Solar Battery Chargers

Solar battery chargers are often overlooked and for good reason; people would prefer solar panels to charge a 12 volt battery and run an inverter to power mains equipment and many people seem to think 12 volt batteries are the only batteries in existence, well they are not as many battery powered items are powered by either AA or AAA dry cells which are 1.5 volts nominally; or the lithium ion cells in tactical torches or lanterns which are nominal 3.7 volts and up to 4.2 volts.

Rechargeable variants of AA and AAA batteries are readily available and these are 1.2 volts nominally and you need to work put the voltages to power dry cell equipment with rechargeable batteries and the classic voltage is 6 volts which means 4 X dry cells of 1.5 volts each, or 5 X rechargeable batteries of 1.2 volts each.

Rechargeable variants come in a range of capacities as do the lithium cells and we see lithium cells quoted with outputs of 10,000 milliamps or 10 amps which is impossible in cells such as the ever popular 18650 cell as they physically don’t have the internal room in the cell to store that much energy density; unlike the rechargeable cells which actually have higher capacities and are generally what they claim to be. Realities are different to claims and generally the standard 18650 lithium cell has around 3000 milliamps of capacity and despite their claims of being 10,000 milliamps they clearly not that capacity and testing shows they are around 2000 – 2500 milliamps and no more.

Many people have standardised their battery to either AA or AAA batteries which is sensible and many have modified them to get the correct voltage which may involve adding an additional rechargeable battery to make a 6 volt system working from 4 dry cells a 5 cell system working from rechargeable cells which can be done in many ways and the most obvious are the battery holders which are cheap and readily available in 1 – 8 cells in AA or AAA sizes. If you do this you need the room to replace a 4 cell holder with a 5 cell holder and the ability to use a standard connecter system so they all get the correct polarity.

Many things are powered by lithium batteries and while tactical torches are the obvious, many other things are powered by them and many tactical torches may come with a rechargeable lithium cell of 4.2 volts, many also come with a 3 cell dry battery holder, usually for 3 X AAA dry batteries as these are similar in voltage as the lithium is 4.2 volts while the dry cells are 3 X 1.5 volts or 4.5 volts and they use an internal voltage regulator to control the voltage to the appliance.

Chargers are fitted with solar panels and they come in different types of solar panel which is the very expensive perovskite type or the other type which is the Zombie panel; the perovskite panel is a new type of panel and is very efficient and also very expensive and not yet on the open market as house panels, or the Zombie panel which is actually ink and much cheaper and adapts to the available light, with a high efficiency
Solar battery chargers have either a single solar panel or multiple concertina panels which fold out like fan feed paper to increase the amount of solar panel area and currently they are the size of a mobile phone and the concertina panel types are thicker, they hold up to 4 rechargeable batteries of either AA or AAA types and many hold both types, while some have the latest switching battery technology which accepts AA and AAA and also lithium cells, which obviously are the best. If you have standardised your rechargeable cells to AA or AAA types you don’t need the more expensive battery charging technology of lithium and the cheaper charger will do.

Many work by charging single cells individually and each having its own monitoring system and they equalise all the cells so the lower charged ones are bought up to the same level as the higher charged cells and from there they are all charged equally so they all complete their charging at the same time. Other types typically charge one cell first, followed by the next cell and so on so you get one fully charged cell which then shuts to a maintenance charge while charging the next cell and then the next cell until all the cells in the charger are fully charged; obviously the cells are charged one at a time and you have to wait before they are all charged and you can remove them from the charger which is no good if you want multiple batteries in a device.

Battery or cell types are important and there are two main chemistries which are:

NiCD or nickel cadmium which are the older technology and are toxic through the inclusion of cadmium and are the lowest capacity cells generally having AA capacities of 800 – 1300 milliamps for an AA size and a self discharge rate of 15 – 20% per month.

NiMH or nickel metal hydride are the newer technology and generally have capacities of 1200 – 2700 milliamps and are the higher capacity cell in the AA size and in both cases the smaller AAA cells have a lower capacity, and they have a self discharge rate of 20-30% per month.

Memory effect is something only NiMH cells suffer from and this is called their memory and in simple terms it means if you continually discharge them to 70% before recharging they learn this and think at 70% charge, they are discharged when they are not. Similarly, if you discharge them and only charge them to 80% charge they think this 80% charge is fully charged and with nickel cadmium cells they need fully discharging and then fully charging to prevent the memory effect; NiMH don’t have the memory effect so you can partially discharge them before recharging them.

