Prius Auxillary Battery Size

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What brand is it, and was there a date code on top of the battery?

Since owning the vehicle, how many hours per week do you drive?

Some here use a 'mobility' battery, with more capacity and less cost : Fitting mobility 12volt AGM battery. | PriusChat

FYI : you're moderated until you've posted 5 times.

 

I suggest that you ask the selling "mechanic" to help you test the battery.
It should be charging (above 13 volts) when running and not discharging when OFF.
It is possible that it has never been fully charged.
It is also possible that it was "used" when installed.

 

From Ed The Fox:

Battery 101.
Here's a minor load:
With car off measure voltage at front jump point. Do not ever measure battery voltage at the battery itself that is meaningless it only matters what the voltage looks like on your dc volt meter at the front black fuse box input which is the front jump point. The front jump bolt is bolting down the main power cable from the battery after it goes through a 100 amp fuse in the black plastic assy that's bolted to the top of the pos post of the battery..
Then with car STILL OFF turn on headlights in high beam for 5 minutes. Shut the high beams off after 5 let it sit a minute then with car STILL OFF measure dc again at front JP bolt. Your car will fail that test big time.
A healthy battery will not drop 1 volt on this very minor load test.[/QUOTE]

 

From the part number, it looks like a Platinum Diehard which is a good battery if it is new.
if you have a standalone battery charger, I’d charge the battery overnight and then test it.

https://www.autozone.com/batteries-starting-and-charging/battery/p/duralast-platinum-agm-battery-340-cca/1080153_0_0

 

20 miles a week does not even come close to keeping that 12 volt battery happy.

And your hybrid battery will not be very happy either. Your right at the point in the cars age when the hybrid battery will start showing degraded performance and not driving it will excite corrosion on those battery connections and eventually lead to failure. Really common on a g2 read all the bad hybrid battery posts on theG2 forum,

keep an eye on the hybrid battery performance onthe mfd. If it’s in purple all the time and takes a long time to get it to 3 blue bars that’s stage 1 of battery failure.

Take a close look at the hybrid battery level before you store the car for the week. Record it. If it shows much lower charge level when you start it again it’s very unhappy.

Btw Batteries Plus sells a 12 volt Duracell that will drop right in. Im over Optima battery’s not impressed at all with those batterys.

 

Don't know much about that charger, but other members will. Best to keep it at or under 4amps, can you select the amperage?

 

I use something similar... As long as its still smart than it's ideal... By smart, I mean the amps are dropped as the battery reaches full charge...

 

The 34B20R-AGM size battery was OEM for the Gen 2 Prius models that DO NOT have the SKS system. Does your car have SKS? If so, your car should have the S46B24R model battery.

If you have SKS, you can help the small battery live longer by turning the SKS off using the small button labelled "KEY", which is located on the dash under the steering wheel. The amount of driving you do may not be adequate to keep the smaller battery charged if it's carrying the load expected of the larger battery.

Your 12v battery is charged from the DC/DC converter in the inverter when the car is in READY mode. Unfortunately, this charge current is relatively small compared to what many people are used to seeing in an 'alternator' type vehicle and will NOT adequately charge a depleted battery in a 'normal' amount of time.

 

When running, you are testing the charging system voltage, NOT the battery itself.

13.6 is right about float voltage and is charging little to none.
That probably means that the charging system thinks it is fully charged.

If it IS fully charged, after a few hours rest it should read about 12.8 volts.

 

The base voltage of Gen2 is 13.8 V. If we replace the 5A fuse near the battery with a p600m 6A diode, the voltage will be 14.4V. This simple procedure will allow you to charge your battery better in winter.

 

Got any links on where to get the p600m 6A diode? Also how to learn more about how it works? Early model Honda Insights have a wire you can snip to provide constant charge to 12v from battery pack rather than a variable charge and this sounds like an even better trick than that. Or maybe I misunderstood? What do other people think?

 

How it works: With this circuit, the ECU monitors the charge level of the battery. A voltage drop of 0.3 - 0.6 V occurs on the diode. The ECU increases the voltage by this value.
You can use another diode, but you need to choose according to the voltage drop.
p.s. This solution has been tested by many.

 

https://www.vishay.com/docs/88692/p600a.pdf

 

My neighbourhood electronics supply, 42 cents.

https://www.be-electronics.com/product_p/6a100.htm

 

Causing the battery to be CONSTANTLY charged at 14.4 volts is NOT a good thing to do.
That is a throw-back to the days of mechanical voltage regulators......and is harder on the batteries than not quite charging them up all the way.

AND.....in most circumstances, forcing more charging current into a battery while it is cold accomplishes nothing useful.
When the charging goes away and the battery gets thoroughly cold again, you are back pretty close to where you started.

So....overall a BAD thing to be messing with.
The circuit has lost it's fuse protection; the diode won't do the same thing.

How many of those "many" who have tested it ended up screwing up their charging circuit costing them a LOT of money to fix. ???

 

1. I want to note that Gen3, Prius C have thermal compensation and the voltage in cold weather reaches 14.6V. If you have an AGM battery, then in the summer you need to remove the diode.
2. The diode can be put in series with the fuse. Nobody messed up their car.
3. All this is done voluntarily.
Ps. Do you have experience operating a vehicle in temperatures below -35 degrees Celsius (-31 F)?