# Ah in and Ah out why don't they balance?



## davidmillin (Dec 14, 2013)

Hi everyone I have been monitoring my amp hour usage with a JLD404 and I find if I reset a fully charged pack to zero and use say 20ah and then fully recharge it the ah rather than being around zero(energy in = energy out) the ah reading is around 1 or 2 ah in other words only 18 or 19 ah has been recorded as going back in yet my voltage records as fully charged. Has anyone else experienced this and what is going on?

Cheers David


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## skooler (Mar 26, 2011)

Shunts arent 100% accurate. They are much mkre accurate at higher currents. Lower currents, such as when charging, can just trickle by unoticed.


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## PStechPaul (May 1, 2012)

I have an idea that this may be due to the difference in the cell voltages when charging and discharging. Although I have heard that the true Ampere-hours are just about fully conserved on LiFePO4 cells, there will always be some losses due to internal and external resistance, so you will need to supply more watt-hours than you get out. This would be seen as the higher voltage of charging as compared to discharging at the same current. But I think there are other effects such as temperature and what might be called "dielectric absorption" which a phenomenon of capacitors. Thus a cell will read a higher voltage while it is being charged and then the voltage will drop after a while, which may be likened to the charge "soaking in" or being distributed more deeply and evenly on the surfaces of the electrodes. Unless there is actual leakage in the electrolyte, the charge remains the same although the voltage drops. So although you may determine that the battery is fully charged by its voltage reading after putting less A-h in than was taken out, you may need to keep current applied a while longer even though the voltage will be higher, but after a period of time it will settle down to what would be expected for a fully charged state.


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## Old Goat (Jun 28, 2011)

I'm having trouble posting a reply. Let me try to send this and if it works, I'll post the "real" reply.


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## Old Goat (Jun 28, 2011)

OK, that seemed to work that time. I tried three times before that and got the error that I was not logged in, even though I was. Anyway, I'm having a similar AH IN - AH OUT problem but mine is perhaps more serious. I also use a JLD 404 AH meter as my fuel gauge. I have 36 cells LiFEPO4 120 AH. The pack is about 1 year old. I drive it every day. Up until this week, I would set the meter to zero after a full charge and after discharging and recharging, it would read 0 AH (plus and minus about 4 AH). But this week, I have experienced numbers like -17AH and today, even worse, it was -22AH after charging all night. I top balance about every 5-6 months. I frequently monitor cell voltages at various states of charge looking for imbalance and there is essentially none. Maybe 0.2V from the lowest to the highest. I will do another round of measurements soon in the hopes of finding one or two cells that are aging more rapidly than the others or otherwise misbehaving, but I don't think I'm going to find any. Is there a chance this is an instrumentation problem? The meter seems to work just fine and has given good, reliable service for a couple of years now. Does anyone have any suggestions?


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## steven4601 (Nov 11, 2010)

Chiming in as I think the devil is in the details.
0.2V variance in cell voltages? That is the virtually the difference between full and empty on LifePo4. Do you have those numbers correct?

Strive for 50mV or less variance at rest after few hours resting from charging if top-balanced.


edit:
AH in and AH-out should match fairly closely unless your plating metals


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## Roy Von Rogers (Mar 21, 2009)

Get another low amp shunt for the charging side, hook both shunts to the negative side of the pack. Put a relay close to the two shunts, so when you turn the charger on it switches the relay to the low shunt, and when the charger is off it will be at your high amp shunt.


Roy


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## dougingraham (Jul 26, 2011)

davidmillin said:


> Hi everyone I have been monitoring my amp hour usage with a JLD404 and I find if I reset a fully charged pack to zero and use say 20ah and then fully recharge it the ah rather than being around zero(energy in = energy out) the ah reading is around 1 or 2 ah in other words only 18 or 19 ah has been recorded as going back in yet my voltage records as fully charged. Has anyone else experienced this and what is going on?
> 
> Cheers David


I have seen very close correlation between input and output. With my iCharger 3010b on 100 AH cells I have even seen over unity as far as AH is concerned on a C/4 cycle. I believe this was on the order of 150 mah in 100 AH cells so an error of 0.15%. Of course this is not possible so instrumentation error was suspected and confirmed by comparing with other meters. There is a slight bias in the current reading in the charger.

Your JLD-404 is not a precision instrument and I would guess you are seeing an accumulation of rounding errors in the readings. It may not even be as good as the "Free" Harbor Freight meters as far as accuracy goes. You don't say what shunt you are using but what you want is the largest resistance you can tolerate to increase the resolution of your readings. Reading 10 amps with a 1000 amp shunt might not give you good numbers on the 10 amp end of the scale.

I am not saying the JLD-404 is bad, you just need to recognize the limitations of the devices you are using to make measurements.


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## Moltenmetal (Mar 20, 2014)

Doug: have you made observations accurate enough to estimate the charge/discharge cycle efficiency? I've seen figures for this in past but no supporting data. Would imagine the charger itself is a significant portion of that in terms of wall-to-stored energy efficiency.

My Fluke 77 is the most accurate meter I have, and the best it will do on cell voltages is +/- 10 mV, i.e. 3 significant figures. But since voltage doesn't vary all that much between full and empty, current measurement must be the most important factor in total Ah accounting. As noted already, the problem is the range of measurement required. Charge occurs at a much smaller fraction of C over a long time, and discharge being 10x that sets the range of the measurement device making it inaccurate in recording the current going in during the charge. With only 3 significant figures to work with at best, that 10x difference means that some error is inevitable if you rely on a single device to measure both, i.e. a JLD404 or my E Expert Pro. You'd think that somebody would solve this with separate shunts for charge and discharge, but I guess that leads to other problems too.

