Ni-Cad versus Ni-MH batteries (Full Version)

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ggeezer -> Ni-Cad versus Ni-MH batteries (5/9/2008 1:25:11 AM)

Jay and I started discussing batteries in the "Ghost" thread which is getting long and we were digressing from the topic so I would like to start a new thread. Jay warned against using Ni-MH for galloping ghost applications because the nominal voltage was 1.1 volts.
Not having too much experience with Ni-MH cells, I became confused when I checked the specs. on the Sanyo web site. The specifications for the HR-3U 2500 AA Ni-MH cell indicates that the voltage stays well above 1.2 v for 2000 maH of capacity with a draw of 500 ma. At this rate, the Rand GG pak should work for at least 6 hours before the voltage drops below 1.2 volts. In fact the voltage curve is better than that for Ni-Cad batteries and the largest capacity AA cell they have is 1000 maH.
If what I read is correct, then battery capacity with "ghost" systems is no longer an issue.
Opinions on this subject would be appreciated. For specs on the Sanyo HR-3U cell go to:
http://sanyo.wslogic.com/pdf/pdfs/HR-3U-2500.pdf



iflyj3 -> RE: Ni-Cad versus Ni-MH batteries (5/9/2008 11:17:31 AM)

I don't know of Jay's reasons but my experience will agree with him. I have not used the Sanyo cells you site, so my experience may be old hat.

I have a pulse system using two Bellamatics, that I build in about 1965, and I have been referbishing. When I turned off the transmitter for a failsafe test, the pulse rate detect single shot kept firing on its own and pulsing the relay that was supposed to cut off the voltage to the Bellamatics. I had never had this happen before. After much head scratching, it dawned on me that I was using NiMH cells. I sub'ed a Nicad pack and the problem went away.

The NiMH was Energizer 2500 AA and the Nicds were Sanyo N700AAC. When I investigated these cells I found the Sanyo's had a 16 millohm ESR. I wrote Energizer and they replied the NiMH had less than 50 millohm ESR. This was apparently the cause as the pulse omission relay pulled in the voltage depression was enough to fire the rate detector.

I did notice that the Sanyo cells you reference list a 25 millohm ESR at 1.0 volts. That is lower than the 50 for the Energizer cells but I do wonder what it is at 1.25 volts as a cell at 1.0 volts is no good at all????

Keep in mind that a motor may draw an average of 500 ma is pulling up to two amps during reversal for a few microseconds. On these pulse systems we are driving hard in a direction, them reversing the voltage to the motor. Since the motor is now a generator before reversing, the voltage it is generating is opposing the applied voltage that is trying to send it in the opposite direction. This is worse than the current draw trying to get a stopped motor started initially.

Anyway, my experience thus far has me going back to Nicds. In addition to the ESR, the NiMH cells do not last as long as Nicds. I bought several NiMH about 2 years ago and now they are going bad. Sanyo Nicds will last 4 to 8 years before losing capacity or internal shorting.

My $.02



ggeezer -> RE: Ni-Cad versus Ni-MH batteries (5/10/2008 8:58:14 PM)

Dan,
Thanks for your .02 worth but your input is far more valuable than that. It goes to show that manufacturer's spec. sheets don't always tell the whole story. You and Jay have had "hands on experience" which will tell a more complete story regarding this application. It is so true that the inrush current can be large and the battery's internal resistance then comes into play. It is also possible that the resistance is a non linear function with respect to current and temperature. My electric flier friends tell me that Ni-MH flight motor packs work best when they are hot. I will stick to Ni Cads.

Orv.



jaymen -> RE: Ni-Cad versus Ni-MH batteries (5/12/2008 6:11:12 PM)

I was going to say.....Ni MH packs work best hot off the charger. I noticed, like Dan, a fall off in performance after my packs were a few years old, and I don't fly that much so it was more age related than the amount of cycles. I would gues I have about 20-30 cycles on all the different packs.

I also noticed Ni MH batteries don't hold a charge over a two week period, meaning you need to charge them right before use, or they don't deliver full capacity. My flight times were about 50% of what they normally would have been when I used a pack that was charged a week before flying.

