Solar Charging system for electric bikes

drsteve744
drsteve744 Registered Users Posts: 5 ✭
in Off Grid Solar & Battery Systems #1
Hello. First time post here, glad to have found you. Totally new to solar systems, although I have done a bunch of reading . . .

I am presently converting 2 mountain bikes into front hub electric pedal assist bikes, with an off the shelf kit.

The idea is to set up a solar charging station to directly re-charge the on-board bike batteries after each use.

I am stumped on how to properly size a system - panels, charge controller, wire sizing and any other essentials. The goal is to maintain a useful near full charge, for a week or so of off grid camping and biking.


Details:

2 bikes, each having- 36V 800W brushless motors, with a motor controller that is rated @ 500W max. I will be maintaining to full charge (3) 12V 14ah SLA deep cycle batteries of the golf cart variety wired in series to 36V, for each bike. The bike's stated range at full throttle is ~15mi @ ~20 mph, or about 45 minute full throttle run time. Volt/amp-hours=504. (36*14)

The actual usage will be running at partial throttle for a few hours in the early morning or evening, coming back to camp to re-charge for 5 or 6 hours, then doing the same thing every day for a week or so. I don't expect to fully run the batteries down to Zero, but stuff happens! I would ideally want to charge both 36V battery packs together, but can manage any other reasonable combination to achieve 2 bikes ready to go the next day. I don't want to use a dc/ac inverter to run the 110 v charger unless absolutely necessary. lots of questions here, for example- is there an advantage to breaking the pack apart to charge the individual 12 volt cells and then re-assembling (or is there a switching unit avaiable to accomplish this? There will be sun -lots of it- here in the Southwest.

A big thanks in advance for getting me going on putting this charging station together.

Steven
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Solar Charging system for electric bikes

    If you have an AC charger, you can get a inexpensive Kill-a-Watt meter (or similar) and measure your "typical/worse case" AC charging requirements.

    Knowing your load requirements is your first goal in designing a cost effective solar PV charging system.

    Next, defining your needs... Obviously, solar panels only "work" in full sun (around 9a-3p is optimum time). For a "pure" Solar to charge controller to bike battery system, you would need to have the bikes in the charging station during (at least some part) of the day.

    So--Your first question--Would it be better for you to build a station with its own set of batteries (and possibly AC inverter) so that you can charge the station during the day, and charge the bikes at any time (day or night, good weather or bad)--Or is your usage such that you can charge the bikes during sunlight (and, will it be for 9-3 or 11-2, 1-4, etc.). Basically, you "lose" charging energy every time you have sunlight with no bike connected (hence, the suggestion for a local battery bank at the charging station to allow charging anytime).

    Also, most battery bank need some minimum amount of time for 80% charging (may be an hour or two, or upwards of 4-6 hours)--Again, a question of when the bikes will be charging vs time of day/available sun.

    Another issue--36 VDC is not a common voltage for off-grid solar power systems. More than likely, you will have to pick a more expensive MPPT charge controller that is programmable to recharge a 36 VDC battery bank (~$300-$600 or so). You could break the batteries back into 12 volt charging (plugs on each battery). And, it may even make sense (one set of batteries for the bike, a second set left in the charging station during the day--Quick battery swap, and away you go).

    Defining the solar array is pretty easy once you know your loads... Call the charging system 77% efficient (panels+controller) and the battery 80% efficient (AGM should be >90%, you pick your numbers) and assume summer in Arizona at >5 hours of noontime equivalent sun per day (this is, more or less, a worst case calculation based on my above assumptions--feel free to change any of them to better represent your needs/usage pattern).

    If the bikes are parked longer than 9a to 3pm assume all sun is being harvested and 100% discharged batteries (just using numbers--change to your needs):
    • 504 Watt*Hour battery bank * 1/0.77 PV eff * 1/0.80 battery eff * 1/5 hours of sun per day = 164 watts of solar panels

    If the bikes are only parked ~noon to 3/4 pm, assume only 1/2 the sun's energy is available:
    • 504 Watt*Hour battery bank * 1/0.77 PV eff * 1/0.80 battery eff * 1/2.5 hours of sun per day = 327 watts of solar panels

    Anyway--I will stop here for now... Questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Solar Charging system for electric bikes

    PS to Steven and any/all new readers/posters,

    By the way, welcome to the forum and I hope that we can help you with your project(s). We attempt to answer all of the questions we can and tend to not be "too chatty" (although, the answers may be long :blush::roll:). And we will never tell you to use the search function to find your own answers.

    We aim to keep the forum "signal to noise ratio" high so that it is easier to find the information folks may need.

    Sincerely,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #4
    Re: Solar Charging system for electric bikes

    Bill caught the highlights, I've looked into an electric scooter or converting my mountain bike for the 9 mile ride to work, so I'll give it a shot.

