Power consumption for a full charge?

[SegwayDan]

Has anyone measured or closely estimated the number of watt-hours consumed for a full charge of an i2?

Also, has anyone measured the peak current flow (amps) during a charge cycle? How about a graph of amps vs. hrs over the period of a charge cycle?

[joe]

Of course: http://forums.segwaychat.com/showthread.php?p=147463
Multiply this with the hours needed for a full recharge (10?) and you'll get the power consumption.

--
Johannes

[quade]

While obviously a meter reading would be better there are several ways of attacking this problem that don't really require it for a rough order of magnitude estimate.

If we knew for certain what the batteries were capable of holding in terms of energy, you could just work that backwards. Whatever the batteries can hold, (kinda obviously) is what you have to put into them to charge them (plus maybe a bit more for losses).

Since I don't actually know that at this moment, I can also work backwards from some of the things we do "sort of" know and get a back of the napkin estimate.

1 -- nominal output is about 750 watts.(* (http://www.leginfo.ca.gov/pub/07-08/bill/asm/ab_0451-0500/ab_470_bill_20070720_chaptered.html))
2 -- nominal speed is about 10 mph.(** (http://www.10mph.com/))
3 -- max range is about 24 miles.(*** (http://www.segway.com/personal-transporter/specs_i2.html))

750w x 2.4h = 1.8kwh

4 -- 8 hour charge from zero to full.

1800w / 8h = 225w

Again, these are just estimates. I have a 80% confidence level that these estimates are within 20% of actual.

[yosgof]

Funny how it works differently in the metric system...

Max velocity - 20Kmh.
Max range - 40 Km
-> Max 2 hours of gliding at top velocity.

For this sort of calculation one must assume that power conversion efficiency of the charger is about 70%. The rest is heat.

If I remember correctly each wheel motor is rated at 3hp so the whole machine is 6hp or 4476W and the motor efficiency is probably 75%.

So 4476W/.75 * 2hr / .7 =~ 17Kw

I have an order of magnitude difference with Quade... I'm probably wrong but am not sure where.

[quade]

I don't think metric has anything to do with it!

I think the main issue is that you assume the unit is using 6hp at all times.




(BTW, don't ever get me started on the "merits" of metric over imperial . . .it's a pet peeve / running gag of mine in engineering and scientific circles. I have a LOT of fun playing with people's minds on that subject.)

[yosgof]

Metric was a joke :-)

Top speed = top power so I guess its a valid assumption.

[quade]

Metric was a joke :-)

Top speed = top power so I guess its a valid assumption.

True if you're talking about race cars, which, as I think we can all agree, the Segway is not. :D

BTW, just got off an IM with a buddy of mine that works for a major cell phone manufacturing company. He's the engineer in charge of designing batteries and chargers for them.

He's estimating that the charger losses into the batteries should be 20%-25% during a "fast charge", but only at most about 5% during a "slow" charge like the one we're talking about on the Segway.

In either case, I stand by my 80% confidence of accuracy within 20%.

Should be interesting to see who proves me wrong first! Certainly there must be somebody out there with a recorder willing to do that.

[hellphish]

Top speed = top power so I guess its a valid assumption.


LOL you might want to look into that.

[bystander]

Funny how it works differently in the metric system...

Max velocity - 20Kmh.
Max range - 40 Km
-> Max 2 hours of gliding at top velocity.

For this sort of calculation one must assume that power conversion efficiency of the charger is about 70%. The rest is heat.

If I remember correctly each wheel motor is rated at 3hp so the whole machine is 6hp or 4476W and the motor efficiency is probably 75%.

So 4476W/.75 * 2hr / .7 =~ 17Kw

I have an order of magnitude difference with Quade... I'm probably wrong but am not sure where.I'm fairly sure 17 kw is way off.

This is the way I figure it:

Each Saphion pack is 400 watt-hours capacity. There are two, for a total of 800 watt-hours. Furthermore, at Segfest 2006, we were told by the representative of Valence that brand-new Saphions may be 20% higher capacity, in order to make the 400 wh spec after they age a little.

So worst (best?) case, one has to get at least 0.96 kw out of the AC mains to fill the packs. And, as you have noted, one must include the charger efficiency when determining the total. In my experience, I think the charger efficiency is approx 86%, but I don't know for sure.

If it were, then 0.96 / 0.86 = 1.11 kw hours.

