Batteries - learn more
Somehow, you'll need to store energy for when there is none available. It's like having food (energy) to sustain you until your next meal. Or filling up your car with petrol (energy) to take you around until your next visit to the petrol station. Batteries are needed to store the generated power from the grid (Eskom) or the sun (Solar panels). Electricity can only be stored in a battery in DC form. The inverter converts the AC power from the grid to DC power and charges the battery. When there's load-shedding, the inverter converts DC power to AC power and supplies the home with power. There are a few things to consider when buying a battery:
Wh (Watt-hour) or kWh (kilowatt-hour)
This is the capacity of the battery or simply put, how long you can run your appliance (at a certain power draw). Say you have a 5kWh-rated battery for your home, it means you can run a 1000W appliance for 5 hours. Or boil a 2000W kettle for half the time - 2.5 hours.
Ah (Amp-hour) or mAh (milliamp-hour)
This is also a measure of the capacity of the battery. Remember the formulae W = V x A ? If you have a 100Ah car battery at 12V, the power generated will be 1.2kWh.
The voltage the battery produces, Examples are: AA batteries 1.5V, car batteries 12V, backup batteries typically don't exceed 48V for safety reasons.
Lead-acid (car) or Lithium batteries
The material and technology used to build the battery. Lead-acid is a tried-and-true technology that costs less, but requires regular maintenance and doesn’t last as long. Lithium is a premium battery technology with a longer lifespan and higher efficiency, but you’ll pay more money for the boost in performance.
This greatly affects the Cycle life, Depth of Discharge, Efficiency, Charge / Discharge rate & Energy Density and ultimately the Price.
- Cycle life - When you discharge a battery (use it to power your appliances), then charge it back up with your panels, that is referred to as one charge cycle. We measure the lifespan of batteries not in terms of years, but rather how many cycles they can handle before they expire.Think of it like putting mileage on a car. When you evaluate the condition of a used car, mileage matters a lot more than the year it was produced.Same goes for batteries and the number of times they’ve been cycled. A sealed lead-acid battery at a vacation home may go through 100 cycles in 4 years, whereas the same battery might go through 300+ cycles in one year at a full-time residence. The one that has gone through 100 cycles is in much better shape.Cycle life is also a function of depth of discharge (how much capacity you use before recharging a battery). Deeper discharges put more stress on the battery, which shortens its cycle life. Lithium ion batteries have more cycle life than gel batteries and they generally last longer.
- Depth Of Discharge - Discharge depth refers to how much overall capacity is used before recharging the battery. For example, if you use a quarter of your battery’s capacity, the depth of discharge would be 25%.Batteries don’t discharge fully when you use them. Instead, they have a recommended depth of discharge: how much can be used before they should be refilled.Gel batteries should only be run to 50% depth of discharge. Beyond that point, you risk negatively affecting their lifespan.In contrast, lithium batteries can handle deep discharges of 80% or more. This essentially means they feature a higher usable capacity.
- Efficiency - Lithium batteries are more efficient. This means that more of your solar power is stored and used.As an example, lead acid batteries are only 80-85% efficient depending on the model and condition. That means if you have 1,000 watts of solar coming into the batteries, there are only 800-850 watts available after the charging and discharging process.Lithium batteries are more than 95% efficient. In the same example, you’d have over 950 watts of power available.Higher efficiency means your batteries charge faster. Depending on the configuration of your system, it could also mean you buying fewer solar panels, less battery capacity and a smaller backup generator.
- Charge Rate - With higher efficiency also comes a faster rate of charge for lithium batteries. They can handle a higher amperage from the charger, which means they can be refilled much faster than lead-acid. This is particularly useful in the Zimbabwean situation where you may get only a few hours to fill up your battery if you are using an inverter backup with no panelsWe express charge rate as a fraction, such as C/5, where C = the capacity of the battery in amp hours (Ah). So a 430 Ah battery charging at a rate of C/5 would receive 86 charging amps (430/5).Lead-acid batteries are limited in how much charge current they can handle, mainly because they will overheat if you charge them too quickly. In addition, the charge rate gets significantly slower as you approach full capacity.Lead acid batteries can charge around C/5 during the bulk phase (up to 85% capacity). After that, the battery charger automatically slows down to top off the batteries. This means lead acid batteries take longer to charge, in some cases more than 2x as long as a Lithium alternative.
- Energy Density - The energy density of lithium batteries is much higher than lead-acid, meaning they fit more storage capacity into less space.For example, it may take two lithium batteries to power a 5 kW system, but you’d need 8 lead-acid batteries to do the same job. When you take the size of the entire battery bank into account, lithium weighs less than half as much.This can be a real benefit if you need to get creative with how you mount your battery bank. If you are hanging an enclosure on the wall or hiding it in a closet, the improved energy density helps your lithium battery bank fit into tighter spaces.