I can't believe I couldn't find this answer -- what is the amp hours rating of the OEM 12V battery?

12.1, but even then, it is not advisable. continuous draining to that level will likely cause irreversible harm. and the car is not designed to charge it back up.
prius was designed to use the 12v very little, effectively not dropping the voltage, and the inverter keeps it charged when in ready.
it takes a long time for the inverter to recharge a battery from 12. 1 to 12.whatever its current voltage is.

 

It is about 40 Ah, I believe.

 

I'd say you'd be fine with any of the first three cutoff voltages. In my experience, even 10.0 won't give you any trouble starting the car, but that would be well into deep cycle territory. Prius aux batteries are better suited to deep cycling than typical car start batteries, but still, the battery ought to last longer if you don't routinely take it that low.

If you look at the battery, you'll probably see a label advising a 5 amp or lower charge rate. Clearly, with a 45 Ah capacity, you'd be looking at (ideally) 9 hours, (in practice) more like 13 hours, for a full charge at that rate. The car's converter isn't really a constant current charger, so it'll really recharge rather faster than that ... at the expense of stressing the battery more.

-Chap

 

As I recall SE Toyota (or some of their dealers) have been installing such a cutoff device on Prius for several years. I wouldn't recommend using such a device, as they caused other problems for owners. They were posting on this site.

Depending on year, the stock Prius 12V battery is 38, 40, and 45 A-Hr capacity. I won't discuss "cranking amps" because it isn't applicable.

When Pearl S' OEM battery was about to fail it would put the car in "ready" at 10V consistently. I now know it was "about to fail" because, while it never actually failed completely, it quickly got flakier and flakier.

 

If you're interested, a good read on 12v batteries and getting better battery performance by using a mobility scooter battery:

Fitting mobility 12volt AGM battery. | PriusChat

 

The maintainer I use for my 12v is rated at 0.8 A. Considering the battery lasted 7 years I'd say 0.8

Indeed my 7 year old 12v battery would put the car in "ready" but it didn't have enough juice to lock/unlock the doors or trunk. I hoped I didn't have issues with the locks, and sure enough after replacing it with a new one the locks worked just fine.

 

It'd be nice if they could put it on the label. Don't most after-market batteries?

I recall trying to find the cold-cranking-amps for our OEM Yuasa when I wanted to test it with a Solar BA5. Good luck wandering around in the Yuasa website. Finally elected to use 325, which I think is close.

 

That's a maintenance charging rate, intended to just hold a battery at full charge, staying just ahead of self-discharge and the small loads that continue when the car is off. When the battery's at less than full charge, you can charge it faster than that (and the car does), but there is a label on the top saying not to go above 4 or 5 amps (IIRC) if you can avoid it.

Think how long it would take to recharge a discharged 45 Ah battery at 0.8 amps (keeping in mind it's only the part of that 0.8 in excess of the constant load and self discharge rate that gets you ahead, so 56 hours would be too optimistic, not to mention you usually need to stuff in something like 1.5 ✕ capacity before you can get 1 ✕ capacity out).

-Chap

 

My 4 amp charger, with battery in good shape to begin with, typically takes 5~6 hours. IIRC, the last depleted battery I dealt with, it took close to 12 hours.

That max amperage warning is in the Owner's Manual somewhere, maybe 4 amp for 3rd gen, and 5 amp for 4th gen??

 

CCA isn't relevant, at least not directly, because the battery never needs to supply a high current. However, a high-CCA battery should accept a higher charging current. That could be good for owners who scarcely drive enough to keep the battery charged.

 

Don't.

My advice is to never consume electrical power except when in Ready. Should you ignore this, I recommend a jump starter in the car all the time.

 

It was relevant for me purely because my Solar BA5 tester was asking for it. It works by electronically load testing, deciding an actual CCA, comparing to spec. It arrives at this value regardless, but I guess bases its verdict on the spec.

 

if i was gonna do anything with the car off, i would install an optima or the like.

 

johnny, you have been here for a year. we can't tell you what will happen in your particular situation. what we cantle you is that prius battery is garbage. i don't know how else to say it. it's barely good enough to start the car, never mind trying to run other things off of it.
we are just trying to make you aware, so you don't get stranded with a dead battery in east overshoe.

 

Your math and logic are correct. A fully charged, healthy Prius battery would have no problem doing what you are asking about. The potential problem comes when the Prius battery is not in a fully-charged state. For one thing, a battery that's several years old will not have the same capacity as it did when it was new. Then put yourself in situation where the car has been used for many consecutive short trips in a short period of time -- not allowing enough "on" time to recover. This scenario could put your Prius battery in a marginal state of charge. Now add the small additional demand your bike charger would put on it, and it might be enough to get you into trouble.

My suggestion would be to get a lithium ion jump starter, and charge you bike, headlight, phone, etc. from that. (I'm not necessarily endorsing the one in the link, just that it seems like a good price on a unit with high mAh rating.) As a bonus, it would provide a safe jump start source for your Prius if it ever needed it, and it would give you a safe source to jump other people's cars without using your Prius. Keep in mind, for your purpose you don't need the Consumer Reports best buy unit. They're going to be testing them as jump starters -- which is way more of a power demand than you'll ever need for your charging or your Prius starting.