how low can you go? safe minimum voltage

EstebanUno

100 mW
Joined
Feb 23, 2011
Messages
38
I'm wondering how low the voltage can go without damaging the motor, controller or battery? Can I run it until the controller shuts off? Is there a built in safety cutoff? Would the bms on the battery shut down discharge at some low value as well? My primary interest is to learn what the actual usable amp hours is so I can estimate range. A second question is what voltage value should trigger a charge cycle from the perspective of battery life and number of charging cycles?

Battery is 48v Lion 13s 5p samsung 29e, 15 ah nominal. It powers bbs02 750w.
 
I believe the 48v bbs02 has a low voltage cutoff of 41v. Your battery BMS may have a different cutoff, and so your system should cutoff when the higher of the two is reached.

This is all from reading here on ES, so no personal experience (yet). I'm waiting for a bbs02 kit at the moment.
 
48V version of S06S controller (and 24V/36V version too for that matter) cuts off at 40V nominal (=45V resting pack voltage 13s2p NCR18650pf). At 13 cells, that's 3.1V-3.5V/cell depending on which value you use. Higher capacity batteries (more parallel strings) are cutting closer to 40V. BBS02 kit may cutt off differently.

Panasonic recommends 2.5V as an absolute lowest value, but NCR18650PF survives 0V without reducing its capacity - although not recommended doing "just because it can"! For Samsung 29E I would keep that absolute lowest point above 3V/cell. Over 3.5V/cell it starts to harm the range dramatically, so it's a fine tuning art. Here, they stopped measuring at 3.0V/cell.
file.php


If you want longer cycle life, charge to 4,10V-4,15V/cell) and never discharge under 3.0V/cell. Motor can not be damaged of too low voltage. Some controllers self-destruct if you play with its internals at low voltage (mosfet shorting because of corrupted MCU), when increasing main resistor without increasing the voltage. Usually they shut down before this happens, if not tampered.
 
OK, I'll run it to cutoff and find out and see where that falls. I have a watt meter installed so the value should be be pretty accurate. 3v x 13 =39, so I should be safe at a cutoff value of 40 or 41, whatever the bbs02 controller setting actually is. I guess I won't know who is doing the cutoff though, bms or controller.

Fellow, thanks for the detailed reply. I learned some valuable points. Particularly to think of V as a function of the individual cells.

Esteban
 
Thanks fellow for the graph. Its a good idea to get a well pictured profile of your battery, and to know its limits, especially on the low and high side. And as you did, for the specific cell that is the makeup of your battery. There is always a data sheet for the cells, from the manufacturer and that data sheet is good reading if you want to preserve the good-health and longevity of your battery.
 
Here is my recommended list of 18650 format cells, as of sommer 2015. Any of this cells is an excellent choice for an ebike. The list is numerical, and can help to fine tune range vs life expectancy. Newcomers LG INR1850MJ1 and Sanyo/Panasonic NCR18650GA are new at the moment, and not yet measured by our friends at http://dampfakkus.de/akku_auswahl.php
Recommended 18650 cells, as of sommer 2015.jpgMy choice of Panasonic NCR18650PF is mainly due to its robustness (zero volt technology?), 18A peak current, and its price. If ordered today, I would probably go for Sanyo/Panasonic NCR18650GA. Talking about tuning for life expectancy, those cells will probably became obsolete before they die. PF's were state of the art last year, and new, better cells are comming all the time.

When using extreme low number of paralleled cells (as 13s2p in my example), you abuse cells more and can not utilize full capacity because of voltage sag. At 13s5p, battery pack resting voltage at LVC should be 41V or something close to that number, and you get more of its low voltage capacity without abusing them with high currents. My practical experiment with NCR18650PF show that 13s2p battery in my configuration have 18km of range, and 13s3p have not "expected" 27km, but 30Km to 40km used with same controller and <1kW peak motor (same LVC)! For those still wandering why i'm using 2p strings despite those drawbacks, click on my signature link;).
 
I agree! Those are the pictures eTrike is refering to:
V Cycles_1.jpg
lithium2.jpgThis picture has a very old graph, multiply by factor 3 to get todays capacity in mAh.

We talked about the safe minimum voltage, how about safe maximum voltage? Last picture explains why some people (like me) can get away without using BMS at all. Some modern cells with safety layers and PTC fuses can manage over 12V without exploding, and then simply shuts down before they reach 50'C. I recommend carefully calculated fuselinks instead of thick wires, and one main 6mm*30mm 125V DC rated ceramic fuse. Do this 12V per cell exercise with RC Lipo, and you will have Romulan Disruptor effect all over the place.
file.php

Someone said: Find me a foolproof cell, and I'll find you a better fool:). This NCR18650PF cell is charged at 12V and 3A PER CELL!!! That must be the most boring scientific test ever made at Panasonic - nothing happend.

For those who prefer RC lipo instead, at 0:36 to 0:38 is explained how to decrease its temperature because they lack safety layers:. This is (balance?) charged with Imax B6:
[youtube]k9mcNvOGKtI[/youtube]
As some youtube comments suggest, the home run around the white Renault is a high speed attempt of creating the vortex, that would put out the flames.
 
I ran my battery down to 3.0 V/cell under load yesterday. I didn't realize I could do this without either the BMS or the bbs02 controller shutting down. Or perhaps my understanding of what constitutes a shutoff is wrong.