I think the real reason for my reccomendation is the voltage though. These silly Galloping Ghost Rand actuators are very sensitive to voltage; too much and they over react and cycle the throttle with a rudder or elevator command. Too low a voltage and they won't cycle the throttle, but more importantly, they receiver glitches.

Dan brings up a very good point I omitted, and that is the huge instantaniouse current draw due to the servo motor reversing, up to 12 times a second at full down elevator. This puts a big load on the batteries and you need plenty of capacity to handle those surges.

One thing of interest that I noted in testing these systems was that I could get more range(300 feet) from a 1200 mAh pack than a 600 mAh pack,(200 feet) and a 350 mAh pack would only give me about 75 feet of range. This must be due to the voltage dropping out during the current surges, and the bigger packs maintained a steadier voltage at the receiver, therefore increasing it's performance. I did not monitor the voltage, but I did note the fall-off in range with smaller packs, even when they were fully charged.

I found most of this stuff out the hard way myself!



paul_TAB4 -> RE: Ni-Cad versus Ni-MH batteries (5/18/2008 11:39:43 PM)

hi everyone I am new to rc and have a battery question. do NiMH batteries have memory like NiCd batteries. can I just top em up?
Also if the mAh are higher, will I get a little more punch? I am currently running NiCd 2400 mAh. I would like a longer run time as I am considering racing. Are there any tips to choosing batteries ie. brands or number of cells?

any help is greatly appreciated[:D]
thanx.



jaymen -> RE: Ni-Cad versus Ni-MH batteries (5/28/2008 8:15:20 PM)

NiMH cells are only 1.1 volts each. If for example you had a 4 cell NiCad pack, 1.2 volts per cell, it would be 4.8 volts. If you replaced the 4 cells with NiMH type cells, you would only have 4.4 volts. This would result in less "punch" due to the reduced voltage. The answer is to use 5 NiMH cells, that would give you 5.5 volts.

For the same physical size and weight, you can almost double the capacity when replacing NiCads with NiMH cells, but you have to add extra cells to make up for the voltage differance.

NiMH cells supposedly have no memory, but in practice, they do benefit if you drain them down and then fully charge them once in a while(cycling). NiMH batteries perform the best and deliver the fullest capacity only when used hot off the charger. If you let them sit for a couple days, re-peak them on the charger for best performance. NiMH battaries do not have the life of NiCads, this is because when you have more capacity in the same size package, the plates and electrolye are thinner and deteriorate faster. Therein lies the trade-off of price, size, and performance.

I used to use a 6 cell NiCad, 350 mAh, 7.2 volt pack for my GWS Tiger Moth park flyer. I have a 7 cell NiMH pack, 800 mAh 7.7 volt pack in there now which almost doubles my flight times.



iflyj3 -> RE: Ni-Cad versus Ni-MH batteries (5/28/2008 8:34:47 PM)

Jaymen,

I have to disagree slightly on your comparison of NiMH to NiCD. They both are rated at 1.2 volts. The reason you have to increase the cell count when using NiMH in high current drain applications is because the ESR of NiMH is higher than NiCD. Since under load the voltage drops because of the cells ESR an additional cell must be added to get the voltage back to an acceptable level. A AA Sanyo NiCD has 16 Milliohms ESR whereas a AA NiMH has 50 milliohms. Quite a difference.

I agree with your comment that the life if NIMH is shorter than NiCD as that has been my experience. However, I have also noticed that the high capacity NiCD's also have a shorter life too. I have found the best NiCD is the Sanyo N700AAC which is a AA pencell. Any more capacity than 700 in the AA package results in a shorter life cell.



HighPlains -> RE: Ni-Cad versus Ni-MH batteries (5/28/2008 9:57:25 PM)

quote:

Any more capacity than 700 in the AA package results in a shorter life cell

I noticed the same thing years ago in my Corvette. When it was new, it seemed like I was replacing the battery every year or two. Rock solid springs, low profile tires, the ride is harsh even on smooth roads. Finally, one replacement I decided to go with the cheapest low capacity battery. It had double the life of the higher capacity batteries. Only think I could think of was the higher capacity batteries are built with thinner components to get more surface area in the same size box. The coarser, cheaper batteries were just more rugged.