BTW, have you looked at my thread recently? I'm interested in your thoughts on this E Expert Pro's prescaler.


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## Old Goat (Jun 28, 2011)

The voltage variance I mentioned depends on when the measurement is taken. When I top balance, I charge every single cell to 3.65 volts. I monitor each cell with the red LED on the mini bms card -- when one starts to glow, I start using my fluke digital multimeter to shut off the current when that cell is right at 3.65V. Then I take that cell out of the charging loop and let it settle, moving on to the others until all have been through the process. So you could claim I am achieving zero delta voltage if you're talking about measurement up to the point the charge is complete. But then the cells settle after charging. The last time I did this, cell 12 settled to the highest voltage, 3.393V and cells 19, 20, 21 25, 27 and 30 settled down to 3.339 (all the others were somewhere in between), making the total variance 54 mV. But if I then take yet another measurement after driving about 50 miles, the delta is sometimes a bit more than that, maybe up to about 65 mV. I mis-spoke when I mentioned a 200 mV variance. BTW, I use only one shunt and it is on the negative side. I don't remember its resistance but I think current measurements on the JLD404 are fairly accurate; I can compare with the hall effect sensor used in the EV Dashboard app and they're always pretty close.


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## dougingraham (Jul 26, 2011)

Moltenmetal said:


> Doug: have you made observations accurate enough to estimate the charge/discharge cycle efficiency? I've seen figures for this in past but no supporting data. Would imagine the charger itself is a significant portion of that in terms of wall-to-stored energy efficiency.


This is one of those things that seems like it should be easy but is actually difficult to do properly. So no, I don't consider the data I have to be good enough to make a claim like 99.99% on the cycle efficiency. However I will make the brash statement that an accurate coulomb count should result in a SOC indication that is far better than the gasoline gauge in your ICE vehicle. The problem is the accurate count. I think if you assume the cyclic efficiency is 100% efficient and then look for the sources of error you will discover the assumption is not the place where significant error lies. If I had not seen the impossible over unity numbers I might have claimed some number measured that was close to 100% and been happy. Instead I had to track down an instrumentation issue. One thing that makes this more complicated is that the capacity of a cell changes a little with every cycle. You need to take that into account.



Moltenmetal said:


> My Fluke 77 is the most accurate meter I have, and the best it will do on cell voltages is +/- 10 mV, i.e. 3 significant figures. But since voltage doesn't vary all that much between full and empty, current measurement must be the most important factor in total Ah accounting. As noted already, the problem is the range of measurement required. Charge occurs at a much smaller fraction of C over a long time, and discharge being 10x that sets the range of the measurement device making it inaccurate in recording the current going in during the charge. With only 3 significant figures to work with at best, that 10x difference means that some error is inevitable if you rely on a single device to measure both, i.e. a JLD404 or my E Expert Pro. You'd think that somebody would solve this with separate shunts for charge and discharge, but I guess that leads to other problems too.


The voltage matters only because you use it to determine the full/empty endpoints. And the voltage changes with temperature and load so they must be closely controlled. For AH counting you would think that the current is the only thing you care about. Using my meter (Fluke 189) with a shunt would let me read milivolts to 5 significant places. With a 50 mv @ 500 A shunt this would mean a resolution of 10 ma if using my meter to do the data collection. I think your meter would be 100 ma resolution if it stayed on the same scale for both directions.

With separate shunts you would have a calibration issue and absolute accuracy enters into the comparison. With a single shunt it is a resolution issue only and since all you care about is comparing the in vs the out the comparison will be good up to the limits of the resolution of the instrument.

If you go back to the original posters question he is seeing a 1 or 2 AH error out of 20. This is 5% if 1 AH and 10% if 2 AH. In the grand scheme of things that is more than I would like to see but there are so many places where the error could creep in.



Moltenmetal said:


> BTW, have you looked at my thread recently? I'm interested in your thoughts on this E Expert Pro's prescaler.


Not sure what thread you are talking about. I guess I will need to go look for it.


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## kennybobby (Aug 10, 2012)

It would be interesting to compare the Ahr going in calculated from the shunt measurement, with the Ahr reported by the charger, e.g. TC/Elcon in the serial output message.

If using only one shunt, the low-current measurements suffer from low signal-to-noise and the high-current mode causes heating of the shunt elements which affects the signal also.

Making an accurate 'fuel' gauge is the holy-grail for an EV builder.


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## major (Apr 4, 2008)

kennybobby said:


> ...... the high-current mode causes heating of the shunt elements which affects the signal also.


A good instrument quality current shunt will use an alloy for the element which will have a zero change in resistance over the range of rated current and temperature. Manganin (alloy of copper, manganese & nickel) has virtually zero temperature coefficient of resistance.

You are correct about battery electric fuel gauges being a bitch  Even in our production EV we call it the guessometer


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## PStechPaul (May 1, 2012)

If the low and high voltage "knees" accurately determine "empty" and "full", then I would think one could recalibrate the actual capacity of the pack by doing a top balance and then run it to exhaustion as determined by the BMS, or bottom balancing and then charging to the full setpoint. The A-h capacity should be as accurate as the current measurement and the time, and it should be easy enough to achieve 0.1% or at least 0.25% accuracy. It's probably most accurate to do the test during charging because the current will be more consistent.


