Minimum Voltage for 52 volt battery before recharge

[diverdon]

   Hello .. I am new to the battery life game and am recharging the battery to full charge to balance it for the first few charges then I will go to charging to %80 ..

    Question is how low do you get a battery before you recharge it .. I have been recharging at 49.5 volts %35 .. I only use and average of %7 per trip so I'm thinking I could go for another ride or 2 before charging ... What's the lowest voltage to let your 52 volt battery get to before recharging ? I have a chart that shows 46.3 volts or %10 as the lowest it should go .. That seems low to me .. Maybe %20 or %25 for recharging ?

   Any help will be really appreciated .. Retired auto mechanic but this battery tech is new to me 

[Tommycat]

I use my battery very similarly to you.
 
To be accurate, my battery is a 52 volt nominal, Lithium Ion, PA cell battery. Rated at 11.5 AH. And every thing stated below will relate to Li-Ion chemistry.

This is the chart that I use for battery charge/voltage comparison...





It seems to go along with recommended voltages per cell at different percentages of charge.

IE: 100% = 4.2 volts per cell X 14 cells in series for a 52 volt nominal battery = 58.8 volts DC.
       50% = 3.6 volts............................................................................... = 50.4 volts DC.
         0% = 3 volts................................................................................. =  42 volts DC.

That said, my recommendations are as follows for longest battery life.

Leave your battery charged at 100% for the shortest time possible. IE: Charge to 80% and then just before using, top off to 100% just before riding on an extended trip.

Do occasionally charge to 100% depending on use, to allow your cells to be properly balanced by your battery's management system. (BMS) Otherwise 80% is fine.

Using your battery in the middle zone of voltages would seem to be to your advantage as seen by this capacity retention/cycles verses voltage chart.





Store your battery at around 50% charge, checking on it occasionally.


And to actually address your question of how low should I go...? 

Our controllers will hit a Low Voltage Cutout of around 3.2 volts per cell or so. (@48 volts nominal) So for a 52 volt nominal battery, this would be equal to a charge of 44.8 volts. This would probably be the lowest recommended usable voltage. 

But as mentioned earlier, staying in the middle voltage range will pay off in the long run. And for riders like you and I that don't need the absolute maximum capacity, is where I'm staying. 



Regards,
T.C.

[diverdon]

 
    Thanks for your reply TC ... The tip about charging to %80 then topping off before the ride is good ... Also about keeping it in the mid range of voltage ....seems like I use the least % of charge on each ride in the middle of the range ... Of course other things like how much wind how much I pedal etc. factor into this ...
 
    I am still "conditioning" the new battery charging to full charge to allow the cells to balance ...heard to do this 3-4 time then just go to %80 for longest battery life ..

     I am trying to determine the lowest voltage to let it go to before recharging ...I have been thinking 49.5 as that is %35 ...on my chart .. You said %44.something ...my chart only goes to 45.1 volts which is 1.0% charge ... I think either my or your chart is wrong? I'm gonna let it go lower than 49.5 volts thinking I can use the charge meter lights on the battery to help me determine when to recharge ... Just hoping I don't trip the low battery switch in it from going uphill or something ..
    Thanks for your help ... I just wonder if my or your chart is wrong ...DJ
 

[Tommycat]

If you could provide a picture or link to the chart your using, we could look at how it's determining it's percentages. As Li-Ion cells do not discharge linearly, some discrepancy's can be found.

[Bikemad]

As Li-Ion cells do not discharge linearly, some discrepancy's can be found.

The table you posted earlier produces an almost linear line from 100% all the way down to 0% and is unlikely to be very accurate for determining a battery's remaining capacity based on battery/cell voltages.
I plotted the following graph using 1/14 of the voltages to represent a single cell:


The typical resting voltage vs. remaining capacity curve for many 18650 lithium cells is quite linear between 85% and 15% but has a prominent curve at either end as roughly shown above.

Unfortunately, the battery gauges I have used that are meant for lithium batteries don't seem to be very accurate, as I the pair of gauges that I plug into my 13S battery to check the remaining capacity give different results.

According to your table, a 14S battery @ 50.4V should have 50% capacity remaining, but if I connect both gauges to my 13S battery when its resting voltage is exactly 46.8V (13/14 of 50.4V or 13x3.6V) one gauge shows 30% and the other shows 45%. 

Alan
 

[Tommycat]

The table you posted earlier produces an almost linear line from 100% all the way down to 0% and is unlikely to be very accurate for determining a battery's remaining capacity based on battery/cell voltages.

Arrrg, precisely what I was pointing out to avoid! 

Thank you for taking the time and plotting that out! Poor chart and unacceptable for people looking for better accuracy.

Does anyone have a better one? Or is it time to draw up a better one extrapolating from a chart such as this for reference?




As seen on this web page...
www.ibt-power.com/Battery_packs/Li_Ion/Lithium_ion_tech.html

Bearing in mind that one chart will not be totally correct in all battery chemistries and usage... but perhaps closer?

Agreed that the manufactures can't seem to get accurate "fuel" gauges. With some displays unable to even give accurate voltage readings... 
Seems the ones that actually track capacity used would be the best solution. But if your like me, just want something simple and direct.


EDIT: Non working link repaired

[Bikemad]

Does anyone have a better one? Or is it time to draw up a better one extrapolating from a chart such as this for reference?

Unfortunately, extrapolating data from a charge or discharge curve is not ideal as we are typically using "resting" voltages instead of "loaded" voltages to determine the remaining battery capacity.
The battery voltage will be higher while it is charging and lower when it is being discharged under load than when it is resting.

I think it would take a fair amount of time to accurately obtain the required information from your own specific battery pack as you would need to either measure the entire discharge process from full, or the entire charging process from empty using a wattmeter, stopping every 250mA to allow the pack voltage to stabilise for a minute or two so that an acceptable "resting" voltage could be measured.

A 10Ah pack would require 40 readings and close monitoring throughout the entire process to ensure that the voltage readings were checked and recorded at the correct intervals.

If a graph was plotted from a table containing the recorded results, it should enable a reasonable estimate of the remaining battery capacity to be obtained for any measured voltage.

With a known battery capacity and a fixed constant charging current, it should then be relatively easy to calculate the required charging time based upon the battery voltage reading prior to charging, so that a timer can be used to switch the charger off at any desired capacity. 

Alan