Solar charger - pet based catamaran

Re: Solar charger - pet based catamaran

Welcome to the forum Asbar Marta.

Looks like a fun little boat there. You have not given us a lot of information--So I am going to make lots of guesses. You will have my sources and equations so you can adjust them assuming that I probably guessed wrong on many of the parameters.

Some assumptions:

• Around Budapest, Hungry
• Summer use with array mounted flat to roof
• Motor draws 40/20/5 amps on High/Medium/Low speed
• Battery is 86 AH @ 12 volt battery
• You want the battery to last for several years
• You want the battery charged quickly (try for daily use, but how much room on roof may limit power)

OK--First thing. Assuming this is a standard flooded cell deep cycle battery--The maximum continuous discharge rates should be:

• 86 AH * 1/20 Hour discharge rate = 4.3 amps "most efficient" discharge rate
• 86 AH * 1/8 Hour rate = 10.75 amps "maximum continuous" discharge rate
• 86 AH * 1/5 Hour rate = 17.2 amps "maximum short term" discharge rate
• 86 AH * 1/2.5 Hour rate = 34.4 amp "maximum surge" (minutes) discharge rate

So--Assuming my guesses on your motor's three speed current draw--The you really do no want to run the motor very often/hours on end at more than medium speed. And ideally at low to medium low speed (if variable speed motor).

Next--There is an ideal range to recharge lead acid batteries with using solar panels. Basically 5% to 13% of the Battery's AH rating. For example, 86AH*0.10=8.6 amp charging current.

If the boat is not used very often--You can get away with a 5% rate of charge. However, if the boat is used daily, then 10% or higher rate of charge is recommended. You have limitations on how much roof space you can dedicate to solar panels--But here is the recommendation (using standard design deratings):

• 86 AH * 14.5 volt charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 81 Watt panel minimum
• 86 AH * 14.5 volt charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 162 Watt panel nominal
• 86 AH * 14.5 volt charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 210 Watt array "cost effective" maximum

You can go upwards of 20-25% rate of charge with solar (or AC charger)--But that would be useful if you are moving every day and want to go as far as you can in sunny weather. At higher charging rates, you should have a charge controller with a remote battery temperature sensor to keep the battery from overheating with over ~13% rate of charge.

Ideally, you should only discharge the battery by ~50% maximum for long life--Although, for portable operation/seasonal/weekend use, and if you are willing to replace the battery every 1-2 years, you can discharge by 80% (good quality deep cycle battery--not recommend for "marine" type batteries--They are not designed for "deep cycle" operation).

Using the SolarElectricHandbook for Budapest, array mounted flat to roof (have to enter "facing south").

[h=3]Budapest
Average Solar Insolation figures[/h] Measured in kWh/m²/day onto a horizontal surface:

Jan
Feb
Mar
Apr
May
Jun


1.18

2.10

3.07

4.24

5.33

5.66



Jul
Aug
Sep
Oct
Nov
Dec


5.64

5.00

3.58

2.22


1.27

0.95



Lets pick 5 hours of average sun per day (May through August long term averages). Assuming battery 80% discharged and you charge during the sunny day and move at night/during cloudy weather (worst case):

• 86 AH * 14.5 volts charging * 1/0.61 off grid DC system eff * 0.80 max discharge * 5.0 hours of full sun per day = 327 Watt array minimum

So--Depending on your design goals, you can run from 81 watt to 327 watt solar panel/array.

Note that for a lead acid battery, this is actually may be an undersized battery bank if it is heavily used. As an example, for an off grid cabin that is lived in 9+ months of the year, we would recommend discharging by 25% per day for two days (2 days of "no sun" storage plus 50% maximum discharge for long battery life).

It really all depends on how much/how often you operate the DC motor. If you just run the boat on weekends and don't discharge too deeply--The smaller panel can work. If you want to go farther, use the solar panel to extend your range, run daily/longer trips (no backup AC generator), then you should look at the higher wattage arrays.

Note that batteries take time to recharge--And discharging below ~75% state of charge (use 25% of battery's energy), it will probably take several days to fully charge the battery even with a larger solar array--It can take >8 hours of full sun to recharge a deeply discharge battery bank--Solar just does not have "enough hours" in a day to fully recharge a deeply cycled battery bank in one day.

If you mostly run the motor on sunny days and want to solar panel to power the motor directly (more or less). Assuming 5/20 amp draw (low/medium speed):

• 5 amps * 14.5 volts * 1/0.77 panel+controller losses = 94 Watt "break even" solar panel during middle of sunny day
• 20 amps * 14.5 volts * 1/0.77 panel+controller losses = 377 Watt "break even" solar panel during middle of sunny day

Anyway--that is how I would approach the problem. You decide what answers will give you the performance you need for your usage (do you use the battery for lights/cell phone/laptop/etc. use?--Means more power to account for).

-Bill