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## Moltenmetal (Mar 20, 2014)

I've got a somewhat puzzling situation and hope you can help:

I have an EE Pro Ah gauge, 32 S 180 Ah cells, arranged in two packs: 10 in the rear, 22 in the front, with the miniBMS.

First charge, from new, was done slowly and carefully to ensure that the BMS charge interlock worked. I used a 12V MR16 bulb with clip leads to shunt around cells which topped off first. I managed to get at least half of the cells in both the front and back packs to the high voltage warning point on the BMS, where the BMS boards switch from a quick blink to a 50% duty cycle flash. I didn't do a full top balance. The odd cell would occasionally have the BMS board go into shunt mode (solid LED with brief off blink) but not consistently.

I entered all the parameters into the EE Pro and "synchronized" it such that it showed "full". Pretty sure all the parameters are correct except perhaps the float charge voltage, since I wasn't 100% top balanced at that point.

I drove the car, taking the pack down to about -135 Ah. No BMS warnings and nothing from my Lee Hart bridge LEDs- everything is working well. Noticed a fairly significant bleed of Ah out of the pack while the car wasn't being driven- presumably the input side of my DC/DC. May have to switch that separately with a relay...

On re-charging, I got back to around -21 Ah and started getting BMS high voltage trips. By -20 Ah, all 10 cells in my rear pack were at the warning point for the BMS- and not one cell in the front pack is there yet...thinking that can't be a coincidence, but puzzled how that could possibly be as the cells are all in series during both charge AND discharge!?

Any thoughts?

I'm nervous about bouncing cells repeatedly against the BMS high voltage limit during charging- my current wiring will do this as long as the charger remains plugged in and no cell stays in alarm. Am I being a worrywart, or is it actually harmful? Wondering if there's a better strategy for top balancing than what I'm using?


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## kennybobby (Aug 10, 2012)

Is the charger connected at the rear pack? Maybe there is a voltage drop on the rear-to-front connection.


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## Moltenmetal (Mar 20, 2014)

Given that the front and rear packs are in series and connected together with 2/0 wire, there should be no significant voltage drop during a ~ 12A charge (which is what my PFC2500 does on 120 V AC input). Since I've been able to pull 400 A while driving (without my throttle pot going full stroke yet) I don't think this is the problem.


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## PStechPaul (May 1, 2012)

There could be a significant temperature difference between front and back, especially while driving or for a while after.


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## tomofreno (Mar 3, 2009)

So the gauge is saying the car used more Ah than it actually did? I have a setup similar to this, 36 CALB SE 180Ah in series, TBS gauge, shunt, 1:5 pre-scaler, and minibms. Most I've ever seen is about 3 Ah difference - i.e. the gauge showing 3Ah used (-3Ah) after the cells were fully re-charged. More typical was 1 to 2Ah. I have 3 boxes, one in the spare tire well, one through the floor where the gas tank was, and one up in front of the motor/transmission. The cells in the front box were always the lower ones, but not by much, typically 3.44V or above when the back ones were at 3.48 - 3.52 (the spread in turn-on voltage for my 5 yr old minibms shunts is 3.50 +/- 0.02V, might be less in newer ones). I assumed this was a temperature effect since the rear cells are in, or 2/3 in the cabin. Even when I would balance them with the rest of the pack they would drift back down to around this offset over time. Only around a 2 Ah difference so not enough to worry about. 

The past year I've been charging with the EMW charger per the CALB spec most of the time, so typically most if not all shunts come on in the last minutes of charging before the charge current decreases to 9A and charging is terminated. The TBS is set to detect "full" as pack V > 124V (float voltage) and charge current less than 9.5A (5.3% float current), so it autosynchronizes and is flashing "Full" just before charging is terminated. EMW and TBS gauge Ah in are typically within 1-2 Ah of each other.
Oh, I also have the DC/DC connected through a relay so it is disconnected from the pack when the ignition is off.

I've always used Ah/mile rather than Wh/mile since it is more stable, so I've become calibrated to how many Ah/mile I use on flats and given %grade hills and can usually predict within about 5% how many Ah it will take me for a given trip by looking at the miles and grade on google maps. 

Regarding guessometer, I think production evs are the worst because they try to predict range based on past energy/mile. At an EVent this year a new Tesla S owner told me he gained 18 miles range coming down off a mountain pass here. He was proud, so I didn't tell him it was likely less than 10 miles range because my car gains about 6-8 (Tesla might be a bit more efficient though). It's about 15 miles down, which is likely a considerable portion of the distance used to estimate past energy/mile usage and predict future energy/mile and range so the estimated gain in range is inflated. I don’t understand why anyone tried to implement such an instrument when energy/mile varies so much with speed and grade.


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## dougingraham (Jul 26, 2011)

Moltenmetal said:


> Given that the front and rear packs are in series and connected together with 2/0 wire, there should be no significant voltage drop during a ~ 12A charge (which is what my PFC2500 does on 120 V AC input). Since I've been able to pull 400 A while driving (without my throttle pot going full stroke yet) I don't think this is the problem.


My first thought was the same as what Paul posted about temperature difference between the boxes. The colder portion of the pack will show the rapid voltage rise before the warmer portion which would trigger your BMS early on that half of the pack. You could also have an instrumentation issue caused by temperature. If the BMS voltage sensors are with the cells are are not temperature compensated they could be misreading the voltage. Check it against your DVM.

Lowering the charge current will probably make this mostly go away. I would also not worry about it too much until you get the pack balanced.


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## Sunking (Aug 10, 2009)

I have not read through all the post so forgive me if this has been brought up.

