Nickel Cadmium (Ni-Cd) batteries are rechargeable batteries (also known as secondary batteries) that can be deeply discharged and have a high cycle life expectancy. Typically, Ni-Cd batteries are used when large capacities and high discharge rates are required.
Apart from the usually higher cost of Ni-Cd batteries (compared to lead acid batteries), the biggest disadvantages of Ni-Cd batteries is their chemical components - nickel oxide hydroxide and metallic cadmium.
Environmental Impact
Nickel Cadmium batteries were invented in the 19th century when environmental concerns simply did not exist. Cadmium is a highly toxic heavy metal and can cause substantial pollution when landfilled or incinerated. While recycling of Ni-Cd is possible it does not exist in Australia – all dumped Ni-Cd batteries are sent overseas for processing – which ironically means that the carbon footprint is increased through recycling!
Other countries have long regulated the use and disposal of Ni-Cd batteries. In the USA, part of the battery price is a fee for its proper disposal at the end of its service lifetime. In the EU, they can only be used for a few predefined purposes.
Ni-Cd Battery Bank
Ni–Cd cells have a nominal cell potential of 1.2 Volt, hence you need 10 cells to make a 12 Volt battery (instead of 6 x 2V lead acid cells). Unlike lead acid batteries you cannot monitor the state of charge (SOC) with a voltmeter or a hydrometer – making it very difficult to determine the state of the battery bank, let alone find a failed cell.
Ni-Cd batteries also have a reputation for suffering from memory effect. Unless maintained properly (periodic full discharges), Ni-Cd batteries seem to 'remember' the amount of discharge for previous discharges, therefore limiting the recharge life of the battery.
Battery Charging
Ni-Cd batteries require a charger with a higher voltage (compared to a lead acid battery charger). While these chargers exist, they are more costly again. A fully charged 12V battery bank can also go as high as 17V, which could cause inverters to shut down or worse, DC bulbs to blow.
In addition, the charging efficiency of Ni-Cd batteries is generally rather low (between 70-75%) – which means of the electricity you put in, only 70-75% can be retrieved. In comparison, lead acid batteries convert about 85% and sealed lead acid batteries are even better (well over 90%).
Conclusion
In conclusion, we strongly advise against Nickel Cadmium batteries on solar systems. The initial cost is too high, the environmental impact is too great, the maintenance is too difficult, and you need special chargers and inverters to integrate them in a home power system. If you need good quality deep cycle batteries, take a look at our flooded and sealed lead acid batteries.