The LiFePO4 Headway cell 38120P TEST REPORT inside
- Thread starter Doctorbass
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safe
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Raw Data Prepared
I've so far managed to get the data prepared enough to be able to start looking at it. This is what a chart of the data looks like at a granularity of one minute intervals. Too much data overloads my limited spreadsheet charting, but I'll do the actual number crunching on the larger data set. They should be pretty close... obviously round off errors are a big deal with this type of analysis, so I'll be aware of the issues.
Just by looking at the data it does look like a 1% Loss MOSFET designed "Smart Battery" system should be very competitive against the LVC approach.
More results are on the way...
I've so far managed to get the data prepared enough to be able to start looking at it. This is what a chart of the data looks like at a granularity of one minute intervals. Too much data overloads my limited spreadsheet charting, but I'll do the actual number crunching on the larger data set. They should be pretty close... obviously round off errors are a big deal with this type of analysis, so I'll be aware of the issues.
Just by looking at the data it does look like a 1% Loss MOSFET designed "Smart Battery" system should be very competitive against the LVC approach.
More results are on the way...
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safe
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Vindication
My intuition was correct.
Yes, in fact a "Smart Battery" could indeed prove to be more effective in getting the most from your pack than an LVC cutoff solution. The results were pretty easy to arrive at and this was with only three cells that were pretty new and in good health. As cells decay the advantage will become even more pronounced.
The cells if used to their full potential would deliver:
Cell 1 - 25.9054 Wh
Cell 2 - 26.9098 Wh
Cell 3 - 25.1866 Wh
But when you subtract the unused portion due to the LVC:
Cell 1 - 25.4805 Wh
Cell 2 - 25.4460 Wh
Cell 3 - 25.1866 Wh
Which means a difference of 1.8887 Wh.
And that translates to a 2.4815% difference.
The "Smart Battery" concept really is "smart" after all...
My intuition was correct.Yes, in fact a "Smart Battery" could indeed prove to be more effective in getting the most from your pack than an LVC cutoff solution. The results were pretty easy to arrive at and this was with only three cells that were pretty new and in good health. As cells decay the advantage will become even more pronounced.
The cells if used to their full potential would deliver:Cell 1 - 25.9054 Wh
Cell 2 - 26.9098 Wh
Cell 3 - 25.1866 Wh
But when you subtract the unused portion due to the LVC:Cell 1 - 25.4805 Wh
Cell 2 - 25.4460 Wh
Cell 3 - 25.1866 Wh
Which means a difference of 1.8887 Wh.
And that translates to a 2.4815% difference.
The "Smart Battery" concept really is "smart" after all...
safe
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Balancing May Make Things Worse...
If your balancing system uses a "heat sink" approach where the higher cells are simply throwing energy down the drain then your losses get even worse. If you took the cell that is the lowest and used that as your baseline on which all the cells need to be brought down to then you get a loss of 3.2330% compared to the full potential of the cells.
However, if the energy is being transferred and used to recharge the weaker cell then this would not apply, it's hard to know how efficient this is in realtime because losses to tend to occur whenever you do anything.
Let me add that the reason for this is that the cells with higher voltage are actually contributing more Watt Hours than the ones at lower voltage. When you balance based on voltage you are actually attacking the strongest and most energetic cell... it's a little like a socialist government... when you destroy your best performers then everyone ends up poor and stupid...
If your balancing system uses a "heat sink" approach where the higher cells are simply throwing energy down the drain then your losses get even worse. If you took the cell that is the lowest and used that as your baseline on which all the cells need to be brought down to then you get a loss of 3.2330% compared to the full potential of the cells.
However, if the energy is being transferred and used to recharge the weaker cell then this would not apply, it's hard to know how efficient this is in realtime because losses to tend to occur whenever you do anything.
Let me add that the reason for this is that the cells with higher voltage are actually contributing more Watt Hours than the ones at lower voltage. When you balance based on voltage you are actually attacking the strongest and most energetic cell... it's a little like a socialist government... when you destroy your best performers then everyone ends up poor and stupid...
Doctorbass said:
Let's try to keep an open mind and be more precise with our words. Especially when testing.
Charging slower MAY or WILL PROBABLY increase cycle life.
There have been indications in other tests that LiFEPO4 life may not be effected much at all until you
really increase the charge rate.
Richard
safe said:[...]
Which means a difference of 1.8887 Wh.
And that translates to a 2.4815% difference.
The "Smart Battery" concept really is "smart" after all...
If you buy 5% or 10% more battery at the outset you may come out ahead. Going after that last 3% may be more worthwhile in a car-sized battery. Simplicity is a virtue.
