Double your battery life

[DickB]

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.

[se-riously]

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

Not questioning the value of your switch, but rather the pricing.  For $95-$100, one could get a brand new charger (of higher quality than the stock RAD charger) that does what your smart switch can do, plus:

1)  Allow the user to choose the charge rate to a perfectly safe, but still faster, 3 amps (stock charger is 2 amps) if they need the battery charged faster or an even slower 1 amp rate for even longer battery longevity, and
2)  Allow the user to have 2 chargers, using one as a backup.

https://lunacycle.com/luna-charger-48v-advanced-300w-ebike-charger/
https://www.bafangusadirect.com/Standard-48V-Bottle-Shark-Battery-Charger-p/42.htm?gclid=EAIaIQobChMIzNGnrpHG8AIVB__jBx18DAPMEAQYByABEgKI__D_BwE

As a side note, I've only seen my battery charge to 53.8V when the light turns green on the RAD stock charger, thus suggesting it only charges to 95%.  Has that been your experience?

[vudude]

I'd buy your SmartSwitch2 just so I could use my Rad charger efficiently. I have the Luna charger, so I would use your switch + Rad charger at a remote location.

[DickB]

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.

[DickB]

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.

[se-riously]

That sounds like a good plug and play solution.  Maybe use higher amp diodes in case someone chooses a faster 3-amp charger (but can still use the plug-and-play device assuming barrel connectors).  I haven't tried yet, but I'm thinking of opening up the charger and seeing if there's a potentiometer to reduce the float voltage.  Of course, this would eliminate the possibility of charging the battery to "full" (95% in RAD's case).

[DickB]

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%.

[se-riously]

Interesting.  My charger floats at 54.0V.

[DickB]

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.

[DickB]

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.

[se-riously]

Great write up showing the step by step.  The one you made with the voltmeter/ammeter is especially pretty.

[DickB]

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

[taskrov]

I made this box and it works as expected. I will add a port to measure the voltage so I can properly label the switch positions based on the voltage.

[DickB]

I found that 4 diodes gets closer to 80%. Also, the diodes got hotter than expected, so I upgraded to 6A diodes with heatsinks.
NTE5852 and NTE5253.

[peanutbutterpope]

I found that 4 diodes gets closer to 80%. Also, the diodes got hotter than expected, so I upgraded to 6A diodes with heatsinks.
NTE5852 and NTE5253.

Any chance you have an updated parts list? I recently bought a Radwagon 4 and extending the battery life sounds ideal considering the cost of these batteries. I wouldn't mind putting together a unit with the display and I probably have a good portion of the parts already.
TIA!