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  1. #1
    RC Racer
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    Jan 2011
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    LVA, LVC and my insight to LiPos

    I just want to sum up some infos on LiPos and the issues with the Summit.

    Every motors needs power and it really don't care if from LiPo, NiMh or other sources. Then it needs a Electronic Speed Controller. You can get those for brushed motors and for brushless motors. Some ESC are programmed to cut off power if the voltage gets too low. The EVX-2 doens't have this option. The VXL (Summit 1:16) has this option, the ESC can be "reprogrammed" using a button to have low voltage cut off (for LiPos) or just let the battery run dead (NiMh).

    One issue with the low voltage cutoff (LVC) or a low voltage alarm (LVA) is that LiPo cells are added serial to a pack like 2S1P. This means you have two cells in a pack. LiPo batteries have the tendency that the cells of a pack don't discharge evenly.

    So you check your voltage on the LiPo and see 6.3V so you asume that each cell has 3.2V and you can drive some - then you check every cell and you see 3.5V and 2.9V - LiPos under 3V often can't be recharged, the whole battery is dead.

    So a LVC should base on CELL info and not on battery info. To prevent some of these issues, LVC are often set "high" like 7V because it's very likely than both cell are close, so even 3V/4V is a lot less plausible than 3.2V/3.8V. So a 7V cutoff is pretty safe.

    Back to our summit: We don't have built in LVC or LVA

    How to handle this? Here's a history of my approaches.

    1. drive till power-loss ($free)
    I did this once or twice, a cell dropped to 2.8V and I was able to bring it back but I didn't feel that confident...

    2. trust and pray and check ($voltage meter or $25 for hk-010)
    I just drive around and after like 30' I pop the hood and connect the balancing tap to HK-010 Voltage Analyzer (basically a volt meter than doesn't connect to the battery tap but to the balancer tap). After some packs, you get a feeling and I always stop around 7V or when one cell drops below 3.5V.

    3. lipo alarm #1 "7.4V-11.1 RC Lipo Battery low Alarm Indicator buzzer" ($3)
    Bought a lipo alarm off ebay. I connected it but as soon as I pull the trigger hard and the truck jumps the gun, the alarm sounds. So the "electronics" of this cheap device can't handle the voltage drop when accelerating. Some $ wasted, I don't mind.

    4. voltage display "Battery Voltage Indicator Checker Tester For 2S-6S Lipo" ($2)
    I am very happy with these, they simply display cell voltage and battery voltage in rotation. I use two zip ties to build two "rings" so they don't fall off when one ring goes around the power cable. The display is pretty accurate, no alarm but using my method of attatchment you can even see your voltage from 10 feet away so after some bashing and crawling I simply turn the truck, look under the skirt and see how the batts are doing. I only connect to the BEC side.

    5. NOVAK (#5470) 2-Cell LiPo Smart Stop ($25)
    I didn't check this out yet, some guys say it's worth, some say that Novak doesn't recommend it, some say to go with 4-cell smart stop while other say to simply us the BEC side. I have one on my desk but didn't feel soldering it yet.

    6. MTTEC Lipo-Wächter BS6 V4 ($40)
    Bought in Europe, this lipo alarm is very "custmizeable": You can chose the voltage and the time the cell has to be below this voltage so you omit the problem with the cheap guards that immediately sound when the battery is under pressure.
    http://www.mttec.de/Akkus/Zell-Ueber...-extra-la.html

    My setup today:
    BEC side: I run the MTTEC (#6) with 3.5V warning and 3.5s delay and 2s "beep"
    Non-BEC side: visual control using voltage display (#4). I like those.
    Last edited by MJP; 04-17-2011 at 02:46 AM.

  2. #2
    RC Racer
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    Last edited by ksb51rl; 04-17-2011 at 09:53 AM.

  3. #3
    RC Qualifier
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    Until recently I was using a very similar setup. I had a Novak LVC / plug in alarms / voltage checker on my truck. Same beeping when I hit the throttle. The audible alarms always went off before the Novak did. The one time I forgot the alarms the non BEC LIPO died.

