HOW MUCH GAS IS IN YOUR EBIKE TANK?
Do you trust your ebike battery gauge? It is common for ebike battery gauges to be less than accurate and you need to know if you have enough charge to get home. If your battery gauge is less than accurate there is a trick to know exactly how much juice you have left. The charts below provide the key (scroll down if you don't want all of the background).
How much battery charge you have left on your ebike is almost as important as knowing how much gas is in the tank in your car. I say almost because at least you can still pedal your ebike when you run out of juice. Sometimes the battery state of charge gauge is very inaccurate on ebike conversions. There are all kinds of reasons for this. Sometimes it is the accuracy of the display, sometimes the display is not configured for the voltage of the battery you are using. This can be confusing and for some it can be aggravating to have a new ebike setup and not have an accurate battery gauge to know how much charge is left. Often times this can be fixed by properly configuring the display with the correct battery voltage but sometimes there is nothing you can do except perhaps get a different display. If everything else on your ebike conversion is working then there is actually a better way to know more precisely than using a gauge or battery percent exactly how much battery is left. This is by reading the exact voltage your battery is putting out.
Lithium Ion batteries vary in voltage from full to empty. This is consistent across all batteries of the same nominal voltage. For example, if you have a 48v battery, 48 volts is the 'nominal' voltage or middle of the range of voltages for that battery. When you charge a 48v battery it actually charges to 54.6 volts and when it is empty it will be somewhere around 40-41 volts. Most ebike displays can show the actual voltage of the battery at any point in the discharge cycle. If you don't see the voltage on your display, go into the configuration menu and chances are you will see an option to enable having the voltage on your display. Sometimes you have a choice between voltage and percent. Choose voltage. Usually you already have a battery gauge that is roughly showing the percent of charge by graphically showing the battery with part of it lit. To know exactly how much battery you have left using the voltage reading, memorize the below voltage values for your battery and you will always know exactly how much charge you have left.
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52 Volt Battery
58.8v – full tank 53v – half tank 46v – almost empty 42v – empty |
48 Volt Battery
54.6v – full tank 49v – half tank 44v – almost empty 41v – empty |
36 Volt Battery
42v – full tank 38v – half tank 33v – almost empty 30v – empty |
That is basically all you have to know. You can stop reading here but I will fill in a few more details below to explain what is going on and a few other things to watch for.
The full tank voltage for your battery above is actually what your charger puts out to charge your battery. When your battery is full it reaches the same voltage as your charger. If you have a smart charger that extends your battery life it will put out about 1 volt less for 90% charge and 2 volts less for 80% charge. This will extend your battery life by 2.5x at 90% and 4x for 80%. Notice that a decrease of 2 volts takes you down to 80%. This is because the above voltage ranges are not linear. There is more energy packed into the higher voltages so the further down the voltage drops the faster it drops. That is why you are almost empty at about 3 volts above empty.
Your bike might start cutting out when you are almost empty. This is because the voltage drops when you ask for power. The more power you ask for the further the voltage drops. Bigger batteries will drop less than smaller batteries. To keep from cutting out when you get this low don't ask for as much power. You may still be able to get home if you ride at the lowest assist level or barely use any throttle. Use too much power and the power will cut off. On some bikes you may need to turn it back on but on most the power will return when the voltage goes back up after the drop from excess power.
The power cuts off because there is a low voltage cutoff programmed into the controller. This protects your battery. Your battery also has a low voltage cutoff that is usually a little under the controller low voltage cutoff as an extra layer of protection. This is because Lithium Ion cells can never be allowed to fully drain or they will be permanently damaged. For most cells the voltage below which will damage it is 2.6 volts. As an example, a 48v battery has 13 cells in series which means you multiply the voltage of each cell by the number of cells to get the output voltage. In this case the lowest voltage before damage occurs is 13x2.6=33.8 volts. For 52 volts this is 14*2.6=36.4 volts and for 36 volts it is 10*2.6=26 volts. You might ask why the low voltage cutoff is set so much higher than these voltages? It is because voltage is dropping fast on the low end of the voltage spectrum and if you don't charge it immediately it may drop dangerously low before you charge it and permanently damage the battery. Batteries that are being stored will slowly lose their charge so Lithium Ion batteries should be periodically charged even when they are not being used.
