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Old 03-15-2007, 11:08 AM   #1
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Default LiPo vs NiMH- Different perspective

So as I am working on some of my stuff, planning for an RC Pro Series race and all, I started thinking about LiPo and NiMH. I know I need to buy a couple more packs before the race, but one of the cool things about the RC Pro Series is the fact that you can run LiPo's in Mod. While looking at some new 4200's, I see matchers with numbers at or above 1.23 volts per cell. Now basic math here, taking 1.23 x 6 cells = 7.4v, which is the same voltage of a 2 cell LiPo. It is seeming that the argument of "LiPo's have higher voltage" is starting to fall by the wayside. Not to pick on anyone here, but Danny from SMC posted this in the Commercial Marketplace not too long ago:

With the latest lots of 4200WC cells being a bit off in voltage we have a good amount of 1.227-1.229 packs for sale at the special price of 32.00

Normally we just stick pack the lower cells but since were seeing more of them I figured we could offer them to RCtech users as these will make great practice/club packs.

Dude! 1.29's as "Lower packs"?!?!?!?! Come On, compared to where things were just a year ago, these are still kick butt batteries at a great deal! But back to my point, with the Not-quite-so-sub-C-anymore NiMH packs making this kind of voltage, it seems like the LiPo/NiMH voltage argument has lost at least one leg to stand on. I know there are people worried about safety issue, but damn, has no-one noticed the number of cell explosions and fires over the last 12 months with the NiMH packs? And there is still a minimum weight to meet, but that's why they sell stick-on lead weights. I'm not meaning this to bash any company here, I just know that, as a racer, I can't afford to buy new packs every couple months. I have year old LiPo's that, as long as I balance them from time to time, work just as well as day one. Now the discharge curve is different between a LiPo and NiMh and you will need to use specialized chargers, but I guess I am starting to really see the line blur here. I just thought this was a different way of looking at things and wondered what everyone thought.
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Old 03-15-2007, 11:24 AM   #2
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Nimh starts higher but I believe Lipo stays more constant where Nimh falls off. I wish we had track times to compare the two. Are people really any faster with one vs the other?? As for safety I guess its a matter of do you want an explosion in your face (nimh) or a fire (lipo) if something were to go wrong.
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Old 03-15-2007, 11:28 AM   #3
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If you can come to an agreement on the safety issue (don't allow cheap cells!) I think you'll find the two are pretty even on the track, with the advantage going to the newest NiMH. I think they're fine in mixed racing with weight limits.
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Old 03-15-2007, 11:30 AM   #4
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Quote:
Originally Posted by or8ital
Nimh starts higher but I believe Lipo stays more constant where Nimh falls off. I wish we had track times to compare the two. Are people really any faster with one vs the other?? As for safety I guess its a matter of do you want an explosion in your face (nimh) or a fire (lipo) if something were to go wrong.
Here's what I don't get with the whole safety concern. The airplane guys have been flying with LiPos for years now. Are we trying to say the airplane guys are all that much smarter or safer than car guys? I just think it's like brushless used to be. It's the new "RC Boogieman"
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Old 03-15-2007, 11:42 AM   #5
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Its the airplane guys that started this mess I believe. I think there were a lot of fires in that hobby which caused the concern on the RC car side of thing.
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Old 03-15-2007, 11:56 AM   #6
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Most of the indoor fly ins require some sort of fire proof container for charging lipos.

I personally have seen a few puff up from being pushed to hard. But I've seen many many take hard dirt dives without any fires. I've dented some pretty good, but never had a problem.

I think most of the problems come from people not taking the time to measure the amps their setup will draw. Or because they are using no name packs that have no C rating or an exagerated C rating.
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Old 03-15-2007, 11:59 AM   #7
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"I see matchers with numbers at or above 1.23 volts per cell. Now basic math here, taking 1.23 x 6 cells = 7.4v, which is the same voltage of a 2 cell LiPo"

Gary, It's really not a relevant test to compare Lipo vs Nimh voltage. The average voltage is just an average voltage at a single discharge rate. In reality, a 2 cell Lipo has a lower nominal voltage but also a much lower DC Internal resistance than a 6 cell Nimh.
In RC applications, that means it you use a Lipo to race a low current mabuchi class, the voltage of a Nimh will far outclass a Lipo. In modified, the higher current drain puts the clear advantage to Lipo. A lipo will maintain most of its voltage under 40, 50, 60, 70, 80...hell, a 20C kokam 3200 can do 128 amps for a few seconds and stay above 6V.
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Old 03-15-2007, 12:03 PM   #8
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Quote:
Originally Posted by linger
Gary, It's really not a relevant test to compare Lipo vs Nimh voltage. The average voltage is just an average voltage at a single discharge rate. In reality, a 2 cell Lipo has a lower nominal voltage but also a much lower DC Internal resistance than a 6 cell Nimh.
Agreed, but when you look at a lot of the arguments against LiPo, one of the first knee-jerk reactions is to bring up the difference in voltage ratings. You and I have talked about this before and how, even though the voltage is really good, the IR is thru the roof and how much better LiPo packs would be if the IR could be lowered. But in a way you also just proved part of my point. You said that a 2 cell Lipo has a lpwer nominal voltage than a 4200 NiMH. This seemingly eliminates the voltage argument, doesn't it?
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Old 03-15-2007, 12:07 PM   #9
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Default The Li-Po thing

