Introduction
Batteries power much of our daily lives, from keeping our smartphones alive to supporting renewable energy systems. Understanding the terms associated with batteries, like amps and amp-hours, can help you make informed decisions about charging and managing your battery-powered devices. In this guide, we’ll explore the relationship between amps, amp-hours, and what it means for typical battery charging times.
Amps vs Amp-Hours: A Quick Overview
Before we dive into what the implications are in the real world, let’s touch on the basics.
– Amperes (A): Amperes–almost always shortened to amps–are a measure of the rate of electric current flow. If you think of electrical flow like water flow, then electrical charge would be litres of water, so electrical current would be like the current in a river. The more amps, the more electric current is flowing.
– Ampere-Hours (Ah): Amp-hours are a measure of electrical charge. If you imagine electrical flow as water flow, then amp-hours are like how many buckets of water filled after leaving a hose pipe on for some length of time. Amp-hours are used to describe the capacity of a battery to store electrical energy.
You can think of it like the total amount of water a storage tank can hold. You could empty a large tank one cup at a time, or all at once by opening the drain valve. The amount of water released would be the same either way, the only difference would be how quickly it was released. In the same sense, if a battery has a capacity of 10 Ah, it can theoretically provide 1 A of current for 10 hours, or 10 A for 1 hour.
Battery Charger Ratings
When it comes to charging, a battery charger’s maximum current rating (in amps) indicates the rate at which it can replenish a battery’s charge. Here’s where the relationship between amps and amp-hours comes into play.
Example: 10A Charger vs. 20A Charger
Imagine you have two battery chargers – one rated at 10 amps and another at 20 amps – and you want to charge the same 20 Ah battery from 50% to 100% with both. 50% of 20 Ah is 10 Ah so the chargers must provide a charge of 10 Ah in total to fully charge the battery.
– 10A Charger: This charger can deliver a maximum current of 10 amps to the battery. To estimate how long this charger would take to charge the battery, simply divide the charge required (10 Ah) by the current (10 A). So, theoretically, it would take 1 hour for the battery to reach full charge (10Ah / 10A = 1h).
–20A Charger: With a 20-amp charger, the charging process is faster. It can provide a maximum current of 20 amps. To charge the same battery from 50% to 100% would take only half an hour (10Ah / 20A = 0.5h).
A note of caution: different batteries have different maximum rates at which they can be charged. Just as you would not fill a glass using a firefighter’s water hose at full blast, high-current chargers are not necessarily well suited to low-capacity batteries (unless the charger is able to automatically reduce current based on battery capacity).
The maximum rate of charging (and discharging) of a deep cycle battery is often stated relative to C. which is a way of describing charging and discharging rates. C is simply the current that would theoretically charge the battery from 0% to 100%.
So, for a 38 Ah battery, C is 38 A. For a 200 Ah battery, C is 200 A, for a 100 Ah C is 100 A, and so on. For lead acid batteries, manufacturers generally recommend no more than 0.2C for charging. 0.2C would be 6.6A for the 38 Ah battery, 40A for the 200 Ah and 20A for the 100 Ah.
In the example setup with the 10 A and 20 A chargers would deliver rates of 0.5C and 1C respectively. This is very high for lead acid technology and would likely lead to reduced lifetime for the battery in question.
From this you can see that it is not necessarily true that the best charger for your application is the one with the highest possible maximum current. Rather, choose the one whose maximum current is around 0.2C for your intended battery setup and save the money from buying an over-sized charger.
However, if you intend to upgrade your setup over time, you can select a higher capacity charger, as long as it is able to automatically adjust its charging current. in that case, the charger will maintain a steady 0.05C- 0.2C rate for a wide range of capacities, allowing you to expand your setup over time. Charge controllers have this capability built-in as standard.
Am-Hours vs Kilowatt-Hours: A Bigger Picture
Battery capacities are sometimes stated in different terms. Sometimes they are described in Ah (or milliamp hours, mAh, for small batteries) or kWh (or watt-hours, Wh, for small batteries). This may be confusing at first, describing the same thing–capacity–in two different ways.
However, the key is that they are not actually describing the same thing. Amp-hours describe the stored charge. Kilowatt-hours describe the stored energy.
Returning to the water analogy, a litre of water falling from the height of a table has a lot less energy than the same litre of water falling from the top of a skyscraper. Remember, the volume of water is like the stored charge. The same 10 Ah of charge can store different amount of energy depending on another factor.
In the analogy, that factor is height above the ground. In batteries, that factor is voltage. The connection between Ah and kWh is the voltage at which the amps are delivered. For a 100 Ah battery rated for 6V, the stored energy is 600 Wh (6V x 100 Ah). For a 100 Ah battery rated for 12V, the stored energy is 1200 Wh, or 1.2 kWh (12V x 100 Ah).
This is how a battery’s ‘capacity’ can be described in two different ways (based on stored charge, or stored energy). The different ways of thinking about battery capacity are useful when thinking about different aspects of how it will be used.
It is easiest to think in terms of stored charge when deciding what sort of charger or charge controller to match your battery up with.
It is easiest to think in terms of energy when deciding what sort of things you might be powering using your battery (100W laptop, plus a few 9W lightbulbs, maybe).
Conclusion
Understanding amps, amp-hours, and watt-hours as well as their relationship with battery charging and discharging will help you make informed decisions about our battery-powered devices and renewable energy systems. The different terms can be confusing but it is helpful to frame the concepts using familiar analogies. When it comes to charging and discharging rates, always follow the manufacturer’s recommendation. If in doubt choose automatic devices that will handle all the difficult decisions for you. Visit the Electric Market and let us help you get power, in your hands.