Safe minimum voltage for "52v" 20ah pack?

[redorblack]

Today I finally did a ride that got me down to 2 bars on a BBS02 750 watt display. I'm running a Lunacycle 52v 20ah Panasonic (ncrb I think) battery pack in order to get a lot of mileage and longevity. I went with the older cell setup since with that amp/hr rating I'm not pushing the pack at all even climbing at peak wattage reported of 1434 watts. Peak draw is about 1.34 C and I doubt that has been for more than a second or so launching or on a serious grade.

But I digress... My question is about voltages. I've never hit shut off with this pack, but today I got it down to two bars. My inline watt meter says I got home with a resting voltage of 49.9v and had a minimum of 46.04 volts. I'm assuming with 14s that means it was sagging to 3.28v per cell at some point.

Can that be used to calculate how much reserve I actually had left? Anyone know what the pack or controller would have actually cut off at? With a Headway cell pack on my Bafang hub motor, if I saw it start dropping the bars on the 'fuel gauge', it was about to cut off due to low voltage, assuming since it was calibrated for lead acid and didn't know what to do with 16s of LiFePo4.

Oh, secondary question. Do these work and do you think it would handle my 52volt pack even though it says 50v? Or anyone know of one that would actually work for measuring capacity on a 52v pack? http://www.ebay.com/itm/50A-Capacity-Tester-Indicator-coulometer-for-8-50V-48V-Lithium-Lead-acid-Battery-/181427862194?hash=item2a3df17ab2:g:moMAAOSw1ZBUtfpA

 

[redilast]

It's difficult to accurately estimate a packs capacity / reserve capacity based upon a voltage under load, unless of course the fuel gauge is doing some pretty complicated behind the scenes math and knows the particular cells voltage sag under a variety of loads or knows the internal resistance. The fuel gauge would also probably need to have a current shunt or be getting amperage information from the BMS somehow. I've only played around with a few battery fuel gauges and they were all just basic + and - connection and would only accurately tell the packs capacity under no or light load.

For example, 3.28v per cell under a 5A per cell load would still be quite a bit of capacity remaining. I would guesstimate around 30-40% capacity remaining. But 3.28v under a like a 0.5A load would be nearly depleted, perhaps around 10% capacity remaining.

The NCR18650B's generally ship at around 3.65v from the factory which is around 30% SOC. So 49.9 resting voltage might be 20-25% capacity remaining, but even then its still hard to know as the voltage will continue to rebound for hours.

The NCR18650B's are rated for and can be taken down to as low as 2.50v under load. So you can definitely go quite a bit lower. But deeper cycles would cause more wear I suppose.

 

[wineboyrider]

Does this pack have or use a BMS? If yes then discharge it until the BMS cuts off and that's where your cells should stop.

 

[wesnewell]

49.9V for a 14s pack is 3.56V per cell average. I have no idea where that would rate for your cells. For NCR18650B cells, that would put soc at about 20%.
http://lygte-info.dk/review/batteries2012/Common18650comparator.php

 

[999zip999]

47 volts is the lowest I would go. Most battery are damage because of being left uncharged and stored for to long. So don't leave below 50% charged for safety. Or don't leave battery un charged.