Yet Another Battery Thread

[legofan623]

I've been lurking around other people's worklogs lately and I noticed everyone has batteries sorted out nicely for their portables, while I do not. I asked about this a while back and didn't really get anywhere because I was quite the uninformed n00b. Now that I have the info that I needed, here's what I have to work with:

Rev C board
Stock regulators
To be on the safe side, 3A draw

I started researching more into Li-ion batteries and I found some interesting information about them. Like the graph below shows, Li-ion batteries drop voltage a steep amount while discharging:

This graph throws the idea of using a 12v battery pack out the window because it wouldn't be able to sustain 12v for very long at all, and with stock regulators I need to stay above 12v. I read somewhere that the stock regulators can go to about 19 or so volts safely though, so I believe that I need to find a battery solution that will keep it's voltage above 12v until the cells are nearly maxed out, which is when I thought of using a 14.8v pack:

It appears that the 14.8v pack doesn't drop out under 12v until it is nearly dead, exactly what I need. To make said pack, I decided that I would need SIXTEEN 3.7v 2500mah batteries in series and parallel to eventually make a 14.8v 10ah battery pack. I don't know what kind of charger I would need, a 3.7v one or a 14.8v one? Haven't figured that out yet.

All in all, I still don't know if this is the best way to go, it it will even work, if I need this much mah, how to charge them, and if I can justify paying over $200 on batteries and a charger.

 

[bentomo]

Let's start with the easiest thing here.

Don't use AA batteries, unless you use store bought lithiums, they won't be able to hold their voltage under a load, so throw that idea out.

If you're using the stock regulator and need to use a 14.8v battery then you'll be purchasing at least 4 cells and put them in series.

However when you increase the voltage on the stock regulator your current draw is going to drop. (This is true for other regulators as well) This is why energy draw is measure in watts (P=AV), because when your voltage drops your amperage increases, and vice versa.

So at the high voltage of 14.8 you will have a low current draw and not need very many batteries for decent run time.

As for a 14.8v battery pack you will need 4 cells, a 14.8v protection circuit, and a 14.8v charger. You can purchase these at batteryspace.com


However with all that said I really recommend getting custom regulators. You can buy them for about $10 each on mouser.com and you only need one for the 3.3v line and the 1.9v line on a rev c gamecube. This will save you cost on batteries as you will only need 2 cells for a 7.4v pack and 7.4v charger. Plus you'll get the most use out of your cells.

 

[Blargaman91]

A stock GC regulator can run on voltage as low as 10.2v. That makes a 11.1v pack mostly unusable and a 14.8v pack a good choice.

10 Ah at 14.8v could very well last you more than 10 hours so you probably don't need that.

 

[legofan623]

I didn't realize how much 10ah would get me lol. I'll drop it to 5.6ah.

Comparing the costs here:

Stock:
Batteries 3.7v 2800mah x8 -> $60
Charger -> $25
Protection PCB -> $26
Misc shipping and such -> $20
Total -> $132

Custom:
Regs x2 -> $20
Batteries 3.7v 2800mah x6 -> $45
Charger -> $25
PCB -> All PCB's were limited to 4.0A or less, assuming $15
Misc shipping -> $20
Total-> $125

I went with 11.1v on custom regs because I don't see 7.4v working all that well, and to stretch the mah on the batteries.

So there are savings to be made with a custom reg set up, even if it's only $7 .
And I guess $200+ was a steep estimate, but I still think that sitting next to an outlet is a better price to pay than $125.

 

[Blargaman91]

If you're just looking for a regular protection PCB, there are ones available for much less than $26 on batteryspace.com. But if you have some special circuit, nevermind I said anything.

I don't know what keeps you from wanting to use 7.4v but it works very well with custom regulators.

 

[legofan623]

7.4v 5600mah doesn't last as long as 11.1v 5600mah if I'm understanding watts correctly. And could I get a link to said PCBs?

 

[bentomo]

Not surprised you didn't see it. I'm not a big fan of how battery space organizes their website.

http://www.batteryspace.com/pcbfor74vli-ionbatterypack8a-1alimit.aspx -7.4v

http://www.batteryspace.com/PCB-for-11.1V-Li-Ion-Battery-Pack-4.0A-limit-----PCM-L03S04-559.aspx 11.1v

 

[Blargaman91]

5600 mAh at 11.1v is 6 batteries. 6 batteries can also hold 8400 mAh at 7.4v, which will last slightly longer because the overall power draw (in watts) decreases slightly with lower voltage.

 

[legofan623]

Ah, ok. Didn't think of it like that.