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Checking Battery with Voltage meter ?

Old 08-22-21, 01:15 PM
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Sempervee
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Question Checking Battery with Voltage meter ?

Greetings, New guy on ebikes with new RAD city 3. I have familiarized myself with keeping 48V battery charged and see the bars light up on the HB info board, but is there a way to tell how much I am really charged up using a digital Voltmeter? I would guess take 48 = 100% minus the reading = is your accurate bank of power at the moment?

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Old 08-22-21, 01:51 PM
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Search on "ebike battery voltage chart" and pick the chart that best works for you. Make sure you're using the 48v battery listing. But in a simple comment ... No 48v batteries are not fully charged when they read 48v. A fully charged 48v lithium Ion battery will read just over 54v.
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Old 08-22-21, 01:52 PM
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That wont be very accurate. The Voltage vs charge/discharge status isnt all that linear.
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Old 08-22-21, 03:17 PM
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The linearity of the voltage to capacity relationship of a particular battery will be dependent on the design and chemistry of the battery, it's age and condition, and how it's being discharged. Yes, it would be surprising to find a battery that exactly matches the chart at all points but in general I've found the charts online to be close for most commercial ebike batteries and a good place for the OP to start from.

For the OP the important voltage points for your battery will likely be something like ...

54-54.6v - full battery. It's a good idea to charge to this level only on an occasional basis (the numbers will depend on how heavily you use a particular battery and it's age/condition). Charging to this level allows the internal BMS to balance the individual cells. You might also charge to this level if you are going to use the battery only within the next few hours and you want the maximum capacity, it's not a good idea to fully charge a Li-Ion battery and leave it unused. In general someone charging up a battery that's in good condition every day or so will probably only want to charge to 100% once or twice every few months.
52-53v - ~80-85% This is probably a good maximum point to charge to on a regular basis to maintain good battery health (long life).
48v - ~ half full (so often a good point to be heading for home). This is also a good level to leave a battery at if you are not planning to use it for a while (weeks or months).
42-43v - this is the "floor" (and is determined by the BMS and/or controller settings and can therefore vary between different vendors and hardware configurations) which is the point where most controllers won't allow the battery to be discharged below (otherwise cell damage can result) ... so essentially "empty" when on the bike.

If someone wanted to determine the capacity of an ebike battery to a more accurate and granular level then looking at the wattage used over time would be the way to go. This would require noting the voltage and amperage draw over time, something that most ebike riders don't have the hardware or patience to do.
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Old 08-22-21, 03:27 PM
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mclewis1 - Thanks for the in depth procedure. I will be following it as you explained.
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Old 08-25-21, 11:07 AM
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Took out my Digital meter and checked that I had the probes in the correct positive and negative positions and both batteries from two different bikes read 20 volts when on the bikes it says a full 100% 4 bars? What am I doing wrong? Is this 48V chart is wrong or I am doing interpreting something incorrectly?
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Old 08-25-21, 11:56 AM
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Sorry to display my ignorance, but don't li ion batteries have a rebound/recovery voltage so SoC isn't well reflected in no-load voltage?
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Old 08-25-21, 12:32 PM
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I had never bothered to check the battery voltage other than to check that the charger and BMS were working properly and bringing it to a full charge. When my old 36v pack would no longer maintain 42v I replaced the pack with a unit of higher capacity.

Yesterday I happened to return from a ride with the display indicating the battery level at about 50%. This thread had made me curious so I checked the voltage immediately before there was any significant rebound. The pack measured just over 36v which is pretty much in line with the 50% level on the chart posted by the OP. I'll give it a few months and check again for comparison.
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Old 08-25-21, 12:33 PM
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Sempervee, Where (what point) are you measuring the battery at ... at the charging port or at the actual output connection (where it feeds your controller)? The charging port is usually managed by the internal BMS and you can sometimes see different voltages at this point that don't always reflect the actual state of charge of the combined cells in the battery unless the BMS believes the battery is plugged into a charger. The output connection will always represent the actual state of charge when the battery is ON, but you have to be very careful about working with that connection, as the full amperage potential is available at that point (so any short across that connector will blow internal fuses or possibly burn/vaporize a probe)..