  4. #4
    RC Qualifier
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    Mar 2011
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    Newport Beach, California
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    A Novak 4S LVC definitely DOESN'T work.
    Didn't try the 2S.
    I put a Common Sense LVA on the BEC side, it plugs into the balancer lead, and fits into the battery compartment with the battery. Hasn't gone off yet...

  5. #5
    RC Qualifier
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    Jan 2011
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    252
    I had the lipo alarm which works great if you have 25+ on your c rating.When I used a large mah battery set with 20c rating it would trigger the alarm when taking off full throttle on pavement.I used my Data logger to confirm that the battery would dip below the voltage setting for a fraction of a second and beep and quickly recover.I did the same with a 30-40c set and it never got close to the alarm level until the packs were almost drained.Keep in mind this was with a Dewalt 14.4 motor which has pulled amp loads of 175a + for a fraction of a second and drops to less than 50a wide open throttle.

  6. #6
    RC Qualifier
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    Yes, I have 2S 25c 8000mAh batteries...

  7. #7
    RC Qualifier
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    Quote Originally Posted by MJP View Post
    I have one of those LVD alarms that beep at 3.3v is that too low ?

  8. #8
    RC Turnbuckle Jr. Jimmie Neutron's Avatar
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    In my opinion, yes.

    I suggest getting one similar to this:
    http://www.ebay.com/itm/1-8S-Lipo-Li...item3f21604c78

    It is adjustable.
    Whatever it is I just said... I could be wrong.

  9. #9
    RC Turnbuckle Jr. pavmentsurfer's Avatar
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    This is an old thread but since its been brought back from teh dead its worth correcting the incorrect info for anyone who reads it.

    A lipo that has been run down below 3VPC is not junk. It just needs to be observed for the next few runs to see if any permanent damage has been done. Its not at all good for a lipo to be run this low but i have revived a number of lipos abused by other users and they have continued to work just fine. If your cells go down much farther I would say your likely in trouble. But if you dont see puffing during charging or running the pack is probably fine.

    An LVC or LVA can monitor either one cell or the whole pack. Any LVC or LVA connected to the main power wires is using the entire pack voltage. So if its set at 3.4 VPC it will go off when the total pack voltage is at 6.8 volts. While the OP is correct in saying this could mean one cell is lower than the other, your cells should not be discharging any more differently than about .02 or .03 per cell. So even at the worst case scenario your pack MIGHT be sitting at 3.1 and 3.5 or something like that. If your cells are alot more out of whack than that, your pack is bad and you should probably stop using it.

    The Novak smart stop 2S has a bad reputation with the EVX speed controls. I fried one after hooking the smart stop up. It was wired correctly and the ESC almost caught on fire the first time teh pack was plugged in. Its a documented issue that the EVX has an on again/off again relationship with the smart stop and for the money there is really NO benefit to using one VS a decent lipo alarm.

    The timing method is not at all advisable. There are so many things that COULD go wrong its just not safe. Not when a good alarm is less than $5.

    Many LVC or LVA's on factory ESCs are set too low IMO. I think the traxxas ESCs are set at around 3.3 or 3.4 which is decent. But many castle ESC's come pre-programmed to 3.2 VPC. Which is too low. Check what yours is set at before you run.

    First, the newest version of the EVX 2 DOES have an LVD built in. Works just like the XL5 with LVD. Best option and very simply but if your truck does not currently have LVD its not the cheapest option.

    Every lipo alarm ive used will sometimes go off under hard accelleration for the first 5 minutes or so of a run, then it stops. If you only ran it one time for a few minutes, it could be concerning. Some of the better alarms have adjustable cut offs as well so you can set them higher if you wish. The cheap ones are not adjustable and I believe are set to around 3 or 3.2 VPC which is unsafe IMO.
    Pede, Summit,
    ERBE, Rally,
    Motley Crew, 1SQ

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