Part 2 – Charging

These are charged with a constant voltage, possibly. in many steps when it comes to the more advanced chargers. A closed 12V battery can for example, be recharged with 14.4V for a limited time (bulk phase), usually around 5-8 hours, then the charger goes on to 13.65-13.8V (equalization or maintenance phase) or it is disconnected to prevent overcharging, gassing and loss of electrolytes. The transition can be timer controlled or take place when the charging current has come down to a certain value. Often the latter is used together with the timer as a safety precaution.

The same type of battery can also be constantly connected to the charger (standby use, parallel use) The charger should then be set to 13.65-13,8V. All voltages above apply at 20 ° C, when temperatures deviate by more than +/- 5 ° voltage should be adjusted, usually by 3-5mV / ° C and cell. The battery manufacturer’s recommendations should always be followed.

With discharging batteries, the current can initially go up to quite high values, the charger must therefore have a limitation that should normally be set to max 0,3C ie., one third of the battery’s nominal capacity.

Here applies inversely with lead that the charging proceeds with a constant current, the voltage will adapt to the number of cells in the battery. One can count on the voltage being around 1,5-1,6V per cell at the end of the charge.

How much current you charge with depends on how fast you want to recharge the battery. Normal charging (14-16 hrs) is being done using C / 10, for example, 70 mA for a 700 mAh battery.

Often you want to recharge the battery pretty quickly, even down to 15-30 minutes occurs in for example, power tool batteries. Then the progress is monitored in one or more ways:

Delta V charging (Δv) means that the charger measures and monitors the voltage throughout the battery. The voltage increases during charging because when the battery is fully charged, it levels off and decline slightly (5-25mV / cell). This the charger recognizes this, and then cancels the charge, or alternatively. transitions to a low charging current (trickle charge).
Temperature measurement directly on the cells. When the battery is fully charged, the temperature has risen to around 45 ° C, whereupon the charger switches off.
The Reflex Principle: Charging with relatively high current, thereafter break, short discharge pulse, measuring of voltage, new charge, etc . The process is repeated continuously, the charging principle is combined with Delta V sensor, is used among other things for quick charging.
Timer in the charger. One can calculate how much time is required to charge a given cell with a given current. Charging status (SOC) must be known before charging starts, the cells may well be completely or only partially discharged.
We take an example: The battery is for 1.7Ah and we have a charger that can leave 1A. The charging factor for NiCd / NiMH is about 1.4. It takes 1.7 hrs to fill the battery with 1A, this multiplied by the charging factor of 1.4 becomes 2.38 hrs, or about 2 hrs and 23 minutes. The timer will turn off the charge after this time.

Note that this assumes that the battery is completely discharged before charging. One often uses several methods of monitoring simultaneously, for example, Δv charging in combination with temp. measurement or safety timer.

Often one builds in some further protection in the battery in order to break the circuit if any fault occurs in the charger or the battery, temperature fuses, Polyswitch (PTC circuit protection) .

These are similar to lead-acid cells when it comes to charging. The voltage at the load is very carefully specified to maintain safety and battery life. Nominal cell voltage is 3,7V and you charge with 4,20V +/- 0,05V. The charging current is also specified and monitored, especially at the beginning of charging a fully discharged battery. You can recharge a Li-Ion battery to about 90% in 2-3 hours, but the last 5-10% of capacity takes another approximately 2 hours.

(Also known as lithium-ion iron phosphate or Li-Fe) is a variant of conventional lithium-ion cells that usually have a higher current capacity and voltage limits. Nominal cell voltage for these is 3,2V and the charging voltage is 3,65V. Charging can take place with a comparatively high current and therefore also in short time.

All Li-Ion, / Li-Pol and LiFe batteries have a add-on active electronic circuit which provides for safety, for example if external circumstances, like if the charger, etc. would go exceed the specifications or if the cell temperature is abnormal. Normally the voltage limits of charging and discharging as well as maximum current are all monitored. Even balancing of the voltages internally between the cells in a series is frequently monitored.

Cell balancing can be done passively in which case one simply cancels or reduces the charge current temporarily and adds a small load of the / those cells that have the highest voltage level. Balancing even remains active if you let the cells that have higher voltage charge those that have less, etc, this technique allows the battery to reach full charge faster.

Never attempt to charge or discharge loose Li-Ion cells without monitoring or balancing circuits, high risk of explosion or fire. The manufacturer’s specifications must always be checked and followed in the selection of chargers.