Clicking relay after starting with a low battery

Yeah move the stuff over in the trunk and put the charger right on the battery I'm not sure why you're getting a relay click from trying to charge under the hood but okay unless the car is sitting in the on mode ACC position something try doing all that with the fob 20 feet from the car like inside the house unlock the door whatever and then leave the remote in the house and walk out to the car set up your charger like you have been do you still hear the clicking No let the charger rip sometimes the tab connections up there are to be desired so I usually charge at the battery terminal post in the trunk like I've always done in all my cars where the battery was up front

 

The car's power source ECU has a funny unexpected behavior, sort of like some desktop computers where you can check a box in 'setup' to have it come back to its previous power state after a power failure. If it was off, it's still off when the power returns, and if it was on when the power went out, it tries to turn on again when it comes back.

I can see times you'd want a desktop computer to do that. It still surprises me they picked that behavior for a car, but they did. If it was on when the 12 volt power failed, you can take the battery out, charge it, leave it sitting for days, whatever, and when you connect the battery back up, the car says "oh! I was on before!" and it turns on again.

So what happens when you have a terrible, horrible, no good, very bad charge on the battery (five hours on a trickle charger is nowhere near enough) is the car says "oh look! the power is back! I was on before! I should turn on again!" and there isn't enough juice in the battery to successfully turn it on, so the power fails again. Then there is less power being drawn from the battery, so it looks like there is power again, so the car says "oh look! the power is back! ..."

You can just unplug the fat cable that plugs in at the battery + post, so the car will stop doing that. Then hook up a charger (not just a trickle charger, but one that'll produce 4 or 5 amps), either right there in the back, or take the battery to a workbench and do it there. Give it 15 or more hours on a 4 to 5 amp charger.

Then, when you plug the car's fat cable back in, the car will again say "oh look! I was on before, so I should be on again!" and now there will be enough charge in the battery for that to work, and you'll be out of the silly clicking loop.

 

Be absolutely sure of your polarity (+ & -) when you connect anything to the 12V system. Have seen too many posts about "oops, got the cables backwards and now the car is dead". Good luck.

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Even when that happens, it's only mostly dead. Usually needs a fuse link and a few other miscellaneous fuses.

All dead, you just have to go through its pockets for spare change.

 

I'm thinking of the one or two threads where the DC-DC converter failed after a botched jump start or battery hook up.

Even so, I prefer a little caution before hand vs dealing with any problems afterwards (hey, I'm lazy that way).

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Yeah I don't do too much jumping unless it's with my noco. Other than that I plug up my 8 amp Schumacher battery charger and it works great fan kicks in when it's on full blast and all that stuff plastic has a hanging piece on it 38 bucks or something I do not like doing the jump thing that's why I bought the noco so I don't have to mess with cables and other cars and all that

 

You need to connect that "trickle charger" for about 24 hours in an attempt to fully charge the battery.
BUT even with that, your grief with the battery may not be over.
How old is it ??
Often an old battery that has been run flat will "self discharge" fairly fast after that and only a new one will fix it.
Note: I hope your little charger really is a fully automatic tender type charger.
That is NOT the same as a trickle charger, which is not recommended for much of anything.

 

You really need to properly charge your battery (& get it tested). Your 12V can "start" your Prius if it's at 12.0 or (or even 11.5V- maybe). But that's way discharged. It can work as it is ("like nobody's business") until it doesn't.

Personally, I try to keep my car as reliable as I can.

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You said that already. That proves almost NOTHING.

The 12 V battery does not start the car, it just boots up all the computers.
The engine actually starts from the HV hybrid battery.

Second time asking how old the battery IS.

I am not the only one offering this advice.
Ignore it at your own risk.