I agree, this is the 21st Century, and I would figure a company such as Lexus would have figured that out.

 

I'm in the Battery industry, though NiMH batteries have improve tremendously in both capacity, cycles and self discharge, it still have a high self discharge if not used, typically NiMH loses 30% of it's capacity per month, today's NiMH loses 15-20%, keep in mind if your car is only half charged when left in storage...it will be in serious trouble in about 3 months. Keep in mind the self discharge is proportional to the capacity , in regards to the above expample it would be 20% loss of the 50% capacity. The above is based on the NiMH chemistry, there are new improvements in NiMH technology where new batteries have a pulse or trickle charge capability, this just came out earlier this year...Lexus/Toyota might have incorporate this into their Hybrid vehicles, I do not know. I hope this help answers your questions.

 

Bongos -- Just saw your reply this evening. Thanks for confirming with your professional experience what I saw after reading some technical literature on NiMH batteries. Can you tell me what would be the impact of letting the 288 volt NiMH battery sit in a discharged state (if we left the car to sit while traveling for six months)?

Specifically, would the NiMH battery be compromised in any way by becoming fully discharged due to self discharge, or would it simply need to be recharged to return to its previous condition?

Separately, I am not sure if the car's computer would even allow the car to start if the 288 volt battery were discharged, even though we could easily keep the 12 volt battery charged up with a battery maintainer. A tow to the dealer might be required.

Thanks for sharing your thoughts.

Regards

 

Volts has little to do with it, it's Amp hour capacity. Six months is a very long time and I would check with your Lexus dealer or go directly with Lexus. For Toyota/Lexus to incorporate Hybrid technology in itself is a huge accomplishment, many factors are involve, outside the charging and discharging of the battery, battery placement where ambient temperature has to be in operation temperature is very important, NiMH are prone to overheating which can damage the battery, to regulate this... cell balancing is involve, I bet there is several computers monitoring each cell, this in itself increases the self discharge. What we do not want is for the battery to self discharge to a point where it is not recoverable, at that state, even if the cells were able to be salvage (through deep cycling), it will only gain back max 60% of it's original capacity, which I suspect will show in MPG.

Based on 288Volt battery, there are 240 battery cells in your vehicle (NiMH are 1.2 Volt per cell), the battery will be fully discharged at 240 volts (1v is industry standard for total discharge, meaning it will need to be charged), unless lexus designed a special cell to go under 1 V discharge, sulfation will start (this is a chemical change where the NiMH turns into a crystalize state), depending on how much the battery sulfate will determine how it is recoverable. Voltage is key in braking down sulfation. In order to brake down sulfation, add voltage (more than you standard charge of 360 V) will be needed to brake down the sulfation. I doubt your 288V battery is actually charged at 360V. Most likely they break the 240 cells into 12 modules, 20 cell per modules. Then charge them in parallel at 36V, then when it discharge, they use some type of device which combines all the voltage to meet the 288Volts required. A single 240 cell pack, though can be done, is very dangerous.

I've work with industry leaders on the EV-1 and sulfation was a major issue, back then we used seal lead acid. I did not work with Lexus engineers on their Hybid so I do not know if they fixed this.

Though I cannot reviel the current automoble manufacture I am working with, the next generation will be a special type of lithium ion. I hope this helps.

 

Bongos --thanks for your reply. My comment regarding voltage was only to distinguish the hybrid pack from the conventional battery, not to suggest that its voltage conferred any other specific qualities to it.

Regarding your message, as I understand it, Lexus has set the min/max voltage limits for the hybrid battery so that it operates in a very narrow range exactly to prevent the sulfation you mention that would require high voltage charging to reverse (and to extend battery life).

It basically means that for now we have to find someone to drive the car every so often while we are out of town for extended periods. Then in the future if it is still necessary to travel like this, we can switch to the next generation of hybrids using lithium ion batteries, assuming this shortcoming is addressed.

Your information is much appreciated as I could not get anyone from Lexus to give me an answer about potential damage to the battery pack if it were discharged. Thanks for clearing that up --You win the gold medal!

 

glad i can help, with lithium, self discharge is 1-2% per month, it can can last for years parked. what also goes with lithium is the dangers of lithium. Unlike NiMH, with Lithium, you have to worry about overcharge, undervoltage, temp, vibration, shock, etc... all within a 1% varience. With NiMH, if a cell were to short, worst thing that can happen is outgassing of hydrogen, with Li, it's a explosion followed by a chemical fire (water or regular extenguisher will only create a larger fire) good thing that a new lithium cell came out which is less dangerous and we have engineers with PhDs to help

 

Just an Updae, one of the Engineers I spoke to said for Toyota, their NiMH has a seperator which reduces the self discharge to about 10% per month, he recommends someone to babysit the car and start it up for 10 minutes every other month

 

That's just about back to where we were when Spinnaker started this thread.

per Spinnaker 9/19/06

 

do you really think that letting a vehicle sit for two months with out starting is good for the rest of the vehicle, i.e. engine seals, tires and tranny? if you trust someone to start your vehicle every two months then why not have them do it more frequently at least once a month to protect those items i mentioned. also i would prefer they acutally drove the car periodically rather than just started it up

 

I have lots of experience letting a Toyota Prius sit idle for months at a time. It's a first generation Prius, bought August, 2002. Since then, it has sat idle every winter and summer, for times varying between 3 and 6 months. We have not observed ANY degradation in the hybrid battery. When we check it periodically, it is fully charged or almost fully charged. The conventional battery, which fires up the car, has been dead a couple times, but this hasn't been a problem since we started disconnecting the conventional battery when we shut down the car.

Dennis

 

Just returned after a being away to see a few new posts to this thread.

Bongos, that's good news about Toyota's 10%/month self discharge limit. Given this new information, assuming no one drove the car, after how many months in storage do you think the NiMH battery would sustain damage from which it would not recover?

DennisL - that's very interesting and good news about your being away all that time and not seeing any degradation in the battery pack of your Prius.

Two thoughts:

1. How do I reconcile the conflicting information of Bongos and DennisL?

2. I am very disappointed in Lexus that they do not make this information available.

 

Thinking about this further...

If the battery loses 10%/month times 6 months in storage = 60% loss of the capacity measured from the time storage began.

If the fully charged state at the beginning of storage is 80% of maximum capacity (guessing), then the loss would be 60%x80% = 48%.

Leaving: 80% less the 48% loss due to self discharge = 32% of the maximum capacity remaining in the battery. This would not seem to be a terribly low level, but it may be for a NiMH battery.

Is this correct?
If it correct, then is it harmful to discharge to this level?
If it is not harmful to discharge to this level, then why does Lexus/Toyota recommend driving every 60 days?

Am I the only one who doesn't understand why this information isn't readily available from the manufacturer?