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Old 09-07-2011, 06:15 AM   #1 (permalink)
 
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Default Battery education

I have read a fair amount on this forum and on the Aptera Forum, but I am still quite ignorant about many battery charging/discharging characteristics. Let me try to capture what I think I "know" with questions throughout. I hope no one takes this as 'knowledge' but merely recapping what I think I have read in order to try to make sense out of it all and I'm hopeful that someone on this forum can enlighten me/correct me where I am wrong. I suspect a lot more experience is needed to obtain definitive answers, but hopefully someone out there can make sense of the questions and provide some insight. I am sure the technology will be changing rapidly, but the current technology is what it is.

I will start with some basic terminology (that will help frame some questions).

Battery pack capacity - number of kWh installed in the vehicle when the battery is new and fully charged

Diminished capacity - as the battery ages, fewer kWh can be stored at full charge

DOD - depth of discharge - % of consumption of the battery pack capacity before you are 'out of gas'

Battery life - Discharge/recharge cycles - # of times/days you can drive the car, discharging, then recharge, before end of battery life.

Rapid charge - a technology to quickly create a battery charge at the expense of shortening the life of the battery.

Recharging cycles - reportedly there are only so many recharging cycles in the life of a battery.

Now some questions - I know the answer to most/all is 'it depends' but I thought there could be some "answers" that provide some general rules about the current technology and/or some "answers" about where people believe technology is heading.

What is the projected battery life of, say, the Nissan Leaf?
How much is that life shortened if you frequently (daily) must quick charge?
Is battery discharge linear? That is, given a uniform route, is 50% of your allowable depth of discharge consumed in the first 1/2 of the trip?
Is battery charging linear - both in terms of time and in terms of electricity consumption? Does it take 1/2 the time and 1/2 the electricity to charge 1/2 the battery?
How much efficiency is lost in battery charging (that is electricity consumed to charge vs. electricity available in the battery to run the car when charging is complete)?
When a battery pack is declared, say 18 kWh, is that the usable juice (say real capacity 20 with 90% DOD) or is it only usable to the extent of DOD (that is, not all of the battery pack's kWh are available for driving unless DOD can go to 100%)?
If your battery pack has a 100 mile range, what would 'proper' behavior for be for recharging after daily 10 miles trips (recharge every night, recharge after 5 days, recharge after 9 days...)?

If I'm not asking the right questions, revise them to ask/answer what I should be asking!
Thanks.
Bob - an uninformed newbie trying to make sense of what it would mean to own an EV in the next 1-2 years.
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Old 09-07-2011, 12:29 PM   #2 (permalink)
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Default Re: Battery education

Bob thanks for the post, the questions you ask are on many people's minds

Quote:
What is the projected battery life of, say, the Nissan Leaf?
Nissan "warrants" the battery for 8 years/100k miles. What they guarantee you is that the battery will deliver power to the motor sufficient for the motor to achieve its peak rated power. How long it will do this, they do not guarantee. Thus, we can see this warranty is worthless.

GM on the other hand guarantees 70% of the original range up to 8 years. No word from Ford on what they guarantee.

Toyota would point to the longevity of its RAV4 EV and hybrids, as does Ford, but those were NiMH. Many feel the Li batteries will prove just as durable if not more so, but the fact is, the data does not exist to say one way or the other, except in lab testing, and those results are not being released. The battery is not going to just stop working one day, it will gradually taper off. I expect at least 10 years of usable life. Nissan has suggested that you might replace just a cell or two should they go bad, without replacing the whole battery. In 10 years, I would bet you dollars to doughnuts batteries will be much improved and much cheaper.

Quote:
Is battery charging linear - both in terms of time and in terms of electricity consumption? Does it take 1/2 the time and 1/2 the electricity to charge 1/2 the battery?
No it is not linear the last 80% takes more time and energy. I would google http://www.gm-volt.com this has been discussed on that site.


Quote:
How much efficiency is lost in battery charging (that is electricity consumed to charge vs. electricity available in the battery to run the car when charging is complete)?
I would say based on what I am seeing with the Volt that charging is 88% to 73%. With variability due to temperature mostly, I think. The Volt will run heat/ac to heat and cool the battery to keep it in the optimum temperature range.

Quote:
When a battery pack is declared, say 18 kWh, is that the usable juice (say real capacity 20 with 90% DOD) or is it only usable to the extent of DOD (that is, not all of the battery pack's kWh are available for driving unless DOD can go to 100%)?
Right the usable portion or DOD is not typically the same as the stated battery pack size. Ford has said that they are using 19-20 kwh of their 23 kwh. GM uses 10.x of its 16 kwh pack. The Leaf is thought by many to use something less than the 24 kwh pack size, but there is some debate on http://www.mynissanleaf.com if that is actually the case, and how much less. Just when it seems settled the debate starts up again. Don't understand why Nissan will not just state plainly how much they use.


Quote:
If your battery pack has a 100 mile range, what would 'proper' behavior for be for recharging after daily 10 miles trips (recharge every night, recharge after 5 days, recharge after 9 days...)?
Discharging and recharging the battery does shorten the life of the battery. If you can drive multiple days without charging you probably should. If you know that you will never have an emergency need pop up that would require greater range and are looking to maximize the life of your battery then you should probably recharge only when necessary. However, for most people, wanting to be ready to deal with the unexpected will mean that they will "top off" as soon as they are able, I know I do. Doing so should not significantly shorten the battery life.



