Is this NIMH enough?  
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Is this NIMH enough? - 5/17/2005 7:05:39 PM   
zope_pope



Posts: 1043
Joined: 1/19/2003
From: Aurora, CO, USA
Status: offline 		Hey guys,
I am using a 2700 mah 6V Expert Electronics battery to power 4 standard servos, 2 high speed high torque digitals, and one high speed micro servo. It will be flying in my DP Ultimate. I have flown this before, but after one flight, my voltwatch type device goes to red when the digitals are under load. This leads me to believe that the battery can't provide the required current for my setup. Should I go with a Nicad instead? I was considering a lipo but they sound like too high of maintenance for me. Please help me get pointed in the right direction. Thanks.
Adam

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RE: Is this NIMH enough? - 5/17/2005 7:49:03 PM   
Rodney



Posts: 4774
Joined: 12/8/2001
From: FL
Status: offline 		First, make sure that the switch and wiring between the battery, switch and receiver is a heavy duty unit. Any impedance in this area will have bad effects on everything else. Try plugging the battery directly into the receiver (i.e. bypass the switch and switch wiring) and see if the problem occurs then. Of course the wiring to the battery itself must be at least 22 gage, preferablly 20 gage, with gold plated connector.

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RE: Is this NIMH enough? - 5/17/2005 8:07:48 PM   
zope_pope



Posts: 1043
Joined: 1/19/2003
From: Aurora, CO, USA
Status: offline 		Well I have the MPI Heavy Duty switch, so it is a heavy duty, but maybe there is something wrong with it. i will go try your advice tonight

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RE: Is this NIMH enough? - 5/18/2005 5:19:18 AM   
Hangtime



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

Last time I checked the 2700 'A' NiMH cell had an internal impedance of about 20 mOhms.. compared to most 'AA' sized nicads at about 10 mOhms this could be the source of the issue.. although you've got plenty of capacity with that 2700 pack, high servo loads can cause a shockingly pronounced voltage drop while the loads are applied.

It's not just a NiMH issue.. high impedance nicads behave the same way. You can get high capacity, LOW impedance NiMH packs, but generally the capacity is a bit less than high impedance cells in the same dimensions. The way most modelers with big birds beat the impedance problem of the 2700's is by running the packs in parallel.. in essence running two packs, two switch harnesses. This cuts the impedance in half (now down to 10 mOhms) and doubles the capacity of the system. An effective move in 25lb 3D airplanes, not such a popular move in lightweight pattern and .60 to 1.20 sized 3D birds due to weight gain.

In your shoes with a small DP Ultimate, I'd switch to a low impedance NiMH pack like a Sanyo 1950 FAUP.. smaller than your 2700 pack, yet VERY low impedance.. about 4.5 mOhms as compared to the 2700's 20 m Ohms. Impedance is like a golf score.. the lower, the better. In your situation you'll have plenty of 'grunt' to handle high drain servo's without the scary voltage drop while the loads are applied, and you'll find you saved a bit of weight also.

Hope this helps!

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Steve Anthony
www.hangtimes.com
NoBS Batteries

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RE: Is this NIMH enough? - 5/18/2005 11:23:40 AM   
Geistware



Posts: 12946
Joined: 4/9/2002
From: Locust Grove, GA, USA
Status: offline 		I did a test and you can make your own conclusion.
I charged a 2700maH NiMH pack and a 1300maH NiCad pack.
The NiMH pack could supply 1 amp for 1600maH then the voltage went below 5.0 volts (5 cell pack)
The NiCad pack could supply 1 amp for 1000maH then the voltage went below 5.0 volts (5 cell pack)

I would get rid of the NiMH Pack.

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RE: Is this NIMH enough? - 5/18/2005 2:12:09 PM   
3D Joy



Posts: 1061
Joined: 12/21/2002
From: , QC, CANADA
Status: offline 		Well said guys.

