C64 DTV laptop project

[Hojo_Norem]

(I have posted this here cause this could apply to most 'console off a battery' projects)

Hi all.  I have begu nwork on my greatest project to date, bigger than my Neo-Geo-in-a-large-wooden-box and more usefull than my 3rd-party-n64-ram-expansion-cooled-with-noisy-ancient-cpu-fan.

The projects started off well, got myself a Gamecube rechargeable battery, decided on the enclosure (the bottom half of two c64 datassettes hinged together) and a 5" PSOne lcd screen (on order from eBay).

However the progect has hit one little snag:

The LCD runs off 7.5v at 6.8w
The C64 DTV runs off 3.3v (regulated from 6v worth of AA batteries)
and the keyboard interface (hacked up PC keyboard gubbinz) runs off 5v


The rechargeable battery inside seems to be 10x1.2v Ni-Mh giving 12v
(not to suprising, seing that the Gamecube runs of 12v)

I have been squeezing Google for all its worth all I have mainly found are sites showing how to do 12v>5v or 5v>15v

Can anybody here point me in the right direction?

PS:  With my limited understanding of wattages and things my calculations make the GameCube Battery's capacity roundabout 6ah (The GC's power rating is 3.25A 39W and the blurb on the pacaging for the battery says is can power a CG with LCD for 1 hour or without LCD for 2 hours)

[Aidan]

You need some regulators. There are plenty of adjustable regulators out on the market. If you care about battery life, then switching regulators are good, but much more complex to implement. If you don't care about battery life and heat, then a standard linear regulator will work, but throws away a lot of energy.

<Darn, didn't mean to hit submit!>

National do a bunch of switch mode regulators under the banner of "Simple Switcher". They even have a web interface that will work out the component values for you, right here

The regulator on the DTV looks to be of a linear type, which is a bit wasteful, especially if you're providing it with 12V.

You might want to see if the PC keyboard will operate from a lower voltage. If it's fabricated with modern processes, then it might well run OK at 3.3V. That will save an extra regulator. You could probably test that by suppling the keyboard with 3.3V and seeing if it still works ok.

[Hojo_Norem]

Thanks for the advice.  After digging around thier site I decided that their LM2576-adjustable switch mode regulator will do the trick and I can order them from my regualr supplier.

Will I be able to get away with running two regulator circuits in parallel to the battery, one circuit to supply the 3.3v to the dvt and keyboard (I can't imagine a modern ps2 keyboard NEEDING 5v) and another circuit to supply the 7.5v to the LCD?

Anyhow I  have decided to use the MAX713 to charge the battery with.  The chip is farily versitile and can run the rest of the laptop while charging the battery from the PSU.  I just shot a sample request at them using my psudo-company name I invented back at 2ndry school (check my WWW, I've kept the name alive up until the present day. ^_^)

[phreak97]

national.com do free samples, they send you 5 of a component for free, no shipping cost. just hope your selected ones are sampleable then make up a fake company name if they are.

[Confused]

Probably not the best way but you do have another choice. Seeing that the battery pack is made up of 12 cells you could tap into them directly to obtain the desired voltages.

If the keyboard runs of 5V than 4 of the cells would produce the required voltage.
4*1.2 = 4.8V

You mentioned that the C64 requires 3.3v derived from 4AA�s so the same 4 cells that power the keyboard could power the C64 board. The remaining 6 cells
6*1.2 = 7.2V could power the LCD.

The only catch is that the LCD side of things will probably go flat before the c64/keyboard side.

[Aidan]

Unless the cells are going to be charged individually (or in two groups), then splitting the battery up like that doesn't do good things to the cells. What will happen is that one of the two groups of cells will discharge faster, and putting the whole battery back on charge will result in one group being overcharged whilst the other group is undercharged. Neither situation is good for the cells. The advantage of running off the full 12V is the extended run time, and the lack of modifications needed to the battery pack!

Moving to a switching regulator instead of a linear regulator will also extend the battery life. If you get really fancy with a boost buck regulator, then you can operate the equipment when the battery voltage is below that required of the powered circuit. That's a lot of work though for marginal benefits.

Don't forget, if you put in place a 3.3V regulator that you'll need to bypass the linear regulator on the DTV.

[Hojo_Norem]

Again thanks for the info guys!

I have just come across another issue with my project, its not a big one but its a little annoying.

According to the MAX713's specs the PSU that will be supplying power to the charging circuit (and the rest of the laptop) needs to be able to supply 1.5v more than the maximum charge voltage of the battery pack, which can reach 1.9v per cell.  There are 10 cells so that 19v, and I need to add 1.5v to that so we are looking about 20.5v.