If your batteries have an on/off switch, turn it on. If your batteries don't have a separate on/off switch you may not be able to measure them without being installed on the bike or charger (without going through other possibly invasive steps). If you don't have an on/off switch on the batteries you may still be able to find a connection point on the bike that will allow you to correctly measure the state of charge voltage of the battery.

For example I have a 52v battery (it's not bike specific, just one of the standard Hailong case downtube batteries). When fully charged if I measure the voltage at the charging port it reads about 32v when the battery is switched OFF, and the correct 58v when it is switched ON.

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Old 08-25-21, 12:49 PM
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Originally Posted by unterhausen View Post
Sorry to display my ignorance, but don't li ion batteries have a rebound/recovery voltage so SoC isn't well reflected in no-load voltage?
Yes they do have a rebound or recovery characteristic, the actual numbers vary a little between different manufacturers and types of Li-Ion cells. You can easily see this if you are monitoring the voltage while riding. On my bike (52v/14.5amp battery using Li-Ion 21700 cells) the voltage will sag under heavy load (accelerating for example which draws 15-20amps) and rebound by about a volt after a few seconds when the load is removed. The voltage may also continue rise another few tenths over the next seconds to maybe few minutes if entirely unloaded (coasting). In my experience aside from this temporary sag the no load voltage readings tend to fairly accurately represent the state of charge (within in half a volt or less).
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Old 08-25-21, 12:55 PM
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Originally Posted by mclewis1 View Post
Sempervee, Where (what point) are you measuring the battery at ... at the charging port or at the actual output connection (where it feeds your controller)? The charging port is usually managed by the internal BMS and you can sometimes see different voltages at this point that don't always reflect the actual state of charge of the combined cells in the battery unless the BMS believes the battery is plugged into a charger. The output connection will always represent the actual state of charge but you have to be very careful about working with that connection, as the full amperage potential is available at that point (so any short across that connector will blow internal fuses or possibly burn/vaporize a probe)..

If your batteries have an on/off switch, turn it on. If your batteries don't have a separate on/off switch you may not be able to measure them without being installed on the bike or charger (without going through other possibly invasive steps). If you don't have an on/off switch on the batteries you may still be able to find a connection point on the bike that will allow you to correctly measure the state of charge voltage of the battery.

For example I have a 52v battery (it's not bike specific, just one of the standard Hailong case downtube batteries). When fully charged if I measure the voltage at the charging port it reads about 32v when the battery is switched OFF, and the correct 58v when it is switched ON.

I plugged it in the battery off the bike with bat in off position electrically at the bottom where the battery connects to the snap in cradle connected to the controller. I have the ability with the key to turn on the battery but when testing I did not turn it on. Apparently no harm from testing as I just returned from a ride with no issues electronically. Now I know to turn it on and careful where I place the probes appropriately. Thanks again mcLewis1!

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Old 08-25-21, 01:14 PM
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Sempervee, That's good news. So with that particular battery you should be able to measure the voltage when the battery is turned on at either the output or at the charge point. It would obviously be safer to use the charge point but it's often harder to get the meter probes in there reliably and without shorting the connections. If you are going to be measuring the voltage on a regular basis you might consider getting a male connector for the charge point (not sure what the RAD battery uses but it should be something easy to source online) and make up a little cable using that connector and leads to your meter. This way it's really simple, solid and safe to just plug your meter into the charge port to quickly measure the battery's state of charge. If you want to get really clever you could build a charging cable adapter that allows you to monitor the voltage with your meter at the charge point while the battery is charging.

I also noticed that my earlier post missed something. The sentence - "The output connection will always represent the actual state of charge" should have the addition of "when the battery is ON". I corrected my earlier post.
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Old 08-25-21, 01:17 PM
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You need a load tester to accurately check the charge and state of a battery. Straight voltage is only meaningful in a way to tell if a cell dropped, or similar.
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Old 08-29-21, 07:17 PM
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Well after riding all day with a combination of stopping for lunch and shopping at the end of the day we had gone 20 miles and had 4 bars left and that correlated to 49.3 + hers was 50.4 volts each or about 65-75%of our battery. Now we know we can do at least 40 miles on a single charge approximately. I wish the RAD dashboard readout had percentage of battery left and not " bars" showing charge life. Perhaps when the warranty is up and it is time to upgrade there will be something available.
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