What kind of battery? Lithium or lead acid?

If lead acid coulomb counting is worthless because Mr Peukert robs you blind. He turns your 100 AH battery into a 40 AH battery without your knowledge. 

If lithium Peukert has some effect and will induce about a 5 to 10% error. Either that or you are seeing normal Current Shunt Errors. Current shunts are only accurate near full scale after they have heated up. At low range they are cold and are only Ball Park meters.


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## major (Apr 4, 2008)

Sunking said:


> .....Either that or you are seeing normal Current Shunt Errors. Current shunts are only accurate near full scale after they have heated up. At low range they are cold and are only Ball Park meters.


I don't know where you folks are getting your instrument shunts from but good quality shunts have served me well over the years (decades) with great accuracy. 



major said:


> A good instrument quality current shunt will use an alloy for the element which will have a zero change in resistance over the range of rated current and temperature. Manganin (alloy of copper, manganese & nickel) has virtually zero temperature coefficient of resistance.


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## Sunking (Aug 10, 2009)

major said:


> I don't know where you folks are getting your instrument shunts from but good quality shunts have served me well over the years (decades) with great accuracy.


Major I agree if you are using commercial or lab grade shunts, but those cost some serious bucks. Personally I would use Hall Effect Transducers on an EV of appropriate scale as it avoids two more point of failure. But if one is using FLA batteries is pointless what you use because it just won't work out well.


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## PStechPaul (May 1, 2012)

I have bought many 1000A 100mV shunts for my Ortmasters, and they cost only about $35 each from a a US manufacturer (Empro), and even less from a Canadian company. I have one that I get calibrated to NIST traceability and that costs about as much as the shunt. I use it to calibrate my Ortmasters and they are typically +/- 0.5% or better, in 8 ranges from 50 A FS to 10kA FS. They are really only rated for 500A continuous (25W) and they get pretty warm, but calibration seems to be good and I have not seen any significant tempco. Shunts are usually calibrated at 25C. The resistive element is typically Manganin or Constantan.


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## tomofreno (Mar 3, 2009)

PStechPaul said:


> There could be a significant temperature difference between front and back, especially while driving or for a while after.


I don't think temperature difference will cause a 21Ah error unless it is very severe, like one box at 20 F and one at 80 F. Even then I'm doubtful. Typically my cells in the front might be about 10 F lower temperature than those in the other boxes after the car is parked outside unheated for several hours in winter because the sun warms the cabin and the cells in there, but not the ones under the hood as much. Even then when the shunts in the back boxes turn on, the cells in the front box are typically 3.44V or greater, so only a few Ah difference. 

I suspect the cells were never fully balanced. I once had a cell that consistently reached about 3.38V when all others were above 3.47V. I charged that cell individually to 3.5V 4 times, but it would always drift back down over time and charge only to around 3.38V - 3.39V. Finally, after more than a year of this, I charged it to 3.55V and held it there for about 10 minutes. It has stayed balanced with the rest ever since (more than 3 years).


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## dimitri (May 16, 2008)

Moltenmetal said:


> I drove the car, taking the pack down to about -135 Ah. No BMS warnings and nothing from my Lee Hart bridge LEDs- everything is working well. Noticed a fairly significant bleed of Ah out of the pack while the car wasn't being driven- presumably the input side of my DC/DC. May have to switch that separately with a relay...
> 
> On re-charging, I got back to around -21 Ah and started getting BMS high voltage trips. By -20 Ah, all 10 cells in my rear pack were at the warning point for the BMS- and not one cell in the front pack is there yet...thinking that can't be a coincidence, but puzzled how that could possibly be as the cells are all in series during both charge AND discharge!?
> 
> ...


There are 2 separate unrelated subjects here. AH counting relies on accurate current readings, which are integrated over time. So, its most critical to make sure that current reading is correct, including zero point when car is turned off. I doubt your DC converter is taking much current when car is off, so it's idle load can be ignored for the most part. So, you need to check the current reading when car is off, when its driving and when its charging, and see if those numbers are off, which would be the root cause of incorrect AH count. I have no experience with your AH meter, so can't tell you how to address wrong current readings. In our EV Display we have functions for calibration and charge efficiency, which allow fine tuning of correct AH count. 

As for pack balance, your initial balance job is good enough, you have nothing to worry about, just let the charger/BMS cycle until all cells line up eventually. There is no harm in tripping HVC, it was designed exactly for this purpose with good safety margin at the top.


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## Moltenmetal (Mar 20, 2014)

Update: I shunted around all 10 cells in the rear pack using a couple of MR16 bulbs. Started at around -21 Ah relative to my last charge, and by -17.5 Ah I had all the front pack cells more or less full. By -17 Ah the pack was more or less full front and back, with only a few cells not at their BMS warning levels. What caused the rear pack to be 3 Ah lower than the front is a puzzle- we'll see if it repeats itself. 

I don't know where the 17 Ah went between the first equalization of my EE Pro and the 2nd, but do know that my pack is full again for sure. Hmm- how many Ah do I use for my first Wh/mile calcs? So I'll drive it another cycle (hopefully, weather permitting) and see if it recurs...otherwise it'll be spring before I can get to the bottom of this!

