Can solar panels overcharge batteries?

Find out whether solar panels can overcharge a battery in your home. Is it dangerous? Should you be worried?

Solar panels can overcharge a battery, but this generally doesn’t happen so long as we understand them and follow manufacturer guidelines. This article gives insight into the damages caused to the batteries due to overcharging and how this can be prevented.

What is a battery?

A battery is a device which allows rapid conversion between chemical energy and electrical energy by means of electrochemical oxidation-reduction reaction between the active materials that are packed in its cell chamber, separated by an ion conducting electrolyte. Batteries can be classified based on whether the stored energy can be recharged or not. A primary battery does not have the capability of being recharged. 

The energy conversion in a primary battery is irreversible, and it is normally discarded once the energy contained in its active reactant is exhausted. A secondary (rechargeable) battery, on the contrary, can be electrically recharged after discharge by supplying current in an opposite direction, restoring the battery to its original status through a reverse electrochemical oxidation-reduction reaction on the two electrodes.

Some of the common battery technologies available in the market are lead acid batteries, lithium ion battery and flow batteries. The common battery parameters considered for selection of battery technology are depth of discharge, C-rate, cycle life, charging/ discharging efficiency, capex and opex costs.

What are the common battery technologies used in residential applications?

The two main types of residential batteries are Lead-Acid and Lithium-Ion batteries.

Lead-Acid batteries

Lead-acid batteries are the world’s most widely used battery type and have been commercially deployed since 1890. Lead-acid battery systems are used in both mobile and stationary applications. They have long been considered the battery of choice for off-grid power systems due to their relatively low cost, reliability, and service life. 

The technology of lead-acid batteries is uncomplicated and manufacturing costs are low; however, such batteries are slow to charge, cannot be fully discharged and have a limited number of charge-discharge cycles. Lead acid batteries are further classified into flooded lead acid batteries, gel batteries and AGM batteries.

Lithium-Ion batteries

Lithium-ion (Li-ion) batteries, on the other hand, is one of the most widely utilized energy storage technologies for portable electronics today. The key characteristics of the battery include high energy density and long cycle life.

Like other solid state electrochemical energy storages, the lithium-ion battery consists mainly of the cathode, anode, and electrolyte. 

Lithium iron phosphate (LiFePO4) and lithium nickel cobalt aluminum battery (NCA) are the leading technologies that are being used for renewable energy application. 

Other lithium-based batteries include lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel manganese cobalt oxide (LiNiMnCoO2) and lithium titanate (Li4Ti5O12). The anode material of a Lithium-ion battery is typically made up of graphite, coated on copper foil.

What is overcharging?

Before we investigate overcharging, one needs to understand how batteries are charged. Both solar panels and batteries operate on direct current. The solar panels produce different voltage and current across the day based on solar irradiation levels. 

The graph below gives insight into voltage and current at different times of the day. Peak sunshine hours in this case are observed between 9:00 and 15:00.  

Environmental Impacts on the performance of solar PV systems research paper Ramadan J. Mustafa, Mohamed R. Gomaa , Mujahed Al-Dhaifallah,and Hegazy Rezk 

On the other hand, the operating voltage of the battery increases with state of charge. For example, in a 12 V battery the operating voltage is 11.75 V at 30% SOC and 12.75 V at 100% SOC.

Lead acid charging voltage, Wikipedia 

Overcharging is when excess Ah (amp hour) is delivered to the batteries. Based on battery technology, batteries behave differently to overcharging.

Is overcharging bad for the health of the battery?

Over charging is bad for the health of the battery irrespective of battery technology. For example, in the case of lead acid batteries, over charging leads to gassing. In open batteries this results in water loss and in sealed batteries this leads to heat generation inside the battery. Gassing starts before full charge is reached and increases as charging progresses. In the case of lithium-ion batteries overcharging can create unstable conditions inside the battery, increase pressure and cause thermal runaway.

What are the steps to prevent overcharging in batteries?

Higher charging voltage from the solar panels leads to higher Ah being delivered to the battery and ultimately leading to overcharging. The easiest way to control over charging of the batteries is to control the output voltage of the solar panel. A hybrid inverter can do this. The role of the charge controller is to limit the charging voltage of the battery to ensure over charging does not occur.

Goodwe Hybrid Inverter


The DC cables from the solar panels are connected to the PV terminals of the hybrid inverter, similarly the battery is connected to the battery terminals of the hybrid inverter. Using maximum power point tracking (MPPT) technology, the hybrid inverter optimizes solar production. The hybrid inverter also controls the battery charging voltage and prevents overcharging.

It is recommended to use a hybrid inverter for both lead acid and lithium-ion battery technology. GoodWe hybrid inverter is a reliable and robust product that can perform this function.

What battery technology can be consistently charged to max capacity?

Both lead acid and lithium-ion battery technology have a limiting factor on maximum charge capacity. In the case of lead acid, its usually around 40% and in lithium ion its around 90%. However, having said this, there is another battery technology available in the market that can be charged to 100% of rated capacity.

Redox flow battery (RFB) is a type of flow-based energy storage device capable of providing reversible conversion between electrical and chemical energy through two redox half-cell reactions. 

The most distinguishable characteristic of an RFB compared to a traditional solid-state battery is that the energy is stored in the flowing electrolyte, typically in two soluble redox couples contained in external tanks sized according to application requirements, whereas in a solid state battery the energy is usually contained in the electrode materials. 

The conversion between electrical energy and chemical energy occurs as the liquid electrolytes are pumped from storage tanks to flow through electrodes in a cell stack. Vanadium redox (VRB) flow batteries and zinc bromine (Zn-Br) flow batteries are commercially available in the market today. Flow battery technology is getting market acceptance and can be seen been used at remote telecom sites and gaining acceptance in residential applications.

http://large.stanford.edu/courses/2011/ph240/xie2/

What is the ideal depth of discharge for home batteries?

The depth of discharge is the percentage of the battery that has been discharged relative to the total battery capacity.

Below is a drawing explaining the depth of discharge for each battery type. Over charging the batteries above the recommended level or draining the batteries below the recommended level leads to reduced operational life of the batteries.

In the case of lead acid battery, you can have a maximum depth of discharge of 40% to 50%, however in the case of lithium-ion the depth of discharge can be as high as 90%.

https://www.cleanenergyreviews.info/blog/simpliphi-pylontech-narada-bae-lead-acid-battery

Can batteries go flat?

The short answer is yes, batteries can go flat. Any battery technology if completely drained goes flat. In case of lead acid batteries if completely drained, there is very little chance of reviving it. On the other hand, lithium-ion batteries can be revived if gone flat, this process is called a black start.

Conclusion

Batteries are an integral part to the residential solar system and the missing link in energy transition. Care should be taken to ensure one does not over charge or drain their batteries completely. The solar panels generate different voltages and currents across the day based on solar irradiance. A hybrid inverter ensures the batteries are not overcharged and increases the life of the batteries.

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