In that range of voltage all I seem to be able to get are 24v PSUs, which isn't realy a problem but the MAX713 needs extra circuitry to be able to handle that voltage.  Like I said its not a biggie as the regulators can handle up to 40v, its just (to me) 24v to run a laptop and charge it's batteris is quite a lot of voltage, but I know not much in the ways of such things (yet ^_^)

One last thing, I peeled back the outer layer of plastic on he battery pac to reveal the individual cells and I noticed a small retangualr metal component with the number 'b555c' printed on it.  The positive power input from the original charging circuit goes into it, then another wire comes out and is connected to the battery terminal.  There is another wire from the battery terminal going to the power switch (selects wether the GC runs off mains or batter power).

The component is taped down suggly inbetween two of the cells.  Could it be some kind of emergency heat triggered charge power cut off?


Phew, that a lot of typing.  I shall retire for the time being.  when I start building this thing I'l be sure to put images up on my WWW.  

[Aidan]

1.9V a cell sounds rather high. Are you sure it's not something lower like 1.65V a cell instead? That would leave you needing 18V, which is pretty close to the 19V that many laptops use. You might also want to check what the original charger puts out, as it should be pretty close to the voltage you need, but the current requirements might be too low, especially as you indicated the battery pack has some basic thermal management.

Batteries always need a higher voltage to charge them than they can provide themselves, especially if you want to do a fast charge. It's less of an issue when you trickle charge, as the charge voltage doesn't need to be so much higher than the battery's own voltage.

[viletim!]

butter_pat_head,
From my limited knowlage of battery charges, they must be current limited to prevent the cells drawing too much current and destroying themselves. The simple 'trickle' charges use just a resistor in series. The more complex ones vary the voltage (while measuring the current) at different times during the charge.

The component is taped down suggly inbetween two of the cells. Could it be some kind of emergency heat triggered charge power cut off?

What you describe sounds like a thermister, there for the reason you describe.

[Hojo_Norem]

Ok.  I have been bangin my head against the wall trying to decipher the important things I need to know about the MAX713 and what it needs to be able to charge my battery.  Lets just say that I have come to the conclusion that I don't have nearly enought battery charger smarts to decipher it.

So I have decided to use the circuitry that came with the battery.  The circuit looks like its some sort of power regulator but anything more than switches and transistors usually confuses the heck out of me, but this is what I have been able to figure out:

All the GNDs are tied together, including the battery's (assumed) - terminal.

PSU power coming into the charger is sent to the main circuit and to the switch PCB.
There is a wire from the charge circuit going to the 'b555c' which is connected to the battery's + terminal (I am assuming as the wire is red).  There is a wire from the battery's +terminal to the switch PCB.

The switch PCB has a 3 position switch with the positions A-B,B-C,C-D.
Position 3 is not avaliable when the battery is in its original enclosure (the thing that bolts onto the bottom of the CG) but when the switch PCB is exposed the switch can be set to position 3.

The PSU power is connected to A, the power to the CG is connected to B and the battery is connected to C. D is GND so if the switch is put into position 3 it will short circuit the battery (not good).  I think the switch can be safely replaced with a 2 position switch.

There are some LED and supporting resistors on the switch PCB, but they arent a problem.

Which leade me to my next question:  One of the thing's I liked about the MAX713 was its ability to drive a load (the laptop) while charging the battery.  It seems the CG charger circuit would be unsuitable for this which led me to think this up:



How I think this would work would be when the PSU is supopost powering the laptop the relay switches on and connects the power line from the PSU to the rest of the laptop, but when the PSU power is removed the relay switches off and the battery takes over.

Would this work?

[viletim!]

butter_pat_head,
That looks like a good soultion.

That metal thing is more likely just a crude thermal fuse, thermisters usualy don't go in series with the cells (they have their own pin). Easy enough to find out. Measure it's resistance when cold, heat it with a hair dryer or something and measure it again. If the resistance changes then it's a thermister, if not, then maybe a thermal fuse.

[kendrick]

I should know better than to bump a six-month old thread, but you might find this item interesting as it relates to retro laptops:

http://www.benheck.com/Games/Atari_800/Ata...00_laptop_5.htm

Ben Heckendorn took the guts of an Atari XEGS and connected a battery and an LCD display. The results are envy inducing. He doesn't spend a lot of time explaining the battery mechanism, but does mention that the DC input charges the battery. Worth a look if your C-64 project is still stalling.

-KKC, goofing off at work.