If it is indeed as bad as 70%, then the figure would be 1.37 kw hours.

Note that this is for a full charge when the batteries are new. As they age, their capacity will drop down to 800 wh, and the amount required for full charging will drop to as little as 0.93 kw hours.

Also note that the amount will be somewhat less when the batteries are not completely discharged.

Concerning the motors, what I heard is that the _peak_ capacity is 2.5 hp each, or about 3.7 kw put together.

Form my point of view, it stands to reason that drawing 3.7 kw out of a 0.96 kw hour device (the batteries) will only permit operation for about 15 minutes.

Since most long distance glides that exhaust the batteries completely last between 2 to 3 hours, I really doubt the 17 kw figure you came up with.

0.96 kwh / 2 hours = 480 watts, and
0.96 kwh / 3 hours = 320 watts

sounds much more reasonable (to me) as an average power draw when travelling at full speed.

[bystander]

Has anyone measured or closely estimated the number of watt-hours consumed for a full charge of an i2?

Also, has anyone measured the peak current flow (amps) during a charge cycle? How about a graph of amps vs. hrs over the period of a charge cycle?You might find the charts on this thread useful:

http://forums.segwaychat.com/showthread.php?t=10079

post #4 has a charging curve and post #1 has a discharge curve.

[bystander]

1 -- nominal output is about 750 watts.(* (http://www.leginfo.ca.gov/pub/07-08/bill/asm/ab_0451-0500/ab_470_bill_20070720_chaptered.html))

750 watts is the amount the device must be less than, on average, to qualify as an EPAMD. It is not the actual amount consumed unless you are going uphill for 20 miles.

I think you'll find the typical power draw is more like 200-300 watts when cruising on the flat. And maybe 300-400 watts if traveling near full speed, or cruising up gentle hills.

The 750 watts isn't a hard limit, the PT will dissipate more than that for short periods of time, such as keeping you from falling when hitting a bump.

Getting back to actual pack capacity, here's the page (http://www.segway.com/personal-transporter/lithium_ion.html) where the capacity of the Saphion and NiMH packs is stated. Figures are for a single pack, and the PT uses two, so NiMH packs are good for 440 wh (per pair) and the Saphion packs are good for 800 wh (per pair).

If you have a NiMH or Saphion pack in front of you, you can read the nominal voltage and ah capacity from the pack's label. And then calculate the wh capacity.

Interestingly, as I noted elsewhere, the representative from Valence that spoke at the 2006 Segfest indicated that brand new Saphions may be up to 20% higher capacity than the rated spec of 800 per pair. This is due to the statistics of manufacturing. They want virtually all Saphions released to exceed 800 _after_ they age for a while, so all are produced at the higher level.

This can make it problematic for those trying to determine the exact capacity, as it throws calculations off by up to 20%, unless you have data logging equipment and means to measure current inside the PT (without breaking it).

2 -- nominal speed is about 10 mph.(** (http://www.10mph.com/))

I'll go along with that. With crossing streets and waiting for lights, my cross-city average is 8-10 mph, even when hitting 12 on the empty sidewalks.

3 -- max range is about 24 miles.(*** (http://www.segway.com/personal-transporter/specs_i2.html))
The 24 mile range, which was discussed at even older Segfests, is the result of a closed, flat course, with little if any acceleration or deceleration. Normal people can't do 24 miles. Unless they weigh very little or travel at considerably less than full speed.

The reference page (http://www.segway.com/personal-transporter/specs_i2.html) you quoted here can also be used to estimate how much power it takes to do a full charge. If you go down to the section entitled "POWER Lithium-ion (Li-ion) Battery Packs", you will find this:

Batteries can be recharged by utilizing any 90 to 260 volt and 50 to 60 Hz AC outlet (typically accessible in most countries). A complete cycle charge will take eight to ten hours...

So, if one looks at the ratings plate of an off board charger, (or removes the CSB from a PT and looks at it's ratings plate) one will find it is rated for 140 watts.

8 hours at 140 watts is 1.12 kwh, and of course 10 hours would be 1.4 kwh.

I have found though, in my experience that 140 watts is a maximum avg. value and not a typical avg. value. In my case I am seeing more like 110-125 watts.

[SegwayDan]

Thanks to all for the imput.

I'm going to take bystander's estimates in the 1-1.5 KW-h range for my cost calculation:

From a recent electric bill, which includes multiple rates, fuel charges, and various taxes: 1550 KW-h cost $170 for a month.

