The literature that I had studied described a delta between float charge and charged battery voltage, but I'm not seeing that much.  I just completed a charge with 4 diodes and ended up with a open-circuit voltage (OCV) of 51.7 V.  That's exactly 80% per my recent test:
https://www.radowners.com/index.php?topic=1016.msg5122#msg5122

In my quest for more information on the Rad battery and charging, I ran an experiment to plot the battery voltage discharge profile and note the accuracy of the Rad battery meters.

For the test, I ran full throttle only on a straight, quite level road (Florida!), stopping every 2 miles to check battery voltage and meter readings.  The meters were read per Rad instructions, with no pedal assist and no throttle, about 60 seconds after stopping.

My test shows that the Rad meters are a bit optimistic.  The LCD meter reads Full at 70% charge. One would expect to see 4 bars. At 50% capacity, the meter shows 4 bars, which you might interpret as 80% remaining.

Found these:
https://www.ebay.com/i/264470802633

Looks like the same, but I have not ordered them.

I finished my geek version. The charger has just come off constant current and entering constant voltage mode, with the current just dropping.

The lower jack on the meter box has the two diodes in series.  The upper jack is a direct connection for 100% charge.

I've also attached a paper showing how to build a simple adapter and more detail on the theory behind it.

Probably just meter accuracy. My meter is specified as +_1.2% accuracy, so that's +- 0.5V.  Probably just a variation in meters.  I have two DVMs that do read within 0.1V on the Rad battery, but I have one that I got for free at Harbor Freight and it's different by almost 1 V.

Next time I order parts I'm going to get a Maxxim MAX6350 voltage reference IC and check it out against my DVMs.

If you find a potentiometer inside and replace it with one mounted on the case, you could still dial up full charge.

BTW this morning I measured a float voltage of 54.3 V on a full charge (no diodes), so that's darn close to 100%.

Although my smart switch works, I have a much simpler solution to partial charging with the Rad charger.

Standard silicone diodes have a forward voltage drop of about 0.8 – 1.0 volts at 1-5 amps. Simply placing two diodes in series in the circuit between the Rad charger and the Rad battery will drop the float voltage to provide less than a full charge. I've tested one cycle with two diodes and achieved an 80% charge level. Other than the reduced float voltage, the charger behaves just like before, charging at a constant 2.03 amps, then tapering off.  The Charge Indicator starts red and then turns green at charge completion as usual.

I've verified the charge level both by measuring rest voltage of the battery after charging and using the Rad Battery Charge Indicator.

The float voltage after the diodes was 52.7 volts, and the rest voltage after my dual diode charge was 52.0 volts.  This correlates to the following table from Benzo Energy, a Li-Ion battery manufacturer (I added the Rad battery column):

  Voltage
Rad    Cell    Charge
54.6   4.20   100.00%
52.8   4.06   90.00%
51.7   3.98   80.00%
51.0   3.92   70.00%
50.3   3.87   60.00%
49.7   3.82   50.00%
49.3   3.79   40.00%
49.0   3.77   30.00%
48.6   3.74   20.00%
47.8   3.68   10.00%
44.9   3.45   5.00%
39.0   3.00   0.00%

On the bike and using the Rad Battery Charge Indicator, when first turned on the battery at rest indicated 100% charge.  However, after a minute or two of use, the Charge Indicator dropped to 80%, indicating true capacity. This is typical operation of the Rad Battery Charge Indicator, as Rad users have probably noticed.

I've been using bare parts and clip leads for this test, but I'm waiting on parts to build a simple cable with male and female barrel power connectors and two diodes to plug in between battery and charger to charge to 80%. Since I'm a geek, I'll add a volt-amp meter, because I like to monitor what's going on.

If you go this route, pick standard (not Shottky) diodes of sufficient current and voltage rating. I'm using 1N5402 diodes rated at 3 amps and 200 V (available of course at Amazon). You can use a 2.1mm x 5.5mm DC Plug Power Adapter Extension Cable of sufficient current rating (18 or 20 gauge) to build. Just cut the cable in half (shortening it is OK), solder the two diodes into the positive lead with the diode bands facing the battery, and insulate with shrink wrap. Less than $15 in parts.

Thanks for your comments.

I'm working on an alternate design that would be about half the cost.

Yes, Rad is using a float voltage of about 54 volts. It suggests that Rad has made a trade-off between longevity and duration by intentionally not charging to maximum cell capacity. Even so, there is still significant battery life extension by charging to only 80% of what Rad charges. (Setting the Luna charger to 80% is actually going to charge to about 90% of what Rad does.)  BTW charging to less than maximum cell capacity as a battery life tradeoff is not an uncommon practice by manufacturers.

