12v battery jump start?

That 14.0 volts is in 'Ready' mode, so is the output of your Prius charging system. What we need to know is the voltage of the 12V battery after the car has been turned off for a long time, e.g. overnight, before it first goes 'Ready' again. Use the very same display, but in accessory mode instead of 'Ready'.

I don't see any harm in leaving them connected for a while, e.g. over an hour. Your Prius is charging both batteries, this will give your battery some recovery time.

The first acid test will be whether or not the Prius stays running when you disconnect the jumpers (and don't let the clamps spark on anything, each other, or the frame!). The second acid test will be your battery voltage tomorrow morning -- or later today, if the battery is a goner.

 

8 hours.

 

you can't use the car to check voltage when it's running, because the reading would be supported by the inverter which is charging the battery around 14 volts.

 

have someone keep an eye on the car so it isn't stolen.

 

I think he meant the 8 hours to be the time the car is sitting turned off before checking the battery in ACC mode.

But for charging the 12V in Ready mode, that also is best to be multiple hours, though I won't put an 8 hour figure on it. Personally, I'd let it go 2-3 hours, turn the car off, then check battery voltage a couple times this afternoon and evening. If it is really really gone, you'll know quickly today.. If it is on the margin between barely gone and barely usable, you won't know until tomorrow morning.

But if your schedule (and safety against theft) allows 8 hours of charging, that will do a better job of getting a charge on it, and will help extend the life of a battery that is aging and weak but not done.

A real battery charger or maintainer will do a better job (and not waste gasoline), but it is reasonable hold such a purchase off until tomorrow, and get it only if this battery appears salvageable. It would not eliminate battery replacement, just put if off another season or year.

 

that seems pretty good.

 

There are various pieces of half truth here, which does nothing to help the OP.

For completeness here is the post to which the OP referred:

First there was never any mention of what voltage the battery was at when it would not power anything, so have to assume well below 10 Volts. If this assumption is correct, 2 Amps +/- is not enough current and will take an age (days) to charge a fully depleted battery. Using a 4-5 A charge will still take 16-20 to fully charge the battery.

No, actually, he meant 8 hours of charging, which in my opinion is woefully too short a time. The car is really only a maintenance charger, so again, like the above mentioned 2 A +/- charger, using the car will need to have it on for 16-20 hours, if not longer.

This might seem like good news, but a dead battery that seems to charge fast on low current (i.e. to 12.7 V in 3 hours) is a sign that battery is in poor condition. The real test is to see how much the voltage drops once the charger has been removed and then the battery left to sit 12-24 hours.

Even more telling is to apply a load to the battery and see what voltage it drops to and stabilises at, and how long it can maintain this voltage before dropping off to under 10 volts.

Only after having some hard empirical data can a determination be made.

Not sure of the source of this:

but it is not really a good chart for AGM batteries (normal flooded Pb/A,yes) as used in our Prii, as its voltages are too low for AGM. If you are going to continually post a chart, please update to data suitable for the AGM batteries used in the Prius.

Here is a better chart:

A fully charge AGM battery will be in the region of 12.9-13.2 V and at 11.7 V is considered fully discharged. Interestingly there will be no adverse symptoms in the Prius until the voltage drops below 10.5 V (on start up) which may only occur briefly and would not normally be seen unless a DVM is connected and observed at start up. In the chart above the top line of the envelope is closer to AGM and more modern "maintenance free" batteries (with, e. g., calcium additives) which typically have a higher voltage when compared to regular flooded Pb/A alternatives. These regular batteries typically follow the lower line.

Having said that, the importance of keeping a Pb/A battery at the full end of the spectrum cannot be emphasised enough. A discharged (or semi-discharged) Pb/A battery suffers damage the longer is stays discharged (or semi-discharged) and therefore affects the longevity of the battery's useful life. If you are happy to replace your battery every 3-5 years, don't worry about any of this, but if you think you should get 10-15 years of life out of a battery, then get a battery charger and use it often.

 

At least it is not plummeting, that is a good sign.

 

It needs to be off and disconnected (to achieve true open circuit voltage (OCV)) for at least 4 hours before taking a reading. All you are seeing is surface charge dissipating. Alternatively, you can bleed off surface charge by turning on your headlights for 30 seconds and then wait until the voltage climbs back up and stabilises before taking the reading.

 

It should be a little higher than 12.3, say comparing to checking with multi meter, car off. Which is good.

Yeah look into chargers? I like CTEK, have their 4.3 model hooked up as often as not, with our low use.

FWIW, my Optima Yellow Top at rest reads around 12.75 when full charged, after surface charge is dispelled, and having sat overnight.

AGM batteries do have higher voltage, but in my experience not that much: 0.2 volt at most.

I monitor also with Solar BA5, all good toys to have.

 

A new AGM should be higher than that, mine was 13.2 V out of the box, and only took 15 mins (or so) on the charger to get to the maintenance charge stage, so took that to mean it was fully charged out of the box. Even now, some 3.5 years later, when fully charged (and surface charge has bled off) it settles at ~12.9 V.

My take on your situation is you were sold a "less than stellar" battery if it was only charging to 12.7 V. Out of interest, how old is your Yellow top now?

Also of note, if your battery sits in your car overnight connected to the car, it is not true open circuit (OC), so it stands to reason that after sitting with a 20-50 mV load 8-12 hours, it will be lower than the level it would be, if it was truly OC.

 

Replace the 12V battery with a good AGM battery and be done. The current battery has suffered a massive MI (myocardial infarction). Its life expectancy has been cut considerably and is no longer reliable.

Price shop a replacement from your local Toyota dealers. Or just go to a PepBoys near you and get the Bosch NOT an Optima.
Bosch) 46Amp Hr (same rating as Toyota batteries for SKS Prius), $143, 4year warranty duration (no proration).
Optima) 38Amp Hr, $238, 3year warranty.
Bosch Premium Performance Battery Group, Size 51 | 647478 | Pep Boys

Read post#5 to learn how to put a larger load on the 12V battery for a better reading. ACC is too light of a load.
Read post#8 to see an AGM battery chart breakdown and some current smart, AGM compatible 12V battery chargers.

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