Alternative Auxiliary Battery

well done! nothing ventured, nothing gained

 

How safe/wise/necessary is it to drill the hole for the vent?
I understand SLA batteries may release hydrogen. Don't want to go Boom.
Since I can't get most battery products shipped (live on island), and I didn't want to go dealer, etc., I wound up purchasing a CSB EVH12390 12V 39AH battery.
I had not seen this thread at time I did this (a month ago) - here's what I did differently -
I didn't drill a hole in battery and I didn't connect the vent hose to anything;
I got a set of battery flag to top post adapters - used 1 on positive terrminal so didn't modify the stock positive battery connection, and I used equivalent of an alternator or starter motor cable to replace the Toyota battery ground cable (old cable available if go back to stock).
The battery I got is shorter, but a bit wider, but all the trim goes back without an issue and the battery is well-secured using the stock setup.

(I saw something on another website that gave me impression there is some sort of temperature sensor in the + battery connection/wiring.)

FYI - My 2014 PriusC with <9000 miles on it (I'm original owner) recently failed to start.
By the time I realized (over a few days - I don't have to use the car) that it was the 12v aux battery, and I decided to try a portable battery car jump starter, the jumpstarter wouldn't even try as the aux battery voltage was under 9V - (I took the 12V battery out, and I was able to get it charged/functional and it would hold enough charge to work - but I thought time to put new battery in.

I also bought a cig-lighter voltage meter so I can monitor voltage.
This CSB EVH12390 setup is working well, I wish I had done some of the voltage readings with original working oem setup before I got the battery discharged to damage-danger levels -
Once the dash shows 'READY' - the car is 'started' & 12v battery is charging off the equivalent of a car's alternator (in this case, it's effectively running off the hybrid battery system)
When car is in ready mode, voltage reading ~14.2-14.4, or ~13.2-13.5 (I'm guessing lower voltage is 'float' charge)
When car turned off/acc - voltage will drop to ~13.2 and fairly quickly drop to 12.9 and continue to drop, albeit much less quickly.
Leave the car for a few days, battery voltage will be around 12.2V. It can also be 12.2V after a couple hours of OFF.
From what I've read about car batteries, I'd expect a higher voltage from a healthy, fully-charged battery - this battery I bought is supposed to be new & it does have recent date code & purchased from reputable local battery dealer.
The spec sheet on this battery, available from official Taiwan mfr's website, says "Nominal voltage" is 12V.
Nominal capacity is 39Ah @ 20hr-rate to 1.75V per cell (which would be 10.5V for the 6-cell 12V battery)

(Once I recharged and 'treated' the 2014 OEM 12v battery, it too will settle down at 12.2V for a few days of vehicle off, and has similar voltage readings when the vehicle is On. 'Treated' = various bench charge/discharge/cycles before I put it back in vehicle. During the life of the vehicle, there have been times where the car is not turned on for more than 30 days - and it has never failed to power unlock/start, etc.)

 

an accident in the battery area is probably the only danger. idk about drilling a hole, sounds dangerous, but i'm not sure.
a lot of people put in cheap tractor batteries and etc., just depends on your level of risk tolerance.