Doing the math, that comes out to 11-16.5 cents per charge.

Now that qualifies as CHEAP in my book!

[quade]

8 hours at 140 watts is 1.12 kwh, and of course 10 hours would be 1.4 kwh.

Ah . . . well . . . my original estimate was 1.8kwh.

Still, not horrible for just guessing based on random stuff.

[dgbint]

It's pretty darn easy, so let's not make it wildly complicated.

My Li-ions are labelled as 73.6V 5.2 AH.

This then means that the maximum energy storable in a single batery is about 380 WHr.

Call it about 800 WHr for a pair.

Depending on the charging efficiency, say 1,000 - 1,200 WHr ( 1.0 - 1.2 KWHr ) needs to be pumped in for the 800 WHr to be stored.

By 'pumped in' I mean 'drawn from the mains'.

So that is the mains load.

I think it has been discussed elsewhere in other threads and others came up with a similar value.

Power costs about 10c - 25c per KWHr depending on your location and plan.

So the cost then will be in the range of 10c - 30c for a full charge.

QED

[PeteInLongBeach]

Doing the math, that comes out to 11-16.5 cents per charge.
Now that qualifies as CHEAP in my book!

Cheap, until you add in $1600 for new batteries every 3-5 years...

[bystander]

Cheap, until you add in $1600 for new batteries every 3-5 years...If one can travel 20 miles per full charge, and the expected cycle life is 1000 charges, that's 20000 miles.

Which is 8 cents per mile, or $1.60 per charge.

Granted, $1.75 is more than $0.15, but that's still kind of low, isn't it?

And if you actually traveled 20000 miles on your PT, and the batteries still worked, any additional miles would be at the 3/4 of a cent rate.

[PeteInLongBeach]

If one can travel 20 miles per full charge, and the expected cycle life is 1000 charges, that's 20000 miles.

Except the Lithium-ion battery life is not just limited by # of cycles, but also age limitations of 3-5 years. I'm not aware of many people that are going to glide 20,000 miles in 3 years. But let's say someone commutes 10 miles per day, 5 days per week = 2400 miles per year = 7200 miles over a 3 year battery life. That's about 22 cents per mile ($1600 batteries).

By comparison, a car that gets 30 miles per gallon is costing about 10 cents per mile (I know, doesn't include maintenance and other related expenses, but just for energy cost comparison).

[bystander]

Except the Lithium-ion battery life is not just limited by # of cycles, but also age limitations of 3-5 years. I'm not aware of many people that are going to glide 20,000 miles in 3 years. But let's say someone commutes 10 miles per day, 5 days per week = 2400 miles per year = 7200 miles over a 3 year battery life. That's about 22 cents per mile ($1600 batteries).

By comparison, a car that gets 30 miles per gallon is costing about 10 cents per mile (I know, doesn't include maintenance and other related expenses, but just for energy cost comparison).Who says Saphions will have the shelf life of Li-ion? If the chemistry is different enough, the shelf life won't be the limiting factor.

On the car, what about the manditory insurance requirements in this state? If you don't drive much, you may be paying 20-40 cents per mile, no?

[guyler]

My i2 consumes 1 amp or about 125 watts during charging. Never did a full charge, so don't know what that takes. My guess would be about 1 kilowatt hour or a little less.

[Michael Taylor]

When we did some testing for the white paper JD Heinzmann and I wrote on efficiency and emissions, we found an average of 1.04 kWh to charge up Lithium Ion batteries.

[hellphish]

My i2 consumes 1 amp or about 125 watts during charging. Never did a full charge, so don't know what that takes. My guess would be about 1 kilowatt hour or a little less.

This is counter to what your owner's manual instructs you to do, yes?

[polo_pro]

Except the Lithium-ion battery life is not just limited by # of cycles, but also age limitations of 3-5 years. I'm not aware of many people that are going to glide 20,000 miles in 3 years. But let's say someone commutes 10 miles per day, 5 days per week = 2400 miles per year = 7200 miles over a 3 year battery life. That's about 22 cents per mile ($1600 batteries).

By comparison, a car that gets 30 miles per gallon is costing about 10 cents per mile (I know, doesn't include maintenance and other related expenses, but just for energy cost comparison).

