Is there a way improve the LVC shutdown
 

First off I didn't do a search for this topic to find an answer before asking. Free time has been limited lately. I hope it wasn't just asked. So apoligies up front.
Setup is pretty basic

e-revo - HV4.5 - spektrum sr3100 receiver
Novak 4 -cell smart stop (12.5 cutoff) connected to the same tabs as the capacitor
using (2) 7.4 2s2p lipos maxamps 6000's or truerc 6400's

The problem I have noticed with my set is the LVC will shut down the RC when it spikes below the 12.5v. It will stutter when WOT then eventully shut the RC down. It just dawned on me the other day. The batteries when I take them out and test them are around the 7.6 volt readings. That's after a couple of minutes. I will have to test the battery voltage as soon as it shutdowns to check the voltage then. There seems that there is still plenty of runtime left but no WOT (not as much fun). Is there anything that I can do to keep the LVC from not shutting down unless the batteries are below the cut off for more then a couple seconds. Would adding another capacitor or a UBEC help? Connecting the LVC up differently?

Thanks for the help.

I think its an inherant problem with the design of some, if not most LVC devices. As soon you dip below below the cutoff, even for a nano-second, it kicks in and you get that bad stuttering you describe. I dont think you can connect it up anywhere different to get better results, being next to the cap is probably the best place I think, since it provides a nice steady flow of power. Only thing I can suggest is to do what I have done- find an LVC that had horn or warning light which tells you your batteries are dipping below the cutoff point, rather than cutting your power.

Thanks Niel. I got a LVC for a 7.4 of e-bay that had a light and a buzzer. The alarm would go off real early. When I contacted the person he said he the were set up for planes but he could set it up any way I wanted. To not go off until the voltage was below for a longer time or the actual LVC was 6v etc. I think I will try and reach him again and see what he can do for me.

What will an the extra HV cap do? I looked at thread you linked to about installng them but it was like 32 pages long. Do you mind re"cap"ing it for me. Less heat?

For my HvMaxx, it resulted in a cooler esc yeah, but much more punch too from my cheapo Nimhs, especially towards the end of a run. Before the extra cap, the truck (txt-1 back then) would sound like it was trying to change gear every few seconds- revs up... dumps.... revs up..... dumps.... and so-on. Now with the extra cap the Gmaxx just runs smoothly right up until the last few minutes, when it slows right down and I bring her home as it were. I would definately recommend adding another novak cap to the hvmaxx, just wire it in parallel to the existing one and you'll instantly see and feel the improvement. $6-10 on ebay I have seen them for, bargain.

I'll give that a shot. I got 2 Novak caps from the escs that I sent in for replacement. Cut them off thinking they might be needed someday. No only if I could use the ton off other stuff I kept that I thought might be needed someday. Thanks again for your help Niel.

Without knowing exactly how the Novak LVC functions, you may be able to help with that by using a diode and a small capacitor (and maybe a small resistor) like in the diagram below:

http://scriptasylum.com/forumpics/lvc_mod.gif

The capacitor would be a small value, like 0.1uF, and is there to store the voltage. When a high current pulse happens, the battery voltage will drop, but this capacitor will slowly drain and the LVC won't respond as quickly. By the time the cap would reach the cutoff voltage of the LVC, the battery voltage is back up. Basically, just adds a delay.

The resistor helps control the rate at which the cap discharges for a faster or slower delay since I have no idea what kind of load the LVC itself presents.

The diode will allow current to flow into the LVC to prevent the small cap from trying to supply the ESC (which would totally negate the purpose of this idea). The diode would have to be a low v-drop type like germanium because the LVC will see the battery pack V minus whatever voltage the diode drops. But since the LVC is not as sensitive to voltage sags, it should be fine.

Delayed reaction LVC, I like it alot. How long before manufacturers start introducing these as a normal item, it makes alot of sense when you concider the load some systems can put on a lesser lipo...

I don't know why they don't have it already. Seems easy to do. The only thing about the arrangement I posted above is that instead of a 4s pack cutting off at 12v (3.0v/cell), it will cut off at 12.7v (3.175v/cell) for a 0.7v drop silicon diode, or 12.3v (3.075v/cell) for a 0.3v drop germanium diode. Anything lower than a 4s LVC wouldn't work well since the diode voltage drop would be too great.

The nice thing about this "circuit" is that it can be contained on a small heatshrink-wrapped PCB with in/out wires (like a UBEC) and can even add a small PCB mount potentiometer to adjust the delay at which the LVC responds.

I'd try it but I don't have a test LVC to play with.

The only other way to make sure the voltage doesn't drop under quick heavy loads is to add a bunch of low-esr caps to help with the surges. It would be larger and more expensive, not to mention the initial hook-up spark due to the large capacitance you'd have to have.

Perhaps a delayed LVC is not a good idea. If it allows the batteries to drop below 3v a cell, even for a split second, it may be enough to do damage to the batteries (or at least shorten their life considerably).

I'd say it depends on how sensitive the lipos are to being over-discharged, but you'd have to go careful, thats for sure. Still, its a clever idea and something very useful for our applications.

Actually, if using this type of outboard circuit, the diode would increase the LVC threshold by 0.7v for silicon diodes, or 0.3v for germanium diodes. So, assuming a normal 0.7v diode, if the LVC is set for 3v/cell, it would actually activate at 12.7v for a 4s pack (3.175v/cell), 9.7v for a 3s pack (3.233v/cell), etc.