If weight is not the top criteria, nothing beats a nicad.



fizzwater2 -> RE: Ni-Cad versus Ni-MH batteries (5/28/2008 10:43:55 PM)

Even not all NiCd's are created the same, though. As highplains said, watch for too much capacity in a small case, especially if you are going to use digital servos (high current drain). You want to find a battery with low internal impedance (ESR) so the voltage doesn't drop out under high load.




jaymen -> RE: Ni-Cad versus Ni-MH batteries (5/28/2008 11:09:18 PM)

Dan, can you explain why all my NiCad 4.8V 500mAh packs work fine in my 2-channel gliders, but when I replaced them with 4 cell AA sized NiMH packs they all glitched terribly and the voltage always reads lower on them than the Nicads , even when fully charged?

I have had this problem since the batteries were new.

Another recent issue I ran into was with the Space Control: It has a pack of 6 "D" cells, NiMH. I had noticed the output of the DC-DC converter which turns the voltage from the 6 cell pack into 135 volts DC for the plate supply of the tubes was low, it was around 112 volts DC. I then noticed the six D cells were NiMH, so I replaced them with NiCads and instantly got the full 135 volts DC. I then checked and noticed I only was getting 6.7 volts from the NiMH pack as opposed to 7.2 volts from the Nicads.

Why am I consistantly getting .1 volts less from NiMH cells, regardless of their capacity/size, even under no load? This I do not understand.

I have noticed glitch problems using 4 cells for receiver packs, which went away when I added a cell. I also noted that 8 cells was not enough to get full output and voltage on the transmitters, yet 9 cells worked fine when using NiMH...what is going on?? Please clarify for me, thanks Dan.



iflyj3 -> RE: Ni-Cad versus Ni-MH batteries (5/29/2008 1:28:43 AM)


quote:

ORIGINAL: jaymen

Dan, can you explain why all my NiCad 4.8V 500mAh packs work fine in my 2-channel gliders, but when I replaced them with 4 cell AA sized NiMH packs they all glitched terribly and the voltage always reads lower on them than the Nicads , even when fully charged?

Another recent issue I ran into was with the Space Control: It has a pack of 6 "D" cells, NiMH. I had noticed the output of the DC-DC converter which turns the voltage from the 6 cell pack into 135 volts DC for the plate supply of the tubes was low, it was around 112 volts DC. I then noticed the six D cells were NiMH, so I replaced them with NiCads and instantly got the full 135 volts DC. I then checked and noticed I only was getting 6.7 volts from the NiMH pack as opposed to 7.2 volts from the Nicads.

Why am I consistantly getting .1 volts less from NiMH cells, regardless of their capacity/size, even under no load? This I do not understand.

I have noticed glitch problems using 4 cells for receiver packs, which went away when I added a cell. I also noted that 8 cells was not enough to get full output and voltage on the transmitters, yet 9 cells worked fine when using NiMH...what is going on?? Please clarify for me, thanks Dan.


Jay,

I think I the main problem is ESR.

I am restoring a pulse system I built in about 1964 that uses two Bellamatics and an ANNCO servo for throttle. On the bench I had a 4 cell NiMH pack (2500mah) that I used. I was getting all kind of funny glitches. After much scoping and measuring I replaced the NiMH pack with a 4 cell Sanyo NiCD pack of 700 mah. All the glitching stopped and it behaved like it used to.

I can't explain why you are measuing 1.1 volts with no load. I just went to the shop and measured a couple of cells that have not been on charge for about two weeks and they measured 1.24 volts. I also checked some new eneloop NiMH cells and the measured 1.34 volts.

I am including pictures os the NiMH pack I have used and you can see the side says 1.2 volts. I also contacted Energizer and asked what the ESR was and they replied less than 50 milliohms.