What is self discharge? Basically it is where the internal materials break down and they lose their capacity until they are recharged and this is based upon monthly rates and an unused NiCD cell will typically lose one fifth of its capacity per month of storage while the NiMH will lose up to one third of its charge, so basically you top them up and create the memory effect with NiCD cells or lose this capacity with both types.

Newer types of NiMH are often called hybrid or sometimes low discharge cells and these self discharge at about 2-3% per month and they are often claimed to be ready for use batteries and they can allegedly be used straight from the packet, but they can take many months to be transported from the manufacturer to the distributor and from the distributor to the exporter and there is the travel time by ship and the receiving countries systems which are transporting to distributing warehouses and finally on to the retailer. As this could take months I always suggest NOT using them straight from the packet, but to give them a top up charge so you know they are at their full capacity and fully charged and even after 3 months of storage they have over 90% of their charge and are useful with useful amounts of capacity. This means you can charge them just before winter and again in mid winter, and at 3 months intervals and they will remain in tip top condition.

Two major types of solar charger exist and this is the way they charge, or more specifically how they gather their light and infra red as this is divided into different colours and the better chargers gather more of what is termed mid light from the spectrum, or blue light; and if the best light is gathered you need to collect more of the mid or blue light to make your charger more efficient. These types of chargers come as outdoor only chargers or inside the window charger and the outdoor chargers are better as many domestic or industrial windows have coatings on them which limits the amount of mid or blue spectrum light coming through the glass and ultimately to the charger.

This now comes down to physical size and the fold out or concertina types have 3 – 6 fold out solar panels and these are generally the best as you insert your cells into the charger and as soon as you fold out the solar panels they begin charging and have the indicator lights to show it is charging and the state of charge, or fully charged. Mobile phone sized solar chargers with just one panel work and struggle to charge batteries and typically they take around 16 hours of bright sunlight to charge a pair of cells which can often be 4 days in winter and the time you most need your emergency batteries. You can get the double fold out panels which power many things and as an emergency stand by I would say no they are too big and not convenient enough and are fine for camping where you may have a vehicle to carry equipment, but not as an emergency stand by back up charger. Many chargers offer other uses and have 1 or 2 USB ports which make them handy for charging power banks or mobile phones, or other USB powered equipment, and many also have charging for C or D cells as well as PP3 batteries making them more flexible for cheap or free charging in an emergency situation.

What do we look for? basically we begin with charging times and a charger which charges 4 cells in around 4 hours is the better option as you can recharge 4 cells in a day and this is convenient for lighting during the longer nights, or for powering most things battery powered and with a second set of rechargeable batteries you can be using one set while recharging another. Some have multiple power input systems and can be charged by plugging into a car cigarette lighter of USB port increasing their flexibility and allowing charging from a vehicle or the USB port of a power bank.

If we choose rechargeable batteries of around 2500 milliamps to 3000 milliamps we will find they have a very useful capacity at competitive prices and they are often readily available in bulk packs of up to 20 batteries and this means we can have 20 charged batteries and if we discharge four batteries we have time to recharge them. Only buy the NiMH types which are either low discharge or hybrid types in a capacity of around 2850 M/a which are currently the largest capacities at reasonable prices as you can get quality brands up to around 8000M/a which may be nice until you see their massive prices and think whoa.

If you have standardised your system on AA or AAA batteries you are not running the risks of fire such as those running lithium cells and you can integrate them into an existing system as I have a mains powered charger with 16 cell capacity which runs from a transformer from the mains, or a charging lead directly from a car cigarette lighter so unless I have a power cut and no diesel for any of my vehicles then I have my small solar back up charger and an integrated system.

What do rechargeable cells run, in most applications they are a direct replacement for a dry cell so switched directly and despite the lower voltage of 1.2 volts per cell, they compensate by using more current or amps and a dry cell of 1.5 volts and generally around 1.5 amps is 2.25 watts and a rechargeable battery of 1.2 volts and 2850 milliamps is 3.15 watts so actually a larger capacity.
In reality most things will work with a direct swap cell for cell but some things won’t such as some electronic items which are voltage specific and replacing a 1.5 volt cell with a 1.2 volt cell may under volt them and they shut down on low battery voltage.

Do they do them for the larger C or D cells? In a word yes, but with C cells having capacities of 6-7000M/a and D cells having capacities of 10,000 M/a they are a good option and will take days to charge them due to their capacity, so I would suggest not.