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RE: RX voltage display - 10/27/2004 9:14:34 PM   
Hangtime



Posts: 346
Joined: 6/5/2002
From: Babylon, NY, USA
Status: offline 		Hi Bill!!

The short answer is capacity ain't the only decison point in cell selection.. impedance is a BIG factor in voltage perfomance under load.

In this situation we were able to reduce system impedance and hence voltage drop by taking out his single 5 cell pack and replacing it with a parallel 1950 FAUP NiMH pack system.. cutting the impedance value from 4.5 mOhm (sub-c Sanyo CP2400SCR) to about 2.5 mOhm. (Since the FAUP's impedance value is about 5 milliohms, using 'em in parallel netted a 50% reduction in system impedance)

Going this route produced some improvement in loaded voltage drop performance. It also bumped up his total capacity significantly (nearly doubled it) and it added switch/connector redundancy he didn't have before. The down-tick is the weight.. the system gained about 5 ounces weight. Of course on his 20 lb (320 oz) airplane that 5 oz didn't affect the performance to any noticeable degree.

(in reply to BillS)
       Post #: 51

RE: RX voltage display - 10/27/2004 9:23:12 PM   
Hangtime



Posts: 346
Joined: 6/5/2002
From: Babylon, NY, USA
Status: offline 		Whups.. forgot to add in links to the MFG's cell specs for the two cell types..

Sanyo FAUP NiMH 1950 4/5 'A'

Sanyo CP2400 SCR Nicad

(in reply to BillS)
       Post #: 52

RE: RX voltage display - 10/27/2004 9:26:38 PM   
BillS


 

Posts: 2988
Joined: 9/6/2004
From: Greensboro, NC, USA
Status: offline 		Hangtime,

quote:

taking out his single 5 cell pack and replacing it with a parallel 1950 FAUP NiMH pack system … bumped up his total capacity significantly (nearly doubled it)


Slow down. In order to double the capacity you must have also switched from NiCad’s to NiMH. Is this interpreted properly? Sorry but the part numbers are meaningless to me

Bill

(in reply to Hangtime)
       Post #: 53

RE: RX voltage display - 10/27/2004 9:29:24 PM   
BillS


 

Posts: 2988
Joined: 9/6/2004
From: Greensboro, NC, USA
Status: offline 		Backfire I am in favor of voltage indicators in the airplane.

quote:

Accurate? No, but better than nothing.


A thousand times better than nothing.

The real problem from a SAFETY standpoint is that ‘Battery Reckless’ and ‘No Battery Knowledge’ do not have a clue. They also are not going to spend money since they don’t even understand a problem exists. Nevertheless they compromise general field safety.

(in reply to backfire)
       Post #: 54

RE: RX voltage display - 10/27/2004 9:31:28 PM   
Hangtime



Posts: 346
Joined: 6/5/2002
From: Babylon, NY, USA
Status: offline 		Hi Bill.. product specs are in the post above. Read slower.

A quickie FYI.. parallel systems give yah the net total of the capacity of the two packs and halve the impedance of one pack. Closest thing to a free lunch we'll ever get in R/C.

(in reply to BillS)
       Post #: 55

RE: RX voltage display - 10/27/2004 11:18:56 PM   
JohnMuchow


 

Posts: 64
Joined: 10/6/2003
From: New York, NY, USA
Status: offline
quote:

ORIGINAL: BillS
But first the manufacturer must be convinced of the need for voltage downlink. The conversation here is less than convincing. There are far too many willing to accept status quo.


I agree. Maxim (Dallas Semiconductor) has an Application Note that might be of interest here:
AN121: Inaccuracies of Estimating Remaining Cell Capacity with Voltage Measurements Alone
http://www.maxim-ic.com/products/power_supplies/battery_support.cfm

But the technology to do it right certainly exists. Using a coulomb counter chip (Maxim has several and Texas Instruments has just released one that is accurate to 1%, still waiting for our evaluation module), you can easily report the used and/or remaining AH of a battery pack. Coulomb counting simply means measuring the charge input to and later removed from the battery. In order to do this, first the charge and discharge current is measured through a low-value series resistor between battery and the load (motor, bulbs, etc.). The voltage drop across the sense resistor (representing the current flowing through it) is then integrated over time to provide an accurate measurement of the state of the charge of the battery.

The charge value is stored using a voltage-to-frequency converter that increments a counter as the current flows through the resistor. The faster the current flows, the higher the frequency and the faster the counter counts. When you want to know how much charge has flowed in or out of the pack, just read the counter!