There are several reasons why a Rad owner might not want to use an aftermarket charger. For one, Rad recommends against it. Many owners will take that to heart. Why would Rad make this recommendation, beyond wanting you to buy only their charger? A non-knowledgeable user might use an aftermarket charger in ways that are detrimental to the Rad battery, such as charging at a 5 amp rate (which is likely higher than the cell manufacturer's recommendation). If you use an aftermarket charger to charge to 100% of cell capacity, you will be reducing battery life versus using the Rad charger. Some users might not know this. And while a small thing for many, others may be unable to change connectors on an aftermarket charger. Many users prefer plug and play.

Li-Ion battery life can be extended significantly by not charging the batteries to their full capacity. For example, charging the battery to 100% capacity, one can expect 300-500 discharge cycles. Charging to 80-85% capacity , one can expect 850-1500 discharge cycles. (Source: batteryuniveristy.com.)

If you are not using the full capacity of your Rad battery when riding, you can take advantage of this characteristic and extend the life of your Rad battery. I typically ride 10-12 miles per day, with minimum level (1-2) assist, and my battery capacity drops only about 20% on the battery meter. On trips into town, I ride about 15 miles at higher assist (3-4), and then use about 40% capacity. Clearly, I could charge to only 80% capacity and have ample reserve capacity at the end of my rides.

The Rad battery charger will on its own charge to 100% capacity.  You can charge to a lesser capacity with it by simply disconnecting the charger before it has completed a normal charge cycle - that is, before the CHARGE LED turns from red to green. But how do you know when to do this to achieve an 80% charge?

I have developed a simple smart switch to do this.  It plugs in between the Rad charger and Rad battery, and monitors the charge. When the battery has been charged to 80%, the smart switch disconnects the charger from the battery. (The CHARGE LED will turn green, just as it does if you unplugged the charger from the battery.)

I have been testing my smart switch for several days now, and so far it is working as expected. I will continue testing to fully validate the design.

I would like to emphasize that this smart switch does not alter the Rad charger or charge characteristics at all. It simply disconnects the charger as if you had unplugged it. Rad even recommends charging to only 75% when storing the battery for more that two weeks (although they offer no way to determine when this level of charge has been achieved!), so it is a perfectly legitimate thing to do.

I am an electrical engineer. I have developed and marketed several add-on electronic devices for automotive and other uses. I use a professional circuit board fabrication company and quality components from a major electronics distributor. If there is interest, I will market my smart switch for Rad users. The cost of the smart switch will be about $50. Considering the cost of a replacement Rad battery, I think it is a wise investment.

I would appreciate any feedback on interest in such a smart switch. I welcome questions.

A litttle arithmatic: (28-26)/26 = 7%!

I like the mounting bracket but found that even a short Polar bottle hit the handlebars. A simple mod fixed that, and put the bottles at a handier angle.

The OP didn't ask any questions, but asked for constructive comments and experiences. I'm offering mine. As well as a very practical solution.

Not everyone cares to buy and use an aftermarket charger. Rad warns against this. I suspect this may be because a user not knowledgeable could run into problems, such as charging at too high a rate and blowing a fuse. I'm not saying that there is anything wrong with using an aftermarket charger; I'm just relaying what Rad advises.  Many people will want to heed this advice.

My goal is to develop a simple-to-use add-on device to stop charging at less than 100%.  My device will have a power jack into which the charger will plug and a plug identical to the Rad plug to plug into the battery pack. It will also have a rotary knob to select charge percentage from 100% down to 80% or so.  To the Rad charger, the device will look like nothing more than a switch. There will be no alteration of the Rad charger nor any change in its operation.  It will be equivalent to simply unplugging the Rad charger at less than 100% charge.

To monitor actual battery voltage, one would need to plug into the power plug at the base of the Rad pack.  (The charge voltage at the Rad charger output is not the same as the battery voltage.) That adds unnecessary complication. It is perfectly legitimate to monitor charge current to determine battery charge capacity. In fact, it what Samsung specifies. Note that Samsung specifies cutoff at 3 hours or .02C, not at a particular voltage. Samsung specifies a CC-CV (constant voltage with limited current) charge, and that is exactly what the Rad charger does. Samsung further specifies that a "Standard Charge" means charging the cell with charge current of 1,700mA and constant voltage 4.2V at 23°C with a 0.02C(68mA) cut-off. For four cells in parallel and 13 cells in series this is 6.8A (as you stated), 54.6 volts, and 272 mA. For a non-standard charge to 80%, simply cut off the charge at greater than 272 mA. That is what I am doing.

