5 things you need to know about importance of correct sizing and purpose

  1. When you select a battery charger you need to first choose a style or form that is appropriate for the task at hand. A desktop charger isn't suitable in an application where the charger needs to be retained or mounted. These form factors may include dust or water ingress considerations or power cables in and out of the unit.

  2. When selecting a battery the capacity required for you to run with a few day's autonomy needs to be matched with a batteries need to be recharged with the appropriate sized current. As a general rule we say use 1/10th batteries AH for its charging rate given you have 12 hours to recharge overnight. If you have less time then you need a larger charger if the battery can accept a higher rate of charge. Or if you are using some of the charging power to run equipment while the charging is simultaneously occurring.

  3. The intended use of the battery is a key factor in the life you will get from it if you use a starting battery in a deep cycle application then be prepared for an early end of life. If you have recently increased your Ah capacity because you required a longer autonomy and used a deep cycle battery but have not increased your charging current to match you can also expect issues. Using any battery charger or regulator which doesn't use the factory recommended recharge voltages of the battery you are charging will lead to an early end of life issues also. These are not covered by the manufacturer's warranty which covers physical defects in production.

  4. Chargers are a logic programmed electrical device that uses timers and measurements of voltage and current to determine the charge cycle. They are therefore pre-programmed with time limits for each stage, or current (amps) and a minimum value before the stage is complete then moving to the next stage of a charging cycle. If you haven't sized your charger correctly to your batteries these rates can be either too long because of undercurrent causing issues with recharge. Or just as annoying if the charger is too big you can reach high voltages quickly but not be able to change modes for a minimum number of hours which can lead to unnecessary overcharging.

  5. Purpose of the system design, simplicity, effectiveness, and efficiency are all core factors in power storage and conversion. Choosing the correct components from the outset might cost a little more upfront but the return well worth the effort in getting it right.

Direct DC Coupling

Morningstar has released a 600V MPPT Solar Controller that allows for direct battery charging in DC rather than going via the  inverter charge controller in a typical grid-tie solution. The intent is to eliminate the double conversion from solar panels being DC power through the inverter to mains then back through a charge controller into the batteries.

This isn't really something new its more of a variable option, it allows you to use your house's power via your distribution power board in the event of the grid going down.

Many people won't know that when the power provider or mains power line is not working if power is cut, that stops your solar panel system from functioning. If power is purposely disconnected because of rolling blackouts or accidental line failure or like in California U.S.A power has been cut to prevent fires from failures in the national grid in dry summer environments.

The Direct DC Coupling allows you to be more efficient and charge your storage batteries directly and keep your grid-tie system live. Other manufacturers call this an "island", an island is where you can operate independantly of the grid and distribute and manage your own resources.

Charging efficiencies

In this article, we will discuss so of the losses when power is converted or transformed or used. Our niche is the conversion of energy and with that there are losses. For example, a light bulb was designed to provide light. It does this by making a small filament of wire glow. The glowing generates the light that's wanted but also heat and that is a wasteful by-product of a light bulb.

With battery charging which is a chemical reaction, there are losses through the chemical conversions of electrons moving between plates and acids. These losses are minimalised with improvements in the technology with over 100 years in development. Losses in charging lead-acid vary from 12% for AGM batteries to about 20-25% for wet cell batteries. That means a 100Ah battery requires 112-125 Amps to recharge and store 100Ah of usable energy.

When you convert DC power from a battery into AC mains power using an inverter the equipment inside the inverter uses power and that accounts for a 5% loss in available power. The same happen when converting AC power into DC power, these devices are called power supplies, or chargers, or power rectifiers. Switch-mode supplies are typically used in all modern-day equipment to help reduce these inefficiencies but they need to be calculated and allowed for.

An example of this might be an off-grid solar system with a small inverter for charging a cellphone.

Solar panels generate DC power 18-20% efficient, that's run through a solar controller 95% into a battery for storage 85% efficient. The mains power inverter converts DC power into AC mains 95% and you connect your phone charger in here. The charger is only 90-95% efficient which then connects to your phone's Lithium battery. Let's just say here it's 99% efficient at charging. All in all, you have four pieces of equipment all with up to 10% losses so your generation of the panels needs to be almost twice the power you want to consume.

When using mains power or grid-tie to power your home but have batteries for backup energy there are losses. You also lose more power in converting them back from DC stored energy in the battery back into usable mains power. This highlights an issue with grid-tie and battery backup. Multiple conversions, the storage, use and generation all have efficiencies and losses.

In future articles, we'll discuss DC versions much like our DC to DC chargers. Direct DC Coupling in a renewable energy system and more.

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