The best setup is when the specs of the cells/pack are programmed into the coulomb counter, but just using it as a elapsed-AH counter works well without that. But, you'll need a microprocessor to read the data and a RF link back to the transmitter (now a transceiver)...or perhaps colored LEDs or beepers on the plane?


_____________________________

John Muchow
CamLight Systems

(in reply to BillS)
       Post #: 56

RE: RX voltage display - 10/27/2004 11:32:28 PM   
Hangtime



Posts: 346
Joined: 6/5/2002
From: Babylon, NY, USA
Status: offline
quote:

The best setup is when the specs of the cells/pack are programmed into the coulomb counter, but just using it as a elapsed-AH counter works well without that. But, you'll need a microprocessor to read the data and a RF link back to the transmitter (now a transceiver)...or perhaps colored LEDs or beepers on the plane?


John.. excellent! All we'd need is a digital counter display on the aircraft.. just peak the pack, reset the counter to zero and monitor MAH consumed. Like they say in that beer commercial..

"BRILLIANT!!"

(in reply to JohnMuchow)
       Post #: 57

RE: RX voltage display - 10/27/2004 11:50:27 PM   
JohnMuchow


 

Posts: 64
Joined: 10/6/2003
From: New York, NY, USA
Status: offline 		Whoops, just noticed the earlier Gas Gauge article reference.
But, some of the earlier problems noted with gas gauging still hold true. Due to load and temperature changes (amongst other things) you never know the real capacity of your packs until you finish discharging (when using the gas gauge just for counting AH going out of a pack). But, at least the coulomb counter/gas gauge could tell that pretty accurately. Your own judgement of how much capacity is left in the cells has to take over from there.

It's a start.

[Edit] BTW, the chip set we're evaluating is the bq20z8x mentioned in the article posted previously:
http://www.elecdesign.com/Articles/ArticleID/8764/8764.html

< Message edited by JohnMuchow -- 10/27/2004 11:55:12 PM >


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John Muchow
CamLight Systems

(in reply to Hangtime)
       Post #: 58

RE: RX voltage display - 10/28/2004 12:08:39 AM   
Hangtime



Posts: 346
Joined: 6/5/2002
From: Babylon, NY, USA
Status: offline 		Whats missing from the ones we've seen up to now is the 'reset' capability.. had to unplug the cottin pickin pack every time, and I'm not charmed by any gadget sitting between the pack and the workload. The furbish lil device would have to be bonehaid-proof and able to handle some pretty significant currnet loads. We've got a nice little digital volt meter you can mount in the plane, heck it even has a 500ma load; essentialy the ESV is on the plane insead of in your flight box getting dusty. Nice as far as it goes.. but wouldn't it be great if we had a easily resettable 'totalizer' that read mAh in and out, flick a switch for voltage, a momentary button for a load and have the whole thing incorporated with a nice redundant HD system switch w/charge port as part of the same device too??!!

(ok, I'm dreaming...)

No matter what we wind up with, a pair of eyeballs and a brain attached to the rest of the pilot will be required to 'interpet' what the info means; not much different than what we do now. The rub will be as it always is.. leading the horse to the trough ain't the tough half of the battle.. it's still no solution for the 'battery reckless' and 'battery clueless' folks that Bill is trying to solve.

My Gramps did have a saying for them too.... 'Kid, the cure for ignorance is education.. the cure for stupidity is death'.

The older I get, it seems the smarter he got.

(in reply to JohnMuchow)
       Post #: 59

RE: RX voltage display - 10/28/2004 12:32:21 AM   
Mike01


 

Posts: 643
Joined: 12/29/2003
From: Brooklyn, NY, USA
Status: offline 		I think some people missed my point.

This votlage myth is not some harmless thing to just smile about and ignore...it's downright dangerous. Seasoned flyers like BillS and others can tell the state of their batteries by using voltage, because they know their batteries and they know how much their planes draw...whether they actually derive their data from the votlage or an intuitive knowledge of their systems remains to be seen (no one has yet volunteered for the virtual voltage detection tests I proposed earlier...I have more packs we try it on).

It's dangerous because it's a well established myth that is sold to new flyers as gospel (at least it was in the two flying fields I've been to). Is it any wonder, BillS, that you have not so experienced people at your field that lose planes to battery failure?

