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Batteries, chargers, and charging methods
This short summary is intended to explain the rechargeable batteries (accumulators) currently available on the market. The technical data of the various cells, and in particular their energy density, serve as reference values only without claiming to be absolutely correct. The various charging methods and Egston's expertise will be explained to you.
Content
Lead battery
Technical properties
| Nominal battery voltage | 2.0V |
|---|---|
| End-of-charge voltage | max. 2.3-2.4V |
| Exhaustive discharge voltage | exhaustive discharge should be no less than 1.8V / battery |
| Energy density | up to 40 Wh/kg, up to 110 Wh/l |
| Charging cycles | min. 300 |
Depending on type, batteries with rapid charging/discharging properties are available on the market. (Up to approx. 0.8C).The charging method for lead batteries is CCCV (constant current constant voltage), i.e. the battery is first charged with a constant current, and once the required battery voltage of 2.3-2.4V has been reached, at a constant voltage. Parallel buffer operation is possible without any problems.
NiCD battery
Technical properties
| Nominal battery voltage | 1.2V |
|---|---|
| End-of-charge voltage | up to 1.8V |
| Exhaustive discharge voltage | exhaustive discharge should be no less than 0.9V / battery |
| Energy density | up to 70 Wh/kg, up to 200 Wh/l |
| Charging cycles | min. 500 |
Ni Cd batteries can be constantly recharged at C/10 (i.e. the charging current is one tenth of the nominal battery capacity), end-of-charge recognition is not required, however, it would enhance the memory effect.
Rapid charge or discharge of Ni Cd batteries is possible at approx. 5/C (i.e. the battery is either charged or discharged within approx. 15 minutes), only with end-of-charge/discharge recognition. For end-of-charge recognition various methods can be used:
- -dU / dt (negative voltage gradient)
- -d^2U / d t^2 (inflectional tangent)
- dT / dt (thermal gradient)
- current integration (gas gauge)
- various other exotic methods
In our experience, the thermal gradient method is most reliable.
Parallel buffer operation with Ni Cd batteries is only conceivable with a C/10 charging current, however, because of the memory effect, the battery will quickly lose its capacity.
NiMH battery
Technical properties
| Nominal battery voltage | 1.2V |
|---|---|
| End-of-charge voltage | up to 1.8V |
| Exhaustive discharge voltage | exhaustive discharge should be no less than 0.9V / battery |
| Energy density | up to 90 Wh/kg, up to 350 Wh/l |
| Charging cycles | min. 500 |
NiMH batteries can also be charged at C/10, in which case the end-of-charge recognition feature is not required.
Rapid charging is possible with NiMH batteries up to 3C, however, an end-of-charge recognition is required. For this purpose, the same methods can be used as with NiCD:
- -dU / dt (negative voltage gradient)
- -d^2U / d t^2 (inflectional tangent)
- dT / dt (thermal gradient)
- Current integration (gas gauge)
End-of-charge recognition with NiMH batteries is more difficult, because the negative voltage gradient is almost negligible (-5mV/battery), and a reliable inflectional tangent can often not be established. The thermal gradient method is the only method for rapid charging of NiMH batteries that has gained acceptance with Egston.
Parallel buffer operation with Ni Cd batteries is only conceivable with a C/10 charging current, and a memory effect does not occur in NiMH batteries as maintained by manufacturers.
LiIon battery
LiIon batteries and Li Ion polymer batteries have, on the whole, the same properties and hence we shall forego explaining these. The polymer is a derivative of the usual battery characterized by increased energy density and a non-liquid electrolyte.
Technical properties
| Nominal battery voltage | 3.7V |
|---|---|
| End-of-charge voltage | up to 4.2V |
| Exhaustive discharge voltage | exhaustive discharge should be no less than 3V / battery |
| Energy density | up to 160 Wh/kg, up to 400 Wh/l |
| Charging cycles | min. 500 |
LiIon batteries can be quickly charged or discharged (up to approx 2C). Just as with lead batteries, the charging method for Li Ion batteries is CCCV (constant current constant voltage), i.e. the battery is first charged with a constant current, and once the required battery voltage of 4.1-4.2V has been reached, at a constant voltage. Parallel buffer operation is possible without any problems.
Problems with LiIon batteries- If the charging voltage is too high, it is possible that too much lithium is released from the cathode, which leads to thermal instability of the remaining cathode material.
- The LiIon battery is thermally instabil. If it is exposed to high temperatures (approx. 150°C (302°F)), this will trigger an exothermal reaction in the battery centre, so the battery may ignite or cause an explosion.
- High temperatures can also be caused by overcharging the battery either due to excessive voltage/output.
- High temperatures may also be the result of a short circuit within the battery.
Nowadays, scientists try to handle thermal instability by using microporous separators, which interrupt the current flow within the battery by fusing the micropores. In addition, thermal safety components are frequently introduced into the battery (PTC, shape memory alloy) in order to handle a contingent external short-circuit.
When using Li Ion batteries, one should not rely on one safety system only, instead safe handling should include:
- Battery
- Battery-pack with safety circuit
- Battery charger
- PSU
The PSU features should be fine-tuned to the battery to be used for charging. Please also note that the customer may use non-original products without giving much thought to whether the safety requirements have been fully met.
Chargers
Chargers for lead batteriesChargers for lead batteries, are devices with a 1% output tolerance, a current regulator is not required, the current limiting will be sufficient.
Models included in the Egston standard range:- N1xxxW1: (With an extensive charging range, max. ambient temperature 50°C (122°F))
- N2xxxW1: (With an extensive charging range, max. ambient temperature 40°C (104°F))
Chargers for Li Ion, Ni Cd and NiMH batteries should always be developed depending on the application they are used for in order to guarantee optimum safety and an extended service life. For this purpose, Egston uses microprocessor-controlled circuitry.
Prospects
The advantages of Li Ion batteries (high capacity, simple end-of-charge recognition, parallel buffer operation) will undoubtedly advance their course. All of the present laptop and notebook models as well as the majority of mobile phones are equipped with this accumulator type these days. Even for do-it-your-self tools or electric bicycles, the Li Ion battery will sooner or later prevail (once the safety problems have been solved).
