Dead 12v battery, can't charge without triggering car alarm

My father's 2005 Prius is overdue for inspection, so my brother and I want to get it in before the end of the week. However, it's been sitting since last April. The 12v is totally dead, and I needed to use the manual key to get in. Here are the steps that I've taken thus far to charge the battery, and the results:

1. Tried multiple times to use a trickle charger on the front terminal, only for the car alarm to trigger every time.

2. Tried using the charger while directly connected to the battery, with the battery disconnected. The charger won't register that it's connected to the battery, and therefore won't charge.

3. Reconnected the battery to the car, tried the charger, and the alarm still triggers.

4. Took out the fuse for the horn, repeated the process, and the alarm still triggers, with the horn.

5. Took out the fuse for the theft deterrent system, repeated the process, and the alarm still triggers.

I'm at a loss at this point. There's been absolutely nothing online that I can find that addresses this problem. The car needs to go in, but our mechanic isn't a Prius specialist, so he isn't going to have a solution outside of having another way to charge the battery that doesn't require it to be connected, or getting another battery.

Does anyone have any ideas on what we can do that doesn't involve a tow truck?

 

Yeah, about that...

It turns out that there are more serious issues than the 12v battery, which our mechanic replaced, including a code P3030. Should I keep discussing it here, or create a new post?

 

According to the mechanic, the dashboard lights will turn on now that there's a new 12v battery, but the car won't start, and the red triangle won't go away, which isn't surprising, considering the P3030. He said that another code popped up, but that he couldn't remember offhand, and would text me later about it. Any suggestions?

 

The other code is P3000, and the reader that his shop is using is a Snap-on Solus that he mentioned cost his shop thousands of dollars. He and the other guy were planning to remove the hybrid battery when I left, so that they could test it.

 

I have an update, and a new problem. There were ten dead cells in the hybrid battery, which our mechanic has since replaced. The car will now turn on and it will drive, but the check engine, brake, master system, and hybrid system warning lights all turn on, and the hybrid battery will stop charging, and the hybrid battery cooling fan is apparently also not working. He thinks that the hybrid battery control module computer may be the problem, possibly due to some corrosion on the connectors, but he's not sure, and while he can, of course, eventually chase down the problem and fix it, the labor costs are already starting to inch toward the "maybe it's time to call it a day" range. There's a rotating cast of codes that have been popping up, two of which are "P0A94 - DC/DC Converter Performance" and "P0AA1 - Hybrid Battery Positive Contactor Circuit Stuck Closed".

Any advice would be appreciated.

 

It's actually my father's car, and exactly what am I supposed to do if not take a car to a shop to have the work done? Are you suggesting that I either let a vehicle either further deteriorate, or do the work myself, neither of which is sensible?

 

Yeah, I can do some things on my own Prius, which is the same generation, but my main skill as far as working on cars is that I'm tenacious, so I can plug away at minor issues by myself with little to no issue, e.g. headlight, taillight, and wiper replacement, as well as changing or adding various fluids, and the spare tire. However, I'm more likely to damage the car if I were to mess with the hybrid battery system, so I have no problems with letting a qualified mechanic work on it.