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I read a thread on the hill-top setting, but one of the things I wanted to figure out was the cost of charging the first 10 percent vs the last 10 percent, in terms of kWh required.

I would appreciate pointers to any threads that addressed how linear the charging process is, and the best way to get the cheapest miles per kWh -- other than charging up at the few places that let you do it for free :)
 

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it should all cost the same - the reason the last 10% charges more slowly is because the bolt tapers off the rate of charge. you're paying for the amount of electrons pushed into the bolt, not the time it takes to do it.
 

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borkbork: So this sounds like it's linear. No reason not to charge it to 100% every time, assuming you don't live at the top of a hill. Right.
 

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borkbork: So this sounds like it's linear. No reason not to charge it to 100% every time, assuming you don't live at the top of a hill. Right.
The charging is process not linear: As stated, the rate slows down for the last 10% due to the computer brain in the charger protecting the battery's chemistry. That does NOT mean it costs more money because it slows down, for electricity is charged by the kWh consumed, not by how long it takes to deliver. So there's no financial benefit to avoiding 100% charges. E.g. 10 kW delivered for an hour is 10 KWh. And 5kW delivered for 2 hours is still 10 kWh.

If you are in a hurry (like on a road trip), skipping the last 10% will optimize your time, provided you can still get to the next charging station. If you are asleep, it doesn't matter.

Since lithium batteries last longest when they're not fully discharged nor fully charged, some people feel they want to use Hilltop Mode to limit their charging to extend battery life. Others feel that the 90% full battery you get from doing so isn't low enough a limit to make much of a lifetime difference. (I've read that Li batteries like to live in the 20% to 80% range.).
 

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The charging is process not linear: As stated, the rate slows down for the last 10% due to the computer brain in the charger protecting the battery's chemistry. That does NOT mean it costs more money because it slows down, for electricity is charged by the kWh consumed, not by how long it takes to deliver.
To give you an analogy, a quart of milk costs the same whether you drink it all up in a day or in a week.
 

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The charging is process not linear: As stated, the rate slows down for the last 10% due to the computer brain in the charger protecting the battery's chemistry. That does NOT mean it costs more money because it slows down, for electricity is charged by the kWh consumed, not by how long it takes to deliver. So there's no financial benefit to avoiding 100% charges. E.g. 10 kW delivered for an hour is 10 KWh. And 5kW delivered for 2 hours is still 10 kWh. QUOTE]

This may be true for Level 2 charging, but all the Level 3 EVSE I have paid for charge by time (usually by the half-hour), not kWh delivered. The first 30 minutes gets you many more kWh & many more mpch (miles per charging hour) than any ensuing 30 min. of charging.
 

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None of these replies are correct, and lasitter has a good question. I haven't been able to find much info on the subject.

Charge efficiency does decrease as the battery approaches full charge. Lithium ion chemistries retain better charge efficiency than lead acid as it approaches full charge, but I've never seen a graph showing the efficiency curve.

From a practical point of view, we can assume charge efficiency is constant, regardless of charge state. BEVs like the Bolt do not discharge their batteries to empty, nor do they charge to full. Instead, they operate somewhere between ~25% to ~80% capacity. For this reason, the charge acceptance efficiency remains high within this range.

The thing that reduces efficiency the most is the fans and pumps that continue to run, despite the reduced charging rate. The cost of the wasted electricity is negligible. What isn't negligible is the extra strain placed on a battery whenever it is charged to 100%. Using hilltop mode will better preserve the capacity, and therefore the number of years the battery lasts before needing replacement.

I have been meaning to test charging efficiency on a LiFePO4 battery and compare it to a lead-acid, but I don't have the necessary metering and logging equipment. Since the 12v battery on all cars is maintained at full charge, a higher percentage of energy is wasted keeping them charged, especially since lead-acid is very inefficient as it approaches full.

Last year a door was left open on the Prius for several days, and it killed the 12v battery. I replaced it with a LiFePO4, which should be more efficient and hopefully longer lived.
 

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I read a thread on the hill-top setting, but one of the things I wanted to figure out was the cost of charging the first 10 percent vs the last 10 percent, in terms of kWh required.

I would appreciate pointers to any threads that addressed how linear the charging process is, and the best way to get the cheapest miles per kWh -- other than charging up at the few places that let you do it for free :)
lasitter, the cost is the cost per kW put in, so no difference first or last or middle kW for me.

To your larger question of maintaining an efficient practice - I am focusing on maintaining good battery life.