    First I take it this is a supported week long trip and not self supported, or your driving to a base then taking day trips?

    Most 36 volt systems run better at 48 volt, these SLA (sealed Lead Acid) batteries don't last very long when they are used for this application, as they are discharged at a higher rate than they are designed too. Most will handle the higher voltage, still not ideal but will help add longevity to the battery pack, aslo 48volt cheaper chargers are available. In truth your battery life would be best with a NiMH or Lithium iron battery pack, NOT cheap...

    If your riding during the day, it will be easiest to have 2 battery packs for each bike. If cost is a big factor, likely a small quiet generator would be the best solution. I'd suggest morning rides and evening walks if you want to do solar.

    Assuming 36 volt and 80% discharge, you have a 14 amp 36 volt battery bank (voltage adds and amperage stays the same for batteries in series) and you want to add @10 amps. You can charge at max c8 (or 1/8th of the battery capacity, normal for SLA batteries, but check your battery) and you'll have @ 6 hours to do this, though you can change the angle at camp before you head out for your evening hike to give you greater exposure. Also I think solar noon is usually around 1pm so 10am to 4pm I think are the prime charging hours.

    So you need 14/8=1.75 amps delivered to the batteries or with 75% system effiecency 2.33amps of solar at charging voltage for 36 volt battery bank, or 52+ volts. The Morning star 45 amp PWM charge controller will work for 36 volts. I'm not sure there is any other of the cheaper PWM type that will handle 36 volts, though there likely are, I stopped looking when I saw that the morningstar CC(charge controller) would work.

    I'm not sure how well the batteries and CC will work together in relation to over charging the batteries since you would want more than the maximum charge rate of array, More expensive MPPT charge controller will allow you to limit the charge rate to the batteries.

    That said you need 2.33 amps, or greater, an array at 52 volts or higher, sounds like 2 of the mid voltage panels, though that will make this a solar charging station for bikes and only 36 volt battery banks, as they will be very ineffiecent for anything else. These are some of the cheapest panel out there now.

    Since the batteries are being abused in this use, you could go ahead and parallel the 2 strings/banks of batteries for a need of 4.66amps. So 2 panels like these 230 amp SolarWorld(the German company that encouraged tarriffs on China made panels from a very small importer of solar, the US, to bring you higher solar costs here with no pressure there) These have a VMP of 29.8 Volts and an IMP of 7.7 Amps. If you hunt you might be able to save a few dollars, These 190 watt panels would work but only save you a few dollars. These are very large panels, and I hope you can drop by Flagstaff, AZ to pick them up, also that you have an SUV to transport them. You could go with 3 or even 6 smaller modules but they would be more expensive for the most part.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • drsteve744
    drsteve744 Registered Users Posts: 5 ✭
    #5
    Re: Solar Charging system for electric bikes

    Wow thanks for the quick and comprehensive answers. Need to get home from work tonight, read and absorb. Then, I'm sure I'll have some questions. Right off, what would an array to charge batteries to serve an inverter be like?
    Their 110 AC chargers are rated at 1.5 amp output, although they can be upgraded to 8 amp unit.

    And thanks again. Steven
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Solar Charging system for electric bikes

    A "simple", rugged, flexible, but not too expensive charging station with batteries and inverter:
    The recommended rate of charge for a battery bank is between 5% and 13% rate of charge, so solar panel wise:
    • 220 AH * 14.5 volts charging * 1/0.77 charging eff * 0.05 rate of charge = 207 watt array minimum
    • 220 AH * 14.5 volts charging * 1/0.77 charging eff * 0.10 rate of charge = 414 watt array "nominal"
    • 220 AH * 14.5 volts charging * 1/0.77 charging eff * 0.13 rate of charge = 539 watt array "max cost effective" (typically)

    Size of charge controller:
    • 207 watt / 14.5 volts = 14 amp
    • 539 watt / 14.5 volts = 37 amp

    So, roughly, the range of charge controller output should be around 14 to 37 amps (at 12 volts). Some controllers to think about (PWM are less expensive vs MPPT):
    • PWM controllers (MorningStar)
    • MPPT Controllers (MorningStar 15/45/60 amp; Rogue 30 Amp; Midnite Classic 60-90+ amps; Outback 60/80amp) (see above controller link for rest of products)

    How much power will a XXX watt solar power system generate... Really need to know (roughly) where the charging station will be, what season(s) it will be used, and if most of the charging will be at night or during daylight hours... Say you are near Flagstaff where there is significant monsoon season. From PV Watts with fixed array mounted tilted to latitude (Las Vegas would be better):
    Month Solar Radiation (kWh/m 2/day)
    1 5.19
    2 5.92
    3 6.27
    4 6.44
    5 6.56
    6 6.61
    7 5.95
    8 5.54
    9 6.59
    10 6.19
    11 5.47
    12 5.07
    Year 5.98

