whats wrong with my line of thought (battery gurus)

[zband]

I really like the idea of the battery operated igo's. For those who are not familiar an igo is an external battery source to charge ipod's camera's even laptops.

Here's my question, why cant a LD NiMh D-cell (10000mAh) be able to charge my laptop battery? Now I understand that the Li-ion in the laptop is 7.2V (or 10.4v gotta check), but its capacity is only 4800 mAh isnt there some sort of circuit that can be put together so that 2 (or 4) D cells could charge the laptop battery? If they had something like that from igo I would buy it right now....

 

[mdocod]

The long answer is long... Maybe later...

The short answer isn't too bad... I'll get straight to the easiest workable solution I can think of:

A car adapter for any of those devices works from ~12V (give or take).

So just set up a 10 or 11 cell NIMH pack (cells in series), this will give you ~12-13.2V nominal voltage to work from, then just put a female "cigarette" lighter plug on the pack and plug your devices into it using the "car chargers" that they either come with or are available.

Exactly how you would go about putting 10 or 11 NIMH D cells in series is up to you, you could buy various plastic "adapters" and wire em up in series to get to the right number of cells, or have a pack cell-matched, welded and shrink-wrapped by a professional operation.

 

[zband]

So you would have to exceed the voltage of the battery you are trying to charge?

If so isnt there some simple circuit to up the voltage from the battery pack in exchange for higher current draw from the batteries? After all, arent nimh batteries all about high current draw?

 

[Lynx_Arc]

You could use a boost circuit but at the cost of efficiency (loss of power). Power (in watts) remains the same so if you were wanting to use 1D cell at a nominal 1.2v to charge a 12v lapotop battery at a current of lets say..500ma you would 12x0.5 or 6watts needed from the battery.... which at 1.2v is 5amp + efficiency losses due to heat etc so it could be up to 10amps coming from a single D cell to try and charge a 12v battery. The battery is 10Ahrs means in one hour its dead and you put half an ampere hour back into the laptop battery which many be just about 1/5 to 1/10 its capacity depending on what batteries it has

 

[zipplet]

Most laptops require a very high current charge too. My laptop power pack puts out as much as 90W (at 19V, so ~4-5A). Even if you found a boost regulator capable of 19V at 5A, the current consumed from a single cell would be insane and probably damage the cell. You may think you can supply a lower current and the laptop will be happy but this isn't the case. The laptop will try to draw a high current anyway, pulling the voltage down and (if it's like my Dell) then reject the power supply.

However... using a boost circuit with a single cell to charge a USB powered device like an iPod can be done, and commercial devices exist that do it

 

[lowatts]

So you would have to exceed the voltage of the battery you are trying to charge?

If you still have the AC adapter for the laptop, you can check its output ratings, if you want to change the battery while it's in the laptop. You need to match the output voltage, or within the voltage range, and match or exceed the amperage rating but you can probably get away with less amperage if you charge with the laptop off.

It's more complicated if you want to charge the battery *outside* of the laptop because you'd have to identify which of the multiple contacts on the battery to connect to and their voltage/amperage/polarity requirements.

 

[mdocod]

I had a big long post all figured out to explain things a little deeper, and got a "server busy" message and lost it all... (stupid me forgot to copy to clipboard before hitting the "submit" button)...

Anyways.. The bottom line is that, from a single NIMH cell, it's basically not possible, there would be no way to get that much voltage boosting and that much current from such a low voltage source to begin with, it would require basically a dead short on the inlet side of the voltage boosting circuit, and a cell with remarkably low resistance, and the amount of available watt-hours would only be enough to charge the pack maybe 5% after efficiency losses. (maybe worse)...

Now, from a 4 cell NIMH pack, it would be possible, but would still require a LARGE voltage boosting circuit with very high current capable components. The inductor in the circuit would likely have to be massive (guessing at this as I've never actually built one, but understand the basic design concept of a simple voltage boosting circuit), to the point of still not being very practical as the voltage boosting circuit would be rather expensive and cumbersome. Enough so that there would likely be an advantage to just using more cells, so that the voltage boost circuit could be smaller and cheaper and lighter...

Even better still, eliminate that efficiency loss and complexity all together, and just build a pack with more voltage to begin with.. 10-12 smaller NIMH cells (say, C size) would end up working better than 4-6 D size NIMH cells as you could eliminate ~10-25% efficiency loss.

Eric