My practice now is to charge up to 90% (hilltop reserve) when I charge unless I know I am going someplace where prudence suggests I should charge to 100%. My daily drives are often only 25-40 miles, so am NOT charging my Bolt up every night, but rather just when I get down to the 15-30% spectrum. (No reason to be compulsive). This should help in terms of keeping battery in that 20-80% or 10-90% range which prolongs lithium battery life. Having bought the car I would like it:nerd: to have best range possible 5 or 10 years from now... :nerd:
 

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Since lithium batteries last longest when they're not fully discharged nor fully charged, some people feel they want to use Hilltop Mode to limit their charging to extend battery life. Others feel that the 90% full battery you get from doing so isn't low enough a limit to make much of a lifetime difference. (I've read that Li batteries like to live in the 20% to 80% range.).
A friend of mine is a walking EV dictionary. An engineer who know everything about EV's, anyhow his range is 30%-70% ideally for your lithium batteries (phone, car, ...). He has a Volt which has some 'hidden reserve', which is used for conditioning. According to him the Bolt has none, so at least use Hilltop Reserve, and ideally in addition to that keep it down in the 30%-70% range for daily commuting.
 

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To your larger question of maintaining an efficient practice - I am focusing on maintaining good battery life.

My practice now is to charge up to 90% (hilltop reserve) when I charge unless I know I am going someplace where prudence suggests I should charge to 100%. My daily drives are often only 25-40 miles, so am NOT charging my Bolt up every night, but rather just when I get down to the 15-30% spectrum. (No reason to be compulsive). This should help in terms of keeping battery in that 20-80% or 10-90% range which prolongs lithium battery life. Having bought the car I would like it:nerd: to have best range possible 5 or 10 years from now... :nerd:
Ok, now that I've had my Bolt for a week, I'm facing this same question. How to maximize battery life? Hilltop reserve is a given. But like Thor above, I also don't need to charge every night, and could probably go all week with just daily commute and errands.

But would it be better to charge very gently every night, keeping the battery in the 85-90% range? Or to let the battery get down to perhaps 50% before charging back to 90%? More shallow charges? Or less frequent charges? Which is better for long-term battery life?
 

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This is a great question, the answer to which I am not sure we know completely. Should we use the 120 volt charger (@ 4 amps) plugged in every night to "trickle" back up to 90%, and only use our Level 2 EVSE when that rpch (range per charging hour) NEEDS to be 25 miles? The higher voltage IS more efficient in terms of "energy paid the electric co./energy retained in battery". But in terms of battery life (degradation), the lower voltage will heat the battery less. Is this good? Is it worth the hassle? I do what was outlined above, drive to 20%, charge @ 240 volts to 90%. Alright you engineers out there, enlighten us.
 

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Alright you engineers out there, enlighten us.
120V*8A=0.96kW ... 0.96kW/64kWh = 0.015C = C/67
240V*32A=7.7kW ... 7.7kW/64kWh = 0.12C = C/8

I wouldn't worry about degradation for anything lower than 0.5C or C/2, hence there's no appreciable difference in expected degradation between Level 1 and Level 2 charging.
 

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borkbork: So this sounds like it's linear. No reason not to charge it to 100% every time, assuming you don't live at the top of a hill. Right.
If you disable the hilltop reserve when charging you will disable the regen for the first part of your next trip. This can be disconcerting if you are use to the regen being there as well as increasing wear on you brakes. This will lead slightly increase cost from reduced efficiency and increased maintenance.
 

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Would like to add that I would expect cell balancing to happen near the end of a charge cycle, so there's likely a bit of energy put into the pack and then burnt off, then put back in order to balance the cells. Not an amount I would be concerned about though.
 

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A note as an engineer that works on hardware/firmware/system hybrid systems, with specialized custom hardware. In general it's best to just stick to a rule of thumb or two. Trying to game the system or baby it is usually a waste of effort, because 1) there is so much software between you and those cells we really have no idea what the optimal behavior is and 2) oftentimes not even the engineers developing it can give you a definitive answer. This is because most of the time there are two many variables to account for.

For example, say you consult an Oracle and charge your battery optimally. That's fine, but what about your driving habits? Maybe the particular way you drive eliminates all of that advantage. Or maybe it's your commute (you have to drive a lot of altitude), or maybe your climate. And maybe your particular driving is a bad combination with your particular charge style, or maybe it's good. Who knows? Oftentimes I've seen situations where people try to game a system and it actually works against them, because they're fighting the intelligent software which assumes the user isn't trying to game it.

For BEV's I think the rules of thumb are to at least use Hilltop Reserve. Keeping it plugged in at night, especially in a cold climate, is also a good idea as I believe it will then make sure to keep the battery in a ready state (temperature mainly) by using the house current (when it's unplugged it doesn't know when you'll drive it again, so I think it mostly shuts down). Finally, depending on your daily drive try to keep it 30%-70% charge, but if you need to go out of don't sweat it. These rules are likely to help and unlikely to cause any trouble.

For me, my workplace has free level 2 charging readily available and I live 10 minutes away. So I have Hilltop Reserve, I try to pull the charge when it gets to 70% or so, and will limit my home chargers to trickle for overnight (but it doesn't get that cold here so I'm probably needlessly gaming it there)
 
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