7/26/2019 Litium Polimer Battery Charger

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Lithium-Polymer Peak Charger

This circuit was developed to charge the Lithium-Polymer cells used in a model aircraft. Lithium-

Polymer cells are incredibly lightweight compared to Ni-cad battery packs of the same voltageand amp-hour rating. Their only drawback is that they require a rigid charge and discharge

regime to achieve maximum life. The most important points of note are as follows

Circuit diagram:

!. They should be charged using a constant-current" constant-voltage method" which stops

the charge once the current has dropped to about the #$!% rate. &or example" for an

'%%m(h pack" charging should be terminated once the current falls to approximately

'%m(.

). They should never be discharged below *+ per cell otherwise they will be permanently

damaged.

*. should not be charged or discharged above their rated current otherwise an explosion and

fire can result,

7/26/2019 Litium Polimer Battery Charger

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To initiate charging" the momentary tart button /switch !0 is pressed" closing the relay

contacts and connecting the battery pack to the output of 123!. The circuit will then charge two'%%m(h cells in series at a constant current of 4%%m(h until they reach a peak terminal voltage

of 5.)+ per cell /nominal terminal voltage for these cells is *.6+0.

123! and transistor 7) form a current-limited voltage regulator. 8hen the charge currentexceeds about 4%%m(" the voltage developed across 16 turns on 7)" which in turn pulls the

ad9ust terminal of 123! towards ground. This shunts the voltage ad9ustment resistance chain

formed by +1) and 1*" thereby limiting the output to 4%%m(.

8hen the battery voltage reaches about '.5+" the regulator limits any further voltage increase" as

set by +1). The charge current will then slowly decrease as the cells reach full capacity. (s aresult" the voltage across 16 also falls" until the bias voltage on the base of 7! is too small to

keep it in conduction. 8hen 7! turns off" the relay also turns off" isolating the fully charged

 battery.

The charger is set up as follows:

!. :t to !)+ ;# and place a digital voltmeter between the output of 123! and the negative

output for the battery pack. (d9ust +1) for a reading of '.5+.

). +1! so that the voltage on the base of 7! is at maximum.

*. Place an ammeter in series with the battery to be charged and press the tart button. Theoutput current will shoot up to around 4%%m(" then slowly decrease over the next one to

two hours.

<nce it falls to around '%m( /or whatever the #$!% rate is for your cells0" slowly turn +1! untilthe relay switches off and indicator L2; goes out.

The circuit should now charge your battery packs to within =6> of their rated capacity.

&inally" note that in most cases" 123! will need to be fitted with a heatsink.

Source: Silicon Chip January 200