    Say 5.5 hours of sun per day, 0.52 end to end system efficiency (AC power, flooded cell battery bank):
    • 414 Watt (nominal) array * 5.5 hours of sun per day * 0.52 derating = 1,184 watt*hours ~10 months of the year

    If you are into Amp*Hours:
    1,184 Watt*Hours * 1/12.5 volt battery bank = 95 Amp*Hours per day (roughly)

    If are into monitoring--A Battery Monitor is great (Victron is another good one). Especially if you have highly variable loads. You want to keep a battery from cycling below 50% state of charge often, below 20% state of charge ever, and get it recharged back above 90% at least once or twice a week (do not let a partially charged battery sit below ~75% state of charge for days/weeks/months--It will help accelerate sulfation of the battery bank).

    Questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: Solar Charging system for electric bikes

    Note, the above system is designed around a 300 watt inverter (and ~C/8 discharge rate) for the 220 AH battery bank:
    • 12 volts * 220 AH * 1/8 rate of discharge * 0.85 inverter efficiency = 281 watts of 115 VAC @ 60 Hz

    So, using the suggested Kill-a-Watt meter to measure your AC battery charger(s), that would tell you the average wattage your charge controller would draw (of course, it is watts * hours (time) for over all work performed... A "slow charger over night" vs a "fast 2 hour charger" should use about the same amount of energy from the battery bank to charge the same battery bank. Note that fast chargers may be less efficient and some batteries do not like "fast" chargers (won't last as long).

    A rough guess about your charge controllers (roughly, worst case design/SWAG--Scientific Wild A$$ Guess):
    • 1.5 amps * 43.5 VDC charging (36 volt bank) * 1/0.80 charger efficiency = 82 watts from inverter
    • 1.5 amps * 43.5 VDC charging (36 volt bank) * 1/0.80 charger efficiency * 1/0.66 PF = 124 VA from inverter
    • 8 amps * 43.5 VDC charging (36 volt bank) * 1/0.80 charger efficiency = 435 watts from inverter
    • 8 amps * 43.5 VDC charging (36 volt bank) * 1/0.80 charger efficiency * 1/0.66 PF = 659 VA from inverter

    So, you can see that if you use the "fast" charger, it is possible that it would exceed the recommended Battery Bank/Inverter continuous output power... A larger battery bank and higher wattage Inverter would be suggested (especially if you charge two packs at once--Which would put you into another class of battery bank/inverter).

    Note that your load requirements can have a dramatic effect on your system design (and costs). Remember that if somebody "does something wrong" and runs the battery bank down below ~20% state of charge, you may have to buy another battery bank.

    So, if you can get away with the slow charger, you will have a less costly system (and if using SLA batteries in the bikes, their battery packs will probably last longer too).

    You should read about batteries from here:

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    And about inverters here:

    All About Inverters
    Choosing an inverter for water pumping

    Regarding adding an inverter to the system--Basically it costs you a bit more power (~85% efficient inverter needs 1/0.85=1.18 times more solar panels/battery bank). And some other complexity (standby losses). However, many times it is worth it (especially if you have to put the solar panels in the sun, but the bike charging is a 100' away in the shade/shed/shop).

    Any way, you need to gather data and do a bit more reading.

    Cheers,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #8
    Re: Solar Charging system for electric bikes
    drsteve744 wrote: »
    I am stumped on how to properly size a system - panels, charge controller, wire sizing and any other essentials. The goal is to maintain a useful near full charge, for a week or so of off grid camping and biking.

    I've done something similar. For portability I use a single 12 volt 30W panel with a boost controller to boost the voltage to the 42 volts that the system needs to recharge. (I use a lithium polymer 36 volt 21 amp hour battery from Batteryspace.) Note that this charges pretty slowly; we're talking days to get a full charge, but I am usually near an outlet so it functions more as a battery maintainer when the bike is outside.

    For a larger system I'd get 3x30W panels and either use a bang-bang (PWM) controller to regulate voltage or a simple buck converter. I don't know of any off the shelf MPPT controllers that have that much voltage setpoint flexibility.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #9
    Re: Solar Charging system for electric bikes

    Sounds like a trip to Black Rock City is happening. Be sure to allow enough water, to wash the panels daily too. I second the motion to have 2 sets of batteries for the bikes, one charging, one in use. And try it out before you depart on the trip, no spares at BRC.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • drsteve744
    drsteve744 Registered Users Posts: 5 ✭
    #10
    Re: Solar Charging system for electric bikes
    bill von novak wrote: »
    a bang-bang (PWM) controller to regulate voltage or a simple buck converter.

