What should low voltage cutoff be on a 52v battery

Hi All,

I am wondering what I should expect to see as the low voltage cutoff on a 52v battery. Right now I am seeing 46v.

I am using a CycleAnalyst which I set to 46v and it shut-off. However, then I lowered the CA to 42v it never got below 46v.

The standard board says low voltage cutoff at 2.5 +/- .07. I naively thought that lvc would be 36.4.

I also have a programmed controller from em3ev that uses the six-wire programmable connector. Could it be the current/voltage settings in here?

mcristiani said:

The standard board says low voltage cutoff at 2.5 +/- .07.

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That would be for a LiFePo4 cell. (and I would not go that low)

What kind of cells are you using?

Good point.

I am running ncr-b or ncr-ga.

The board I have from batteryspace.com says over discharge voltage 2.5 +/- 0.07v.

What should I expect the low voltage trigger to be for these cells?

If I were to use a BMS, I'd be using something closer to 3.5v myself, but some of these BMS go a lot lower than that. It depends on the load you're going to have the cell under when it gets that low, and how far you want to be discharging them, and the specific voltage curve of the cells you're using.

You can probably find the discharge curve for them on the manufacturer site, or here on ES (I've seen lots of cells' data posted in various pack build and repair threads, and threads about specific cells too).

edit; example link
http://www.batteryspace.com/prod-specs/NCR18650B.pdf
see chart on there for Discharge Characteristics (by rate of discharge)
shows around 3-3.3v for empty depending on discharge rate

All depends on the cells you are using. Rated LVC could be anywhere from 2.1V (most lifepo4) to 3.0V (most lipo, 18650 cells, etc.). You need to know what cells you have and what the rated lvc is for them, then add 0.5V to that to be safe.The rated lvc for most cells is way to low for practical purposes.

Interesting. Thanks for the specsheet amberwolf.

I see on there that at about 3.26 - which is where the ncr-b seems to be cutting off - the discharge curve drops really quickly. Makes sense this is where it cuts off.

If the BMS says 2.5v as the lvc, what is controlling the cutoff though?

BMS is a cell level controller, meaning any of the cell groups drop below the cell level lvc, it shuts the battery off. The controller is a full voltage lvc, meaning it shuts it off when the pack voltage drops below a certain level. Typically, 31V for 36V kit, 42V for 48V kit, etc. Most displays allow adjusting this to a degree. If the bms shuts it off, you usually have to disconnect the battery for it to reset. If the controller shuts it off, it will usually turn back on when the voltage level rises above lvc.

Interesting. Thanks.

Not sure if I should start a new subject for this question, but will ask it here first.

If my 58.8v battery only charges to 58.1v what does that mean? I rode it and it still registered around ~8.3ah - out of 10.5ah so it has plenty of juice.

If my 58.8V battery only charges to 58.1V what does that mean?

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4.1V per cell 14S X 4.1V = 57.4V, sooo...

At 58.1V I would call that charged to 95%. I believe that anything less than 100% is better for the battery, and I use 90% all the time now.

I will take it! Thanks.

mcristiani said:

If my 58.8v battery only charges to 58.1v what does that mean? I rode it and it still registered around ~8.3ah - out of 10.5ah so it has plenty of juice.

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Just like your other batteries:

What are the individual cell voltages? That's how to start to tell what that means. If some banks are not as high as others, then those banks are not as fully charged, which means either the BMS has channels that are draining cell groups, or some of the groups don't have as high a capacity as others.

Often to balance them and get full voltage on the pack you must leave the battery on teh charger for hours, sometimes days if it's a bad imbalance. 0.7v low from full is not that bad unless it's all on one single group.


It coudl also be the charger simply isn't high enough voltage to fully charge the pack.

mcristiani said:

If the BMS says 2.5v as the lvc, what is controlling the cutoff though?

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The BMS's electronics are setup for a particular voltage usually for a particular chemistry.

If you use a BMS that runs down to 2.5v on cells that are empty at voltages higher than that, and you use the battery until the BMS shuts off, then you are destroying the cells by running them down too low. The damage may be slow, or it may be quick, depending on the particular cell characteristics and chemistry, but it is happening.

So you should change that BMS to one with an LVC that is for the correct chemistry of the cells.

The one you have now is not made for those cells, it is made for a different chemistry, probably LiFePO4, as has already been pointed out above. It should not be used with the non-LiFePO4 cells you have.

Thanks Amberwolf.

#1 - It is painful to check each cell grouping, no? I have to expose the balancing harness at a minimum, no. On an enclosed battery, this is a pain.

