In it's most basic form, a solar battery recharger could be built as below (and this is what you get with a lot of low cost off-the-shelf packages)...
What we have here is a panel connected to a battery via a diode. The job of the diode is to let the current flow only one way (i.e. from the solar panel into the battery). This prevents the battery leaking current back into the solar panel at night. The charged battery can then be used to provide current to various devices such as low voltage lighting. The problem with this method is that there is no control over the charging. So long as the solar panel is generating electricity it will continue to pump it into the battery even when the battery is full and this could lead to battery damage through overcharging where the battery electrolyte can literally be boiled off. The battery can also be damaged from being drained too heavily as there is nothing to cut off heavy loads from draining it.
My set up differs in that it is like the one pictured below.
As you can see, a regulator is in place of the diode and performs the controling functions including reverse current blocking (stopping the battery discharging into the solar panel), undervolt threshold (so the load is cut off when the battery voltage drops too low), overvolt threshold (so the current from the solar panel is shut off once the battery reaches maximum charge voltage), lightning, overload and short circuit protection (via an in-built fuse).
You will also notice the two Ammeters to read current and two Voltmeters to read Voltage. These are for a control panel and will indicate the voltage/current being generated by the solar panel, the voltage of the battery and the current consumption of the load. These items aren't necessary but should look pretty cool and will help me to work out how well everything is operating and whether or not I am making the most of my installation. My Ammeters have built-in shunts but not all do so be careful if buying some of your own.
So the project begins and it's time to go shopping. Although I'm going to try and produce a customised installation, complete off-the-shelf products are avilable as with this example from CPC.
My costs break down as below (prices include VAT and delivery where applicable):
18W Solar Panel
Purchased from online auction site at £71.00
100Ah Leisure Battery
Purchased from online auction site at £68.45
Solar charge controller
Purchased from online auction site at £21.98
Purchased from online auction site at £20.49
Mains Power Meter
Purchased from online auction site at £13.60
Control panel Ammeters and Voltmeters
Purchased from online auction site at £41.14
Low voltage plugs/sockets
Purchased from CPC at £7.79
12v CFL bulbs (x2)
Purchased from CPC at £19.81
Backboxes, faceplates, connectors, cable, etc. £30.00
The battery is one of the most important components of a project like this and I did quite a lot of research into mine before I bought it. The important factors in sourcing a suitable battery are the Amp/Hour rating (Ah), the Depth of Discharge rating (DoD) and the type of battery technology.
The Amp/Hour rating gives you an idea of the capacity of the battery. A battery rated at 100Ah should provide 100 Amps of current for one hour before becoming discharged. In practice, battery discharge is not linear and you should avoid complete discharge of your battery anyway, but it goes without saying that the higher the Ah rating of your battery, the longer it will last before it is either discharged or reaches the undervolt threshold and is shut off by a regulator.
The Depth of Discharge (DoD) rating shows how much your battery can be safely discharged without damaging it. Usually this is 80 to 60% which means you should ensure your battery is not overloaded to the point where it is discharged below this. Heavy discharging and recharging is known as Deep Cycling. If you know your load is going to be burning off 50Ah then you should buy a 100Ah (or higher) battery to ensure it can cope with the load and isn't going to be completely discharged by it.
When it comes to battery technology, there are four types you are likely to come across...
1. Lead/Acid car battery
Cheap and readily available - but not suitable for this kind of project. The problem with car batteries is that they are designed to give a short high current kick to a vehicle starter motor and not to provide constant current for long periods of time. Starting your car takes about 5% of the batteries capacity after which the vehicle's alternator takes over generating electricity for the equipment while also recharging the battery. Car batteries don't have a high DoD and are vulnerable to damage more easily than other technologies. Lead/acid batteries are often semi-sealed meaning the electrolyte may need to be replenished (using distilled water) occasionally. These batteries may also give off some hydrogen gas while charging so care must be taken to ensure there is suitable ventilation. Care should be taken when handling and transporting because of the corrosion hazard.
2. Lead/Acid leisure battery
These mid-range batteries are the ideal price/performance compromise. While similar to a car battery, leisure batteries can withstand higher DoD and come in relatively high Ah ratings. Leisure batteries are designed for constant drain applications as they are meant to be used in motor homes and on boats providing electrical power for items such as fridges and televisions. They can also be used for emergency power such as starting an engine if the main battery has failed and as such, they sometimes have dual terminals providing standard engine battery connections as well as screw post connections. Like car batteries, leisure batteries are readily available from high street automotive outlets. Leisure batteries are usually semi-sealed, may require electrolyte top-up and can give off hydrogen gas while charging. Care should be taken when handling and transporting because of the corrosion hazard.
3. AGM battery
AGM stands for Absorbed Glass Mat and is a new type of lead acid battery offering longer life, complete seal, gas free, maintenance free, low self discharge operation. Because they are completely sealed, they can be used in any orientation with no risk of leaking. AGM batteries are less sensitive to temperature changes than traditional lead/acid types so there is less of an efficiency drop in low/high temperatures. Expect the cost of an AGM battery to be about twice that of a normal leisure battery.
4. Gel battery
Similar to AGM technology and with the same advantages, but especially suited to deep cycling. Again, prices are about twice that of a normal leisure battery.
I went for the leisure battery option because of budget constraints and also because I read that AGM and Gel batteries need a careful charging scheme or they can become damaged. I need to do more research into battery technology and charging regimes but that can wait until such time as I decide to upgrade my installation (if that happens). My battery is a 100Ah 60% DoD Energy Squadron.
Armed with the parts, it was time for installation to commence...