Technical Information

Rechargeable nickel cadmium (Ni-cad) and nickel metal hydride (Ni-MH) cells are never charged when you receive them. To obtain best results follow the manufactures instructions. Always remember that new cells will require 6-8 full charge / discharge cycles before reaching full working capacity.

Batteries with light indicators [gas gauges] such as Bosch 24v/3.0ah and Makita 9.6v/2.0ah can only be used as a guide to indicate the remaining working capacity or charging capacity and therefore should not be used/ accepted as a true reading.

Business experience to date indicates that our cells will not affect the working performance of the power tool – Rebatt only increase the working capacity.

Rule of Thumb
Please note that you cannot always use nickel metal hydride cells in Ni-Cad chargers unless stated by manufacturer. A charger designed to service Ni-MH batteries can also accommodate Nickel Cadmium batteries but NOT the other way around. A Nickel cadmium charger could overcharge the NI-MH battery pack thus causing overheating and physical damage.

Quick Charger
Or rapid charger is one of the most popular. It positions itself between the slow charger and the fast charger. Charging time for power tool batteries usually takes between 1-3 hours and the charge rate 0.3C. Charge control is required to terminate the charge when the battery is fully charged using time information. The quick-chargers are made to accommodate either nickel based or lithium based batteries which cannot be interchanged in the same charger.

Fast [smart] chargers
Is the latest trend on the power tool market which is best suited to nickel-based batteries. A slow charge rate is known to build up a crystalline formation known as “memory effect”.

There are several advantages with this type of rapid charge – Smart chargers monitor battery current by a rise in temperature to determine when the battery is fully charged, thereby preventing over-charging by sensing the rate of temperature increase over time rather than respond to an absolute temperature.

To terminate the charge, a temperature increase of 1degrees C [1.8 F] per minute with an absolute temperature cut-off of 60 degrees C [140 F] “works well” states Buchmann. Some battery manufacturers encode the cell pack electrically with more expensive control circuits to identify their chemistry and rating. The charger then sets the correct charge current and algorithm for the battery intended.

When a battery is fully charged, some chargers switch to a topping mode governed by a timer that completes the charge cycle at a reduced rate until fully charged before changing to a trickle charge or maintenance charge which compensates for any self-discharge. This prevents any damage to the battery cells.

One cycle = one full charge and discharge before needing a full charge again
Cycle charging is a type of charging in which nickel-cadmium batteries are forcibly cycled one time [a full discharge coupled with a complete recharge] Cycle chargers normally perform this operation automatically without any operator initiation.

The useful life of Ni-cad and Ni-MH cells is usually 1000 charge / discharges which can increase up to 1400 cycles in certain situations. However, battery life ends when the battery pack can no longer provide 80% of its rated capacity after been fully recharged. Approx life, 18 – 24 months with proper use in the commercial and professional trade.

Memory Effect
This adverse condition occurs with nickel based battery packs when the battery cells are only partially discharged many times therefore a periodic full discharge is so important that “if forgotten” large crystals will; form on the cell plates referred to as memory effect.

The crystalline build – up reduces working capacity and the battery pack will eventually lose its performance if below the 80% needed before been recharged again very quickly. To prevent memory effect and increase longevity the user should occasionally [every 120 cycles] fully discharge the battery down to 0.1volts per cell to remove the crystalline formation from the cell plates.

REBATT UK can offer the reconditioning service on batteries every 120 cycles or every 3 months. The price is £ 2.00 / battery pack plus p+p. Optimum results are achieved when nickel-based batteries are reconditioned from NEW.

Lithium – ion / Polymer chargers
Today, the Li-ion is the fastest growing and most promising battery chemistry. The energy density is typically twice that of the standard Ni-cad and Ni-MH with the potential to increase the energy density [voltage] up to 3 times. This type of battery is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory effect and no scheduled cycling is required to increase longevity. In addition, the self discharge is less than half when compared to other chemistries.

No trickle charge is applied because the li-ion would be unable to absorb the overcharge as it could cause lithium metal plating on the anode, a condition that renders the cell unstable.

These type of chargers have stricter controls in terms of charging methods and charging time demanded by these batteries. The Li-ion charger has voltage-limiting device similar to the lead acid battery. The majority of modern cell packs designed for maximum cycle life use an end-of-charge voltage threshold of approx 3.8v/cell up to 4.20v/cell when the voltage threshold is reached which is protected by the protection circuit. The tolerance on all battery packs is a tight +/- 0.05v / cell.

The li-ion typically discharges to 3.0v/cell. The spinel and coke versions can be discharged to 2.5v/cell. Caution should be exercised not to discharge a lithium-based battery lower than 2.5v/cell as this may cut off the batteries protection circuit. Similarly, if the voltage threshold is exceeded during charging the protection circuit will cut of the voltage to prevent damage and the possibility of overheating and further damage.

The pulse charge method for Li-ion has no major advantages and the voltage peaks wreak havoc with the voltage with the voltage limiting circuits. While charge times can be reduced, some manufacturers suggest that pulse charging may shorten the cycle life.

The loss of charge acceptance of the li-ion / polymer batteries is due to cell oxidization which occurs during usage and age. Lion batteries cannot be restored with cycling or deep discharging like nickel based cells. The capacity loss is permanent because the metals used in the cells are designated to run for a specific time only. Service life approx 18 – 24 months on average.

According to Isidor Buchmann fast charge methods do not significantly decrease the charge time. A charge rate above 1C should be avoided because high current can induce lithium plating. With the majority of modern Lithium ion packs a charge above 1C is not possible due to the protection circuits limiting the amount of voltage the battery pack can accept. Li-ion batteries tend to have a slow metabolism and therefore need to take its time to absorb the energy.

The charge and discharge current of a battery is measured in C-rate. Most portable batteries, with the exception of the SLA are rated at 1C. A discharge of 1C draws a current to the rated capacity. For example, a battery rated at 1000mAh provides 1ah of use if discharged at 1C rate. The same battery charged at 0.5C provides 500mAh for 2 hours usage. At 2C the same battery delivers 2000mAh for 30 min. IC is often referred to as one-hour discharge.

Lithium-based batteries have a defined age limit. Once the anticipated cycles have been delivered, no method exists to improve the battery. The main reason for failure is high internal resistance caused by oxidization. Operating the battery at elevated temperatures will momentarily reduce this condition.

When the temperature normalizes, the condition of high internal resistance returns. If discharged below 2.5v/cell, the internal protection circuit often opens. Many of the lithium-ion chargers cannot apply a recharge and the battery appears to be dead. This can even happen to fairly new batteries early on during the service life.

Check with the battery charger manufacturer to establish if the charger features a booster or wake-up function, to activate the protection circuit to enable it to recharge if the battery has been discharged too low. However, if the cell voltage falls below 1.5v/cell and lower for long periods, a recharge should not be attempted because of safety concerns with the protection circuit opening thus cutting off any charge / discharge cycles.