Discussions of batteries and solar panels can involve a lot of jargon. Let’s try to clear some of it up.
Voltage is how hard electricity is “pushing”, how much force it has.
Voltage is measured in volts, or V for short.
A high voltage is not, in itself, dangerous. When you walk across carpet and pick up a static charge, it can easily be a few thousand volts – but when you touch something and feel the ‘zap’ as the static discharges, it doesn’t hurt (much!) because there’s only a tiny electrical charge involved. But touch the mains power in a wall socket, which can supply plenty of current, and it’s a different and potentially deadly story.
On the other hand, even though a 12 V car battery can supply a lot of current, the voltage is too low to send any appreciable current though the human body, so in this sense it’s safe to touch.
Charge means how much electricity is stored somewhere, such as in a battery.
Scientists measure charge in coulombs. But battery capacity (how much charge a battery can store) is almost always given in amp hours, and to understand that we’ll need to know what an amp is.
Current is how much electricity is being transferred each second.
It is measured in amps (short for amperes), or A for short.
One amp is one coulomb of charge transferred per second.
Battery capacity is usually specified in amp hours, or Ah for short.
If a battery has a capacity of 100 Ah, and it was fully charged to begin with, then it should be able to supply a current of 1 amp for 100 hours before it is completely flat. Or 4 amps for 25 hours, or 5 amps for 20 hours, etc.
Power refers to how quickly energy is being supplied or used. If something is using a lot of power, it means that a lot of energy is being transferred in a short time.
Power is measured in watts, or W for short. Or kilowatts (kW), where 1 kW = 1000 W.
Power isn’t restricted to electricity – it relates to any energy transfer.
But in electrical terms, power equals voltage times current: watts = volts x amps.
So, for example, a 200 W solar panel could (in theory) deliver 10 amps of current at 20 volts.
The “in theory” is because the power ratings on solar panels are best case (full sunlight, not too hot), and the actual power delivered depends on the load (e.g. battery charger) connected to the panel.
Energy is a slippery concept – physics defines energy as the potential to do work, where work is any activity, whether moving something, or providing light or heat. The more energy you have, the more you can do.
Scientists measure energy in joules (J), and a watt of power is one joule of energy per second.
But more commonly, when talking about electricity, energy is expressed a watt hours, or Wh.
Power in watts times time in hours equals energy in watt hours.
So, a watt hour is the energy delivered by one watt of power in one hour.
And a kilowatt hour (kWh) is the energy delivered by 1000 watts in one hour (or 1 watt in 1000 hours).
Household electricity is generally billed in kWh.
Battery capacity vs energy
Battery capacity is a way of expressing how much energy a battery can store.
Battery capacity is usually expressed in amp hours.
For lead-acid batteries, the capacity is usually specified at the 20 hour discharge rate. That is, if you’re drawing a current at a steady rate that would fully discharge the battery in 20 hours, multiply that current by 20 to get the battery capacity. For example, if a 5 A current will discharge a battery in 20 hours, the capacity is 5 x 20 = 100 Ah.
The capacity of lead-acid batteries depends on the discharge current, falling for higher currents.
Again using a 100 Ah battery as an example, it should be able to deliver a continuous 5 A for 20 hours. But if you draw a much larger current, say 50 A, it will last less than the 2 hours than the 100 Ah rating would suggest.
Strictly speaking, amp hours is a measure of electrical charge, not energy. We (the whole industry) get away with specifying battery capacity in amp hours because (for most battery types) batteries continue to supply a reasonably constant voltage as they discharge.
For example, a lead-acid battery may start off at 13.7 V when fully charged, and do down to 11.8 V when almost fully discharged. But it averages around 12 V, which is why we call it a 12 V battery.
If the voltage is constant, a battery’s energy storage capacity in Wh equals its capacity in Ah times the voltage:
A 100 Ah 12 V battery can store 100 x 12 = 1200 Wh (or 1.2 kWh) of energy.
State of charge
A battery’s state of charge (commonly written as SoC) refers to how “full” the battery is, as a percentage.
For example, if a 100 Ah battery’s state of charge is 70%, it should have 70% x 100 Ah = 70 Ah of charge left in it.
A related concept is depth of discharge, which is simply how much of a battery’s capacity has been used so far, as a percentage. In the last example, the depth of discharge is 30% – we’ve used 30% of the charge and 70% is left.
All materials have some resistance to the flow of electrical current*.
Resistance causes a voltage drop across whatever the current is flowing through. For example, if you have solar panels with a long cable connecting to a battery charger, the charger won’t see the full output voltage from the panels, because of the voltage drop due to the resistance of the cable. Shorter cables have less resistance. So do fatter ones.
This phenomenon doesn’t only occur in cables. It also happens inside batteries.
Every power supply, whether battery or solar panel or generator, has some internal resistance.
Since resistance causes a voltage drop when current flows, the internal resistance of a power supply means that the voltage it’s supplying will drop (or droop) whenever higher current is drawn from it. Stop drawing current, and the supply voltage goes back up.
This effect is more pronounced in some types of batteries (such as lead-acid) than others (such as lithium).
It means that when your battery voltage is low, it’s not necessarily low in charge. It might simply be that something is drawing current from the battery.
*ok, that’s not necessarily true at really low temperatures, like -200°C. But it’s true in the everyday world.
A voltage regulator converts a variable unregulated input voltage into a specific fixed regulated voltage.
For example, a regulator could convert the 16 to 22 V output from a solar panel to 13.7 V, for charging a 12 V battery.
That’s not the best way to charge lead acid batteries, but it’s a low cost option that works ok.
A smart battery charger is designed to charge specific types of batteries, measuring the battery voltage to determine its current state of charge, and automatically adjusting the charging voltage and current during charging to quickly charge the battery, allow it to absorb that charge, and keep it topped up – without overcharging, to maximise the battery’s life.
Recall that power is voltage times current. For any power supply, as the current changes, the voltage will change too. If there is no current, the voltage will be high, but zero power because there is no current. With a really high current, there might not be much voltage supplied, and hence not much power.
In between those two extremes at some point, for some specific current, the power available will reach a maximum, at the maximum power point.
A maximum power point tracking (MPPT for short) solar charger automatically adjusts the current drawn from a solar panel to ensure that the maximum power is available for delivery to the battery.
The ideal current for maximising power from a solar panel will vary according to the amount of sunshine, and also temperature. An MPPT charger will automatically vary the load current as those conditions change, to ensure that as much energy as possible is converted from sunlight and ultimately stored in the battery.