Battery Volts, Amps & SOC

[Graydon]

Hello, new member.
Im trying to get my head around my battery voltage's, amps and state of charge.
I have 2 Shark 52v batteries for my two Photon converted bikes (a Salsa Fatbike. and a Titanium/Ultegra Di2 road bike). One battery is the 16Ah model and the other the 13 Ah model.
I have been monitoring the batteries voltage via the display, im not sure how accurate the voltage readings are, are they coming from the BMS (which I would trust) or some other voltage meter built into the display (which I wouldn't trust)?

A lot of the 'charts on Lithium Ion batteries show a charging state of 3.65v per cell and 3.4 resting charged state. So how does this add up to 52v (14s5p and 14s4p) on my batteries? 14s = 47.6v? I am thinking the nominal volt is 3.72, or something close?

What I see, when charging and charged, it's showing 60.1V; and 59.1V when resting charged. It drops down to 55v after 45km of hilly climbing (650m), still showing 2 of 4 bars. Riding 30 km further, it dropped to 50.9v and down to a single bar. The bike was till running fine at 80 km, down to 50.2V and still a single bar showing. I'm guessing I'd get another 20-30 km range out of the battery at this point.

So what is the shutdown voltage of these batteries? and what voltage should I be stopping to ride so as not to damage the batteries?

thank-you 

[Bikemad]

Hi Graydon andto the forum.

The Shark batteries use 18650 cylindrical cells:



These cells are LiIon (not LiFePO₄) and therefore have a nominal voltage of 3.6/3.7V.  The maximum charging voltage is typically 4.2V per cell and permanent damage can occur if they are discharged below 2.5V per cell.

Your 52V batteries each have 14 paralleled cell groups connected in series, therefore the 52V comes from 14S x 3.7V (Number of cells in series multiplied by the nominal voltage of each cell) which is actually 51.8V, but this figure is typically rounded up to 52V for simplicity. Similarly, a 48V 18650 LiIon pack is actually 48.1V (13S x 3.7V) and a 52V LiFePO₄ is actually 51.2V (16S x 3.2V).

52V batteries are not actually 52V. They are 50.4V as they use 14 series of 3.6V nominal cells (14*3.6V = 50.4V).

Unless you are connecting wirelessly to the battery's BMS using an App, the voltage shown on a wired display would probably be calculated from the battery voltage feeding the display unit itself, and from the readings you have mentioned, your display is either reading slightly high, or your charger is outputting too much voltage, as the charging voltage should never exceed 58.8V for a 14S LiMn pack.

The battery's BMS should cut the power when the total pack voltage (or an individual cell group's voltage) reaches the preset low limit to protect the cells from permanent damage.

I prefer not to let my cells go below a resting voltage of ~3.5V (i.e. 49V for your packs) however, under heavy load the voltage will typically drop much lower than this without activating the BMS's low voltage cutoff.

I have no experience with the Photon mid drive kits, so I don't know whether their controllers also incorporate any automatic current reduction or cut off function when set low voltages are reached, but the Shark battery should definitely have it's own protection provided by it's built in BMS.

Unfortunately, I don't know what the shutdown voltage of the Shark battery is, but I'm sure you'll find out if it ever gets that low. 
I've not seen the Shark BMS's Bluetooth App, so I don't know whether it might allow the Cutoff voltage to be customised to suit your preferences.

Alan
 

[Defuzo]

I have a 48v battery, that once was unbalanced almost 1/2v on two groups. Its a 13s11p (cells: ICR18650-26F Samsung), dono Bms (still original cheesiness thingy).

I have bean unable to balance it to less than 20mV difference (with some budget hobby charger, and a active balance card 4s, -that I moved around). Wanted an estimate of remaining capacity, before ripping it apart, -wana make another 36v batteries). 

Made a 40km test ride. Read 54.2v at start (with real "multi-meter"), and 51.7v now afterwards (that is less than 5% drop, so I may do the trip 5 more times, without charging?). Biggest cell-voltage-difference are roundabout 30mV.

Anything to worry about?

[Graydon]

Hello Alan, thank-you  very much for that answer an information, it is exactly what I was looking for

[Bikemad]

Made a 40km test ride. Read 54.2v at start (with real "multi-meter"), and 51.7v now afterwards (that is less than 5% drop, so I may do the trip 5 more times, without charging?). Biggest cell-voltage-difference are roundabout 30mV.

Anything to worry about?

When my 10Ah 13s battery is at 51.7V it takes 1.686Ah (~17%) to bring it back up to its fully charged voltage, but it doesn't necessarily mean that it would be able to do the same trip another 4.88 times (83% remaining capacity divided by 17%).

The battery voltage will continue to drop as it is further discharged, therefore you will need to pull more Amps from the battery in order to produce the same level of power assistance (Power in Watts = Volts x Amps).

A battery voltage of 54V would require 10.29 Amps of current to produce 500 Watts of output power @ 90% efficiency.
The same battery with a voltage of 45V would require 12.35 Amps of current to produce the same 500 Watts of output power at the same 90% efficiency, therefore it would be draining your battery 20% faster just to produce the same power.
Unfortunately, with a higher current draw, the overall efficiency of the motor is typically lower, therefore your remaining battery capacity is likely to be used up even quicker.

Although you have been able to complete a 40km test ride using just 17% of your battery's capacity, I think it is extremely unlikely that you would be able to do the same trip 5 more times using the remaining 83% battery capacity.

I think the 30mV difference in your cells is nothing to be concerned about. The 8S active balancer that I tested didn't have any noticeable effect unless there was more than 100mV difference between cells. 

Alan