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Battery School Part 2 – Charging

Charging of batteries

Charging the various battery types is easiest and safest with those chargers which are normally supplied with the equipment containing the batteries.

Sometimes you have to choose or recommend a charger for a given battery system. The criteria applied in selecting the charging system is usually:

  • Desired time for recharging the batteries, require the shortest time possible or would the standard 12-24 hours be enough?
  • Cyclic use or stand-by / parallel use?
  • Financially, how much can the charger cost?
  • Charging from electricity network or from the voltage source, for example the vehicle or both parts?
  • Opportunities for monitoring and alarm for malfunctions?

The different chemical systems have completely different systems and principles for charging and chargers for example, lead-acid batteries can not be used for NiMH . In addition to the fact that the batteries can be destroyed, it is a security risk, for example the risk of fire or explosion.

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.

Tricks and tips

NiCd and NiMH-batteries – not charging full capacity

New NiCd and NiMH batteries, or those which have been some time in stock do not provide full capacity after the first charge, they are also usually more or less exhausted when you buy them (except NiMH cells of Low Self Discharge type, LSD cells ). This is not because of quality problems, but lies in the battery's chemical "nature". 

  • This is sometimes misunderstood, and the customer goes back to the place of purchase to return their battery. Here's how you can get the battery going again in the best and fastest way:
  • Charge the first time at least 16-24 hours, even if the charger indicates "fully charged" after a shorter time. 
  • Use the battery until it is empty, the unit switches off, etc.
  • Now make a new charge according to the charger's manual, after 2-4 times the battery should give full capacity and you get the best life.
Ladda inte ur batteriet för bra

Many know that you should discharge the the battery (NiCd and to some extent NiMH) before you recharge again, but not seldom do they discharge out "for much". One should discharge to about 1 V / cell, for example, 4V for a 4-cell battery that is nominally for 4.8V. Thereafter must the load be removed, the easiest way to do this is by discharging with, for example, a phone or device in use, or alternatively with a charger with a discharge function. If one otherwise applies a load in the form of lighting or resistance, and forgets about it, the battery will be destroyed.

Lead and lithium-ion – avoid battery discharge

Lead and lithium-ion need not and should not be discharged unnecessarily, applies in particular to deep discharges. A lead acid battery is destroyed (sulfation on the plates) relatively quickly if left empty.

These types of batteries can advantageously be recharged before they are completely exhausted, unlike NiCd / NiMH. They should not be left with the charger connected unnecessarily long time after full charge with the exception of lead-acid batteries that are specifically intended for stand-by operation.

Batteriet verkar ”dött”

The protection circuit in lithium-ion batteries prevents among other things, deep discharge but if you still have "crossed the line" the charger will try to charge the battery again, but it will take longer than normal.

Sometimes this is misunderstood; the battery seems "dead". Note that this does not have to do with the charger's capacity, but the protection circuit inside the battery has just done its job. 

Lagra med 50-60 procent SOC

Lithium-ion / LiFe cells and batteries are shipped and stocked typically with about 50% SOC (State of Charge). One achieves then the best possible lifespan and the cells are degrade very little internally purely chemically. If you want to store a Li-Ion / LiFe system for longer, 3-6 months or more, you should therefore make sure that you have around 50-60% SOC before putting away the battery.

Extend the battery life

During operation and use one wants obviously to get as much as possible of the battery's capacity and charge up therefore usually the system as well as discharge to the limits.

However, one can assume that the total life of a battery system gets longer if you do not use the entire capacity of each work cycle.

In some contexts and in larger and with more expensive systems one often has advanced control of charging and discharging level. One dimensions the system to some excess capacity and thereafter uses the range between for example, 20 and 80% of the battery’s total capacity (60% DOD, Depth Of Charge) and can achieve around 8000 cycles, while 80% DOD with the same battery might give around 1500 cycles.

State of charge (SOC) of lead or Li-ion batteries

The voltage measurement can roughly determine the state of charge for a lead acid battery. Applies to 12V (6 cells) batteries at 20 ° C, the battery must have been at rest for at least three hours after charging. Divide or multiplied by voltage values for different numbers of cells.

11,92 V 0% 12,24 V 40% 12,56 V 80%
12,00 V 10% 12,32 V 50% 12,64 V 90%
12,08 V 20% 12,40 V 60% 12,72 V 100%
12,16 V 30% 12,48 V 70%

The charging status of lithium-ion cells can be determined in a similar manner, the table shows the voltage per cell, multiplied by the voltage of the number of cells that the battery contains in the case of higher voltage, for example, by 4 for a battery with 14.4V nominal voltage, etc:

3,0-3,3V 20% 3,5-3,8V 60% 4,1-4,2V 100%
3.3-3,5V 40% 3,8-4,1V 80%