Earlier this fine day, ax0n, asmdianX, and myself (and my daughter) went down to Surplus Exchange to look for a replacement Sun machine for ax0n. Turns out they had already sold the Ultra 10 I had seen there earlier in the week. While there I looked though a box of old laptops and found a pair of Blackbirds, and a 500 series power supply. (And a Compaq Contura Aero 4/33c 8/255) They are a 540c 12/500 and a 540 12/250. And they came with tw battery packs each. With the aid of IBR and EMMpathy I've got a t least one pack that seems to be showing the full original capacity of 1752 mAH! It's been running for a while on the battery while I have it jacked into my router and use ti to make this post. OS 7.6 and IE 2.0 makes for a functional, but limited surfing platform. I've still got 50mins of run time with 59% charge left. It's got a draw of 1000-1300 mA, depending on how many programs are running and if the HDD is off or not. It's interesting to watch the power draw drop as I quit running programs. 
With this pair, I'm up to two 520c's, the new 540 and the new 540c. I'm probably going to sell off the worse ones for parts. (ie a banged up 520c that runs fine, and the 540 with a funky LCD) I'm just happy that of the 5 battery packs I have, at least one takes a decent charge! I also discovered how to take the empty bay covers apart and use them to replace the broken clips/cover on battery packs. 
That's a very useful trick to know. Maybe I shold get some photos of the process and post 'em up.
I just pulled the RAM card from the 12/500 and found out that it has 8MB on 4* SEC KM48C2100AJ-7 chips, and there are pads for 12 more chips. I woudl love to bump my 540c to 36MB, so would it be possible to just nab some chips from a 16MB SIMM with 8 chips, and some more from an 8MB with 4 chips, and solder away? I'm betting that I might find a SIMM with the exact chips, but if not, ship of the right size/speed/layout should be fine, right? All I would be out is time and experiance, and not the cash for a large RAM module...
Not only are 32MB cards scarce, and pricey, but also are they the way to go. I've seen two on eBay in four years. The most recent was GBP30.00. Retail cards start (new from some sellers) at all-but-prohibitive prices (USD80) for 32MB. I have seen USD190 asked.
The expansion RAM cards for the early PBs, at least, use RAM chips that differ from model to model and from desktop RAM, and even differ from themselves when they feel like it. Compare the two sides of a 160/180 card. PB500 series RAM is PSRAM, used elsewhere in the backlit Luggable, 140/145/145B/170, 160/165/180, 165c/180c and Duos, at speeds from 100ns to 60ns. Even if you have the time and the gear to desolder and resolder the ICs, you have the problem of getting hold of the correct densities also.
If you upgrade a 540 to a 117 or 166MHz 603e processor you need faster chips, with different capacitor loadings to avoid crippling wait states. On balance, no.
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Thanks for the help! I hadn't thought about the PPC upgrade being an issue. I did search for info on the chips themselves, and I did noice that they are different from the other 5xx RAM cards I have. I did find that there were regular 72-pin SIMMs made that used those exact chips, so I am hoping that I can find one for next to nothing and move them to the 5xx card. I can get SMT chips off fairly easily, either with the toaster oven method to loosen the chips, and a little ChipQuik to help or use the stove burner trick to get 'em off quick-n-dirty. I've done the stove burner one before, but only on parts that I didn't care about at all.