The full tank voltage for your battery above is actually what your charger puts out to charge your battery. When your battery is full it reaches the same voltage as your charger. If you have a smart charger that extends your battery life it will put out about 1 volt less for 90% charge and 2 volts less for 80% charge. This will extend your battery life by 2.5x at 90% and 4x for 80%. Notice that a decrease of 2 volts takes you down to 80%. This is because the above voltage ranges are not linear. There is more energy packed into the higher voltages so the further down the voltage drops the faster it drops. That is why you are almost empty at about 3 volts above empty.
Your bike might start cutting out when you are almost empty. This is because the voltage drops when you ask for power. The more power you ask for the further the voltage drops. Bigger batteries will drop less than smaller batteries. To keep from cutting out when you get this low don't ask for as much power. You may still be able to get home if you ride at the lowest assist level or barely use any throttle. Use too much power and the power will cut off. On some bikes you may need to turn it back on but on most the power will return when the voltage goes back up after the drop from excess power.
The power cuts off because there is a low voltage cutoff programmed into the controller. This protects your battery. Your battery also has a low voltage cutoff that is usually a little under the controller low voltage cutoff as an extra layer of protection. This is because Lithium Ion cells can never be allowed to fully drain or they will be permanently damaged. For most cells the voltage below which will damage it is 2.6 volts. As an example, a 48v battery has 13 cells in series which means you multiply the voltage of each cell by the number of cells to get the output voltage. In this case the lowest voltage before damage occurs is 13x2.6=33.8 volts. For 52 volts this is 14*2.6=36.4 volts and for 36 volts it is 10*2.6=26 volts. You might ask why the low voltage cutoff is set so much higher than these voltages? It is because voltage is dropping fast on the low end of the voltage spectrum and if you don't charge it immediately it may drop dangerously low before you charge it and permanently damage the battery. Batteries that are being stored will slowly lose their charge so Lithium Ion batteries should be periodically charged even when they are not being used.
Note that the data we use has the energy density skewed toward the top end a little more than this chart does. In other words, the voltage declines faster as the battery drains. This discussion has been about how full your tank or battery is but it says nothing about how big the tank is or how much range your will get from your battery. That requires you to know the watt hours as the measure of capacity. Motors are all pretty close in efficiency provided you pull the same watts from each of them, and they are geared efficiently for your riding conditions. Mid-drive motors are somewhat more efficient because they can use the gears on the bike to be more efficient. Whereas hub motors are always in a fairly high gear and aren't very efficient on hills or with headwinds.
More powerful motors can drain a battery more quickly; however, you will be traveling faster such that if there were no wind resistance, the range would be similar to smaller motors. However, wind drag increases dramatically above 16mph and hence has a significant impact on the range. Overall, range is highly variable due to many factors such as hills or grade, wind resistance, weight of the bike, weight of the rider, pedaling effort of the rider, efficiency of the motor, etc.
If I have a 52v battery.
I start my ride with a full charge of 58.8v.
At 20 miles (as an example) my voltage says 53v (1/2 full).
Is it reasonable to assume I can ride 20 more mile (same terrain + same speed) before I’m empty?
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Electrify Bike replied:
Yes, you will empty at about 42v. However, the voltage may drop to 42v and cut out when using a lot of power. It would help if you used very little power to minimize voltage drop when you are that low.
Very usefull and understandable explanation! So if I combine 48 v battery, 48 v controler and 36 v geared motor all I have to do is not overheating the motor?
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Electrify Bike replied:
Yes, you will have a higher top speed out of that motor, but you must be careful about heat buildup.
Thanks for this info. It really gave me reassurance as to my battery status. My 52 volt battery reads 90% when the voltage indicator reaches 50 volts. I trust the voltage read more than the percentage reading. Thanks again
Hi, i have a 48v battery and last few months it charges to 53.2v. As i see from this article, it should be 9 or 10% degrading. But my range is drastically reduced by %50. I don’t understand much of it. Can anybody give me an answer? This article is really helpful and clear. Really good work. Thanks.
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Electrify Bike replied:
Yes, that is an indication that your battery has lost capacity. The storage energy is denser at the top of the voltage range, explaining much of the loss. You could try a full charge from a mostly drained battery and leave the charger on for a few hours after it is full to see if the cells will balance at a higher voltage level. This doesn’t help if the battery is getting worn out.
Solid information and very helpful! Will keep this bookmarked for the future to refer back to
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