Battery question

The 12v battery should hold it's charge for a couple of weeks given the low drain due to electronics that don't fully shut down. If it appears to be draining too fast, shut down the SKS (sensing the dongle at a distance) or even better, disconnect the negative lead to the car body and consider getting a BatteryMinder when storing your Prius.

The HV battery is fully disconnected when a Prius is turned off and the self discharge rate of the NiMh modules is very slow in a good battery. It should still be at a sufficient SOC after several months of sitting idle.

JeffD

 

You mention the number of bars, which makes me think you are talking about the HV battery. As JeffD says, that should be 100% disconnected when the car is not started. Leaving the lights on will have absolutely no effect on it. (Believe me, I speak from experience...) I have left both a Gen1 and a Gen2 Prius for weeks with no problems.

So that's practical experience. OTOH, I am curious, theoretically to hear from JeffD or anyone else: It was always my understanding that the self-discharge for NiMH is quite high relatively speaking (relative to, say , alkaline or LiIon). Of course that info is mostly for AA-sized batteries. Can you explain in more detail? Thanks...

 

When my 2004 HV battery failed at 195k miles, I replaced it with a ReInVolt and was allowed to keep my old battery. I then disassembled the failed battery and found that 27 of the 28 modules were still good (the 28th had one bad cell of the six in series). After educating myself (with aid from some helpful PC gurus) and buying a SuperMate DC6 charger/rebalancer I rebalanced each of those modules for resale (used to successfully repair 6 other HV batteries). This took several weeks to complete (more than a day per module). My findings were:

1. All of the modules were restored to more than 6.5 amp-hour capacity (their factory rating)
2. When fully charged a module gets to over 8 volts (nominal is 7.2 volts), but the "surface charge" quickly dissipates and the voltage drops to about 7.8 volts overnight.
3. The module voltage of the last group of 27 good cells to be sold was still above 7.6 volts two months after they were charged. So the self discharge rate is reasonable.

Perhaps LiOn batteries have an even slower self discharge rate (as seems to be the case with my 18v LiOn drills), but I am certain that our HV batteries will hold most of their charge for a 3 to 6 month idle period.

JeffD

 

For the OP, there are two possible causes that I know of, both are normal. The first is the "oh, duh" moment that nobody ever admits to - they left their car running all night. This will drain the HV battery to power the car's computer, but the engine will run periodically to keep it from going below 2 bars. The second cause is just that gauging and displaying the battery SoC is an inexact science. The car takes a best guess while you're driving by counting how much electricity is going in and out over time, but it's not perfectly accurate. When the battery has a chance to rest overnight, the car can more accurately judge the SoC of the battery, which may be very different than what it thought while you were driving last. Also, there is hysteresis on the display; so when the battery is being drained from a higher SoC, 56% will be displayed as 6 bars, but when it's being charged from a lower SoC, 56% will be displayed as only 5 bars. And furthermore, that hysteresis is quite close for some of the levels; so you might power off the car at 52% SoC, being displayed as 5 bars. When you power it on the next morning, the car decides it's really 51.5% SoC, which could be displayed as 3 bars.

As for the self-discharge of NiMH, it can be all over the place. In general, it's a tradeoff with capacity. If you buy high-capacity NiMH batteries, the self-discharge will be through the roof. If you buy the "ready-to-use" or "precharged" type of NiMH (such as Eneloop AAs), then the self-discharge rate is similar to that of an Alkaline. I wouldn't be surprised if the batteries Toyota uses are more towards the low-self-discharge range than the high-energy-density range of designs.

 

JeffD: Interesting stuff when you disassembled the battery. Thanks!
Macman408: I'm guessing that your last paragraph may explain the self-discharge of AA NiMH. They are trying to jam more amp-hours in and making insulators or whatever thinner and thinner. Maybe they Prius batter does not have to be quite so dense.

Interesting discussion...

 

The readout for the HV battery WILL change overnight. This does not reflect a change in the HV battery. It is the metering system that is changing. Ignore it. It's meaningless. It is virtually impossible to tell how much charge there is left in any battery. The best that can be done is an "estimate". The HSD does this by measuring the charge into and out of the battery. It also uses the voltage as a "second opinion", but the voltage changes a lot with temperature. Overnight the temperature drops a lot. Hence the readout changes. Keeping in mind that the HV battery is -completely- disconnected (both +ve and -ve terminals) there is only internal leakage to worry about. The Prius HV NiMH battery has very low internal leakage. DO NOT rely on the meter to tell you the state of charge! It can't tell the state of charge to any reasonable accuracy.

This change will NOT affect the vehicle starting. The 12V battery is the one that can go dead if you leave the lights on, for example. That WILL affect your ability to get to "READY".

And of course the 12V battery only powers up the systems. The HV battery is the one that actually turns over the engine.

 

BTW, I would add that at least for my 2007 (Gen 2), if the 12V battery is weak, *very* bizzare things happen when you try to start. For example, I got panel lights coming on, along with the display and maybe the radio, but a totally bizzare behavior and messages. As I recall not the "warning triangle of death" with a normal error code. It took a good 1/2 hour on the charger before I could start, and after that all was fine.

 

has long warranty on those batteries if it is defective (out of the factory) it will show up soon .... if not it will last quite a bit we should start some survey or something but 200k miles are not unheard of with normal use.

Before someone corrects me I know the miles are not exactly indicative of battery life. It depends more on cycles temperature and loads ... so only city driving probably uses up the battery "faster" .... more cycle within the same mileage.

 

One detail I found while experimenting with many diferent Prius vehicles is that batteries are working better and more stabile providing more energy for EV mode on the vehicles that had more miles on meter compared with those with less miles. Most energy I have found in cars previusly owned by company that drove regularly and long distance... Worst battery capacity I had was the Prius from a lady that drove a total of 4000 miles over 4 years, that one managed less than 30seconds in EV mode when I bought it and it took a while before battery gained capacity...

 

I believe you but what sort of theory are working on here?

The first part seem to make some sense but not that much. But the second that you re-gained capacity not at all.

Is it not more likely that you learned how to use the battery in a better way ... I mean 30 second is not a measurment of capacity what speed what else were running ... what SOC to start with etc.

Using it in EV on purpose is not the best way unless the energy came from regen ...