    Can you translate what these 2 devices are for me?
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  • drsteve744
    drsteve744 Registered Users Posts: 5 ✭
    #11
    Re: Solar Charging system for electric bikes
    mike90045 wrote: »
    Sounds like a trip to Black Rock City is happening. Be sure to allow enough water, to wash the panels daily too. I second the motion to have 2 sets of batteries for the bikes, one charging, one in use. And try it out before you depart on the trip, no spares at BRC.

    Yup, it's happening . . . 2 1/2 months to put it all together
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  • drsteve744
    drsteve744 Registered Users Posts: 5 ✭
    #12
    Re: Solar Charging system for electric bikes
    BB. wrote: »
    A "simple", rugged, flexible, but not too expensive charging station with batteries and inverter:
    The recommended rate of charge for a battery bank is between 5% and 13% rate of charge, so solar panel wise:
    • 220 AH * 14.5 volts charging * 1/0.77 charging eff * 0.05 rate of charge = 207 watt array minimum
    • 220 AH * 14.5 volts charging * 1/0.77 charging eff * 0.10 rate of charge = 414 watt array "nominal"
    • 220 AH * 14.5 volts charging * 1/0.77 charging eff * 0.13 rate of charge = 539 watt array "max cost effective" (typically)

    Size of charge controller:
    • 207 watt / 14.5 volts = 14 amp
    • 539 watt / 14.5 volts = 37 amp

    So, roughly, the range of charge controller output should be around 14 to 37 amps (at 12 volts). Some controllers to think about (PWM are less expensive vs MPPT):


    -Bill

    Thanks! the calculations are starting to make sense to me. I need to price out my components for both types of systems, carefully evaluate my usage for Black Rock City and beyond, then press the start button.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #13
    Re: Solar Charging system for electric bikes

    A bit about Solar Charge Controllers:

    All About Charge Controllers
    Read this page about power tracking controllers

    Bang-Bang controllers is an engineering term... Basically instead of a smooth ramp up and smooth ramp down, proportional output, etc., a Bang-Bang controller is just "on/off"...

    Like getting into your car and full gas until 60 mph, then zero gas to 50 mph and full gas 'till 60 mph--etc... (note: there is a lot of engineering and math behind types of control and negative feed back loops--This just one small aspect of bang/bang and overall PID--Proportional/Integral/Derivative controller theory/design/implementation).

    PWM controllers are similar to "Bang Bang" controllers... Pulse Width Modulation... Basically, if you want 0% output from the controller, the "switch" is off. If you want 100% output, then the switch is "on". If you want 50% output, the switch is on for 1 second and off for 1 second (some controllers are "slow", others will run a "cycle" at 100's-1,000's of Hz (cycles per second).

    PWM controllers are less expensive, reliable, usually fairly simple (not a lot of bells and whistles).
    An MPPT charge controller is much more sophisticated... They typically use "buck mode" power supplies internally (switch/inductor/diode and usually some sort of digital control to limit output voltage and current as required).

    One of the big advantages is they are like a "DC Adjustable Transformer" in operation. You can put 12 amps at 100 volt input (I*V=P=1,200 watts), and get 100 amps at 12 volts output (sometimes called "down converting"). Just like a transformer.

    Two places where MPPT works well for solar power systems... One, if you have very long wire runs (over 10-20 feet or so) from the solar array to the Charge Controller/Battery Bank/Shed--You can run your solar panels in series for Vmp-array at upwards of 100 VDC (depends on controller and location) and use much smaller wires to carry the same amount of power.

    The second reason--Newer solar panels (typically >100watts) don't come in "12/24 volt flavors" anymore... A "12 volt" panels should have Vmp~17.5 to 18.5 volts or so to charge a 12 volt battery bank with a solar charge controller. If you have a 35 volt array + PWM controller charging a 12 volt battery bank, you will lose over 50% of the panels output power (usable wattage).

    But, because the >100 volt panels are so "cheap" this days (because of solar grid tie system that don't "care" about Vmp vs battery bank voltage), you can buy a 200+ watt panel with Vmp~30 volts or so, and use a MPPT charge controller to efficiently (upwards of 95% efficiency) and down convert to higher current/lower voltage to recharge your battery bank.

    In the end, small/simple/low cost systems tend towards PWM controllers (call it less than 400 watts of solar panels). And larger/complex systems tend towards 800 watts and above.

    However, there are some very nice, lower cost, MPPT charge controllers (small MorningStar and Rogue) that also work very well for their class of system (typically, more programming/logging options).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #14
    Re: Solar Charging system for electric bikes

    By the way, I should add that MPPT controller, when they start cutting back on current flow (as the battery nears full charge in Absorb stage), they reduce output current and begin to operate in a PWM type mode... MPPT is designed to get as much energy from the solar array as possible when demanded by the battery bank/loads attached (i.e., inverter, DC loads, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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