#2 - Could you look at this page. I purchased the 14s board from here.
http://www.batteryspace.com/pcmforli-coli-mn-nibatterypacks.aspx

What lvc should I be looking for? 2.9?

What lvc should I be looking for? 2.9?

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I am frequently reading that the batteries that have achieved the longest life-cycles do not live in a cycle profile that pushes the upper and lower limits. Running from 3.0V per cell up to 4.1V is better than going farther away from the center, and if I was in a contest for life extension, I would size the battery so I could achieve my desired range while using 3.3V to 4.05V per cell.

I don't think there is any evidence that operating in an even smaller envelope between 3.4V and 4.0V is any better, but....I do not recommend a daily charge/discharge outside of 3.0V to 4.1V...for 14S, that would be:

14S = 42.0V to 57.4V

More to your point AmberWolf.

Is this a better-suited board? It has an lvc of 2.75 for each cell.

http://www.ebay.com/itm/Balance-BMS...422771?hash=item33d3f5a333:g:2FkAAOSwxGRZgCO8

Take your pack and charge it to your preferred amount. I've come to believe that 95%soc may be best for longevity while having no real world detectable difference in performance.

While riding your pack and you reach around 50% remaining capacity, keep an eye on the resting voltage and voltage drop you get with your riding style. Take that voltage drop and divide it by the number of parallels cells in your pack. This will give you the per cell drops you are seeing with your riding style. Take this per cell drop and evaluate how close it puts you to 3.0 per cell (or some slightly higher voltage to give you a small safety buffer. )

Ride your pack until you notice your pack voltage just slip below nominal 3.7 per cell at rest. Note where you are at this point in relation to your total capacity. Compare where your previous measured voltage drops may put you in relation to 3.0 or preferred safe voltage.


This will give you an idea of the lowest voltage that you should approach given your riding style. Just be aware that the voltage toward the end of your pack capacity tends to drop off quite quickly and you could be left with no juice to get you where you want to be if you don't also consider where you are in relation to remaining capacity. (This it's also where you can start damaging cells. ) This can vary significantly between brands and differing battery ages, and even slightly between similar packs. Keeping an eye on cell balance and charge acceptance when you hook back up to your charger can allow you to gauge which cells may be more inclined to sag voltage under load more than the rest.

Note that nominal voltage is 3.7 per cell. It's best to base calculations on this rather than hot if the charger voltage.

I tend to consider my battery discharged when I have 20% remaining capacity. I think this is pretty safe and allows me to ride how I like for my whole trip and not worry. If you are planning a battery build it may require a 20% bump in planned pack capacity to play this way.

mcristiani said:

Thanks Amberwolf.

#1 - It is painful to check each cell grouping, no? I have to expose the balancing harness at a minimum, no. On an enclosed battery, this is a pain.

#2 - Could you look at this page. I purchased the 14s board from here.
http://www.batteryspace.com/pcmforli-coli-mn-nibatterypacks.aspx

What lvc should I be looking for? 2.9?

Click to expand...

You controller is usually set for 3 volts for LVC. I'd want a similar value on the BMS.

I looked briefly at your above link and sure enough, they state 2.5 volts for your controller, but they're listing 2.5 volts for almost all of their 18650 type BMS boards, even the little button boards used on a single 18650 cell. I even saw an LVC listed at 2.3 volts for a 10S board advertised specifically for 18650.

I thought nobody actually used the rated LVC, and no one goes under. Perhaps the spec on your BMS is a misprint, their web guy is illiterate, or their designer is crazy.

I have since paid attention to the LVC field in the GUI (eXtended Parameter Designer) making sure to take into account battery voltage for a profile.

Thanks.

I had originally ignored the LVC field in the gui for programming my controller.

I have since made sure to set it 1-2v above the lvc per the voltage.

I have an EM3ev 52V Jumbo Shark with Samsung 35E cells and am using it with a TSDZ2.

The cells discharge cut-off is rated at 2.65V and EM3ev tell me the BMS LVC is set at 2.75V.

From what I can gather here, setting my controller's LVC to 0.5V over the cells rated 2.65V should be considered safe. So that's 3.15V, or 44.1V for the pack.

I never run it down this low in daily use, but would like to safely maximise range for occasional long days when touring.

Can anyone please confirm this is ok?

i run my packs down to 3.0v under throttle/sag all the time, sometimes i can only use half throttle to get up the hill to my house after a long ride.
the cells usually bounce back to about 3.2v once you stop

your 14s so 3.0v is 42v under throttle