First let me say that your numbers look reasonable. Having actually done this (though for only 1.5 years), I may have chosen slightlly lower numbers like 8 miles and 4 days per week. But I'd be nitpicking.

One thing I'd like to say is that anyone who's commuting like both of us described doesn't need Li-Ions (assuming they can charge up at work if needed). NiMH are fine for this user.

The NiMH batteries won't last as long because they have only 500 full charges before needing replacement. So to get the 720 charges over the three year period, you'd need 2 pairs. However, used pairs of NiMH batteries can be found for $200 that have more than half their charges left. So as a careful consumer, you could probably bring your total battery replacement cost down to $400 using used NiMH batteries. Using these numbers with your above numbers, the cost should drop a bit more than a nickel a mile (plus another penny for the electricity costs) for the "energy costs" as you put it.

I know you wanted to ignore maintenance costs, insurance costs and of course registration costs. But I will point out that those numbers heavily favor the segway given that you've already handled battery replacement costs.

ps - One annoying fact about NiMH is that you have to "condition" them every few months. But just like trying to buy used NiMH with alot of life left in them, it's an annoyance you have to live with if you want to keep costs under control. Just like you take your car in for maintanence regularly, eh?

[bentbiker]

This is counter to what your owner's manual instructs you to do, yes?
I believe he means he never totally discharged the machine -- not that he unplugs prior to being fully charged.

[quade]

I believe he means he never totally discharged the machine -- not that he unplugs prior to being fully charged.

Or possibly that he's never had a meter on the machine for the full duration of a recharge cycle.

[quade]

My Li-ions are labelled as 73.6V 5.2 AH.

Where is this label? Would I have to disconnect the batteries to see it? Can anybody provide a snap shot?

[guyler]

Since my i2 has never been wholly discharged I really don't know what it would take to fully charge it. As previously stated it draws 1 amp at 123 volts during the charging cycle, so I guessed 1 kw hour for a full charge.

Incidentally, I always keep the i2 plugged in when not in use, but nothing can be learned from this as to what a full charge entails.

[bystander]

When we did some testing for the white paper JD Heinzmann and I wrote on efficiency and emissions, we found an average of 1.04 kWh to charge up Lithium Ion batteries.Yes, that's a great reference for this thread.

Here's the link to the info page:
http://www.segway.com/solutions/environment.html

And here's the link to the white paper itself:
http://www.segway.com/downloads/pdfs/energy_efficient_segway_whitepaper.pdf

My only curiosity would be, what was the average watt-hour capacity of the battery packs actually used in those tests? I'm trying to get a more accurate figure on charger efficiency.

If 800 wh, then 800/1040 = 77%
If 850 wh, then 800/1040 = 82%
If 900 wh, then 800/1040 = 86%
If 950 wh, then 800/1040 = 91%

One presumes these tests would be performed with freshly produced batteries and the avg. measured battery wh rating would be in the middle of that range.

[polo_pro]

Where is this label? Would I have to disconnect the batteries to see it? Can anybody provide a snap shot?

Quade, bite the bullet. Pop the batteries. They were meant to be removed. They YEARN to be removed, just so you can see all the wonderous stuff hidden back there. Like for instance how your engraved serial number doesn't match what's on the barcode on the back of your segway!

No, don't ask me how I know. Remember when you were taking pictures at the polo field and you THOUGHT I was playing. Nope...that was my double. I was actually over at your segway verifying that the serial numbers didn't match...

8^) 8^) 8^)

[polo_pro]

Ah...it's like the meticulous/mathematic side of my brain is back. As always, I enjoy reading bystander's references and extra info. It was sorely missed for many months...almost felt like I had a lobotomy! 8^) 8^) 8^)

Yes, that's a great reference for this thread.

Here's the link to the info page:
http://www.segway.com/solutions/environment.html

And here's the link to the white paper itself:
http://www.segway.com/downloads/pdfs/energy_efficient_segway_whitepaper.pdf

My only curiosity would be, what was the average watt-hour capacity of the battery packs actually used in those tests? I'm trying to get a more accurate figure on charger efficiency.

If 800 wh, then 800/1040 = 77%
If 850 wh, then 800/1040 = 82%
If 900 wh, then 800/1040 = 86%
If 950 wh, then 800/1040 = 91%

One presumes these tests would be performed with freshly produced batteries and the avg. measured battery wh rating would be in the middle of that range.

