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If I am charging a battery from a solar panel, how do I switch the current away from the battery when the battery is completely charged?

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  • \$\begingroup\$ Usually you don't switch the current "away from" the battery; you just disconnect the solar panel from the battery so no current flows. \$\endgroup\$ Commented Aug 2, 2016 at 1:37
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    \$\begingroup\$ Best solution is to use a maximum power point tracking charge controller (for your battery chemistry) that will pull the maximum possible power from the solar panel AND manage the charging of your battery. \$\endgroup\$
    – John D
    Commented Aug 2, 2016 at 1:40
  • \$\begingroup\$ @JohnD: But that doesn't answer the question -- you still get to the point where the battery doesn't need any power from the panel. \$\endgroup\$
    – Dave Tweed
    Commented Aug 2, 2016 at 2:09
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    \$\begingroup\$ @DaveTweed: "AND manage the charging of your battery" would include knowing when to disconnect the PV array. \$\endgroup\$
    – EM Fields
    Commented Aug 2, 2016 at 2:24

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The simplest way is to use a FET in series between panel and battery. Add some circuitry to sense battery voltage (VBATT). When VBATT > VBATT_MAX, you turn off the FET. When VBATT < (VBATT_MAX - V_HYST) you turn the FET on.

VBATT_MAX depends on the battery chemistry. V_HYST could be maybe 1 to 3V, depending on many factors. The reason for using V_HYST is to avoid switching fast. When you disconnect the charger from the battery, the battery voltage will fall a certain amount rapidly, and you don't want to instantly turn it back on.

Using a MPPT solar charge controller is a decent idea, though, so give it some consideration. An MPPT can let you use a smaller panel and get the same amount of power out of it as a larger one, utilized less efficiently.

"HYST" is meant to stand for "hysteresis," which you can look up if you are not already familiar with it.

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