Charging Batteries with Solar Panels

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Charging Batteries with Solar Panels

When charging your batteries with solar panels you need time for the electrical supply to be held in the battery. The amount of charge current available depends on how long the sun shines and its intensity (irradiance) on the solar panels. The current from the panels will vary with location and time. The available irradiance depends on the latitude of the location, the season of the year, the angle of the solar panels relative to the path of the sun adjust the sun intensity. The direct sun on the solar panels may vary from 5 hours up to 10 hours in a day.

The amount of charge needed for the batteries will depend on how you use the batteries. The following takes no account of current drawn from the batteries during the day. You can charge them in the day and use them at night (like security lights). The operation for charging is different to run a household during the day and then use lights at night. There will be less power in excess to charge the batteries. Days are longer in summer than winter and the sun is higher in the sky. Design the charging panels for the shorter days and lower sun. If they can do the job in that environment they can be more than enough for the rest of the year.

The higher capacity solar panels will charge the bank of batteries more quickly. We measure the battery capacity in Amp Hours. Let’s assume a 200Ah battery at 12 Volts. In this case, we will set the number of batteries at 24. We need to find the number of solar panels to charge those batteries. The variable is charge time, which we can fix at say 5 hours.

DOD for batteries

When charging batteries with solar panels we need to top up the battery from the DoD to full charge. The depth of discharge (DoD) for a battery is the percentage of the available battery capacity that can be used up. While the DoD of 80% might be possible for a lithium battery, it’s not going to work for a lead-acid battery. The higher the battery DoD, the more stored energy capacity you can consistently get from the battery without doing long-term damage. This means at 50% DoD you need to have almost double the number of lead-acid batteries as you would for lithium batteries.

Completely discharging Lead Acid batteries will reduce their usable life. This is due to the DoD limitation, which for a lead-acid battery bank is 50%. DoD limitations mean that for lead-acid batteries there is only 100Ah of useable energy per battery. Let’s say we have a battery bank of twenty-four (24) x12 volts of lead-acid batteries where each is 200Ah. That would mean 4800 Amp-hours of storage at 12 volts connecting all batteries in parallel.

Calculation for Charging Batteries with Solar Panels

If the solar panels are 24 volts, we might connect pairs of 12-volt batteries together in series which gives (2 x 12v) 24 volts. We can have strings of 24 volts batteries (12 volts x 2 in series) and together in parallel. The result of the combination is 12 ‘batteries’ x 200Ah at 24 volts, so they need the same amount of charge.

A battery rated at say 200 Amp-hours at 24 volts should deliver 10 amps of power at 24v for 20 hours. It could also give 20 amps of power at 24v for 10 hours (remembering the DoD).

In the calculating times of charging batteries with solar panels, we need some details and make some assumptions. We set the battery voltage at 12 volts x 2 batteries = 24 volts, so 12 Units of 24-volt batteries can be connected in many combinations. Let’s assume the following information for the solar panel used. A common brand solar panel rated at 250 Watts runs at about 8 to 9 amps at between 25V – 30V. Note that solar panel voltages are in the item description and information on the manufacturer’s website or on the unit itself.

The Solar Panel

We measure Solar Panels power generation in Watts, and here we assume panels of 250 Watts. To calculate the amps it can supply to the battery, divide the Watts by the Voltage of the Solar Panel. We take these 250 watts (for this particular solar panel) and divide it by the voltage of the panel, which in this case is 24 Volts. That calculation gives 10.4Amps which is close to the actual 8 or 9 amperes that the common brand 250 Wattspanels indicate.

In series within a string, you add the voltages (while amps stay the same). However, for panels in parallel (or in separate strings), you add their amperage (and voltage stays the same). Using multiple strings there will be an addition of voltage and amperage, depending on the set-up.

All Solar Panels 30 watts and above need a Solar Charge Controller or Regulator. A charge controller is necessary to protect the batteries from overcharging. It supplies the correct flow of current to the battery to prevent any damage to the battery from charging too quickly.

How many Solar Panels?


We intend to charge batteries with solar panels using an array of 250 watts panels for 5 hours. We say 5 hours to cater for those shorter winter days.

Let’s consider that 12 batteries in parallel x 200Ah will hold2400 Amp-hours at 24 volts. We want to charge the batteries for say 5 hours (to 50% DoD) and 8 amperes. Remember that you can access the charge to DoD for the particular type of battery (50% from lead-acid batteries).

12 batteries in 2 strings of 6 x 24v series x 200Ah hold 2400Amp hours per string. That means there are 6 x 24v =144 volts and 2 x 8 =16Amps for achieving full charge in 5 hours.

Let us consider that the suggested battery voltage is 144 Volts.

Each string of solar panels must deliver in the 144 Volt range (no partial shading).

Each panel works at about 8A and between 25 V to 30 Volts. So, 6modules in a series would be 150V. For the current requirement, 2 strings of panels connected in parallel would achieve 16 Amps.

The maximum charge rate will be for 5 hours.

(2400Ah x144v) Batteries / (5h x 250watts x 30v) solar panels

345,600 / 37,500 = 9.2 panels per string for 2 strings.

18 Modules (2 x 9) could work. If charging batteries with solar panels the charge period is longer than 5 hours fewer panels are needed.

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