I am an electrical engineer.  I have built and marketed several specialty electronic add-on devices for automotive use. I will likely market my cutoff device.  Just like the average Rad user need know nothing about C, CC-CV, current and voltage, the user of this device will also need not bother with all that. Just plug in the device and set the desired charge capacity. Simple and practical.

Rad instructs users to plug the charger into the battery charge port first, then plug the charger into a power outlet. This is correct. Any spark at the AC power plug will not be transferred to the battery. You'll notice that the Rad charger power and charge LEDs do not illuminate until a second or so after the AC power is connected, implying that the charger electronics are stabilized before any power is routed to the battery.  This is common practice in many consumer electronic devices such as audio amplifiers.

BTW I have measured no difference in charger output voltage or current regardless of the position of the key switch.

I agree that the battery pack or controller likely has a low voltage cutoff to protect the batteries.

The low "trickle" current that I measured after several hours may be just powering the battery management system electronics in the pack, with no current actually going to the cells.  Hard to tell without taking the battery pack apart. (Which I may do someday. I'm that way - I like to see how things work.)

This article is informative:

https://www.powerelectronics.com/markets/mobile/article/21859861/proper-care-extends-liion-battery-life

It describes two methods for terminating Li-Ion charge.  I suspect that Rad is using something akin to the second:

"The second method is similar, but it requires monitoring the charge current. As the battery charges, the voltage rises, exactly as in the first method. When it reaches the programmed voltage limit, which is also called the float voltage, the charge current begins to drop. When it first begins to drop, the battery is about 50% to 60% charged. The float voltage continues to be applied until the charge current drops to a sufficiently low level (C/10 to C/20), at which time the battery is approximately 92% to 99% charged and the charge cycle ends."

I base this on my measurements out of the Rad charger on an early charge cycle:

Hours     V       A
     0    48.0  2.03
  1.5    49.0  1.77
  2.5    50.5  1.42
  5.0    52     0.80
12.0    53.8  0.20

Note that the voltage and current seen at the charger output/battery pack input is not necessarily what the batteries are seeing, as the pack may have intervening electronics. However, it's reasonable to assume that the bulk of the current is going to the batteries themselves. Monitoring the charger output current should give a good indication as to how fully charged the batteries become.

"A C/10 termination level will only bring the battery up to about 92% capacity, but there will be an increase in cycle life. A C/5 termination level can double the cycle life although the battery charge capacity drops even further to approximately 85%. A number of charger ICs provide either a C/10 (10% current threshold) or C/x (adjustable current threshold) charge termination mode."

While it is safe to charge Li-Ion cells at up to the C rate, the Rad charger charges at considerably less than the C rate. Perhaps that is to prevent overheating the batteries.  (The Luna charger also charges at less than the C rate, although higher at up to 5 A.) The C/10 rate for Rad is 1.4 A. C/5 is 2.8 A, actually higher than the Rad charger provides, so it appears that there's not a practical way to limit charge to 80 or 85%, as that level may be reached while the charger is still charging at 2 A.  I propose cutting off the charge when the current reaches C/10. That will extend battery life while still providing a decent charge level.

I will actually make the cut-off current adjustable, so that it could be shut off at say 1.7 A.  I will be experimenting with cutoff current versus battery full percentage to see if I can reliably charge to 80%, but of course it will take me some time to go through multiple charge/discharge cycles.  I do ride every day however.

The stripes that I used have the narrow and wider stripe on a slightly wider clear tape that maintains separation between the stripes. The Rad Rover fenders have a crease at the edges. Pretty easy to align the clear down along the crease.  Then you pull the clear off.

Deep discharge does also reduce battery life. batteryuniversity.com cautions against it.

Battery charge limits can be set by monitoring either voltage or current. I've monitored both while charging to get an idea of how the Rad charger operates. Of minor concern to me is that the charger continues to supply current at a low level even after the charge light turns green.  Voltage peaks at 53.8V. Current drops to only 20 mA but doesn't shut off after several hours - this may be in order to achieve the balance charge. If not balance charging, Rad recommends disconnecting the charger as soon as possible after the green light, but that is not always convenient.  I have asked Rad via email why they don't stop charging altogether; I have not yet gotten an answer.

I'm going to build a simple circuit to plug in between charger and battery to monitor current and automatically disconnect at a desired current, and give that a try.