[PeteInLongBeach]

Who says Saphions will have the shelf life of Li-ion? If the chemistry is different enough, the shelf life won't be the limiting factor.

On the car, what about the manditory insurance requirements in this state? If you don't drive much, you may be paying 20-40 cents per mile, no?

Whereas I read that Saphions are a safer chemistry and design, I have yet to find any documentation that says the they have X% better shelf life than generic Li-ions. What I have read is that Li-ions in general will deteriorate over time regardless of the number of cycles. With the current cost of this battery technology, such deterioration is a concern. If there is documentation or proof that our Saphions have a shelf life that is significantly better than 3-5 years, I would feel much relieved to know about it. I'm already facing the possibility of buying a new $800 battery to replace one that appears to have failed in less than 18 months, so I'm a bit sensitive to this.

Insurance cost was not part of my analogy - I was only comparing the energy costs.

[bystander]

Whereas I read that Saphions are a safer chemistry and design, I have yet to find any documentation that says the they have X% better shelf life than generic Li-ions. What I have read is that Li-ions in general will deteriorate over time regardless of the number of cycles. With the current cost of this battery technology, such deterioration is a concern. If there is documentation or proof that our Saphions have a shelf life that is significantly better than 3-5 years, I would feel much relieved to know about it. I'm already facing the possibility of buying a new $800 battery to replace one that appears to have failed in less than 18 months, so I'm a bit sensitive to this.

Insurance cost was not part of my analogy - I was only comparing the energy costs.I understand and can relate both of your points.

I'm sorry to hear about the problem you are having with your 18 month old battery. But unless it was sitting on a shelf, and not being discharged and charged in your PT all that time, chances are it failed for reasons other than the shelf life degradation problem.

I have yet to find any documentation that specifically states that Lithium Ion Phosphate definitely has the shelf life decay problem.

I too have read about plain Li-ion deterioration, but the things I've read where it discusses such things are many years old. This deterioration is not documented in data collected in the last couple of years. I don't know for sure if the problem is really gone or not, but I'm not seeing fresh data that supports that notion.

Back when NiCad was a bigger segment of the market, and Li-Ion was new, the shelf life deterioration was a large comparison point, and NiCad manufacturers would be remiss if they didn't publicize it widely.

A battery consumer that needed lots of backup batteries (such as a Telco) could save money and guard against inflation by buying 5 sets of NiCad or Lead-Acid batteries, each lasting two years, for a total of 10. And the battery seller could make a big lump sum sale because the Telcos had hundreds of installations that needed the batteries.

Back then, "old chemistry" battery manufactures wanted to make it clear that the newfangled "Li-Ion" battery users could not take advantage of this buying pattern. This was done in order to promote sales of obsolete (and otherwise difficult to sell) technology. With Li-Ion, battery sellers had to make smaller sales every couple of years instead of a decade sized lump sum. This is less convenient for the seller, of course.

But that was quite some time ago. The Li-Ion shelf life deterioration was true and documented a decade or more ago. But is it true now? Are you completely sure?

I agree that buying a spare Li-Ion, phosphate or not, and not using it until your everyday in-use Li-Ion needs replacement may not be a good idea. But on PTs, most everyone has only one set, and those who have more than one set (playing polo, long distance gliding, etc) cycle through them. A battery that is being discharged and charged several times a month is not prone to "shelf-life degradation".

The point being, Saphions that we are all using have not significantly demonstrated the shelf-life decay symptom, mostly for the reason that none of we users have them "on the shelf". So I don't see why we should make such a big deal about it.

As for the insurance, I realize my comment is OT and I understand you do not want to compare this. However, for many, it is a real "real world" expense that can be part of the equation of what device is most economical. For those that have no choice but to own a car, I'm sure that they don't want to hear about it. But for those who car ownership is an option, it can be significant.

I'm not trying to tell you that you are incorrect, rather, I am presenting some ideas that others reading this thread may be interested in.

[polo_pro]

I understand and can relate both of your points.

I'm sorry to hear about the problem you are having with your 18 month old battery. But unless it was sitting on a shelf, and not being discharged and charged in your PT all that time, chances are it failed for reasons other than the shelf life degradation problem.

