# Please check my solar outlay for Lithium ion battery

I'm in the processes of 'solarising' my electric bike, and would appreciate a more experienced opinion on how I intend to do it.

The motor is a 350W running off a 36 volt battery. I intend to get three 150W panels, each rated at 12V, and wire them in series to achieve 36V.

I've found an MPPT controller unit that charges Li ion batteries.

Would this be sufficient voltage to charge the battery with?

• Where are you located. ? – Russell McMahon Jun 15 at 13:45
• Also do you plan to use 3 panels because you think you need 36V or do you want to use 3 panels because you charge much faster? – Oldfart Jun 15 at 14:04
• A little bit more information on your battery would help. The majority of the capacity is charged with a constant current, which you need to get right for your battery (Though most of the time is spent in constant voltage, this only charges the last 3rd of the battery). Do you know what the cell configuration is? At least add the Ah rating and the current you normally charge it with. – hekete Jun 16 at 6:51
• I edited ,y answer to use the correct panel wattages. The answer is now correct technically. The 3 panels in series WILL give adequate voltage. – Russell McMahon Jun 16 at 22:45
• Many thanks everyone for you comments. The battery is 17.5 Ah - I'm unsure of the cell configuration. I will be cycling mainly along equatorial routes - this is a round the world bike trip. So I can deduce that my intended setup would work. 450 watts total from 3 panels would be sufficient charging power (considering the variables), and the MPPT charger will sort the voltage for the battery. Many thanks. – Alex Thomson Jun 17 at 10:40

Updated to use correct 150W panel sizes.

Given:
- 350w 36 volt motor.
- 36V battery
- Three 150w 12V panels, each rated at 12v, wired in series
- MPPT controller unit

Would this be sufficient voltage to charge the battery with?

The panels will charge your battery.
A more important issue is, how much charge will the panels deliver to the battery?

"12 Volt" panels are typically rated at ~= 17 to 18V Vmp. The Voc is higher again.

A nominal 36V LiIon battery has 10 x 3.6V mean voltage cells. in series (and perhaps multiple strings of these in paralllel.
Vmax per cell is ~= 4.2V or 42V/string.
The 50 Volt Vmp panels will provide more than enough voltage to charge the battery and the MPPT converter will optimise the voltage.

In full sun, optimally aligned and clean and not too hot the panels will deliver
Power = P = 3 x 150 Watts = 450 Watts.
Under typical arrangements you may get 2/3 of this into the battery - more with care, or say 2/3 x 450 = 300 Watts.
At a mean voltage of 36V that's about 8 A battery charging current. If using typical 18650 cells you'll need typically about 3 strings of say 3.3 Ah LiIon cells charging at 1C to handle that current.
Or a battery of at least 30 cells configured as 10S3P say. = 36V x 10 Ah = 360 Wh.
That's not a large battery by ebike standards and you will probably wish to add more parallel strings. I'll assume a "reference" battery of double that capacity - 10S6P, 60 cells, 36V 20 Ah, 720 Wh.

The above 10S6P 20 Ah battery charging at 20A would take usefully under an hour to charge to about 75% capacity and then several hours total to finish the last 25% due to the current tailoring off in the CV part of the usual CCCV charging cycle.

So you could charge one 10S6P battery above to ~~= 75% of capacity per SSH / Sunshine hour / daily equivalent hour of full sunshine.

Depending where you are and season you could get about 0 hours/day (Alaska in winter), about 2 hours/day (not quite snowing places in winter) and 5 or 6 or almost 7 hours per day in summer.

The site www.gaisma.com will give you SSH per day (shown as kWh/m^2) for most locations on earth.

Where are you located?
Here are [dozens of USA locations Gaisma knows about] (https://www.gaisma.com/en/dir/us-country.html). Each of thos paes lists dozens of sub locations. There will be one near enough you - IF you are in the US. Similarly for Europe.

Here is the page for Atlanta in Georgia USA as an example.
This image from that page shows SSH/day in kWh/m^2 for Atlanta, Georgia. The average daily average SSH by month is shown.
In December there is about 2 SSH/day and for 5 or 6 months over 5 SSH/day. With a 3.5 kWh battery (rather good) ~= 5 x the above 720 Wh example, you could charge it to 75% capacity for 5 or 6 months of the year and about 30% capacity (still useful) on an AVERAGE mid winters day. In winter a clean snow free (if you get it) panel optimally pointed would be wise.

• Each panel is 150 W, not 350 W. – Chupacabras Jun 16 at 17:39
• @Chupacabras Thanks. Edited. – Russell McMahon Jun 16 at 22:38

PV panels meanly recognized by the short-circuit current and open-circuit voltage. They have a characteristic curve like the one shows bellow in half sun and full sun (It is the simplified curve). Full sun means that the PV module absorbs enough energy from the light to produce its rated power. When some PV cells are connected in series with each other, the open circuit voltages of them added together, while the short-circuit current does not change. In order to estimate the behavior of the PV cell, the intersection point of the charactristic curve of PV cells and the battery should be found. If the battery is modeled as a constant voltage element, i.e. without considering its internal impedance. If the voltage of the battery is almost 35.5V when discharged completely, then your battery stars to charge until its voltage reaches 36V. Since the open-circuit voltage of a fully charged 36-volt battery is more than 36V, the battery is not charged completely. In this condition, you have two choices: 1) use other PV cells with more Voc voltage. 2) use a converter between PV cells. The first choice is better, in case of simplicity, and the second one is better if you want to implement an MPPT or a current controller for the charging process. Edit: Appreciating Russel for his answer, I had posted his answer while I was writing my answer.

Good luck

• 1. An MPPT controller is (usually) capable of boosting or bucking Vin so IF the output was in fact 36V then the required 42V + would (usually) be available. 2. However, an industry standard 12V panel is actually almost always rated at around 17 to 18 V Vmp - ie optimally loaded voltage. This is partially due to need to meet max lead acid battery voltages plus diode drop + wiring drop + . This is usually achieved with 36 Silicon PV cells in series. 3. A nominal 36V LiIon battery (10 series cells) has a full charge voltage of about 42V (4.2V/cell) in most cases. ... – Russell McMahon Jun 16 at 22:44
• ... A very few allow slightly higher and usefully longer cell life is achieved at 4.1V or even 4.0V/cell. – Russell McMahon Jun 16 at 22:44
• Thanks for your answers AM.H and Russell. I will put my faith and trust in an MPPT controller! – Alex Thomson Jun 17 at 10:44