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I would like to use my homemade battery charger, rated 15VDC 7A, to charge a 25Ah lead acid battery. Would there be an easy way to limit the charging current to 2.5A (Ah/10)?

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  • \$\begingroup\$ modify the charger. my battery charger uses SCRs in half the rectifier and the SCRs control the charge rate. \$\endgroup\$ Jul 16, 2020 at 7:52
  • \$\begingroup\$ You'll need to advise the brand and model of charger. It is unlikely to be practical to do this. A smaller appropriate charger is a better choice. You could add a 2.5A constant current source. You could modify the circuitry. \$\endgroup\$
    – Russell McMahon
    Jul 16, 2020 at 11:03

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I would like to use my homemade battery charger, rated 15VDC 7A, to charge a 25Ah lead acid battery. Would there be an easy way to limit the charging current to 2.5A (Ah/10)?

As you did your own battery charger, if done with analog electronics, you might have done as a 1, 2 or 3 stage charger, as I will explain further ahead.

But to answer your question, yes there are some ways to limit the charging current - crucial for “bulk charging” mode:

  • As you did your own charger, you might already have a current limiting circuit, usually included to protect the series pass power transistor. In this case, recalculate the shunt resistor and switch to this larger value to further limit the current.
  • Another way, very simple and non-invasive to your charger would be to use a series resistor between the charger (positive) and the battery (positive) to further limit the charging current. A few incandescent lamps, as used in the cars, rated as 10W or 21W, can be joined in parallel until the desired current passes to the battery. Incandescent lamps are great non-linear (non ohmic) resistors, maintaining the current less dependent of their voltage drop - if you allow an heresy, it would be a poor man’s “constant” current circuit.

As a reminder, these are the 3 stages or modes applicable for normal charging of lead acid batteries:

  1. Bulk mode: Charging current is limited up to a “safe“ value, while the battery voltage increases. It is a constant current (CC) mode. When current starts to reduce, the battery is charged at aprox. 80% of rated capacity.
  2. Absorption mode: When the battery voltage reaches the “absorption charging voltage”, it enters the absorption mode, operating in constant voltage mode, typically at 14.4V (@ 25°C). Depending of literature sources bulk mode shifts to the next mode when the charging current reduces to about 10% to 20% of bulk current value or 3% to 5% of AH. At this point, battery would be fully charged (100%).
  3. Floating mode: The constant voltage mode value is lowered to the “floating charging mode”, typically at 13.8V. It can be maintained for extended periods (days) without overcharging the battery, but for that, the precise value should be fine tuned for existing ambient/battery temperature.

Additional information about how to charge, or when the absorption charging mode should be terminated?

The recommended current for bulk charging (first mode) is 10% of rated AH. Some articles and manufacturers cite up to 20% as maximum to be considered still within “slow charging” class.

References:

Recommended charging for lead acid batteries - Battery University

Chargetek’s charging and equalization - additional information

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