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.