Battery % Readings - Is it really that hard?

[smorgasbord]

We're in the third decade of the 21th century, folks. I can't believe we still have to mentally map a reading of, say, 50.7 volts on our 48 volt battery (that alone is crazy) to a 75% battery charge - and that the 75% battery charge itself doesn't actually relate to usable battery percentage!

Here's a chart that maybe came from Luna originally:

The first issue is that you can't run it to 0 % (39 volts). The motor controller will turn off at something like 42 volts (probably controller specific). So, first thing to do is to map 54.6 volts to 100% and then, say, 42.9 volts to 0%. That should be really easy, even small 8-bit controllers could have a tiny 4-bit lookup table to produce 16 values, which would be far superior to the 5 bars most displays show today. A 5-bit table would yield 32 values - still really small.

Now, there's the issue that voltage drop against usage (miles ridden is the better proxy) isn't linear. That is, a 0.5 volt drop when the battery is almost full is the result of more miles being ridden than a 0.5 volt drop when the battery is half or less full. This is why so many people report, for instance, that they get 15 miles before going from 5 bars to 4 bars, but going from 3 bars to 2 bars happens after only 6 miles. That kind of Range Anxiety is scary when you're on a heavy ebike that's really hard to pedal without assistance, particularly if there's a hill between you and home.

Let's say the true range of an ebike with a 200 lb rider on flat ground in PAS 2 (or equivalent) and going, say 15 MPH, is 48 miles. With the 4-bit/16-value table, I could empirically build the values in a few hours. Just find a flat road that's a few miles long on a calm day and measure the battery voltage every 4 miles. I think the only problem would be whether I'm willing to ride the bike until it literally gives up, in case that damages the battery. Would throw it on a charger right away, of course.

Anyway, this 4-bit table would give me enough gradations (6.25% apart) going from full to zero, where zero is bike gives up. Now if I've got more or less load, more or less hills or wind, etc., I would still know how close I am to running out of battery on this bike and could compare mileage ridden that day to the percentage display.

What I don't get is why no manufacturer even enables this, much less does this. (Or maybe there are manufacturers that do?)

Surely all controllers with a better than multi-segment display, from Bosch to Bafang, can handle a 16 entry table lookup for battery percentage - they're just not set up to do this. Matter of fact, the Bafang controller is so lame that even when set up for 52 volts, the battery percentage reads as if a 48 volt battery is being used.

Bafang and others, at least give us a way to load entries in that table.

 

[Stefan Mikes]

Funny thing, my BLOKS (Specialized) display reads the battery charge in %. Or, perhaps I missed the point.

 

[rich c]

You don't need a calculation with Bosch, it has the range left for every assist level you are in. I have no concern what the percentage is, I just want to pull in my driveway with 2-4 miles left. If you only get the percentage of battery, you still can't calculate how many miles it will go. Bosch calculates all the time so if you know you are 20 miles from home, you better be in an assist mode that tells you there is 24 miles left in the battery.

 

[harryS]

It's not too hard, but the public is too dumb.to appreciate any of this. A lot of auto owners I know only know their MPG because the car's computer calculates it. Otherwise they say "The little Honda is nice. It only cost me $20 to fill the tank, Dad. Can I have some money".

It could also lead to upset customers. Temperature. Wind. Weight. Speed. All of that effects power consumption and if a bike designer starts trying to be more precise, it does get hard to give an answer that won't have users calling Customer Service to find out why their tthree thousand dollar ebike is acting up.

 

[PDoz]

No issue with giant either - I can see % battery, an optimistic estimate of range remaining, plus a bar reminding me how much assistance I'm using with each pedal stroke. Getting actual voltage is a lot more complicated - perhaps the big manufacturers know they need to dumb it down for us simple folk?

 

[smorgasbord]

Funny thing, my BLOKS (Specialized) display reads the battery charge in %. Or, perhaps I missed the point.

Yeah, unless you can ride down to 1% AND the miles/percentage ratio remains constant, it's not doing what I'm suggesting.

You don't need a calculation with Bosch, it has the range left for every assist level you are in.

Cool - is that for all Bosch displays or just some? Do you find that the range in miles left is accurate?

 

[Stefan Mikes]

The Specialized TCD display does the same as Bosch. You can also activate a special mode that will automatically and seamlessly adjust the pedalling support so you are guaranteed to reach your target (as distance) or you will be able to ride for given amount of time — with defined amount of battery left. Users say that mode makes the Specialized e-bikes sluggish and underpowered but the feature is there.

 

[smorgasbord]

It could also lead to upset customers. Temperature. Wind. Weight. Speed. All of that effects power consumption and if a bike designer starts trying to be more precise, it does get hard to give an answer that won't have users calling Customer Service to find out why their tthree thousand dollar ebike is acting up.

Nothing could be worse than today's situation, at least with Bafang, Dapu, and other Asian motor manufacturers. At best you get a percentage, but the bike craps out well before it reaches 0%. At worst you only get 5 bars and the miles you can ride in the first bar greatly exceed the miles you can ride in the second to last.