Can't think what else would be consuming power from the pack when the car is just sitting there with the contactor open OTHER than the input side of the BMS. The only other things on the pack when the car ignition is off are the EE Pro and its prescaler, my two green LEDs on the Lee Hart bridges, and the output side of my charger. The BMS celltop boards of course consume a little out of each cell, but that current doesn't flow through the shunt. When the car is sitting there, a current large enough that the EE Pro distinguishes it from zero is flowing out of the pack. Hmm, perhaps I'll put a smaller shunt in there and measure that current accurately- perhaps it's a mis-reading of this leakage current over the past couple of weeks that has tricked the EE Pro into thinking I'd drained the pack more than it actually had. I noted down the EE Pro pack reading day after day between first charge and 2nd, so I can compare the values and see if they're similar. With a 500 A shunt, reading less than 0.1 A is a tough business to do accurately, so that might be the issue right there.

The main charge occurs at around 12.5A which you'd think the EE Pro can measure very accurately. So I'm confident that the number of Ah I put back in, minus my shunting lightbulb losses of course, are accurate. 

My first drive cycle consisted of nothing but hard accelerations followed by 100 A of off-peddle regen, block after block, since the car isn't safetied yet and I dare not take it outside of my secluded little neighbourhood. I was mostly interested in having fun with the car, with a secondary motive of wanting to test reliability of the build- to break it in. If anything is going to break, I want it to break while I'm close to home! So perhaps the EE Pro doesn't do such a good job of integrating the positive and negative current flows under those driving conditions- it should, as it was bloody expensive! Once the car is in service there will be long stretches of highway speeds so this should be much less of an issue.

I do have my EE Pro sharing the shunt with an analog panel ammeter provided by CanEV. Since the EE Pro no doubt has a very high input impedance when measuring the voltage across the shunt, having these two measurements being made in parallel shouldn't matter.

The EE Pro plus prescaler seems to be measuring total pack voltage correctly, at least within the accuracy of my Fluke 77 for sure. I should really check my Fluke against the good meters we have at work, just to be sure.


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## Moltenmetal (Mar 20, 2014)

Sunking: LiFePO4 and Peukert exponent is set in the EE Pro as 1.0. The set-up parameters for the EE Pro were given to me by CanEV, and Randy's on vacation at the moment so I can't ask him for advice.


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## dimitri (May 16, 2008)

Moltenmetal said:


> Update:


Such a detailed update, but no mention of what current you read on EE Pro when car is turned off.

None of Ah numbers matter if your current reading is wrong.

Assuming EE Pro has current reading display, why would you need an Ammeter in parallel on the same shunt? I bet having 2 meters on same shunt is not a good practice, and since you are having measurement issues I would remove Ammeter for now, to see if that helps EE Pro measure the current correctly.


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## Moltenmetal (Mar 20, 2014)

Dimitri, and others: thanks for the suggestions and the help.

The current reading on the EE Pro when the ignition is off and the charger not plugged in, bounces around between zero and 0.1 A. Steady leakage via the EE Pro amounts to about -1.1 Ah/day, which if it is to be believed, I calculate to be around 5 watts of parasitic losses- enough to flatten my pack in just less than half a year- not really enough to worry me, but enough to irritate my calcs of Wh/mile etc. 

About 1.5 W of that loss is my Lee Hart bridge resistors and LEDs. The other 3.5 W is going somewhere, or isn't, depending on whether or not I trust the EE Pro's current measurements down this low. Doesn't really matter where that extra power is going right now- what matters is whether the EE Pro is accurately measuring whatever is being lost. Over the 15 days between my two charges, the amount of Ah mismatch between charge and discharge on the EE Pro is suspiciously close to my total parasitic losses...16.5 vs around 17.1 Ah. I suspect this is coincidence though, as I do have real losses of at least 1.5 W.

I intend to measure the real total parasitic loss by inserting my multimeter in the circuit temporarily- just haven't done that yet.

I have an analog ammeter because it is a useful tool for driving. I could disconnect it and see if the EE Pro's readings are different, but doing that while the controller is pulling significant current would be pretty tough the way I've wired it. 

The EE Pro can display Ah, voltage, current etc. at the push of a button but it has only a 1 line display, so I use it to display Ah. Attention when driving should really be on the road rather than on pushing buttons. I don't want to share my "fuel gauge" with my ammeter, which from an EV driving perspective is rather like a tach on an IC engine car- except far more useful. My Priuses both have what amount to "power gauge" displays and I use them, in the same way I'll use my ammeter. 

Similarly the Curtis Spyglass display scrolls through a list of parameters including current and voltage, but the display is small and takes attention to read. Useful for programming and diagnosis but not for driving really, at least with my eyesight.

CanEV does wire the ammeter and the EE Pro's current sense taps in parallel off the same shunt and finds that to be acceptable, but again he is unavailable to discuss for another couple weeks. But thinking about it logically for a minute: the shunt is what- 75 mV at 500 A? That makes it 
0.00015 ohms. Any resistor of realistic size in parallel to that is not going to affect the voltage across the shunt, even though the voltage itself is very low. So how would two ammeters, one analog and one electronic and hence probably very high impedance, in parallel across that shunt make a difference?


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## tomofreno (Mar 3, 2009)

Moltenmetal said:


> ...I have an analog ammeter because it is a useful tool for driving. I could disconnect it and see if the EE Pro's readings are different, but doing that while the controller is pulling significant current would be pretty tough the way I've wired it.
> 
> The EE Pro can display Ah, voltage, current etc. at the push of a button but it has only a 1 line display, so I use it to display Ah. Attention when driving should really be on the road rather than on pushing buttons. I don't want to share my "fuel gauge" with my ammeter, which from an EV driving perspective is rather like a tach on an IC engine car- except far more useful. My Priuses both have what amount to "power gauge" displays and I use them, in the same way I'll use my ammeter...


