Voltage drop as battery drains



  • As my battery drains, the voltage reported by diagnostics drops. At below 40% battery, I can feel the power loss. By 35% battery, riding starts to become dangerous. The folks at Onewheel claim power loss is not an issue. Physics tells us that (power=voltage*current). Unless the current increases, how can the Owewheel folks claim that the power does not decrease as the voltage decreases? Both physics and experience tells me that it does.



  • I took my XR down to 1% battery today on purpose, to see how far I could go,, barely made 1 mile on the last 10% at 4-8 mph carving smooth pavement, didn't have my lord helmet so I chickened out on going till shut down. I'm 200# and 6'; I felt the power loss and chilled my riding accordingly.
    got no clue what the voltage was



  • You are right that the performance of the the OW drops as the battery discharges. The OW has a programed feedback warning system. You will notice that as the power available drops pushback happens at a slightly lower speed. But the change is really quite minor. This is due to the discharge characteristics of lithium chemistry batteries.
    Li-ion Discharge Voltage Curve Typical.jpg
    Notice that from 80% down to 20% the voltage is essentially constant. Also if you discharge a lithium battery below a critical voltage you will damage the battery, hence almost all lithium batteries come with a BMS (battery management system) that cuts off the battery at around 30% capacity. Your phone may say that it is dead, but in fact the BMS has just cut you off. The Onewheel is no different. In fact if you want to get the best life out of your Li batteries you should avoid charging them all the way up. Pushing the voltage past 3.8v per cell significantly shortens the life of the battery. Most BMS won't stop charging at 80% since everybody want their phone to hold a charge for for as long as possible. (and most phones are obsolete before the battery quits) But if you want that iPhone to last 10 years you should really take it off the charger at about 80%.



  • @Sponge315
    Soo, your saying I shouldn't do overnight charges and stay in the top 50%?



  • I don't ride my boards past the last 20% of battery. It gives me a maximum safe usable range. I've been into electric vehicles for a number of years now and it's a common practice not to take an electric vehicle beyond the last 20% of battery power mostly because of the previously mentioned damage that it can cause to the batteries and also under 20% the electronics can become unstable.



  • @Sponge315
    Very good info shared here, Thank you.

    I feel @Future-Motion has done an excellent job on the BMS side of the OW design. It follows what you have shared and allows the OW models to be charged by different systems but still optimize the cells for best performance and life without much concern on the charge rates. The BMS handles the low and high side voltage concerns you have noted.

    Again this great info for all to read.....



  • @b0ardski
    That is a good question. I don't know exactly how FM has programed their BMS. Do they stop charging at 3.8v, 4.0v or do they let the charger go all of the way up to 100% of the batteries capacity? Nor do I know where exactly they do a cut off voltage. But it is a pretty safe bet that when your app says you are out of battery (0%) The battery still has 20-30% left and the BMS has shut it down to protect the batteries. You should be fine running it all the way out, the BMS isn't going to let you hurt the batteries.
    I know that most cell phones let the battery charge all the way up to about 4.2v per cell since they don't really care about max battery life. They are more concerned about max run time.

    There is another concern here if you are digging into battery tech. Li batteries don't all charge and discharge at the same rate. Over time you can get quite an imbalance between cells. I did a little checking and found that the voltage reported to the app is the voltage from the last cell in the pack, not all the cells. If this is a slow cell it could be giving you a higher reading than normal, or if it a faster cell it might be a bit low. This is why FM advises that you should occasionally let the batteries charge overnight, or longer, to assure that all of the cells are up to the same level. I guess you could open it up and measure each cell, but I am having far to much fun with my OW to take it apart and look for such things.



  • @Sponge315 but I am having far to much fun with my OW to take it apart and look for such things.
    0

    me too



  • @Sponge315 just adding to the conversation. XR has 15 cells and BMS charges up to 63v so each cell is charged to 4.2v. Good summary of the NMC battery here from the battery university site.
    NMC Batt

    https://batteryuniversity.com/learn/article/types_of_lithium_ion

    Not sure I have the image link working. Sorry first time for everything. 🙄```


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