How to know the battery's charge level

So I have a 48V 12ah ebike battery (Panasonic NCR18650PF cells) that I use with my ebike. The battery is in a dolphin case and the case has a small battery meter which lights up some lights when you press a button.

What I'm wondering is, how do I really know the percentage of battery left? I don't really trust those indicator lights. Is there a formula for taking fully charged voltage and the voltage at some point along the discharge curve and determining the percentage of capacity remaining?

Please let me know if you need any more information in order to answer my question.

Thank you for any advice.

You can make a pretty close guess, by adding a simple voltmeter to your bike. The battery voltage will slowly drop as you ride, and with some experience, you can then find out about where half is for your particular pack.

Then you can look up your cell, since you know exactly what is in your battery. The discharge graph will show expected voltage at 50% discharged. But this may not be perfectly accurate, since your battery may not be exactly charged the same as in the test, and your battery has a bms that may result in an earlier shut off than the test. it will be close enough most likely.

You can use the odometer to make some test rides, preferably at least two. Ride where you wont have to do a ton of stopping and going, or climbing huge hills. Small, not so steep rolling hills ok. The idea is to cruise at a fairly moderate speed for most of the ride. Stay near the house as possible towards the end of the ride, and go till the battery stops. Take note of battery levels every 3-5 miles or so.

Now you have an odometer reading of full, empty, and some readings along the way. the voltage you had at half of the distance is your half full voltage, for your pack, the way you ride. Half full voltage will vary depending on how your pack is built, when the bms shuts off, when the charger shuts off, etc.


The best way to know your pack status is using a watt meter. This will actually count the watt hours as your battery discharges. Again, it takes testing till it's empty to know the real world watt hours your battery delivers. After the test that says perhaps, your battery can dish up 350wh, you would then know that half is 175 wh.

Lastly, if your riding is repeated enough, like a daily commute,, you can just use your odometer to know pretty much when to turn for home on a weekend ride. It's when you change the type of ride or the speed you ride that your distance changes a lot. Like you ride in a flat city, if you go to the mountains your distance can change by 50%. Speed is the thing that really changes your expected distance. Slow down when you think it's going to be tight.

Thank you so much. It's awesome that you would write up such a thorough explanation!

Over time you will get a gauge for it. But I feel a drop of power. Just don't go there as the rule. Yes DoHero is a Hero of mine.

True, the display battery meters can lie, but when you feel that abrupt drop in speed and power, you are in the last 5%.

Presice to 1/10 of a volt.

Or rather, reads to .1v. Plenty accurate enough at .5v though, if that is the best it can do. You simply adjust to whatever it actually reads, and then that is still reading consistently to .1v. three bucks for that kind of voltmeter. all you really need.

Old thread I know but would this type of watt meter be something I could check my state of charge with.?

https://a.co/d/7ezBA7u





My LED lights on my battery do not work (have not worked since new) and I am running a headless set-up (no computer, no display.

I don't want to unnecessarily charge my battery all the time, but I lose track of how far I've ridden it. It'd be nice to do a quick test before heading out.

Also does anyone know how I'd use it to test.? Does it have to be inline between battery and load.?

For a wattmeter to be useful as anything other than a voltmeter (which you can just use a cheap multimeter for), you'd need to leave the wattmeter wired in series between the battery and the controller all the time, so it can count up the ah / wh used, and then if you know the actual ah / wh capacity of the battery, and start out with it fully charged, then you can subtract the ah / wh used from the known capacity, and know how much you have left.

You don't want to leave the wattmeter connected to the battery unless you're using the system since it can drain a typical ebike battery in a few days or so, especially if it's closer to empty than full. However, most of those wattmeters don't retain info when not powered by the battery, so then you can't keep track of the ah / wh used unless you write everything down and add it all up manually. Some have a place for a secondary battery to connect to to keep this data there, and then you can just reset it when you charge back to full. Some, like the Cycle Analyst, do remember the data when powered off, but since the CA is not just a battery meter, but rather a whole ebike computer, it's a lot more expensive than the cheap RC wattmeters--and as a wattmeter, it *also* measures current both directions, so it can measure recharging too if that is done thru the same port you discahrge with (some batteries have a separate port, some don't).