A friend of mine flied a 2100 mah Nimh pack (AA size) which had an internal impedance of 25. The plane is a WH Extra 300 at 28% (I think) after 3 flights from a full charge, he made a snap and the receiver went into failsafe! Current needed made the voltage to drop considerably...

He doubled up the packs.

The voltage of my batteries (Nimh 2300mah 2 in parallel through their own HD MPI switch) with ONE coreless digital servo stalled drops down more than 1 volt! That is with 2 batteries!

I ordered 2 4/5FAUP at 1950 Mah at 6V for my plane. Yes they are heavier and of less capacity and more expensive but I will NEVER get 2300 mah from my 2300 batteries anyway.

Hope this helps.

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RE: Is this NIMH enough? - 5/18/2005 3:59:42 PM   
zope_pope



Posts: 1043
Joined: 1/19/2003
From: Aurora, CO, USA
Status: offline 		Thanks guys,
I was thinking about going with NoBS batteries for their very informative site, and their owner even decided to help me. Last time I will buy an Expert Electronics pack. Thanks for the help guys!
Adam

< Message edited by zope_pope -- 5/18/2005 6:11:54 PM >

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RE: Is this NIMH enough? - 5/18/2005 6:05:34 PM   
JoeAirPort



Posts: 7130
Joined: 8/11/2003
From: Milwaukee, WI, USA
Status: offline 		Rodney so you 2nd Geistware's comment on not using NiMh?


< Message edited by JoeAirPort -- 5/20/2005 1:39:56 AM >

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RE: Is this NIMH enough? - 5/18/2005 6:45:04 PM   
marked23



Posts: 377
Joined: 3/2/2004
From: Lynnwood, WA, USA
Status: offline 		I've tried three different brands of the "blinking lights" voltage meter type devices. The Voltwatch brand was, by far, the worst at giving me a good impression of my NiMH battery status. A fullycharged GP3300 pack (from Steve no less) could be brought down to the yellow by just moving the sticks aggressively. I tried a similar volt meter that HeliProz sells under the Mavrikk brand. That one is a lot better for reading the health of my 4-cell nimh. The best of the three was the one CenturyHeli sells. It looks just like the Mavrikk one except it has two micro switches on the back for setting different voltages. The one from Century gives me an accurate representation of my battery state.

I think the Voltwatch is just too jumpy. If the voltage dips a bit, the Voltwatch lights move a lot. It was really disconcerting. I quit using it.
I'm talking about the regular Voltwatch. I haven't tried the new (slightly smaller) Voltwatch.

This is just my experience, your mileage may vary.

-Mark

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RE: Is this NIMH enough? - 5/18/2005 7:04:17 PM   
RCAddiction



Posts: 901
Joined: 3/29/2002
From: Waxhaw, NC, USA
Status: offline 		2 observations:

1. Consider using a decent loaded battery tester. I use this one from I4C which can load your pack to 500ma, 1000ma, or 1500ma. It is $50 but saves the cost of buying Voltwatch devices for every aircraft you fly.

http://www.i4cproducts.com/battery_tester.htm

2. Nobody seems to have mentioned that besides wiring connections and/or the battery pack, that there may be one or more bad servos or linkages which are causing excessive current draw due to binding, etc.

3. Zope....in addition to using a loaded battery tester, you should cycle your battery (discharge then charge it on a cycle charger) to see how much capacity you've used up after 1 flight. It will also tell you how much capacity that pack can provide when it is fully charged. If you have a problem with a high impedence connection, binding servo, etc, you might actually be draining the NiMH battery and the Voltwatch may actually be doing its job by warning you.

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RE: Is this NIMH enough? - 5/18/2005 8:23:18 PM   
zope_pope



Posts: 1043
Joined: 1/19/2003
From: Aurora, CO, USA
Status: offline 		I know that the airplane used about 400 mah range checking, adjusting the engine, and on the first flight. I used my trusty accucycle to get that number. It could also be a bad servo, but not a bad linkage as I have done the same test by hand (loading the servo arm with my fingers).