 

[Alex M]

Getting actual voltage is a lot more complicated - perhaps the big manufacturers know they need to dumb it down for us simple folk?

It's not complicated, DC voltmeters cost less than $1 on Ebay. Adding DCV measuring/displaying circuit in ebike will cost a few cents. However, voltage is not very informative because it's sagging under load and is also changing with temperature.

Manufacturers are well aware of the need to dumb it down for most users - this is why they provide a simple bar graph. Yes, 5 bars doesn't cut it, some models have 10 bars. Some also have estimated range at the given PAS, but this is just an estimate and for some users could be misleading or unncecessary because they want to know when the battery is down to, say, 30-35%, not whether the motor will still be whirring when you pull up to your garage.

 

[Deleted member 4210]

Problem is voltage by itself to measure the remaining capacity (in amp hours) has several shortcomings and is inaccurate when you are below full capacity or not at zero capacity. Battery performance varies from cell to cell, and during your ride there is no cost effective way to measure which battery is the least performing one or vice versa. That is why during charging you have a process called cell balancing.

To try to dynamically measure capacity during a ride, and then to also do it over the life of the battery as each cell will deteriorate at different rates, makes using voltage not as accurate as you are wanting. These OEM's use voltage to keep things simple.

So whether they display percent or use bars, the inaccuracies will exist on nearly every ebike. And it will change in terms of accuracy over the life of the battery. Besides that, if you think batteries are already too expensive, you would certainly not want to pay the premium associated with obtaining cells that are perfectly identical not only in their voltage capacity, but also in each cell's individual discharge rates. The industry gets grade b and c cells from the process of 'separating out' the cells that come out of production that are out of tolerance and specification. So even with their current process of making batteries as costly as they are now, isn't good enough to produce chemistry that discharges identically or re-charges identically. That's just chemistry. Voltage also varies based on temperature.

Now Tesla's own batteries that are 21700 might be produced under even better manufacturing tolerances, and have tighter scrutiny at the end of the production line. But with $45000 to $100000 vehicles they can also build in much more sophisticated software for charging, battery management, and even measurement capabilities that just aren't in the purview of something that is only $1000 to $10000 in price.

 

[Deleted member 4210]

Batteryuniversity.com/learn/article/how_to_measure_state_of_charge

 

[rich c]

Yeah, unless you can ride down to 1% AND the miles/percentage ratio remains constant, it's not doing what I'm suggesting.



Cool - is that for all Bosch displays or just some? Do you find that the range in miles left is accurate?

I'm only familiar with the Intuvia. I've found it to be a bit conservative. Getting home with 1 bar showing, but saying only 2 miles range left.

 

[Alex M]

Batteryuniversity.com/learn/article/how_to_measure_state_of_charge

Coulomb counting described in this article is an old and accurate method. It measures the "charge" flowing in when charging and out when discharging, and does the math. Off the shelf devices based on this method have been on the market for decades, cost from $25 but they are standalone, not "ebike ready", it needs to be integrated permanently into battery - like those primitive LED indicators on ebike battery.

 

[Ebiker01]

Dapu

Dapu as the way BH is using it does have the % , and also other Battery info on the display.

 

[harryS]

Coulomb counting described in this article is an old and accurate method. I

I sometimes put a wattmeter on my bikes while riding. Measures the AH used during a ride. It's very repeatable for the same route, Mounts between the battery and controller. Can't watch it while riding. Probably a good thing. Better to keep eyes on road. They're inexpensive.

If one does not want to void warranty on a store bought ebike, the alectrically adept owner can make a harness to plug it between the charger and the battery (no changes needed to either). I've found, as expected, that the AH going into a battery during charging is about the same as the AH that left it during use.

 

[Keith Johnston]

We're in the third decade of the 21th century, folks. I can't believe we still have to mentally map a reading of, say, 50.7 volts on our 48 volt battery (that alone is crazy) to a 75% battery charge - and that the 75% battery charge itself doesn't actually relate to usable battery percentage!

Here's a chart that maybe came from Luna originally:

View attachment 44295

The first issue is that you can't run it to 0 % (39 volts). The motor controller will turn off at something like 42 volts (probably controller specific). So, first thing to do is to map 54.6 volts to 100% and then, say, 42.9 volts to 0%. That should be really easy, even small 8-bit controllers could have a tiny 4-bit lookup table to produce 16 values, which would be far superior to the 5 bars most displays show today. A 5-bit table would yield 32 values - still really small.

Now, there's the issue that voltage drop against usage (miles ridden is the better proxy) isn't linear. That is, a 0.5 volt drop when the battery is almost full is the result of more miles being ridden than a 0.5 volt drop when the battery is half or less full. This is why so many people report, for instance, that they get 15 miles before going from 5 bars to 4 bars, but going from 3 bars to 2 bars happens after only 6 miles. That kind of Range Anxiety is scary when you're on a heavy ebike that's really hard to pedal without assistance, particularly if there's a hill between you and home.

