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Solar recumbent (tandem) trike


10 µW
Jan 8, 2016
This is going to be my first attempt at an ebike build. It's an ambitious project, but I've got enough of a background in both bikes and basic electronics that I'm hopeful! I've spent the past couple days reading everything I can find here on ES and elsewhere. Unfortunately, I haven't seen many detailed specs from Sun Trip or other solar ebike riders about what solar and e-assist gear they're using. I know it varies pretty widely, but it would be nice to hear about peoples' experiences with different rigs...

Anyway, here's the bike (ok, trike(s)) I'm planning to electrify, loaded up for a recent 4-day trip:

The Bike: For now, it's this homebuilt recumbent trike, which will mostly be ridden with another delta trike (currently a Whike) linked behind it as above to make a tandem. With two riders, both trikes, all our touring gear, and the dog (not pictured), it's probably 400-500lbs. I'm planning to build a new tandem at some point soon that should shave off some weight, but might also build a little "camper trailer," so it's not going to get that much lighter.

The Goal: Bike across the US starting here in California in early 2022, visit my partner's family in Pennsylvania, and bike back. Maybe it'll coincide with the 2022 Sun Trip! We're not interested in going terribly fast, but we want some help maintaining speed as we haul all that weight over hills and mountains.

The Assist: I'm getting ready to order a Grin All-Axle hub motor kit. With 12mm adapters it should easily replace a rear wheel on the existing trike, and it's versatile enough to be used in any future builds. Torque-sensing bottom bracket because I really liked the feel of the torque-sensing ebikes I've test-ridden. For the cross-country trip, I may add a second motor depending on how things are going with the first one.

Not sure about yet: (please advise?!)
  • Controller - it seems like the Phaserunner is top-of-the-line, but I'm wondering if it's overkill for this application? How does it compare to other controllers? I don't really know how to evaluate controllers, and would appreciate suggestions/input.
  • Battery - My first thought is to just get Grin's LiGo packs (a few to start with, and more as we do longer trips with the assist). They seem like well-designed packs with an abundance of protection against failures due to variations in voltage/current/temperature/etc. On the other hand, I get the impression that LiFePO4 has a longer usable life and is a little easier on the environment than lithium ion, which I like. Lower energy density, though. PingBattery (for example, since I've seen them recommended here) seems to sell them for ~$1/Wh, which is comparable to the LiGo packs. Does anyone know if the Ping packs have comparable protection features?
  • Voltage - Extra-complicated on a solar rig, since (as solarEbike explains here) you have to keep the maximum panel voltage below the minimum battery voltage and keep the maximum panel current below the MPPT controller current limit, among other things. My first thought was: Use a really high-voltage battery, so you can push more power through the MPPT controller without hitting it's current limit or having to get more MPPT units (which are expensive if you go with the nice GenaSun ones; see below). But that probably means putting more panels in series to get the voltage up, and panels in series are more severely impacted by partial shade than panels in parallel. I haven't been able to find much info on what Sun Trippers or other solar ebike riders are choosing re: voltage, and why. Can anyone tell me more?
  • MPPT Charge Controller - GenaSun has quicker response times, but a maximum input current of 8A. That means for each 100W solar panel with ~20V/5A output, I need a $200 MPPT controller. That adds up when you're aiming for 500W of solar! Also, the GenaSun comes from the factory with a fixed output voltage which (I think? please correct me if this is wrong) would work with Lithium Ion batteries but not LiFePO4 cells, which have a different charging voltage. The alternative would be Grin's cheaper ($75) adjustable-output MPPT controller, which has a 15A maximum. It costs less, you can connect more panels to each MPPT unit, and the adjustable output means it would work with virtually any battery, instead of locking me into lithium ion packs of a particular voltage.
  • Solar Panels - I rather like these rigid panels that were mentioned in another build thread here on ES. They weigh about what a flexible panel does, but don't need much support framework. I contacted the manufacturer, and they wire their panels with "several zones, and bypass diodes" to mitigate partial shading. Much like the "Shadow Optimized" panels from Sunbeam Systems, which I was comparing them to. Both are in the $3-4/watt range. I know there are less expensive panels out there, but I get the impression they may not have the shadow optimization. "100W" panels with maximum output of a little over 20V and 6A would be safe for use with 36V batteries. Grin allows you to order a kit with their 170W panel and a 36V MPPT controller, but based on my understanding of solarEbike's explanation, I think it would be best suited for a 48V or higher battery... right?

Please forgive the wall of questions - I've been reading up on all this for the past few days and am still wrapping my head around all the options - and I know I've really only just scratched the surface.

Any thoughts or suggestions are much appreciated!


We're not interested in going terribly fast

What speed is not terribly fast? How many miles a day do you expect? Does your tandem setup rely on just one disk break?
Don't think you will want to ride back after getting there anytime soon.
ZeroEm said:
What speed is not terribly fast? How many miles a day do you expect? Does your tandem setup rely on just one disk break?
Don't think you will want to ride back after getting there anytime soon.