 My TBS gauge has a 5 bar readout for Ah, so I would guess your gauge also does. That's all I use for a fuel gauge in typical driving, and set the display to read Ampere. Usually no need to track Ah any closer than that. I'll occasionally change the display to Ah, easy to do for me since it is mounted on the windshield post at eye level, but usually just leave it on Amp. I have it set to alarm for low battery at 35% SoC, so still can drive quite a bit at that point if necessary. I'd disconnect the other Ampere display for a while just to check if that is the problem. The documentation for the TBS says it draws 9mA at 12V. It, and the minibms main board are drawing power from my 12V accessory battery, so there are no loads on the main pack other than the distributed minibms boards when parked. I've left it for a couple weeks no problem, hardly any change in pack V, so to me 21 Ah is really anomalous.


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## dimitri (May 16, 2008)

Moltenmetal said:


> Dimitri, and others: thanks for the suggestions and the help.


Your zero point reading is quite acceptable, so that is not the cause of ~20Ah discrepancy between charge and load. That leaves measurement errors at higher values, i.e. registering too much current during heavy loads, which is then not compensated during charge, since charge happens at a low and steady rate.

While I agree that parallel ammeter on the same shunt should be OK in theory, I don't know if there are some adverse effects in practice, for example some dampening of the voltage due to mechanical inertia in the ammeter, especially when current fluctuates wildly when driving. This is just a wild guess, I have nothing to support this crazy idea 

Also, you should get a trusted DC clamp meter and confirm that steady charge current during bulk stage is accurately registered by EE Pro, since its critical for correct AH count back to zero point.


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## Moltenmetal (Mar 20, 2014)

Excellent advice, both. I'll see what I can borrow for measurement purposes without the risk of blowing it up, and report back.

Agree that the only way to really know if the parallel ammeters is the problem is to disconnect the analog one and check the readings on the other- at low currents, because doing so at high currents is going to be impractical.


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## Moltenmetal (Mar 20, 2014)

OK, guess I have to disconnect my dash-mounted ammeter for a time and see if I get better matching between drain and charge Ah, then reconnect it and see if I get the discrepancy again. Otherwise I won't really know the cause.


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## PStechPaul (May 1, 2012)

Many shunts are calibrated with the expectation of an analog meter being used on it and the leads to the shunt are also taken into account. But a typical analog meter is 50 mV and 5 mA, or 10 ohms, and will not significantly affect the reading of a high impedance electronic meter. 

One possibility is that the electronic meter may read a heavily filtered average DC value, while the waveform at high current draw may have some high frequency ripple or other AC component. You may need to use a true RMS meter.

If the problem is the low mV reading while idle, caused by current draw from the DC-DC converter and/or BMS, perhaps those components could be wired through a much smaller shunt to get a better reading.


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## azdeltawye (Dec 30, 2008)

PStechPaul said:


> …Although I have heard that the true Ampere-hours are just about fully conserved on LiFePO4 cells,...


 My data would agree with the above statement. 

My converted 2001 Jetta has been in service since May 2010 and has accumulated over 46k electric miles. I recorded ALL Ah in and out from a Brusa BCM-400 Ah/Wh meter: 
Amp-hours in = 107,335.8
Amp-hours out = 106,666.7
Coulombic efficiency of battery = (Ah out)/(Ah in) = 99.4%


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## dougingraham (Jul 26, 2011)

azdeltawye said:


> Coulombic efficiency of battery = (Ah out)/(Ah in) = 99.4%


That is really interesting data assuming there is no offset in the amp reading. I don't have a counter on the output of my battery, just the charger AH and it is hardly what I would consider accurate (probably about 5%). I will have to look at the Brusa meter. I was not aware of it.


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## Moltenmetal (Mar 20, 2014)

azdeltawye: obviously Ah in/out and Wh in/out are not the same thing. Ah in/out MUST balance or else those extra electrons are going into side reactions that must ultimately destroy the battery. As to Wh in/out, there has to be some overvoltage during charging that goes into heat- necessary to provide the driving force for diffusion within the cell- and that loss goes to the ultimate energy efficiency of the vehicle from wall socket to wheels. There is also of course the charger efficiency sapping some wattage. Do you have any good measurements of Wh in from the wall vs Wh out of the pack? 

Totally separate from my issue which is a measurement problem of some sort which I'm sure I'll ultimately sort out- eventually- when I get the car road legal and need to charge it twice daily. The first charge was very puzzling. Hoping to get a 2nd in before winter but that'll be it until spring.


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## Roy Von Rogers (Mar 21, 2009)

Not sure why to worry about whats going in, if your charger shuts off, and the voltage is where it suppose to be your full. Just look at the ah going out, that's your fuel gauge.

You got to remember (JLD404) uses a shunt, that's nothing but a 500/1000 amp resistor, that drops 75mv at full limit of shunt. If you are charging at say 10 amps, the millivolts are so small, there will be an error, for its not going to be that accurate at such low value, it has to travel trough the wire, which also has some resistance from the shunt to the meter.


Roy


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## azdeltawye (Dec 30, 2008)

dougingraham said:


> … I will have to look at the Brusa meter. I was not aware of it…


 The Brusa unit is designed and built very well with a processor board mounted right on the shunt which converts the mV signal to pulses that are sent to the dash-mounted gauge via cat 5 cable. Unfortunately I think they discontinued the BCM series gauges. But they were grossly overpriced anyway… 




Moltenmetal said:


> azdeltawye: obviously Ah in/out and Wh in/out are not the same thing. Ah in/out MUST balance or else those extra electrons are going into side reactions that must ultimately destroy the battery. ….


