Battery technology

What types of rechargeable batteries are used in your MAL Lighting lamp and what are the differences? This is a short selection that is designed to give you a feeling for the characteristics of the various battery types:

There are many different primary and rechargeable battery types, but only a few meet the characteristics needed for our lights. The first step is the difference between primary and secondary batteries. A primary battery is, with a few exceptions, designed to be used once and cannot be recharged. A secondary battery is designed to be used more than once and is rechargeable. There are several types of secondary batteries with an explanation of each.

The various rechargeable battery types differ in the chemical composition of the electrolite, the electrodes, and shape. The electochemical process of charging and discharging remains the same. Differences in the usable current, temperature characteristics, charging and discharge characteristics determine the selected use.

Lead Acid

Lead acid batteries are one of the oldest and most used battery types. The cell voltage is approx 2 volts. A 12V battery consists of 6 cells connected in series to make 12V. They are charged with constant voltage that is less than the “gassing” voltage (this is the voltage at which the electrolyte boils off the electrodes) of 2.4V per cell. The charging process must be completed before the gassing process starts. Fastcharging of lead acid batteries is complicated in comparison to NiMH batteries, since the charging current is usually limited by the internal resistance of the battery, and the charging voltage must stay under the “gassing” voltage of 2.4V per cell. The maximum charging current is defined by these characteristics. A further characteristic of lead acid batteries is that the charging current from constant voltage chargers starts out low and climbs during the charge. The charging temperature is also critical since the voltage at which gassing takes place varies according to temperature. The batteries should not be discharged below 1.75V /cell since this deep discharging drastically shortens the batteries lifespan. Lead acid batteries should not be stored uncharged for long periods. The leadsulfate created during the discharge will crystallise out and the battery drys out. A big advantage of lead acid batteries is their long lifespan of over 5 years.

NiMH-Batteries

The  NiMH battery has a cell voltage of 1.25V per cell. The electrolyte does not take part in the chemical reaction, as it does in the lead acid battery, it’s only function is to transport the ion stream between the electrodes. The electrodes also do not lose mass to the electrolyte during the charge/discharge cycle as happens in the lead acid battery. Since the NiMH does not undergo physical changes or change the specific weight of it’s electrolyte, the output voltage is relatively constant and it has a long lifespan. NiMH batteries are lighter and usually smaller than comparable lead acid batteries.

NiMH batteries are charged with constant current which is a fraction of the value C. If a battery with a capacity of 1Ah is charged with a current of 1A, this is a 1C charge rate. Due to efficiency and parasitic losses during the charging process the battery is fully charged after a 1.4C charge instead of the ideal case 1C charge. Most manufacturers define a normal charge at a C/10 rate for 14 hours, in our example of a 1Ah battery this means 14 hours at 100mA.

NiMH batteries can also be fastcharged (many at rates of up to 4C), but you have to make sure that the battery is not overcharged. Most retail fastchargers switch the charging process off as soon as the charging voltage drops slightly (so called Delta-U process). This Delta-U process is based on the effect that as soon as the battery is full the charge no longer goes into the battery but is electrochemically transferred into heat and gas. This effect is noticeable by a small drop in the battery voltage at the end of the charging process that can be electrically detected and used to swith the fastcharging off.

NiMH batteries can, in comparison to lead acid batteries, be deeply discharged. Unfortunately this is only valid for single cells, since due to variations in the individual cell capacity of the cells in a battery pack some cells will be deeply discharged earlier than other cells and can electrically flip-flop or change polarity, which has a major negative effect on battery lifespan.

If a NiMH battery is repeatedly only partially discharged before charging the battery can acquire a memory effect where only part of the battery capacity is available for use. Every time the battery is charged too early, or remains on a standby charge (a charge of C/20 or less to compensate for the battery’s self-discharge) too long, it loses a portion of it’s capacity. This loss can be reversed by multiple complete charge/discharge cycles.

Li-Ion-Batteries

The newest battery technology on the market since the mid 1990s is the Li-Ion battery. Specific capacity, size and weight are unbeatable but also expensive.

Due to it’s cell voltage of 3.6-3.8V you can use less cells to get the voltage needed. Memory effect does not exist, you can charge this battery at any state of it’s charge.

The internal processes of a Li-Ion battery are similar to those of NiMH batteries, the energy is produced by a transfer of electrons between the electrodes. The electrolyte is composed of a Lithium salt solution that stores Lithium ions during the charge. The positive electrode is made of carbon.