Let's say the true range of an ebike with a 200 lb rider on flat ground in PAS 2 (or equivalent) and going, say 15 MPH, is 48 miles. With the 4-bit/16-value table, I could empirically build the values in a few hours. Just find a flat road that's a few miles long on a calm day and measure the battery voltage every 4 miles. I think the only problem would be whether I'm willing to ride the bike until it literally gives up, in case that damages the battery. Would throw it on a charger right away, of course.

Anyway, this 4-bit table would give me enough gradations (6.25% apart) going from full to zero, where zero is bike gives up. Now if I've got more or less load, more or less hills or wind, etc., I would still know how close I am to running out of battery on this bike and could compare mileage ridden that day to the percentage display.

What I don't get is why no manufacturer even enables this, much less does this. (Or maybe there are manufacturers that do?)

Surely all controllers with a better than multi-segment display, from Bosch to Bafang, can handle a 16 entry table lookup for battery percentage - they're just not set up to do this. Matter of fact, the Bafang controller is so lame that even when set up for 52 volts, the battery percentage reads as if a 48 volt battery is being used.

Bafang and others, at least give us a way to load entries in that table.

Thanks for sharing this table. I have a new e-Trike with a 500W Bafang motor and 860C display. Terrific, EXCEPT that the display always says battery is at 100%, plus the 'range' display is blank. Apparently this is a known problem with Bafang and Utah Trikes has been asking Bafang for a resolution, w/o success so far. I sent Bafang in Shanghai an email with same request but no word.
Hopefully this chart will help me avoid being stranded b/c my health isn't good enough to bike unassisted home if I'm miles out w/a dead battery. Any other insights into Bafang issues like this? Any solutions? Thanks!

 

[fooferdoggie]

Cool - is that for all Bosch displays or just some? Do you find that the range in miles left is accurate?

usually it is conservative by a couple of mile.

 

[Novice]

I am probably not that experienced to give a reply, I have only had an ebike 10 months. I have found with my riding technique I can get 30 miles comfortably and have 35v left from 42v battery. So I always get back home, I realise we’re all different and 30 miles may not be enough.

 

[Stefan Mikes]

A small update after I have ridden Specialized Turbo Vado with new TCD-W display for 6 months now (I had the older BLOKS display before):

You might ask why you need the 10-bar State of Charge indicator if you already had the Battery % field. It is because screens 1-5 might have Cadence or Heart Rate instead of Battery % (fully customizable). (I cannot understand why some manufacturers cannot put Battery % on their displays...)

In January 2020, I could not use the Mission Control app. Now I can and will tell you just one thing...

The Smart Control feature in Mission Control is one of the best "Auto" pedalling assistance solutions found in the market. Reliable, allowing ride a Turbo e-bike pleasantly, and with absolute reliability you'll reach your target on a single battery.

 

[Jon A]

We're in the third decade of the 21th century, folks. I can't believe we still have to mentally map a reading of, say, 50.7 volts on our 48 volt battery (that alone is crazy) to a 75% battery charge - and that the 75% battery charge itself doesn't actually relate to usable battery percentage!

Here's a chart that maybe came from Luna originally:

View attachment 44295

The first issue is that you can't run it to 0 % (39 volts). The motor controller will turn off at something like 42 volts (probably controller specific). So, first thing to do is to map 54.6 volts to 100% and then, say, 42.9 volts to 0%. That should be really easy, even small 8-bit controllers could have a tiny 4-bit lookup table to produce 16 values, which would be far superior to the 5 bars most displays show today. A 5-bit table would yield 32 values - still really small.

Now, there's the issue that voltage drop against usage (miles ridden is the better proxy) isn't linear....

I share your frustration, I was thinking about this the other day trying to figure out what was going on with my Bafang displays. Now I understand why everybody says to use voltage!

First, I made a new chart like the above but took into account the voltage drop:

You can see the voltage is dropping more for each increment toward the end of the chart than at the beginning. It's still probably far from perfect, but it should be a bit more accurate than the above chart. Here's one for 42V if you or your bike wants to call it quits at 42:

With my battery at about 47.5V, I tested three Bafang displays (two 500C's and one DPC-18) and an Eggrider. They all gave voltage accurately (within a couple tenths of a volt or so, "close enough"). For percentages, the Eggrider said 39% which is right in line with my chart for a 42V cutoff (and of course the Eggrider lets you adjust it).

The Bafang displays said 24%, 24% and 25%. So they're precise, if not accurate. Interestingly, the percentages were more consistent than the voltages they displayed. But it appears they're mapping to about a 45V cutoff:

Since I have my controller set to let the fun continue until 39V, that's basically leaving about 35% of my battery capacity unused, even running the display to 0%. I don't want to abuse the battery, but that's a bit excessive.

Unfortunately after all that I don't have a solution to the problem you describe--the Bafang displays aren't adjustable. One can use an Eggrider which can be accurate...but the numbers are so darn small I can't see them when I'm riding. So, I guess I'll do what everybody else does--use voltage on the display and keep one of those charts on my phone for quick reference until I have it memorized.