Higher pinions = safer MMM's? Or no? Someone's lying.... - Page 10 - RC-Monster Forums

lutach · 11.01.2009, 08:24 PM

Quote:

Originally Posted by BrianG

I thought it was sex that sells? They should start displaying scantily clad women on the boxes of their products.

I was going to say that....But if you're buying a vehicle, most would go for speed and power. Example: The new Ford car with ECO boost. I don't know where the ECO come in, but the car seems to be fast.

Bernie Wolfard · 11.03.2009, 11:57 AM

How do you tell if your battery is a good one and how to do track your batteries ability to discharge as they age? Several Lipo chargers can also measure battery resistance. The ones I know of are several Hyperion and NeuEnergy chargers. I am sure there are more that have this feature, I am just not aware of them. Internal resistance is what controls a C-Rating, the lower the internal resistance the faster the battery can discharge before it overheats. A batteries internal resistance (C-Rating) also determines the amount of ripple current the controller will have to deal with. The lower the batteries internal resistance the better the battery. With one of these chargers you can simply plug the battery in and records its resistance. The lower the number, the better the battery. If you track a batteries internal resistance over time you will see its resistance going up as the battery reaches the end of its lifespan.

With a charger that measures internal resistance it is interesting to plug in different packs of the same size and type from the same battery seller and see how different they are. If you do this you will find that the better battery companies have much less variance in their batteries because they QC the cells they use. You can also go through several cheap batteries and separate the good from the bad. Another interesting thing to track by measuring internal resistance is how many good cycles you can get out of specific packs. Again, you will find that the better batteries (except for bleeding edge supper high discharge cells, which typically don't last very long) will last much longer than cheapos. If you then calculate cost per run you will find the more expensive better packs are actually much less expensive in the long run.

The equipment it takes to measure battery internal resistance makes doing it yourself expensive and impractical, but it can be an easy job for a charger. Look for this feature in your next charger.

Bernie

zviper · 11.03.2009, 12:11 PM

bernie i dont have a charger that reads internal resistance. can i use a volt meter to get the internal resistance ?

Pdelcast · 11.03.2009, 12:12 PM

Quote:

Originally Posted by zviper

bernie i dont have a charger that reads internal resistance. can i use a volt meter to get the internal resistance ?

You could, but it's difficult. You need a large constant current load to put on the batteries to see what the voltage change is under load.

07.30.2010, 02:48 AM

Has there been any new developments or info in regards to this lately since this thread ended?

zviper · 11.03.2009, 12:15 PM

ok so its a little more complicated then i thought. i guess when i get new batts ill pickup a new charger. thanks patrick, awesome product btw

BrianG · 11.03.2009, 01:02 PM

I'm curious how chargers do this. Resistance can change even over the course of a charge/discharge cycle so that would mean the charger would have to monitor pack voltage while applying the charge current in on/off cycles over the course of the charge, apply the resistance formula (r=(V_unloaded-V_loaded)/A), and then plot the curve. Or do these chargers simply measure the ir at the start of the charge?

Also, doesn't the ir change dramatically at near the cells max discharge rating? So, the ir value you see when charging at 1C may not be even near the value at 20C.

JThiessen · 11.03.2009, 02:17 PM

Bernie - that's terrific information. I asked quite some time ago what the resistance info meant to me, but it apparently stumped all of us at that time - either that or I couldn't understand BrianG's EE language when he explained what he knew!!

BrianG · 11.03.2009, 02:24 PM

lol, I'm not an EE, just an enthusiastic hobbyist.

J57ltr · 11.03.2009, 02:49 PM

The ICE charger I have only measures resistance when doing a discharge. After a few minutes during the discharge cycle you can see the graph rise then fall back down, so basically you have what looks like a cosine pulse. To me it's the charger dropping the load and allowing the voltage to rise then applying the load again, then it calculates the resistance. It doesn't seem to change too much with loads between 1 and 10 amps. I have noticed that different packs have different readings and they do change but they are usually within about 5% from what I have seen on this charger. this is the only charger I have experience with that has this function, but I thought that all chargers only did this during discharge.

One question I have is how much resistance is too much? The other thing I see is that the internal resistance will increase with the number of cells. So basically all things being equal a 6S pack will have a higher internal resistance than a 4S pack made from the exact same cells.

Jeff

snellemin · 03:18 PM

Quote:

Originally Posted by J57ltr

The other thing I see is that the internal resistance will increase with the number of cells. So basically all things being equal a 6S pack will have a higher internal resistance than a 4S pack made from the exact same cells.

Jeff

This is why you would use parallel packs to bring the resistance down. Or use a bunch of caps to compensate.

J57ltr · 11.03.2009, 03:40 PM

Quote:

Originally Posted by snellemin

This is why you would use parallel packs to bring the resistance down. Or use a bunch of caps to compensate.

As I see it if ripple is the problem then caps would help, but if the voltage is rising to a dangerous level then you need the TVS as well. To me since we are talking about IR of battery packs the problem would be that you couldn’t shove the electrons in to the battery fast enough so the voltage “backs up” (or rises) to an unsafe level. So then you need something to clamp this voltage to a safe level, enter the TVS that’s what its job is that’s what it does. So basically both are needed if the IR of the pack is too high. What is too high?

Jeff

BrianG · 11.03.2009, 04:18 PM

TVSs and caps have different functions:

Caps will help filter ripple current on running and somewhat on braking. The only trouble with adding lots of capacitance is the enormous hook-up spark, but if you know what you're doing, you can use a no-spark resistor, even on certain ECS.

TVS devices will shunt excess voltage spikes generated during braking. But, they do have limits to what they can safety shunt. The power they have to dissipate is (V_spike - V_tvs) * braking_A. So, if the voltage spike is 25v using a 14.8v battery and the braking current is 15A, that's 153w on that itty-bitty device. Granted, the spikes are very short in duration, so that helps immensely, but if you're going a lot of hard braking, they will overheat. So, using multiple TVS devices can be used to spread this load out a bit making each last longer. Vehicle weight, braking time, gearing, and voltage all play a role in how large the spikes are.

J57ltr · 11.03.2009, 04:25 PM

That’s pretty much what I said. My question is what is to high of an internal resistance?

Jeff

BrianG · 11.03.2009, 04:29 PM

lol, ok.

The amount of resistance depends on the setup. A low current setup won't be affected as much by battery resistance as a demanding one. Obviously, the lower the resistance, the better. Ideally, it should be 0 ohms.

You can get an idea of the effect that resistance has on a setup by playing with this little demo I made: http://scriptasylum.com/rc_speed/_vdrop.html