If you just want to know the voltage, you can use any cheap voltmeter or multimeter, and connect the probes to your battery output when you want to check it. Or permanently wire it in, since the multimeter has it's own battery and won't drain your ebike battery just sitting there connected as a voltmeter. (though it only tells you voltage, not actual used capacity).

There are also simple "bargraph" voltmeters for specific battery types / voltages, which you could wire to a pushbutton on the battery wiring (or install in the case), to just "push to test" whenever you wanted to know approximately how charged it is (like the LED meter that isn't working in your battery right now). This image search shows a number of types; I don't have any specific info on any particular one shown there
https://www.google.com/search?q=ebike+bargraph+battery+voltmeter&tbm=isch


Regarding your existing battery LED meter, it's probably just disconnected inside the battery (broken wire, bad solder joint, etc). Some battery cases have a place for a meter but it's not actually installed (cheap manufacturer).

Thanks.! My current LED lights always show a full charge (even when the battery is fully dead.!)

Sounds like I could just use my voltmeter and, with some trial and error, figure out when I’m down to about 1/2 charged.

The actual measurement of Wh can be done with a Wattmeter, You can also use a receiver such as a bulb whose power you know and you measured the time it takes to go from 100% to 0%.
If it takes 1 hour to go from 100% to 0% with a 48V and 100W bulb, this means that the battery has delivered 100wh. In reality your 12Ah battery it would take 5h45 to go from 100% to 0% with the same bulb.
... to estimate capacity of the battery, you can measure the voltage when the battery is 100% charged and measure the voltage when it is discharged. This will help you to know the health status of your battery and know if your battery is well charged to 100% and if it is still discharging well until the same voltage.
It is also necessary to measure the total distance traveled during the duration of the discharge.
If for example you have traveled 40km with all of the 12Ah of your battery, this means that the consumption is 48x12=576wh/40km or 14.4wh/km.
If over time you see that the voltage at 100% is lower than before and the km traveled are lower, it means that your battery is starting to weaken.
If you used the wattmeter above connect it so that it cuts off when you turn off the bike but before turning it off you will have to read the value of the wh consumed and add it to the previous wh. This wattmeter has no memory. For it to keep the memory, you must add a small battery connected to the JST plug which is on the side of the display. The ideal way to measure the autonomy of a battery is to have a controller or a smartbms or even a power meter that does the work for you.

OK..!! Thank you..!

I know I can go close to 60 kilometres in one trip with a less than full battery (tried that).

So, if I’m just using it intermittently for errands I’ll just charge it every, say, 30kms.

I might try testing just voltage and see if I learn anything about the battery’s habits.

The voltage needs to be with cells isolated and at rest in order to accurately reflect SoC% once you have a calibrated table of that correspondence.

In the middle of a ride it only lets you guesstimate.

Starting at a precise benchmark of your definition of 100% Full and using a shunt based coulomb counting wattmeter to count down

is still approximate but much more reliable.

Resetting that 100% point each charge cycle is required, do not try to base SoC measurement off both-ways discharge and charging Ah flow or it will quickly drift

OK..!! Not sure if I’m going to bother learning all this stuff or just stick to bolting things together (which is what I enjoy).

I might just mark down on my whiteboard each time I ride the bike and the approximate distance.

I read something somewhere about charging when I’m down to about 40% and not storing it with a full charge.? Does that seem like a reasonable strategy.?

The bike is being ridden 2x per week when weather is mild, and 0x per week on the really cold and awful weeks (winter riding pattern).

Yes it's a good strategy, if you charge 100% it's to use it immediately. If you need to store the battery it can be at 80%. Also avoid charging the battery when it is very cold. For a battery to last a long time, it should always be at its nominal voltage.