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.

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

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.

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!!

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

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

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

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. :whistle:

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.

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

Jeff

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. :smile:

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

Unfortunately LiPos don't work well when submerged in LN2. :na: hehehe

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

Not here too freeze, sheesh. I just read 12 pages. You are getting decent batteries. Decent batteries paired with a sensible top speed (gear for 40-45) will keep you and your esc happy.

I surmise the 20t pinion failures were noobs with cheap batteries and those running the 25t pinions had decent batteries. There were simply too many variables to make the "20t vs. 25t" debate. It was all hogwash IMO.

Everybody here has had success running good batteries and gearing conservatively.

You are gettng the good batteries now gear for 40-45 and you are set.