Technology 2017-08-04T09:19:04+00:00

Power Solutions for lithium batteries

Kaitek Flash Battery has designed and developed an innovative control and management system for lithium cells aiming to optimize the energy exploitation and to minimize the decay over time of the battery’s performance.

Lithium batteries

Lithium based accumulators give considerable higher performances with respect to the other mainly employed technologies (Pb, NiCd, NiMH), as demonstrated by their large and nearly exclusive usage in today’s portable electronic devices. In facts lithium batteries have:
• High energy density (>150 wh/l)
• Small weight (< 10 kg/kWh) • Long life (> 3000 cycles)
• No memory effect – higher usage flexibility
Their higher cost results to be cheap when considering their long life duration.
All these characteristics make lithium batteries particularly suited also to power applications for traction of vehicles or for static electric energy storage, where until now they have found little usage due to the more sophisticated recharge and management system they require.

Power batteries, which voltage typically ranges from few tens to several hundreds of Volts, are made connecting in series many cells (each having 3.2V or 3.7V typical nominal voltage), and these should be controlled and managed individually to ensure that none of them will work beyond normal operating conditions, thus avoiding their early decay, which would affect the whole battery.

Without single cell management:
– In discharge phase, the duration of the battery is limited by the cell that first reaches the minimum operating voltage
– During charge phase, as soon as a cell is fully charged, no more current can be injected in the battery and the charge must stop, even if most of the other cells still didn’t reached 100% of charge

The combination of both these effects with cycling charges and discharges increases the difference between the cells and heavily reduces the amount of energy the battery can source.

Performance of Lithium Batteries compared to others (Pb, NiCd, NiMH)

0Wh/l
Elevata densità di energia
0kg/kWh
Peso contenuto
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Lunga durata nel tempo

Discharge phase:

During a constant current discharge phase the cells have a quick voltage drop when their energy is too low to sustain the discharge current. Frequently happens that in a battery pack composed of many cells, few of these reach the complete discharge before the others, so when most of the cells would still be able to source some energy. In this case, if no active balancing is used (the only applicable during discharge), it is necessary to stop sinking current to avoid a damaging over-discharge of few cells, even if the battery supplied less energy than it should be able to do considering nominal capacity.
The active balancing performed by the BBS can support the weakest cells during discharge and limit their voltage decay. This extends the discharge phase until all the cells are empty, so the battery can source the maximum amount of energy.

During a constant current discharge phase, cells experience a sharp voltage drop when the energy in them is no longer sufficient to deliver the required current. Often, in a battery pack made up of many cells, some of the cells run down before the others, i.e. when most of the other cells still have some energy to deliver. If no active balancing is used (the only effective one for discharge), it is necessary to prevent irreversible damage to the cell that has already reached the minimum voltage level, by minimising or blocking discharge, sometimes well before the time provided for by the battery’s theoretical capacity.

The combined balancing of the BBS is able to support the weakest cells during discharge and keep voltage drops in the same to a minimum. This lengthens discharge until such a time as all the cells are completely flat, making it possible to use the energy pack’s entire nominal energy.

Charge phase:

With a recharge the target is to have at the end all the cells 100% charged. End of charge voltage for lithium cells is considerably higher than their nominal voltage (e.g. for Li-Ion it is in range 3,65V – 4V) and a cell can be said to be fully charged when it is possible to maintain its end of charge voltage with only a very small charge current (compared to the nominal capacity). In this condition, an excess charge current would rapidly lead to an overvoltage that can permanently damage the cell itself.
The following graphs show the typical curves of the minimum and maximum cell voltages measured in a Li-Ion battery that is being charged with 3,8V [this parameters can be changed in BBS settings] end-of-charge target voltage, respectively: with no balancing system (a); with passive only balancing system (b); with BBS combined balancing system (c).

If no balancing is performed, as soon as the voltage of a single cell rises and reaches the target voltage, the charge current must be stopped and the charge phase ends. Most of the cells are not fully charged yet. The difference between the cells may be negligible at the beginning of the battery life, but after each cycle they become more and more unbalanced and the effective battery capacity rapidly decreases.

With passive balancing techniques it is possible to charge every cell to the target voltage, but these can only use very low charge currents during last phase of charging, so the time required to complete a charge is very long and largely depends on the conditions of the cells. The continuous energy dissipation may be so high that overall system efficiency is affected, and problems due to high temperatures and heat dissipation may occur.

The technology of the BBS, with its high power combined balancing system, is able to fully charge all the cells in much shorter time. In facts, balancing phase typically requires less than 30 minutes for every kind of battery, so the charge time is easily predictable and almost constant regardless the health of the cells.
The overall energy efficiency is very high thanks to the active balancing system, as the passive system is only used when it can speed up the charge process without affecting efficiency.

The conclusions:

BBS technology has demonstrated to be able to exploit the characteristics of lithium batteries at their best, and in some applications it showed it could also restore batteries that appeared to be no more usable after being previously managed with an improper traditional management system.

Excellent Results!

Therefore, with BBS, lithium batteries can be used effectively in all types of applications, many of which could benefit much more from using lithium as opposed to conventional batteries.

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