 The reason for the difference in Ah in and Ah out is the Ah consumed while top-balancing. Every couple weeks or so I deliberately extend the charge cycle to allow the BMS shunts kick on and top balance the pack. At the end of a top-balance charge cycle the Ah gauge will have a negative reading instead of zero.




Moltenmetal said:


> … Do you have any good measurements of Wh in from the wall vs Wh out of the pack? ….


 As for the DC Watt-hours in/out here are the totals as of 9/5/2014 (46063 miles):
DC Watt-hours in = 16,228.8 kWh
DC Watt-hours out = 15,150.7 kWh
Energy storage efficiency of battery = (DC Wh out)/(DC Wh in) = 93.4%

Now using the AC Watt-hour totals to calculate overall efficiency of the system:
Total AC Watt-hour from the wall = 17,695.2 kWh
Overall battery charger and storage efficiency = (DC Wh out)/(AC Wh in) = 85.6%


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## Moltenmetal (Mar 20, 2014)

Thanks Darren- excellent information!


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## Moltenmetal (Mar 20, 2014)

I trust the Ah in far more than I trust the Ah out at present. Far easier to measure a steady 12.5 A in one direction than to integrate a large number of +400/-100A spikes each lasting ten to twenty seconds...


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## Moltenmetal (Mar 20, 2014)

OK, (facepalm)- feeling like a moron, yet again...

Just now, prior to plugging in the charger, I went through the parameters in the EEPro carefully and noted down each one and (sigh) the Peukert exponent was set to the default of 1.25 instead of to 1.0. I swear I changed it- I have the value noted down...All the other parameters are where I set them, with the value noted in the margin where I set the parameters to something different than the defaults-

So I'm confident that explains it- it would definitely give a difference in the direction I'm observing. Will report back if my charge doesn't match my most recent discharge...


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## tomofreno (Mar 3, 2009)

Thanks for informing us of the probable source.


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## Moltenmetal (Mar 20, 2014)

I took the pack down 26.7 Ah today and recharged it. Pack topped out at -2.0 Ah, ie. it took 2 Ah less to recharge than the EEPro calculated to have been drawn. That's still significant but much less of a worry.


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## Moltenmetal (Mar 20, 2014)

Sorry to bump an old thread, but for the benefit of future "searchers"- and to save TBS more questions on the same topic, I got to the bottom of this.

EEPro function f5.6, the "charge efficiency factor", adjusts the Ah in value recorded during charging (and presumably also adjusts the Ah in during regeneration- not clear on that). I had mine set to AUTO, which is the default value, and somewhere along the line it got messed up in its calculation and started using a value of greater than 100%. Once I set that value to 100%, and of course with f.5.4 (Peukert) set to 1.0 also, my Ah in and Ah out started matching within 1%.

Running the analog ammeter in parallel with the shunt going to the EEPro is NOT a problem.

Note that the shunt sense wires must be a twisted pair for noise rejection. I ran mine with shielded twisted pair cable but didn't ground the sheath, which is a no-no- because it wasn't clear to me what "ground" I should use in this case. They recommend connecting the sheath to B-, at the shunt end of the cable only.


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## Sunking (Aug 10, 2009)

Moltenmetal said:


> Note that the shunt sense wires must be a twisted pair for noise rejection. I ran mine with shielded twisted pair cable but didn't ground the sheath, which is a no-no- because it wasn't clear to me what "ground" I should use in this case. .


As with any shielded cable, they terminate at one end only at the source. As with EV's the chassis are not bonded or referenced with the exception or 12 volt circuit, so you would use one of the battery polarities as the reference point assuming it is what the controller uses as Reference. 

Ground is a horrible word as it it not used as a conductor except by utilities which operate at high voltage. Low voltage under 1000 volts is forbidden to use earth as a conductor because it is a very poor conductor. It would be deadly to do so.


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## Roy Von Rogers (Mar 21, 2009)

Shielding for the pupose of emi will always be referenced to the offending item i.e controller. So the shield needs to be connected to the controllers chassis and the other end left open, doing so, the shielding will act as a wave trap.


Roy


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## Tesseract (Sep 27, 2008)

Sunking said:


> As with any shielded cable, they terminate at one end only at the source....


Shielding is a complicated topic and you (and, by extension, I) do it a disservice by applying blanket rules like the venerable, "ground the shield at one end only" rule; that rule is only true for low frequencies - think DC to audio. If the signals undergo rapid level transitions and/or are high frequency (ie - rise time of ~1us or faster; 100kHz or higher) then you do, indeed, "ground" both ends of the shield (ie - terminate it to the chassis of both/each device).

To prevent DC/low-frequency currents from circulating between devices (ie - the dreaded "ground loop") the chassis of each device will often be AC-coupled to its internal signal or power ground (via a low-value capacitor with minimal self inductance).

NB - "ground" has a very different meaning in electronics vs. "NEC" wiring: in the former case it is a common reference for all other voltages/currents in a circuit; in the latter it is an emergency/protective sink for fault currents. Do not make the mistake of applying rules intended for the distribution of electricity to the board/subsystem/product level.


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## Sunking (Aug 10, 2009)

Tesseract said:


> Shielding is a complicated topic and you (and, by extension, I) do it a disservice by applying blanket rules like the venerable, "ground the shield at one end only" rule; that rule is only true for low frequencies - think DC to audio. If the signals undergo rapid level transitions and/or are high frequency (ie - rise time of ~1us or faster; 100kHz or higher) then you do, indeed, "ground" both ends of the shield (ie - terminate it to the chassis of both/each device).


