circuits in the megasticky.
(Press back. It's second from the top.)