On the day I first arrived at my local club I had to wait an hour for my instructor to find a plane someone had crashed in the bushes...a big expensive plane that went down because of battery failure. One of the first things my instructor taught me was "check the voltage before each flight so you know the batteries are charged".

I started to say "but you can't tell the state of charge from the..." but thought better of it. He was old, stubborn, and set in his ways. Ways that had led to him combing the woods for an hour looking for a thousand dollar plane while I sat patiently waiting. Of course it was not his plane, but that is a moot point. I'm sure the guy that lost the plane checked the voltage...this particular club is so anal he would never have flown without it.

(in reply to Hangtime)
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RE: RX voltage display - 10/28/2004 12:36:47 AM   
JohnMuchow


 

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From: New York, NY, USA
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quote:

ORIGINAL: Hangtime
The older I get, it seems the smarter he got.


Amen....funny how that works, isn't it?

OK, how about this...
Reset is easy since we have a micro reading the counter on the gas gauge (GG) and can zero it out at any time. We only need a sense resistor inline with the pack, and that's pretty reliable (only as good as the connections to the circuit board it's on though).

Having the unit alternate between accumulated-AH and real-time voltage display is easy (that micro just sits around while the GG is counting, easy to measure voltage), perhaps with a bicolor LED to indicate which is being displayed?

Adding a couple of solder pads for a load (or have it already mounted) is certainly doable...perhaps better to have it optional so weight is lower?

0-100A discharge current range is doable, but the resolution and total-AH counting range drops significantly if we try to have it all in one device. If a setting could be changed or new resistor used, it's easier to offer better resolution.

Not a perfect solution, but one with lots of possibilities.


_____________________________

John Muchow
CamLight Systems

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RE: RX voltage display - 10/28/2004 12:45:33 AM   
JohnMuchow


 

Posts: 64
Joined: 10/6/2003
From: New York, NY, USA
Status: offline
quote:

ORIGINAL: Mike01
I think some people missed my point.
This votlage myth is not some harmless thing to just smile about and ignore...it's downright dangerous. [clip]


PMJI...
I agree but IMHO it's going to have to be a long drawn out education process. We can do things like pass around the Maxim article I posted the link to, point people to supporting web sites, attempt to explain the science, etc., but no matter what we do or how right we are, some people just won't care....even if they believe you.

Think about how hard it is to get the other half of the country to wear seat belts (or whatever percentage it is). There's probably nothing with clearer evidence as to how it can save lives, but millions of people won't/don't do it....no matter what.


_____________________________

John Muchow
CamLight Systems

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RE: RX voltage display - 10/28/2004 1:15:43 AM   
JNorton



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Joined: 1/29/2002
From: Montague, MI, USA
Status: offline 		Mike01
Interesting how we draw a conclusion from are own experience. My experience is different from yours. In my club it is taught - very clearly that voltage measurements of batteries are only done on known fully charged packs. Having a fully charged pack as a starting point - voltage measurement will show a range that is safe to fly. We agree that voltage measurement in and of itself is worthless.

JohnMuchow - Reds Battery clinic is a very good source of credible information concerning usage of RC batteries. If more people would read it, it would help. As far as your analogy about wearing seat belts a better one is people who still smoke cigarettes, including my Father-In-Law who uses oxygen at night in order to breathe!

John

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Saito Member #467

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RE: RX voltage display - 10/28/2004 1:32:37 AM   
Flying Geezer



Posts: 996
Joined: 4/12/2003
From: Bowling Green, KY, USA
Status: offline 		OK, I'm in. The corelation between battery voltage and state of charge is not a myth. It takes a little knowledge to use it correctly because there are factors to be considered. For example: a quarter scale plane with two 1400 mAH packs reading 4.9 volts has a substantial amount of current delivery left. A 500-800 mAh pack at the same voltage in the same aircraft is a plane I wouldn't want to fly. In addition to being aware of the batteries capacity, (tested for capacity every couple of months), the loaded voltmeter should be connected and watched for 30 seconds or so, to see if voltage is fading. It's not an absolute measurement of capacity, but it defines a safety range if you know what you are doing, and the condition of your batteries, and you are astute enough to provide enough excess battery capacity for the airborne system you are flying. It's not perfect, but the corelation of loaded battery voltage, and remaing charge is NOT a myth. I don't care who says so.

NOW, let me have it!

"You can lead a horse to water, and you can make him drink, but it's really tough to get him to rinse out the glass."

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(in reply to JNorton)
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