Richard
safe
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I think once the circuit is developed that you could get this level of performance with no real difference in compelexity. One of the advantages of the Smart Battery approach is that you can also improve charging... you can more easily charge on a "per cell" basis... so when you think of the collective design it's looking pretty good.rf said:
Obviously most people are using a LVC and some flavor of a BMS or many individual chargers... but all I've proven is that there is a better way if you face the "data" honestly.
safe
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rf said:
No, I have proven it using math.It's like when the astronomers discovered black holes out in space, you can't physically go there and try it out to make sure it's for real, but all the previous theory that predicted the behavior turned out to be validated when the measurements were made.
That's how science works.
First you propose a theory, then you use scientific measurements with approved methods to gather the data and with that data you either validate or invalidate your theory.
I have proven the theory based on real data.
The next step is to turn the proven theory into a commerical or at least experimental product or project. That's where things are now. Maybe next winter when I have a lot of time and am not spending my days welding and otherwise building ebikes.
The question that was answered was really very simple and let me redefine that so you aren't confused. The issue was MOSFET resistance and how much of a negative that would cause in using the Smart Battery. The best you can hope for using MOSFET's is an overall loss of about 1% of your packs energy due to resistance through the MOSFET's. Since even in this idealized three cell pack you get 2.5% in losses due to an early LVC cutoff (some cells extra capacity is always wasted and the weakest cell worked the hardest) you have plenty of room to use the 1% loss MOSFET's and still come out ahead.
I suspect in real world conditons if you have 15 cells then it's very possible that the weakest cell could be as much as 10% down from the best and that means that you lose that extra capacity.
So the Smart Battery is going to show better and better advantages when your pack is in a more damaged (older) condition. The more the pack ages the more the Smart Battery helps to preserve it's full capacity for use.
Theory Proven Next Step Build Prototype(and maybe discover that there is some other unrelated problem that crops up that is presently unforseen that causes the theory to be impossible to realize)
Safe, your "smart battery" idea really belongs in its own thread. Please don't trash up this thread with off-topic stuff.
EMF
100 kW
*bites tongue*
well I for one want to hear want you have to say.EMF said:
so by all means, spill it at the aforementioned thread.
Jeremy Harris
100 MW
Well, I hope these cells work out to be OK, as I've just taken the plunge and ordered 64 of them...............
According to Victoria at Headway, the cells don't have tabs. The next project is therefore a CD tab welder so that I can make a really neat pack. I don't fancy the idea of soldering directly to the case of the cells much, the risk of unseen damage seems a bit high.
BTW, the price was £9.56 (about $19) each for the 38120 10Ah cells.
Jeremy
According to Victoria at Headway, the cells don't have tabs. The next project is therefore a CD tab welder so that I can make a really neat pack. I don't fancy the idea of soldering directly to the case of the cells much, the risk of unseen damage seems a bit high.
BTW, the price was £9.56 (about $19) each for the 38120 10Ah cells.
Jeremy
Jeremy Harris
100 MW
That's the plan, 48V at 40Ah, at least to start with. I can fit this many cells inside the frame, in front of the motor, so will still have some space left for another pack, either under the seat or in the dummy fuel tank if I find I need more volts.
I'm deliberately limiting the motor voltage at the moment, as I want the bike to remain registered legally as a moped. This means officially limiting the speed to 30mph, but the current plan is to see if the powers-that-be will accept the Cycle Analyst speed limiting function as a 30mph limiter (without telling them that it can be over ridden with a switch...........).
The advantage of the 38120s is that they can be assembled into an odd shaped pack, which will make the finished bike a bit neater.
Jeremy
I'm deliberately limiting the motor voltage at the moment, as I want the bike to remain registered legally as a moped. This means officially limiting the speed to 30mph, but the current plan is to see if the powers-that-be will accept the Cycle Analyst speed limiting function as a 30mph limiter (without telling them that it can be over ridden with a switch...........).
The advantage of the 38120s is that they can be assembled into an odd shaped pack, which will make the finished bike a bit neater.
Jeremy
recumbent
100 kW
bikeraider said:
You guys are surely holding lots of battery weight for a reason? I was hoping that 20 AH would be enough range for me to do all the riding i want for a day.
I just ordered a 12ah/48v lifepo4 from Justin, if this worked well perhaps order another and this would be plenty to go touring long distances without worry. I peddal some like most cyclists.
But 40 ah? that's lots of batteries, Past experience told me that 10ah is good only for shopping trips and any enjoyable rides under couple hours, which is enough 80% of the time.
Jeremy Harris
100 MW
Good news!
I got home today to find a big box had arrived from UPS. Deep joy and bliss, I'm now the proud owner of 64 Headway 38120 10Ah LiFePO4 cells.
Delivery from China was quick and painless, I'd happily deal with Headway again.
I just need to build the pack now and see how they perform.................
