I have a 12V 7Ah SLA battery with a smart battery charger for SLA batteries with the maximal output of 0.8A.

I would like to use the battery to charge an Arduino board with few other things (at most 0.5A current consumption). Several times a day, the battery must be used for few seconds to power a motor (which takes few amps).

The battery must be contantly used for powering the board. Is it safe to charge and use the battery constantly? Is it a standard use case, can't it reduce its lifetime?


You aren't really charging and using it at the same time in reality. This is the same when you use your smartphone, whilst plugged into a charger.

Any power required by the system (in this case the motor, and your aurdino board) is taken from the power input (the smart battery charger input). Anything remaining in the power budget is then used to charge the batteries. This means you are charging the battery, but powering the system straight from the main input and not the battery.

The trickier situation is when the power required by the system is greater than the main power input. In the common example is when charging a phone from a PC (only 500mA) and the phone is using more than 500mA (especially common when using GPS). This situation requires sourcing power from the battery at the same time as using main input power, and is requires some kind of load balancing. In this situation the battery is not charging - it is discharging.

This has no real impact on the battery life, beyond the usual charge/discharge fatigue you would expect.

  • \$\begingroup\$ Maybe, maybe not. A good quality, 3 stage, smart charger is required. The charger must know the type of battery and or the max charge voltage and nominal cell voltage. An SLA battery is sensitive to over charging. A simple charger will likely over charge an SLA. \$\endgroup\$ Aug 12 '18 at 14:11
  • \$\begingroup\$ Agreed, but perhaps outside the purview of the question. It was regarding simultaneously charge/discharge not appropriate SLA chargers. \$\endgroup\$
    – Oliver
    Aug 12 '18 at 18:57
  • \$\begingroup\$ Except if a lesser charger is used your comment "no real impact on the battery life" is not true. Many chargers will over charge an SLA and greatly impact the battery's lifespan. The give away is when the charger does not have voltage or battery type settings. In many cases this does not matter. It matters a lot when the charger is left on more than 48 hours after the SLA is fully charged. Not any old charger can be used. There is a specif type of charger for an UPS battery back up. \$\endgroup\$ Aug 12 '18 at 19:02

Is it safe to charge and use the battery constantly? Is it a standard use case, can't it reduce its lifetime?

Depends on the charger. A good charger will will have a setting for type of battery, battery cell voltage, and have three charging stages.

  1. Constant current
  2. Topping
  3. Floating or hysteresis

Chargers charge beyond the nominal battery voltage then float down to the nominal voltage. Keeping the battery at the topping charge for more than 48 hours will harm the battery.

Many cheap chargers deliver only a topping charge. This would not be good for your application. A topping charger should be disconnected after 60 hours to prevent over charging. If your charger has a floating or hysteresis stage you should be okay.

enter image description here

Once fully charged, the battery should not stay at the topping voltage for more than 48 hours and must be reduced to the float voltage level.

This is especially critical for SLA
because they are less tolerant to overcharge than the flooded type. Charging beyond the specified limits turns redundant energy into heat and the battery begins to gas.
Source: Battery University


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