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Old 09-07-2011, 01:42 PM   #3 (permalink)
 
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Default Re: Battery education

Wow, thanks - VERY helpful straightforward answers. I appreciate it.
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Old 09-07-2011, 04:26 PM   #4 (permalink)
 
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Default Re: Battery education

Quote:
Originally Posted by Charged Up
Discharging and recharging the battery does shorten the life of the battery. If you can drive multiple days without charging you, know that you will never have an emergency need pop up that would require greater range and are looking to maximize the life of your battery then you should probably recharge only when necessary. However, for most people, wanting to be ready to deal with the unexpected will mean that they will "top off" as soon as they are able, I know I do. Doing so should not significantly shorten the battery life.
It's true that the discharge/recharge is what shortens the life of the battery, but the idea that you drain the battery and recharge only when needed is only true for dumb batteries.

With smart batteries - the ones in cars - it's better to keep them charged at least to 80%. This allows the battery's logic to cycle through the battery's cells more efficiently thus increasing the life of the battery.
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Old 09-07-2011, 11:46 PM   #5 (permalink)
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Quote:
No it is not linear the last 80% takes more time and energy. I would google http://www.gm-volt.com this has been discussed on that site.
Opps, not what I meant to say. It is the last 20% that takes more time and energy.

Quote:
It's true that the discharge/recharge is what shortens the life of the battery, but the idea that you drain the battery and recharge only when needed is only true for dumb batteries.

With smart batteries - the ones in cars - it's better to keep them charged at least to 80%. This allows the battery's logic to cycle through the battery's cells more efficiently thus increasing the life of the battery.
I was not recommending that he only charge when the battery was completely drained. He gave a scenario of 10 miles use each day, and asked what was the "proper" recharging schedule. "every night, recharge after 5 days, recharge after 9 days?"

If the goal is to save electricity. Then he should recharge only once it is necessary, because as noted above, getting the last 20% of charge into a battery consumes a disproportionate amount of electricity. Also each discharge/recharge cycle does reduce the life of the battery, so reducing the number of recharge cycles will prolong the longevity of the battery. How significant that is for this situation remains to be seen. If the user is certain of not needing additional mileage it may be worth it, if only for the electric bill savings.

Your response seems to allude to the "memory effect" associated with other batteries. Li batteries are not very prone to this, and it should not be a concern for EV buyers.

I have not read that keeping an 80% SOC is advantageous, if you have a source on that I would be happy to read it. I have read that a 50% SOC is recommended for long term storage of a Li battery.



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Old 09-08-2011, 06:24 PM   #6 (permalink)
 
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Default Re: Battery education

Quote:
Originally Posted by Charged Up
Your response seems to allude to the "memory effect" associated with other batteries. Li batteries are not very prone to this, and it should not be a concern for EV buyers.

I have not read that keeping an 80% SOC is advantageous, if you have a source on that I would be happy to read it. I have read that a 50% SOC is recommended for long term storage of a Li battery.
No memory effect alluded to. What I was getting at is the batteries have logic to know which cells are the best/next/least used to be cycled next. If the battery is not charged to 80% then it limits the logic built into the car's battery to determine which cell to use next - this is very close to how solid-state hard drives work. --I do not have a source readily available, it was from MNL forum IIRC.

It is correct that it's a bad idea to drive constantly on lower battery charge (<20%) because as a battery's charge goes down, the Volts outputted goes down, requiring the Amps to rise which discharges the battery at a faster rate.

For long term storage 40-60% is the range. That's why when you get any type of rechargable battery items - toothbrushes / screwdrivers / etc - you can turn them on and use them, though this is a bad idea as they need to be recharged to prevent a deep discharge causing capacity loss.
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Old 09-08-2011, 09:07 PM   #7 (permalink)
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Quote:
If the battery is not charged to 80% then it limits the logic built into the car's battery to determine which cell to use next
So your recommendation would be charge the battery everyday even if you only use 10 miles?

Are you saying cells are used one at a time? Or maybe what you are saying is that cells are "pulled" from differently, depending on their SOC?



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Old 09-09-2011, 09:52 PM   #8 (permalink)
 
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Default Re: Battery education

With dumb batteries, such as those in a laptop, it's a good idea not to charge every time and cycle down to 20%. A manual approach to cycling batteries.

The smart EV batteries solves this overhead and does it all for you. This allows the battery to choose which cells to use be it because they're at the best temp, they havn't been used in many cylces, etc. The key is to assist the battery in managing itself in the best way possible.

So, yes, it is best to charge the car to 80% even if you drive only 10 miles.
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Old 09-11-2011, 10:50 PM   #9 (permalink)
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Default Re: Battery education

Quote:
Originally Posted by travisty

So, yes, it is best to charge the car to 80% even if you drive only 10 miles.
Well, I know we will be able to set start and stop times for the Focus E. But not sure if you can charge to a certain percent. If not, maybe Ford is listening to this battery saving idea???
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Old 09-12-2011, 01:26 AM   #10 (permalink)
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Ah OK now I see Travisty is not talking about the same thing we are talking about.

The original question was, if after day one I have 90% of my SOC left should I recharge? The answer was, although not a big deal either way, if you want to save electricity and make some minor contribution to extended the life of the battery then no.

Travisty chimed in with:
Quote:
t's better to keep them charged at least to 80%
When he said "keep" them at least 80%, I thought he was saying don't allow them to dip down below 80% SOC if possible, instead I now think what he was saying is, when you recharge don't recharge to more than 80% of the pack capacity?

At this point I am not clear, but if the latter, then that is not necessary because Ford already limits what portion of the pack you can use to protect the battery. Unlike the Leaf, which apparently defaults to use 100%, but recommends that the owner select the 80% option. Again there is debate about whether the 100% really is 100%.

So it should be unnecessary for Ford to give us an 80% option because they are already limiting the top end to protect against overcharging, and on the low end against discharging to deeply.



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