I've been looking at the motor simulator with some ballpark figures and it seems like 15mph going up a 1% grade is viable. That's showing a consumption of ~11 Wh/mile. With the 10 or so LiGo packs that I'd use in the final build, it would then have a 70ish mile range on a single charge. Based on Justin's review of 2018 Sun Trip data that's low - tandems averaged 14Wh/mile. Based on that figure, we'd expect a shorter range - more like 50 miles per charge.

I believe 2018 Sun Trip riders averaged 4Wh per 1W of solar panel, so with 500W of solar panels, we'd average 2000Wh per day from the solar (I think Justin said their 550W panels produced 1000-3000Wh depending on the weather). That give us 1-3 charges worth of range or 50-150 miles if the timing works out right, but realistically... I'd be very happy to get anything above 50 miles most days.

There's currently a disc brake on the front wheel of the front trike, and the Whike has disc brakes on all wheels but the front wheel is removed in tandem mode. That gives us 3 brakes in tandem mode, but only one under direct control of the captain. (For the other two, the captain has to shout "Brake!" and hope the stoker can hear over traffic. On long downhills we do a lot of "Half brake!" "3/4 brake!" - but we got some practice with that on our previous tandem, where the stoker had the drag brake lever.) Installing another disc brake or two on the rear wheels of the front trike is on my to-do list for the next few weeks, because this rig really needs more braking power! The hub motor will add regen braking as well, which should help reduce pad wear.


kg6gfq said:
For the other two, the captain has to shout "Brake!" and hope the stoker can hear over traffic.
Might want to add some brake lights 8)

Some people making 12 Volt LiFePO4 (4S) batteries just use standard 12 Volt Lead Acid chargers.
The biggest difference is the float charge. A SLA battery needs a float charge during storage. A LiFePO4 needs to be stored at less than full state of charge and topped up before use. As you will be actively using the batteries, the storage issues should not come into play until you get home.

With four Li-phosphate cells in series, each cell tops at 3.60V, which is the correct full-charge voltage. At this point, the charge should be disconnected but the topping charge continues while driving. Li-phosphate is tolerant to some overcharge; however, keeping the voltage at 14.40V for a prolonged time, as most vehicles do on a long road trip, could stress Li-phosphate. Time will tell how durable Li-Phosphate will be as a lead acid replacement with a regular vehicle charging system. Cold temperature also reduces performance of Li-ion and this could affect the cranking ability in extreme cases.
Progress has been made! The e-assist is installed and pretty well tested. Solar panels and charge controller have arrived, and will be installed as soon as I get some more MC4 connectors and finish rigging up a "roof" structure to mount them on - probably in a couple days.

The Grin All-Axle motor is working great. We managed to overheat it briefly on a 10%+ grade going out to the coast, so probably we'll need a second one for touring. That was a 5-hour round trip with a mix of flat and hilly terrain, about 40 miles, at an average speed of 8mph. We used 16.4Wh/mile, and got 24% regen. [Hauling a mattress to the recycler (no real hills) at 10.5mph brought us up to 19Wh/mi. Riding a flat bike path at 7.5mph took only 6Wh/mi.]

I got a PAS sensor and am still not sure if I want to upgrade to a torque sensor. There's definitely a learning curve, and I spend a lot of time adjusting the assist level. We ride with a speed limit of 10-14mph (usually 10 for multi-use paths and 14 for roads) and often with proportional regen enabled, so the speed limit is actually enforced. It's nice to have the automatic braking on long descents, but sometimes it's laggy - the I'd like some more buttons/controls for the CycleAnalyst, but the solar current sensor replaces the analog aux input, so that's probably not going to happen.

MPPT Charger: I found out that there is a GenaSun charger specifically for 36V (12S) LiFePO4 batteries - the GVB-8-Li-42.6V-WP, available directly from Sunforge. I wound up going with that charger, a 36V battery, and two of the 170W SunPower panels from Grin (which are a much better deal now than they were when I initially posted!).

The battery is a 36V/24Ah model from Sun-Cycle and seems pretty generic. If anyone can suggest tests I should run to determine the quality of the cells and/or BMS, I'd be happy to try anything non-destructive. So far it seems to work fine. It came with the large Anderson-type connector on the output so I made a converter to the small Anderson connectors that Grin Tech uses. The charging connector is the same kind used on computer monitors and power supplies, so I can scavenge one off an old computer cord for solar charging. It also came with a little USB charger that plugs into the charging connector, which is kind of cute/clever.

The roof structure is a bamboo frame, and I'll be wrapping the joints with casein/linen composite - a nice non-toxic, zero-VOC alternative to epoxy and fiberglass. I'll write up something about that on my website and/or do a video of it sometime soon.