At the end the resting voltage was 42v. Which / 13s = 3.23 V. But watching the watt meter, the voltage under load varied from 39.0 to 39.7. The motor would occasionally hum and lose power for a second or so and resume, perhaps once every 5 minutes at some point starting around 41V under load, 43+ resting. I interpreted this as a low voltage warning, with cutoff to follow at some lower V value. But full stop never came. Didn't want to risk it so at 3.0 V per cell under load I stopped.

fellow said:
Here is my recommended list of 18650 format cells

My Samsung 29e followed the table pretty well. Starting value was 4.1 V. Volts column value is end reading under load. That seemed to best match fellow's table.

Volts .. mAH cumulative
3.6 ... 0650
3.4 ... 1030
3.0 ... 2660

Don't know how the watt hours were calculated in the original table. The voltage multiplication factor used is not the value from the same row.
 
Don't know how the watt hours were calculated in the original table. The voltage multiplication factor used is not the value from the same row.
I guess some kind of integral, using samples every 2 seconds.
integralCota1.jpg

The motor would occasionally hum and lose power for a second or so and resume, perhaps once every 5 minutes at some point starting around 41V under load, 43+ resting. I interpreted this as a low voltage warning, with cutoff to follow at some lower V value. But full stop never came. Didn't want to risk it so at 3.0 V per cell under load I stopped.
Those power cuts occur when the voltage sags below LVC, even for a very short period of time, say one millisecond. I'm not sure if it's easy to catch it every time without an oscilloscope or some kind of super sampling/sensitive Vmin capable multimeter. You can add one or two more cells in series and test the LVC again.

What was the difference in range (Km or Miles) between 3.5V/cell and 3.2V/cell? According to that table it looks significant?
 
fellow said:
What was the difference in range (Km or Miles) between 3.5V/cell and 3.2V/cell? According to that table it looks significant?

The readings are from 3 different rides and routes, from full charge to nearing LVC over the total of the three. But the motor was used almost exclusively for climbing, though grades varied. I've found that the grade changes power usage almost imperceptibly per unit of elv gain. 1000 feet elevation gain at 5% uses the approx same wh as 1000 feet at 10%. Of course miles vary. This revelation has made it quite easy for me to estimate range over varying terrain. Flat would be factored in using miles.

Here's the table with the average elevation gain (from google maps) column added. I don't have a value for 3.2, since that value was mid-ride.

Volts . mAH. wh calc. elv gain. grade. wh/1000'
3.6 ... 0650... 159... 1578'.... 6%... 101
3.4 ... 1030... 148... 1603'... 10%.... 98
3.0 ... 2660... 284... 3317'.... 4%.... 86 (more conservative power use as battery neared LVC)

(mAH is cumulative; wh calcs and wh/1000' are for each trip and NOT cumul. Wh = ah * avg V under load.)

So the range from 4.1 to 3.4 was roughly equivalent to 3.4 to 3.0. So yes the difference was significant.

Good to know about the momentary low voltage cutoff. Apart from this test, I should only rarely encounter this.
 
if your battery has allowed one cell to hit the LVC then it must be recharged right away. the voltage sag drops the cell voltage from 3.5V down to 2.8V because of the lack of available capacity left in the battery and if you continue using a battery after it has hit the LVC then you have to cut way back on power to less than 10% until it can be charged. this is where batteries die and the reason they use the LVC is to warn you when to stop and recharge. ignoring it will destroy your pack very early.

that picture of the RC plane on fire has nothing to do with charging beyond the HVC and is because of a short in the wiring. it has nothing to do with charging.
 
you know nothing about batteries so how can you justify misleading people like that by making such ignorant statements. it is as if you have never seen a real battery outa balance.
 
dnmun said:
if your battery has allowed one cell to hit the LVC then it must be recharged right away. the voltage sag drops the cell voltage from 3.5V down to 2.8V because of the lack of available capacity left in the battery and if you continue using a battery after it has hit the LVC then you have to cut way back on power to less than 10% until it can be charged. this is where batteries die and the reason they use the LVC is to warn you when to stop and recharge. ignoring it will destroy your pack very early.

This was a one time test of range. Typical discharge will be to 3.2v or higher. Only went as low as I did because I was expecting a full shutoff. I guess that would happen when V stays below the LVC point, which didn't happen. There was never an error on the 961 display like 06 or 22 indicating a low voltage related shutoff.

It was good to see discharge capacity in line with what I should expect from Samsung 29e. Sure they are labeled Samsung, but from China, one wonders ...?
 
eTrike said:
@Esteban Do you have a way to see cell-level voltage? Some BMSs can be fishy, so having the ability to check that periodically can save your pack.

No, been counting on the BMS. Individual cell values I have reported have been mathematically derived.

Also, do you notice a drop in power as your pack drains? I ask because my experience with cells like this left me with a need to decrease throttle towards the end in order to keep LVC alarm from buzzing, so I am curious if that is your experience with this pack.

No drop in power on the lower end. But higher discharges will increase the difference between resting and under-load voltage, bringing the LVC into play. I think the voltage drop is similar to higher charged values. It's just that it's noticeable because of the LVC.
 
riba2233 said:
Samsung 29E are Li-NCA, so nickel-cobalt-aluminium. You can discharge them up to 2.5V per cell if you really want.

Good to know. But looking at the discharge curves etrike posted (thanks), not much is left below 3.1V.
 
Back
Top