In normal planes I was using two parallel packs so the effective ESR would be less than 25 milliohms.
However, after two years they are starting to go bad so I replacing them all with NiCD's.
I also used 8 cell packs in my transmitters with no voltage problems.



Mad Man Marko-RCU -> RE: Ni-Cad versus Ni-MH batteries (5/29/2008 3:37:56 AM)

The biggest problem I see with the Nimh batteries is the "self discharge" factor. I have been using the sanyo 2700 mah cells in my RC sailboats and you do have to keep them on some sort of trickle charger or charge just before every race to prevent them from loosing their charge. Good old nicads have a great tolerance for charging on Sunday night and being able to go to a Thursday night race but the Nimh have to be charged the night before or the do not last. My new batteries are all Nicads I'll just buy an extra pack and swap them out if I need more capacity. The 2700's also have a fairly high voltage drop using digital servos. Love that 1960's technology.

Peace

Mark O



jaymen -> RE: Ni-Cad versus Ni-MH batteries (5/29/2008 4:48:23 PM)

Aha.....I see the differance now....I'm not using Eveready Energizers, I'm using some no-name brand AAA size and AA size cells that the hobby shop was selling.

I have checked a fully charged Nicad AA 600 mAh pack and found each cell at about 1.35 volts as compared to about 1.25 volts for the NiMH cells. So it seems there is about a .1 volt differance between the two types, but not as low as I thought, which was 1.1 volts. This maybe because I was testing under load, which would account for a lower voltage measured.

Dan, it makes sense now that the higher electro-static resistance, or internal resistance of the NiMH cells are higher than NiCads. I guess this means they have higher admittance or MHOs rating too. It must be that when you cram more capacity into the same size cell, it can only be done with thinner plates and less electrolyte between them. This means the electro-chemical action is more limited at higher current draw due to the smaller volume of electrolyte. So the NiMH cells are probably great where peak current demand is low. The additional cell I have been using is therefore compensating for the higher voltage drop across the NiMH cells under high current as compared to a NiCad. This means my observations of 1.1 volts per cell under load is due to the current demands, and the higher resistance of the NiMH cells.

The other phenomina I have observed is reduced range of the airborne system with a Rand LR-3 running when using smaller capacity packs... this must be due to the pack's ability to supply peak current and maintain voltage. At first I could not figure out why a receiver with 2 microvolts sensitivity as measured on my signal generator only had 30 feet of range. Extending the transmitter antenna only increased the range to 50 feet! Then I tried a 600mAh Nicad in place of the 350 mAh test pack, voila, 300 feet of range with antenna down!!

I'm afraid to use Lithium cells to power my Rand Galloping Ghost actuators because the voltage curve shows they drop off at a fairly steady rate. This means I would experience quite a shift in the actuator trow, and neutral as the Lithium pack disscharged. NiCads on the other hand have a pretty flat voltage disscharge curve, and the only shift one sees using the Rand LR-3 is during the initial first few minutes before the surface charge burns off. I suppose one could use a regulator with the Lithiums, but I bet an LM317 would get pretty hot driving a motorized actuator.



iflyj3 -> RE: Ni-Cad versus Ni-MH batteries (5/29/2008 9:59:43 PM)


quote:

ORIGINAL: jaymen

Aha.....I see the differance now....I'm not using Eveready Energizers, I'm using some no-name brand AAA size and AA size cells that the hobby shop was selling.


I agree with your conclusions. As far as cells from various places. I don't buy NiCD's any longer unless they are Sanyo. I have been burned too many times on no label cells.

Around here most RC pilots think a 2500 maH battery pack is better than a 700 maH pack. They just go by the numbers being bigger is better and totally do not under stand the effective series resistance (ESR) part of the equation.

As you know all of the new spread spectrum receivers are microprocessor controlled and are sensitive to a sudden voltage drop. They can't understand why a 2500 maH pack would be a problem when a 700 pack is fine.

In stead of trying to use a single 1100 to 2000 maH pack I use two 700 maH packs in parallel. That gives me about 8 milliohms of ESR and redundant batteries incase one gives up.



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