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How to charge a 4V 1000 mAh SLA Battery, can I use a 5V 2A charger? Answered


never use that directly, instead use a 5v 350ma power supply with a 1N4007 diode,

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2.25 - 2.3V per cell, which means you only need an absolute max of 4.6V - anything above and your battery will start to gas out.

If you have a box of junk electronics see that you can find a diode with suitable voltage drop of around 0.4V- 0.5V.
If your charger provides a bit over 5V with no load a standrand 4007 diode should do the trick.

You should also consider a resistor to limit the charging current.
With that added to the diode and the output voltage between 4.5 and 4.6V you are good to go.