Okay, kids, gather round.
A battery pack is rated three ways and it needs the appropriate connection to fit your computer. The connection is probably the simplest to deal with, its a common connector, so its just a matter of figuring out what wires go to what pins. There are schematics available so that can be figured out as well. Not a big deal. Actually, a bit of checking with a multimeter would do as well, but the schematics would help eliminate some of the options.
So we go to how to determine if the pack has the goods. The SAGERs currently are using an adaptor that puts out 6 amp at 20 volts, that's 120 watts. It can do it for ever, well, as long as you pay the bill. So anything that can deliver at least 20 volts and 6 amps will do. If it can deliver a higher amperage, not a problem. Perhaps a tad less will do, but that's iffy. If anything, your battery would probably charge slower than it does now. Operating power for these buggers is probably around 90 to 100 watts.
The watt hours or amp hours are sort of similar measurements. The watt hours is the more correct rating, what it means is how many watts the battery can deliver for how long. So an 100 watt hour battery would produce 100 watts for one hour or 1 watt for 100 hours or 10 watts for 10 hours. But when you add in the wattage rating for the battery, ie - volts time amps, in the case of the SAGER, 120 watts, a 100 watt hour battery would deliver its 120 watts for 0.833 hours.
Amp hours short circuit the rating, so to speak, they take into account the voltage the battery delivers its power at, in this case, 20 volts, so if you have a battery rated at 8 amphours, it means it will deliver 8 amps at the required voltage for one hour.
The battery in my 8886, which I believe is the same as the 8887, is rated at 6000 mAH with 14.6 hours. That means it puts out 87.6 watt hours. I think the 8890 is 6600 mAH and the voltage is the same so its good for 96 watt hours.
You will notice that while the charger is rated for 120 watt hours, the batteries are only 87 and 96 watt hours. That's because the charger is also charging the battery in addition to running the machine.
I don't know if this means you can could use a battery pack that puts out 14.6 volts, though if you have a plug that matched the battery itself and you removed the battery and plugged in there, you definitely could get by with 14.6 volts as long as it can put out 6 amps or so.
So, to recap, you are going to have to have something that puts out 20 volts. Its going to need to be able push 6 amps. You can figure of about an hour per 100 watt hours of listed capacity when you are cranking, give or take. If you are just cruising, surfing, doing email, you would probably get 1-1/2 to 2 times that. With 140 watt hours (again, assuming its putting out 20 volts) you should get 1.4 hours of cranking and 2.1 to 2.8 hours of cruising. If your battery is already charged, you won't be drawing for that, so maybe more time. The battery charging looks like it takes about 30 to 35 watts or about a quarter of the power the AC adaptor puts out.
One of the reasons for the 20 volts on the charger vs the 14.6 volts for the battery is it takes a higher voltage to drive the juice into the battery.
Hope this little primer helps out.