Even so, this would just prevent the LVC from reacting too quickly. Bursts usually last a couple mS, so all you need to do is average the voltage out so it doesn't dip at the LVC at quite the same speed as at the ESC. Basically, make it have around a 1/5 to 1/4 second delay.

Is it perfect? No. Then again, it is a simple and cheap alternative. :smile:

I'm with you on this one. If the battery is dipping below the 3V/cell mark then the battery isnt up to the task and you will be damaging it if you run it this way. Gear down to reduce the load to a level that the battery is capable of supporting or get a better battery.

I say this with the exception of the MGM controllers which just flat out have much to sensative of a LVC.

I would say that the best cure would be a pair of trakpower lipos. They cured my issues with poor performance on a high draw system. Gearing down slightly may help too.

I only skimmed over the technical jargen but I undestand what you are trying to do I can donate a LVC for you to try it out on. Suddenly I have one that is not being used at the moment,

http://i212.photobucket.com/albums/c...o/DSCF4181.jpg

Now I need to get a hold of Mike. Ten minutes before this happened I ordered the Gorilla Maxx tranny conversion and the other half of the FLM tranny case from him. I am hoping he will let me cancel that order so I can order a MGM controller and Neu motor. It doesn't look like he has many Nue's in stock though.

Well, if you don't mind donating it for wa while, I wouldn't mind trying it out. And for your trouble, you can have the modified circuit (assuming it works as well as I think it will). :smile:

Let me know via PM what you want to do and we can make the arrangements.

Oh, that looks rather nasty, what happended there exactly?

I'm not sure. It was recommeded that I add another capacitor to it and bam there goes my esc. If I could only remember who it was.....


JK it was running fine with the extra capacitor. I had 3 or 4 packs with the extra capacitor installed. Would water shorting out the board cause a melt down like that? I had it out for a run and it was wet out. Didn't wrap it in the usual plastic bag though. Came in and turned it and the controller off. I didn't uplug the batteries though I had to help the nieighbor. Later on I smelled the smoke. So it was quite a while after I had shut it off that this happended. I thought lipo fire at first. Threw those batteries in the barbie outside. The batteries are fine. I had another HV esc do this to me while I was running it. It was bone dry out no chance of water doing it to that one.

Maybe it was me when I added the extra capacitor. If I got the wire too hot adding the other cap. Maybe it worked itself loose from the tabs on the esc. I don't know it sat there for a long time before I noticed any smoke smell.

:whistle:

Water would certainly cause a dead short on the exposed PCB, if it joined any of the solder joints together and such. Could be condensation also, going from cold/wet outside to warm inside. I would have thought though that if the capacitor or soldering was bad, it would have gone up in smoke as soon as you turned it on.... nasty either way.

:whistle::lol:

Yeah, turning into one of the month like I had last fall. Power supply for my charger died today. There another 50 to replace. Got a supply that will get me by. Unlike last fall when I replaced the esc with the same ones this time were upgrading. That what you suppose to do when something breaks right? Otherwise why break it.

OK, I did a little playing around with the Novak 4s LVC. The internal resistance of the unit is 200k ohms. So, depending on how much of a delay you want, you pick a capacitor that will discharge to ~12v using 200k ohms as the discharge resistance.

Since the internal resistance is constant (and it is because I checked), you don't need the additional discharge resistor shown in my original diagram. So, the new diagram looks like this:

http://scriptasylum.com/forumpics/lvc_mod_new.gif

I created a little Excel spreadsheet which helps to pick out the proper capacitor for the delay you want. Just fill in only the green boxed areas.

• Resistance (ohms): Enter the LVC internal resistance, 200,000 in this case.
• Capacitance (farads): I used 22uF. Other common values for ~25v are 10uF and 47uF, but don't provide the right delay IMO, but that's up to you.
• Battery Voltage: Self-explanatory. Use the nominal battery voltage. For a 4s pack, use 14.8v.
• Time Constants to show: Caps are accepted to be charged/discharged in 5 time constants, but if you want to see more detail in the lower time ranges, use a lower number.
• Euler's number: This is a constant for natural logarithm base. Leave as-is unless you really want to mess with it.

The table will show the time, discharge capacitor voltage, and charge capacitor voltage. And the graph will reflect this.

I figured a delay of ~1 second before the cap reaches LVC threshold of 12v should suffice, while still retaining full LVC functionality. So, for a 22uF cap, the discharge voltage is at 12.033 at 0.91 seconds (use 0.3 for the "time constants to show" to view this). If you use a 10uF capacitor, the cap will discharge to 12.033v after 0.41 seconds if you wish.

That's about it. Don't forget the diode or this will not work. Germanium works better because of the lower voltage drop (0.3v compared to the 0.7v of a normal silicon diode). Also, don't forget that the LVC will trip a little earlier because of this voltage drop. Any diode with a PIV value >= 100v will work fine, so 1N4002, 1N4002, etc.

Incidentally, the formulas for finding capacitor voltage at any time (not just during the 1,2,3,4, or 5 time constants) are:

Charge:
capacitor_v = v_battery X (1 - 2.7182^(-t / (R X C)))

Discharge:
capacitor_v = v_battery X 2.7182^(-t / (R X C))

capacitor_v = capacitor voltage at "t" seconds
v_battery = battery voltage
2.7182 = Euler's constant
t = time in seconds
C = capacitance in farads
R = Resistance in ohms