If you were looking for possible other reasons why Pete's batteries fail, I dug around (because I remember reading something earlier this year) and found this:

http://forums.segwaychat.com/showpost.php?p=138973&postcount=35

Now I'm not sure how this might have affected things. But at times Pete wasn't taking the best care of these Li-Ions that failed. I will also point out that Pete did rectify the situation (by taking off the handlebars) as soon as he found out. But still those Li-Ions were not being charged for weeks at a time even if the CS wasn't draining them through that period. The manufacturer recommends "keeping the segway plugged in at all times (or make sure the Li-Ion batteries are on an off board charger)".

[bystander]

Steve, see PM.

[PeteInLongBeach]

Steve, there are a couple of conditions related to my battery failure issue that suggest that the battery was defective:

-This is the rear battery that has gone bad. It is the front battery that would be most sensitive to being unplugged as it supplies standby power to the CS/keyport.

-This failed rear battery has always been much slower to charge, even when new. Had I realized this might be a symptom of a defective battery, I would have dealt with it much sooner.

This is certainly not the first reported instance of premature failure of an early Saphion battery. I think if Segway wants to maintain an image of quality & reliability, draw new customers to this technology and repeat customers to new models, they should consider a longer warranty period for their most expensive consumable part.

[Sharkie]

Actually, as I understand it, it's the rear battery that goes dead if the machine is left unplugged. At least that's the one that went dead for the local dealer when a Gen1 GT was left unplugged.

Jim

[bystander]

Thought we were talking about shelf life here.

I also recall that the trouble with the idle power drain was on the rear packs. It was not a shelf life issue. It was an issue with a subcontractor tweaking the design of the electronics in the handlebar (blinky & support electronics) who wasn't aware they weren't allowed a trickle drain. The problem was resolved by increasing the smarts and capabilities of the battery packs to inhibit such drains.

But here's some good news about shelf-life.

A123 systems, another company (besides Valence) that makes Lithium Ion Phosphate batteries has published a pdf file (http://www.a123systems.com/newsite/pdf/tech_life_03.pdf) that explains why Lithium Ion Phosphate chemistry batteries have superior cycle life.

It turns out that the Phosphate based Li-Ions have a slightly less nominal voltage than the metal oxide based Li-Ions. This is often cited as a disadvantage, as the power & energy densities are proportionally reduced.

But, with respect to cycle life, the lower nominal voltage is not a liability. Rather, it is an asset. The higher nominal voltages of conventional metal oxide Li-Ions contributes to a higher oxidation potential, which causes those types to consume themselves over long periods of time on the shelf.

A123 systems is stating that (http://www.a123systems.com/newsite/pdf/tech_life_01.pdf) their batteries are expected to retain 80% of their original capacity for up to 7000 cycles.

More pdfs here:

http://www.a123systems.com/newsite/pdf/

Yes, I know that we all have Valence batteries in our PTs, not batteries from A123. But they are the same chemical reaction. If there is a cycle-life lengthening effect due to reduced nominal voltage for one manufacturer, then anyone who manufactures a battery based on the same chemical reaction will obtain similar results. This is not to say it is 100% certain. If a manufacturer only expects the battery to last 3-5 years instead of 20 years, other components (terminals, housings, etc.) may not have been designed-in with the 20 year possible lifespan in mind.

[polo_pro]

Actually, as I understand it, it's the rear battery that goes dead if the machine is left unplugged. At least that's the one that went dead for the local dealer when a Gen1 GT was left unplugged.

And if it is the rear battery (which I'm not claiming since I don't know), then I'd expect that same battery to take longer to charge (even if it hasn't been drained to the point of being unuseable). Can any of the dealers chime in and tell us whether the Gen 1 battery drains the front or rear battery when the segway is not being used and is not plugged in for weeks at a time?

[PeteInLongBeach]

And if it is the rear battery (which I'm not claiming since I don't know), then I'd expect that same battery to take longer to charge (even if it hasn't been drained to the point of being unuseable). Can any of the dealers chime in and tell us whether the Gen 1 battery drains the front or rear battery when the segway is not being used and is not plugged in for weeks at a time?

When I install the front battery first, the CS beeps. If I install the rear battery first, it doesn't beep until I install the front battery. So I assume the front battery is maintaining the keyport.

Also, the rear battery was much slower to charge under ALL conditions & circumstances (after a long glide, after being removed from the machine, etc.), and this was noticeable long before I started keeping it in storage on the RV. And, it was never in storage so long that either battery was significantly discharged. The i180 reference manual cautions that the batteries should be charged at least once per month in storage, and I never let them go that long.