I have worked in Telemetry/Instrumentation/RF/Data/Telecom professionally for 35 years. We use a lot higher frequencies than EV's motor controllers or signal I/O like DS1, DS2, DS3 10/100/1000 Mb/s Ethernet, RF up to several Ghz, and even audio. You start bonding shielded cable at both ends especially over such short distances is asking for trouble. As the manufacture recommended, one end, at the source reference. Any AV tech will tell you if you run shielded cables and bond at both ends you get a LOUD HUM. Single Point Ground is the key to Quite. Bond both ends and you are going to introduce Common Mode Noise.

Personally Optical transducer would be the preference which is acutally pretty easy to do today.


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## PStechPaul (May 1, 2012)

The reason why audio cables often cause hum is that the shield is used as a signal return, particularly for the RCA phono plugs and coaxial cable. This was often corrected by connecting the frame of the turntable to a solid earth ground at the amplifier using a separate fairly heavy ground wire. Two-wire twisted pair conductors inside a grounded shield is preferable in many ways, especially where high magnetic fields are present and the twist cancels out their effects.

The choice of which end to ground may depend on where the noise may be coming from and where the most solid earth or chassis ground may be present, which IMHO would usually be the instrument itself and not the sensor. But then again, if the noise originates near the sensor (such as in the motor), it may be better to shorten the path for the noise in the shield, so a ground at the motor frame may be better.


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## wb9k (Apr 9, 2015)

PStechPaul said:


> The reason why audio cables often cause hum is that the shield is used as a signal return, particularly for the RCA phono plugs and coaxial cable. This was often corrected by connecting the frame of the turntable to a solid earth ground at the amplifier using a separate fairly heavy ground wire. Two-wire twisted pair conductors inside a grounded shield is preferable in many ways, especially where high magnetic fields are present and the twist cancels out their effects.
> 
> The choice of which end to ground may depend on where the noise may be coming from and where the most solid earth or chassis ground may be present, which IMHO would usually be the instrument itself and not the sensor. But then again, if the noise originates near the sensor (such as in the motor), it may be better to shorten the path for the noise in the shield, so a ground at the motor frame may be better.


In audio, the shield of a signal-carrying conductor should always be grounded at the source end of the cable by default. This generally works the best for minimizing noise.


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## wb9k (Apr 9, 2015)

Sunking said:


> I have worked in Telemetry/Instrumentation/RF/Data/Telecom professionally for 35 years. We use a lot higher frequencies than EV's motor controllers or signal I/O like DS1, DS2, DS3 10/100/1000 Mb/s Ethernet, RF up to several Ghz, and even audio. You start bonding shielded cable at both ends especially over such short distances is asking for trouble. As the manufacture recommended, one end, at the source reference. Any AV tech will tell you if you run shielded cables and bond at both ends you get a LOUD HUM. Single Point Ground is the key to Quite. Bond both ends and you are going to introduce Common Mode Noise.
> 
> Personally Optical transducer would be the preference which is acutally pretty easy to do today.


Dereck,

We may disagree strongly on some things, but this is not one of them. I've also worked with RF and high-quality audio for many years and agree 100% with what you're saying here. It's all about avoiding the potential for loops. Same with data in small systems I've worked with. Lay ground conductors in a "star", or "wheel and spoke" configuration, not a long single daisy chain, and never ever in a loop that is tied to "ground" at both ends. These are the cardinal rules.


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## dougingraham (Jul 26, 2011)

ekthor said:


> Hello guys,
> 
> I had the same concern, I'm using 36 LiFePo cells and a home-made charger, no BMS, no DC-DC, bottom-balanced cells. After full charge I get -1.5A.h to -4.5 Ah and I WAS a bit worried. I'm using JLD with a 500V, 75mV Shunt.
> 
> 2 months ago I was really worried because the JLD went down to -15.5Ah !!  Still I can't find an answer for this issue. Any idea?


There are probably a couple of things going on here mostly having to do with rounding errors. I have done careful measurement at rates up to 30 amps with 100 AH cells (basically C/3) and the measurement errors appear to exceed the the cycle losses in the cells.

Your discharge currents are going to vary between a few amps and several hundred amps. Your charge currents are going to be fairly constant. With 36 cells your peak charge voltage would be around 126 volts (3.5 vpc) If you charge from 120volts on a 15 amps circuit the most you could charge at would be 90% of 1800 watts or 1620 watts. When pack is nearly full the charge current could be 12.9 amps and when empty it could be 15 amps. With a 75mv 500 amp shunt this is a voltage between 0.001935 and 0.00225 volts across the shunt. Assuming 500 amps is 0.075 volts those charge currents are a tiny fraction of the discharge currents. Around 2.5% of the peak currents. The rounding errors will occur over the entire charge interval. On the discharge side you have short duration high current events. How often does the JLD measure the current? Once per second? 10 times per second? How often does it update the AH counter? So while the high current readings during the discharge have a chance for greater accuracy because of their varying nature and the update rates you can see wide variability on this side as well.

One other thing to consider would be things that are not being measured by the JLD 404. There must not be anything that is connected to the battery that does not go through the shunt. It would be easy to hook the DC-DC converter to the wrong side of the shunt and have it taking energy away from the battery that is not being seen by the JLD. Look for any wires connecting to the battery side of the shunt or the battery on the side where the shunt is connected. Anything there would draw current without being measured.

Best Wishes!


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