Talking to a friend of mine who has been flying since real airplanes had propellers , he said that when Li-Po came on the set, the guys used to try and charge them like regular cells..which caused the fires. As they realized you cant peak em like sub c's, fires diminished and the packs lasted for what seemed like forever. I myself have been running Li-Po for months. My Pro 4 has been run over not once but twice by a real car. No boom, no fires, no major damage (chassis cracked, shell mangled, body post and shocks crushed. Nonetheless, if my car can be run over and the li-po pack still give me 35 minutes run time, then the whole Li-Po arguement is invalid!
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Old 03-15-2007, 12:09 PM   #10
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Quote:
Originally Posted by bxpitbull
Talking to a friend of mine who has been flying since real airplanes had propellers , he said that when Li-Po came on the set, the guys used to try and charge them like regular cells..which caused the fires. As they realized you cant peak em like sub c's, fires diminished and the packs lasted for what seemed like forever. I myself have been running Li-Po for months. My Pro 4 has been run over not once but twice by a real car. No boom, no fires, no major damage (chassis cracked, shell mangled, body post and shocks crushed. Nonetheless, if my car can be run over and the li-po pack still give me 35 minutes run time, then the whole Li-Po arguement is invalid!
Sorry this is off topic but how does your car get run over not once but twice by a real car??
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Old 03-15-2007, 12:12 PM   #11
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Quote:
Originally Posted by or8ital
Sorry this is off topic but how does your car get run over not once but twice by a real car??
There is a spot where we do speed runs (mostly nitro). In not looking where I was going, a car was coming in the opposite direction. Had I gone straight, I wouldve gone directly under the car. I panicked and crunch. Fixed the car, the following week, testing and making sure all was good, didnt adjust the screw to my servo horn...ooh, she's a little run away.That one was as bad, just grazed the inside of the sidewall of the cars tired, mangling my front bumper.
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Old 03-15-2007, 12:16 PM   #12
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Quote:
Originally Posted by or8ital
Sorry this is off topic but how does your car get run over not once but twice by a real car??
He must broke a mirror with in the last seven years....
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Old 03-15-2007, 03:11 PM   #13
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I've been talking to airplane guys about LiPo, and each one has a horror story about packs igniting. But they also admit LiPo cells have gotten much better than the early days.

Nickel batteries explode, that is when enough pressure builds up inside the cell the end cap pops off, usually with a loud bang. Saw one at my local track do this a few weeks ago. And if nickel packs are shorted they get very hot and usually start smoking, but the cells themselves don't iginite, anything in the vicinity will.

LiPo cells don't explode, they ignite, that is they begin to burn themselves. And that fire is extremely hot! So anything around it is going to get torched pretty good. Like a wooden workbench, or a car's interior.

There's no doubt LiPo will make it's way into cars, but let's not be complete idiots about this.
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Old 03-15-2007, 03:25 PM   #14
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They are great in airplanes. The lighter weight makes a big performance difference and they seem to really hold up in terms of giving consistent run time. They are not that fragile, as I've crashed and seen others crash and not yet experienced damage to the cells. I do charge, transport and store my cells in a metal can as a precaution. The only reason you wouldn't see someone run brushless and lipo in an electric rc plane is initial cost.

As for the comment about repeaking, you can certainly throw a cool lipo back on the charger after partial or full capacity use and not have to worry about only full cycling it. If its already charged and you put it on the charger again accidentally, it should do no harm as the charger shuts off after the voltage hits a predetermined threshold. I don't believe its stressing the cell like you do on a nimh where repeaking means running enough current through it that it experiences another delta peak.
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Old 03-15-2007, 09:25 PM   #15
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This past weekend (3/11/07) three of us ran Novak's 3.5 and 4.5 BL with Peak/Orion 3200 LiPo's with one of us switching to 4200 NiMH in one of the heats. Guess what happened, NOTHING. Lap times didn't change enought to talk about and NOTHING BLEW UP OR CAUGHT FIRE.

Can you believe that....

By the way, the one that switched to the NiMH is a sponsered driver and he ran the LiPo's in the A-Main even though it wasn't fully peaked. Reason for that is that he was going to run the NiMH in the main but changed his mind.

One thing we did learn is that the 3200 Peak/Orion LiPo's have no problem making run time with the 3.5 or 4.5 BL Novak. But you will have to recharge the pack between heats.
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