How accurate is dislpay"remaining ampere hours" ?
Posted by catboy519@reddit | ebikes | View on Reddit | 8 comments
Reason for why I'm asking: I want to figure out and calculate how many watts my bike truly uses from the battery, because those specs arent given as information.
Method: 1. Check both the voltage + "remaining ampere hours" on the bikes display. 2. Ride at constant max power for let's say 6 minutes , check voltage 3 minutes in also because of voltagesag. 3. Check the voltage again to determine what the average voltage was, check the ampere hours to see how many have gone out of the battery, then just do the math of what the average power was.
My bike doesn't show typical peak vs continuous power behavior: when I run my bike at max power, it can do so for ATLEAST 30 minutes. I was not able to test it for longer, due to reaching traffic lights where I had to stop so then from that point it wouldnt be a fair test anymore, and also because 30 minutes at max power drains about 50% frmo my battery.
My questions: 1. How accurately does an ebike count the current on a single ride? Let's say it reports 11.0 ampere hours and then 9.0 ampere hours, how accurately can we be sure that 2.0 have gone through it? 2. How good is my method to figure out the actual power of my ebike?
geekroick@reddit
It depends on how the battery has been constructed and how the voltage/current is being calculated by the display/controller.
Most batteries aren't measured with any degree of real accuracy, the display just takes a voltage reading and calculates a figure based on that reading. So in the example of a 36v battery the voltage will reduce from 42v when fully charged down to about 30-32v when the battery is depleted.
If your display actually shows the AH figure either that's an adjustable setting somewhere in the menu, or it's measuring the consumption in another way.
If you want a really accurate measurement then use a coloumb meter in between the battery and the controller/motor. EG https://a.aliexpress.com/_EJPDBg8
catboy519@reddit (OP)
My ebike explicitly shows both % and AH as 2 different values so I wonder if for the AH it literally counts how much current is passing through regardless of voltage?
You say coulomb meter but doesnt the controller already use a built in coulomb meter in order to show those remaining ampere hours?
DonnPT@reddit
My feeling is that, if the manufacturer is trying to show you how many amp-hours are left on the battery, the best info is resting voltage for percent, then apply the percent to a preset battery total storage value.
Sure, in principle, using the same preset storage value, the computer could arrive at that more directly using an amp-hours discharged value - but that would fail as soon as some charge leaks out of the battery without the computer's knowledge. At that point, it would be off until a full reset - where it would initialize using the resting voltage.
Not that resting voltage gives you more than the crudest idea. It's OK for what we use it for, but for a test of consumption rates, not sure it's worth the trouble.
The controller should have an amperage rating, like 30A or something. You can multiply that times the battery voltage for a theoretical maximum power. In practice voltage will drop, so a 48V battery might read 51V in front of the house, but when you ramp up to full power and try to climb a hill, it's likely to drop to under 48V. Anyway, multiply V × A = W.
catboy519@reddit (OP)
My b Theres 3 things to consider when looking at the amphours stated by the display. * Start amphours * Finish amphours * The difference between the 2 after a ride, .
Now the numbers on the screen may be off (showing 11 amp hours when you really have 12) but then they should both be off by roughly the same amount so therefore, the difference won't be off much. But thats just me doing math but I don't know how well basic math represents the truth of physics and engineering and hardware/software. Sensor stuff you know.
I maybe just rambling but I think when we look at the difference of the amphours after a ride it might read an very accurate value.
But I don't know how the bike keeps track of or calculates the amphours. Does it literally count how much current passes through or does it estimate with the voltage or does it rely on the battery BMS communication maybe
My controller sticker says 14A rating +/- 1 but that isnt clear if thats nominal or peak power... and also! That doesnt necessarily say if the software of the controller truly allows for 14a current.
But I have just gotten home fromm a quick cheap inaccurate test where my controller seems to do 13.33333 ampere so that checks out it could easily be 14.0, I will redo the test in a more accurate way soon as soon as there is enough headwind for my bike to run at max full power continuously.
DonnPT@reddit
I think I already have addressed your questions to the best of my ability, which is not much since I can only speculate based on what makes sense to me.
I actually have a relatively fancy computer that shows average consumption rates and totals, etc., and it tries to do fancy logic to improve on the charge level problem with LiFePO4 batteries. They have a flat discharge voltage curve, so voltage at 40% is hard to reliably distinguish from 60%, so the computer throws in some guesswork based on usage history. It isn't very good. You charge it up, with the computer off, and then turn the computer on, and it has to figure out what happened: the charge went up, maybe, but it also has this disharge history ... no. Resting voltage is the best bet, if your batteries are common lithium cobalt etc. cells. Anyway, it's a curve. Whether the the computer is correctly interpreting the curve at one charge level and not another, or all, or none, is not an answerable question. It's good for a rough estimate.
catboy519@reddit (OP)
I know that my battery or the BMS keeps track or the amp hours. My display just takes the information then shows it. So maybe, just maybe, the bms counts charge both when charging and dis charging.
geekroick@reddit
Quite possibly, yes. Without seeing the wiring or construction it's impossible to say.
But if you want more detail than what's currently available, fit an external one.
catboy519@reddit (OP)
Nice. Thanksyou