Jeremy
I got home today to find a big box had arrived from UPS. Deep joy and bliss, I'm now the proud owner of 64 Headway 38120 10Ah LiFePO4 cells.
Delivery from China was quick and painless, I'd happily deal with Headway again.
I just need to build the pack now and see how they perform.................
Jeremy
Ypedal
100 TW
http://endless-sphere.com/forums/viewtopic.php?f=1&t=4086
Hmm.. smells like fish ! :wink:
Lol.. me thinks you got Andy's cells ! :lol:
Got pictures ?.. like battery pron !
Hmm.. smells like fish ! :wink:
Lol.. me thinks you got Andy's cells ! :lol:
Got pictures ?.. like battery pron !
Jeremy Harris
100 MW
Here you go, a small sample of battery porn..........
For what it's worth, I found Headway easy to deal with and honest.
Jeremy
For what it's worth, I found Headway easy to deal with and honest.
Jeremy
RLT
10 kW
Jeremy... I'm pleased to hear that. I'm getting really tempted to order some myself. (Getting sick of the fragility of my LiIons).
That "£9.56 (about $19)" each that you paid... Did that include shipping? If not, how much extra was your shipping?
That "£9.56 (about $19)" each that you paid... Did that include shipping? If not, how much extra was your shipping?
Jeremy Harris
100 MW
The price I gave was excluding shipping, import duty, tax etc, which seems to vary a lot depending where you live. Shipping cost to the UK was around $220 for 64 cells, plus I got hit for another $100 or so by UPS for their fees, tax etc.
Overall, these cells still seem to be reasonably good value, about on a par with Thundersky cells, but better suited for my motorcycle conversion as they are an easier shape to work with when it comes to making the pack fit in the space available.
I get the feeling that Headway are still finding their feet with direct sales. They tried to set up a Paypal account, at my suggestion, but somehow this didn't seem to work for them. Payment has to be by bank transfer at the moment, which does present a possible risk. My guess is that if demand was there from the direct sales market they may well gear up to be able to support it a little better.
Jeremy
Overall, these cells still seem to be reasonably good value, about on a par with Thundersky cells, but better suited for my motorcycle conversion as they are an easier shape to work with when it comes to making the pack fit in the space available.
I get the feeling that Headway are still finding their feet with direct sales. They tried to set up a Paypal account, at my suggestion, but somehow this didn't seem to work for them. Payment has to be by bank transfer at the moment, which does present a possible risk. My guess is that if demand was there from the direct sales market they may well gear up to be able to support it a little better.
Jeremy
RLT
10 kW
OUCH! That shipping and UPS fee hurts!
I think, probably, in USA it would be much less. My last batch of 110 32650s came in 2 days from Hong Kong, Via UPS, with no extra fees.... I'm not sure what the shipping was because it was included in my price of $11.34 each.
Seems like they should be able set up PayPal unless there is something kind of 'hinky' in their immediate past. ... I have three suppliers right in their neighborhood that use it, and they are just small scale 'jobber/retailers'.
I think, probably, in USA it would be much less. My last batch of 110 32650s came in 2 days from Hong Kong, Via UPS, with no extra fees.... I'm not sure what the shipping was because it was included in my price of $11.34 each.
Seems like they should be able set up PayPal unless there is something kind of 'hinky' in their immediate past. ... I have three suppliers right in their neighborhood that use it, and they are just small scale 'jobber/retailers'.
hi Doc,
thank you very much for all the info!
you posted discharge graphs at various C rates. do you have similar graphs (or the data) for charging? (ideally at 1C or higher.) the charging graphs at http://www.zeva.com.au/tech/headway/ are obviously flawed.
thanks again!
thank you very much for all the info!
you posted discharge graphs at various C rates. do you have similar graphs (or the data) for charging? (ideally at 1C or higher.) the charging graphs at http://www.zeva.com.au/tech/headway/ are obviously flawed.
thanks again!
Doctorbass
100 GW
Lanchon said:
No charging graph ... not tested from now...
they hold 10A charge no prob.. no important temp increase while charging..
flip_normal
100 W
I just got this reply back from Victoria at Headway sales re. the 10Ah cells:
I've been quoted 9.25GBP per cell (~18US$) on 100 x 38120L (10Ah) cells.
Re. cycle life the Headway site says "70% capacity left after 1000cycles". Even though this is not the industry standard way of measuring cycle life (normally measured to 80% of original capacity) I'm seriously considering going for these 10Ah cells.
I've been quoted 9.25GBP per cell (~18US$) on 100 x 38120L (10Ah) cells.
Re. cycle life the Headway site says "70% capacity left after 1000cycles". Even though this is not the industry standard way of measuring cycle life (normally measured to 80% of original capacity) I'm seriously considering going for these 10Ah cells.
