Stall-out/Recall

Unfortunately, the charge rate of the 12 volt from the Prius' DC to DC converter is very slow. AFAIK, it's correct that there's no difference depending on whether the ICE is off, running or at a high RPM. Prius has no alternator.

http://www.autoshop101.com/forms/Hybrid03.pdf has a mention of keeping charging amperage below 3.5 amps on page 3-8 (I'd copy and paste but the browser's not letting me).

Another problem is during the time the car's left off, there's a parasitic drain in addition to the drain from running the CHS pump and the evap check that runs ~5 hours after the car is turned off. There's also the drain from booting the car up.

 

Because voltage on the Prius 12V bus is no higher than 13.8VDC and in some cases may be even lower than that (for example, 13.2V measured by some owners under certain conditions, for example, cabin ventilation system off, and the inverter thinking that the 12V battery is fully-charged via the voltage sense line), you should not expect that charging time of the 12V battery will be improved vs. using a lab power supply set to appropriate constant current / constant voltage settings as I had done.

When the Prius is READY, there is no difference in charging time between the engine resting, 1,100 RPM idle speed, and 4,000 RPM. Please stop thinking of a conventional gas engine with alternator, where the charging current has a positive correlation with engine speed and where the charging voltage may reach 14.4V or higher. The Prius 12V bus is powered by the DC/DC converter within the inverter, and voltage is maintained quite precisely to not exceed ~13.8VDC (or may be lower as previously discussed).

Obviously it would be best if the owner got a new, properly charged battery. One would think that when you spend $200 on a battery you could always expect that. However based upon recent PC posts it has become painfully obvious that such a simple, easy outcome cannot always be achieved.

If the owner is going to drive the car over a couple of days so that 8 or more READY hours are logged, then peace. Nothing else needs to be done.

However, some owners log very few miles. In some cases, that is why they had a 12V battery issue to begin with. The battery is always in a state of semi-discharge, its capacity declines, and eventually fails.

In such cases, I suggest the owner use a real battery charger overnight - not one of those wimpy wall warts masquerading as a battery tender. If that is not available, then it is better for the owner to leave the car READY for an extended period (assuming they can do so safely) than to park the car, forget about it, and then come back several days later only to experience a no-start situation.

 

Thanks for the full discussion guys! We all have legitimate concerns about fully charged batteries (especially when it is cold). This is a good reason to buy your battery locally, rather than via mail order. I was indeed, thinking old-school alternator. Can you do the math on a amps per hour chart that shows when your Prius is X volts too low, charge for Y length of time?

 

There is only a rough correlation between battery voltage and capacity so it is hard to be precise with a chart.

I'll give you a rough data point for the chart: I typically will charge the 12V batteries on my less-used vehicles 2x per month. Starting with a battery voltage of 12.4V, it usually will take the charger overnight to get to the point where the current flow is only 0.1A and the resting battery voltage becomes 12.7V or better (measured 24 hours after the charging process is stopped.)

If the battery is in great condition, the charging time might be less. If the battery is in poor condition, the charging time will be longer. The battery in my HiHy is relatively small, in pretty good condition and may take around 4-5 hours. The battery in my Mustang is larger, older, and not in great condition and will take 10+ hours. The battery in my Prius doesn't need routine charging because the Prius is the daily driver. My daughter drives the other Prius daily so routine charging also is not an issue there.

When using a lab power supply, the concept is to keep charging until you see that the current flow is no longer declining as time passes. At that point, if the current flow remains relatively high (like 0.2A or more) then you know the battery is not in great condition. However it doesn't matter that the battery is not in great condition if you periodically charge it.

When using an automatic battery charger, you hope that the control system within the charger will properly charge the battery. That hope might be reasonable - or might not be...

 

Finally, what is the likely difference in charging times between a fancy pants lab charger, a modestly priced charger from Target/PepBoys/Murrays and a cheap trickle charger from Heartland?

 

The main difference is in the current capacity of the charger. The Prius 12V AGM battery is supposed to be charged with a maximum charging current of 4A. So if you get a charger with that capacity, then in principle it should be equal in charging time to other chargers with similar capacity.

I think that Optima allows 10A charging current so if your charger can handle that current, then you can charge at a faster rate. However a higher charging current results in heating of the battery which over time may result in somewhat reduced service life.

Note that the charging current will decline as time passes as most chargers will use a constant current, then constant voltage control philosophy.

 

There is no difference in charge rate from high revs to engine not running. The Prius does not!! charge the 12v battery from the engine via an alternator, it does not have one. Yes the maximum safe charge rate for the Prius battery is 4amps no matter what charging system is used, meaning the fastest you can charge a discharged battery is 8hours.

Patrick is correct in saying the Optima battery may be charged at 10 amps provided the charger has battery temperature monitoring, this would still have a 3.5 to 4hour charge time.

A trickle charger is very good for a battery but with a charge rate of typically 500ma (1/2amp) would take 70 to 80 hours to charge a discharged battery fully.

John (Britprius)