I attempted to remove multiboot and installed u-boot and upgraded from power kernel 51 to 52. Unfortunately, I must have done something wrong, because I have no entries in U-boot besides the "Attached Kernel", which doesn't work of course (probably because I'm on Thumb-CSSU.)

Now, messing around with rescueOS and trying to take a look at things, I've drained my battery. Is there any way within the U-boot console to get the battery charging? I figure I can try to flash the power kernel directly (replacing U-boot), but can't get the device into update mode with the battery so low.

That's the first question. Secondly, any way to get a working kernel so I can boot into maemo and hopefully fix things up? Would flashing just the kernel work? (worth a try?).

Yes. Just flash the kernel.

I think it would charge itself even if nothing loads, instead of just dropping dead. I think the charging chip should have a fallback mechanism that charges the battery when some timer/watchdog doesn't get reset correctly.

Get a power source (3.8-4v, at least able to kick out 1a) and wire it to the battery terminals on your n900. Could be pretty much any li-ion single cell, or a ac-dc adapter rated for that voltage/amperage. DIY a fake battery out of balsa, or another easily cut material (non-conductive), and wire some leads to female terminals that give the right polarity to the n900's battery pins.

Or, any li-ion charger (either single cell, or a customizable one that lets you select the cell count/ah rate. Diy leads to the battery's +/- and charge the battery like that. You can also charge a bl-5j in a n810, or a $10 ac wall charger specifically designed for that battery (what i use for my n900 that i fried the usb/usb charging hardware, and can only charge the battery externally.

Just depends on what you have lying around, if you want to diy it, or buy a wall charger for bl-5j's. I've done both. Even used a rc smart battery charger, with a broken n78 board, and soldering a few leads to the same pins the n900 uses. You could do that with any broken nokia phone that has the same type of pins the n900 has.

The easiest thing, and a nice upgrade, is an extra battery(s) ( i have 4bp-5l's lol), and a wall ac charger, along with a usb powered external charger (yoobao yb007). I save the usb ports for data only now. Charge all my battery's externally. The ac one for home use, and the yoobao for on the go lipo packs/5v regulator & a female usb output (or male so you dont need another micro/mini usb cable to attach the yoobao).

And the cool thing about a lipo/regulator with a female usb, is you can charge/power anything 5v to the regulators amperage max. MY nook color pulls 25w through a small $10 ubec (rc voltage regulator) and a 4 cell 5500mah lipo pack. That could charge 5 n900s at their max charging current of 1000mah at once with a usb hub.

I have an external charger ordered and on it's way.

The BL-5J doesn't fit into the n810 without cutting some plastic either on the battery or the n810, so I didn't bother with that. We'll see how long I can hold out though... hopefully the external charger gets here soon!

I think just flashing the kernel will get me going again.

Thanks!

Happens to me all the time...
What I do is:

Connect it to a pc usb port, and leave it there... don't try to turn it on... It might turn on by itself. Leave it be, it'll shut down again...

After 15 or more minutes... unplug, turn it on... then continue charging...

You need to install KP for uboot from the repo, not the normal kernel... It will download the kernel image and update the boot screen.

You could try flashing with the (I seem to recall) -l (that's an L, a small one) option, which will only load the kernel. Follow the guide for flashing A KERNEL ONLY (no rootfs or emmc) and flash KP51r1 with the -l option, which will only load it for one boot. Then keep it plugged into the PC so that it detects a charger (try a quick change to a wallcharger if that doesn't work) and it should just boot. From there you can try fixing uboot without needing to reflash it

good luck

I had done something like this, but decided to try again, and it worked! I was able to flash the kernel to kp52 and reinstall u-boot properly this type.

Thanks!