RV Solar Calculations to recharge electric scooters

Re: RV Solar Calculations to recharge electric scooters

Hi Sovlentgreen,

soylentgreen wrote: »

Proof my calculations please? Be nice!

System:
Batteries: 2 X Trojan T145s in series (approx 220A x 6V *2) = 2640Wh

That is good--Note that is at a (typically) C/20 or 20 hour discharge rate. Or ~22 amp discharge (264 watt) rate of discharge. If you discharge faster, your battery bank will appear to have less capacity.

Solar: 3 x 85w solartech spm 85s (3*85 = 255W nameplate)

The "typical maximum" charging current during the middle of the day will be:

• 255 Watts * 1/14.5 volts charging * 0.77 panel+controller derating = 13.54 amps

The rate of charge for this battery bank:

• 13.54 amp / 220 Amp*Hour = 0.062 = 6.2% rate of charge

We typically recommend charging at a 5% to 13% rate of charge--So your current array is a bit on the "small" side--But will work fine (ideally, you recharge the T145 batteries during the day and recharge the scooters at night). If you want to recharge a lot/mostly during the day, you should have an array that is larger by the charging current for the scooters (i.e., 5% minimum charge + XX amps for scooter charging--To make sure the battery bank is correctly recharged).

Load:
Recharge 2 currie ezip 750 electric scooters once per day?
battery packs: 24v*12AH = 288Wh

Scenario:
Desert Southwest in August (PVWatts suggests 7.0 hours of insolation). Panels on tilted roof (south facing at just about latitude angle)

That 7 hours sounds a bit on the high side (except for a couple months a year). But using your numbers, assuming you use an AC inverter + AC battery charger (guesses at typical efficiency):

• 255 Watts * 0.77 panel+controller derate * 7 hours sun * 0.80 flooded cell eff * 0.85 inverter eff * 0.85 scooter charger eff = 794 Watt*Hours per day (summer?)
• 794 WH per day / 288 WH to battery pack = ~2.8 charges per day (charge T145 during day, recharge scooters at night)

Calculations
Power: 7.0 hours * 255 W = 1785Wh nameplate. Derate by 85% = about 1500Wh/day.
Load: 288Wh * 2 scooters = 576WH. Derate by 80% (???) for charging inefficiencies = 720Wh/day.

There are lots of deratings--And it it is not unusual to derate from solar array to 12 VAC output (through inverter) of 0.52 (52% efficiency). You can look at my above equation and change it based on your setup (i.e., if you charge direct from 12 VDC--no AC Inverter, and your scooter charger is more/less than 85% efficient, etc.).

And--if you charge during the day, you could save much of the 80% of T145 battery losses.

Answer 1: this could recharge each scooter as many as 2 times per day if no clouds, indefinitely.
Answer 2 : if willing to discharge the RV battery system to 50% this would give another 1000WH-approx, so could still manage about 1 (maybe 2) days of overcast?

Depending on your location/weather (much of Arizona has the summer Monsoon season). Typically we use 4 hours of sun for 9+ months of the year. Few places are >6 hours of "noon time equivalent" sun per day during the summer.

Solar panels only work well in full sun/no shade. If you have a week of cloudy weather, it is very possible to have 50% or less energy collection... Very dark/stormy days, you can be less than 5% of rated winter output.

So--either you have to assume that you will not recharge during extended bad weather--and/or use backup power sources during this time (grid, backup genset, etc.).

Worst case, no sun:
220 AH * 12 volts * 0.85 inverter eff * 0.85 charger efficiency * 1/ 288 WH per charge = 6.6 charges to drain T145 by 50%

Of course, you probably will get some sun, so you will recharge when the sun pokes back out. Note that Lead Acid batteries should be recharged back over >~75% state of charge after a couple of days. If you let lead acid batteries sit for weeks/months/etc. below ~75% state of charge, they will sulfate and lose capacity (and fail after a few months or year of such cycling).

Questions:
1. I'd guess the scooters stop working before exhausting their battery packs fully, so the 24*12 estimate is conservative.
Any ideas what a normal "full discharge" of an electric scooter is? (These use SLA 12V batteries)

2. Is my 85% derating for the solar system realistic (this is a PWM charge controller, not a MPPT)

3. My 80% estimate at charge efficiency is a SWAG.

4. Ambient temperatures may be in the 90Fs or 100Fs - does this monkey-wrench anything?

Your best bet--Get a Kill-a-Watt type meter (if doing AC charging) and/or a DC Watt*Hour/Amp*Hour meter for DC charging (and measuring battery charging numbers directly). If you are going to be doing a lot of DC off grid/battery charging--I would also suggest an inexpensive DC Current Clamp / DMM (much safer and faster than measuring current with a typical DMM where you have to "break the wire" to insert the meter).

I would use ~0.82 derating for solar panels, and 0.95 derating for charge controller.

PWM vs MPPT, for a first approximation with properly selected solar panels (Vmp~17.5 to 18.6 volts or so), there is not a huge difference in "efficiency". The operational details of the controllers are quite different, if you want to discuss them more.

Regarding high temperatures--A couple of issues. Vmp for the panels "falls" as temperature rises... Can drop as much as 20% in very hot/sunny weather. In some cases, the "low" Vmp may not be high enough to full charge/equalize a battery bank. If you are operating in very hot temperatures--Using a higher voltage Vmp-array with a MPPT type charge controller can take care of the low Vmp / battery charging voltage issue (MPPT controllers take high voltage/low current and efficiently down-convert to low voltage/high current for the battery bank).

The other issue is batteries are usually rated (life time, voltages, etc.) at ~77F. Hot batteries will "age" faster--Approximately 1/2 life for every 18F over 77F). Also, there are remote battery temperature sensor options for many charge controllers--Reduces the charging voltage set point for "hot" batteries. For fast charging, a RBTS can be a good safety device (prevent thermal run away).

I will stop typing here--Don't want to go too deep on the first reply.

-Bill