I am not actually using a voltwatch. I am using the MPI HD Switch with the voltmeter lights built into it. I am not sure how accurate it is, but since this is my favorite airplane, one can never be too careful.

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RE: Is this NIMH enough? - 5/18/2005 9:29:36 PM   
Geistware



Posts: 12946
Joined: 4/9/2002
From: Locust Grove, GA, USA
Status: offline 		I have used the voltwatch and I4C units.
The Voltwatch is accurate with NiCad's
On NiMH, it gives you a false impression because the discharge vure is flat until the pack is almost dead.
I never knew the internal loading of a NiMH pack was so high until I measured it.
Almost half the pack is useless for high load applications.

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RE: Is this NIMH enough? - 5/18/2005 9:51:10 PM   
famousdave


 

Posts: 2263
Joined: 9/16/2003
From: Bradenton , FL, USA
Status: offline 		1> Voltwatch and Expert power monitors are worthless, they are way too sensitive to transients to be of any benefit whatsoever.. they are a gimmick
2> Use a low impedance pack like Steve Suggested (Expert packs are cheap high impedance packs) and don't be fooled by big capacity numbers
3> Use a H9 Load test meter and apply a 500mA load (single pack) and make sure it stays above 6V (i.e. no-fly voltage is 6V)
4> Fly and don't worry about NiMh, it is a lot better than NiCd.


PS: Li-Ion is far LESS maintenance than either NiMh or NiCd. The only bad thing is you need a special charger which costs about $100 and you have to use regulators which could introduce another point of failure. Other than that there is ZERO maintenance.

NiMh cells discharge themselves over time so they require topping off every so often
NiCd cells need to be cycled regularly or they get memory effect

With Li-Ion you just charge and use, kind of like filling a gas tank - you only need to be religious about read the charge state with the load meter between flights. I have literally gone 4 weeks and flew 20 flights on a single charge. NO BS!


Many times in smaller planes NiMh is the best solution as you need the battery weight to tune CG anyway - why go with a light battery if you have to add weight elsewhere??!
DP


< Message edited by desertpig -- 5/18/2005 9:56:42 PM >

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RE: Is this NIMH enough? - 5/18/2005 11:56:07 PM   
RCAddiction



Posts: 901
Joined: 3/29/2002
From: Waxhaw, NC, USA
Status: offline 		Zope, I'm very familiar with the MPI switch with built-in LED's. It was actually my idea, which I provided to Jarvis (Maxx Products owner) one night when he visited Venture Hobbies. He lives nearby and I see him periodically.

The MPI switch has a wire you clip for 4.8V or 6V. Make sure it's set for the right voltage. I believe you said your pack was 6V, right? Are you running a regulator between the battery and switch which might reduce the voltage and be fooling the battery indicator?

Anyhow, as several have indicated, you want to check any pack with a decent loaded voltmeter to be sure what you have. It is possible that the MPI indicator is defective and that your battery is fine. Cycle the pack to check its total capacity. It should be close to 2700. If you get something like 2000 or so, it's probably a failed pack. Maybe a bad cell.

I am a bit surprised that people don't care for the 2700 Expert Sanyo NiMH pack:
http://www.horizonhobby.com/Shop/ByCategory/Product/Default.aspx?ProdID=EXRB301

I don't know why people are dissing it. I've used the 2700 mah Expert pack (Sanyo A cell) on a 120 sized Giles with all digital servos and it performed very well. Dave Thacker (Radical RC) mentions the Sanyo 2700 NiMH pack on his site "One of my good flying friends uses one of my 2700 packs in a 1/3 scale Aircraft International Extra with 9 JR coreless servos. He has about $4000 in this ship and is one of the most respected pilots in the club. With NiMH, he has a huge capacity reserve and a light weight pack."





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