External alkaline battery pack for a laptop ...

[OzCDN]

I am looking to put together a simple external battery pack for a small laptop I have.

The external power supply puts out 19 VDC and it is this port that I want to bring my battery power in through (i.e. I am not looking to replace or run parallel to the laptop battery). My thought is to use 12 Alkaline 'D' cells in series to give me 18V.

So my question is: Is the fact that I am 1V low a problem? Is the voltage drop off going to be an issue or is this going to be good until the batteries are pretty much done?

Alternately, I can get 19.2V from 16 rechargeables. I'm pretty sure this will work but I am not sure what I am going to need to recharge this. I am assuming that my charger that does 4 NMH AA cells won't have the power to do 16 D cells.

I am trying to avoid any electronics if I can (i.e. keeping it very simple) but I do not want to risk the laptop at all.

Anyway, this is not my area of expertise so I am hoping that someone can give me some advise to let me know if this is reasonable or if I am way off base.

Thanks,
Dave

 

[carmusic]

yes it should work at 18v, but i wont last long with alcaline battery, i would go with lead acid battery one 12V and one 6V (or maybe just a power drill battery!) , it will cost maybe a little more but they are rechargeable with a simple wall mount ac adapter

 

[OzCDN]

This is an interesting idea that I am going to look into tomorrow. 18V power tool batteries are pretty common now and, as you pointed out, they come with their own solution for charging.

So now I am trying to get an idea of how much extra time a given battery will give. Laptop batteries are usually quoted in mAH. The external battery I am applying replaces the charger so it actually runs at 1.6x the battery voltage (11.1V vs. 18V).

So, if my internal and external batteries are rated for the same mAh (for simplicity), the external should last 1.6x longer. Right?

 

[Diver300]

So now I am trying to get an idea of how much extra time a given battery will give. Laptop batteries are usually quoted in mAH. The external battery I am applying replaces the charger so it actually runs at 1.6x the battery voltage (11.1V vs. 18V).

So, if my internal and external batteries are rated for the same mAh (for simplicity), the external should last 1.6x longer. Right?

Probably, but it depends on how the laptop has been designed. The charger voltage is more than the battery voltage to make the charger simple.

If all the voltage conversions are switch mode power supplies, then the efficiency will be nearly constant and the bigger input voltage will result in less input current.

If any voltage conversions use linear power supplies, then the larger voltage will result in the same input current. It is unlikely that the laptop works like that as it would simply get too hot.

 

[OzCDN]

Done

I ended up going with three 6V lead-acid batteries which measure out at 18.75V ... pretty near perfect. They are 5 Ah so they offer over 9 hours of power and the internal adds another 2.5 hours. More than enough.

I was running off them last night and I am very happy with the result.

I just need to package this Frankenstein up and it will be all good.

Thanks for the help.

Dave