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Trickle charging a battery.

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cobra1

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I have the need to build a basic UPS.

I have found a circuit that is basically what i want and i will modify it to suit my additional needs.

**broken link removed**

I will be changing the transformer to a secondary winding giving 18v.
The battery will be a 15v NiMH Pack.

How do i calculate the resistor required to trickle charge this battery???

I dont anticipate the power to constantly trip out (once ever 6 months or so maybe) so it doesnt matter how long it takes to recharge the battery.

After the battery, there will be a 12v and a 5v regulator, 12v outputs and 5v outputs.

When mains power is lost i still want both 12v and 5v power outputs.

From what i can find this is the simplest way of doing this, is there any better way??
Also if any of you guys know anything about switching power supplies i would like to hear your thoughts, the transformer i plan to use is huge delivering just over 1 amp, even then with everything on this will be almost at its limit. Could a Switching power supply be made fairly easy handling more than 1 amp???
 
The required resistor value would be (Vs-Vd-Vb)/Ic, where Vs is the supply voltage across C1, Vd is the series diode voltage (typically 0.65V), Vb is the battery voltage during charge, and Ic is the desired charge current.

If Vs is close to Vb then it may be difficult to get a reliable controlled charge current. You may need to increase Vs and add a regulator, to provide the 12V output.

Switching supplies operating from 230VAC are not trivial to design so, unless you need high efficiency or small size, I would stay with a regular transformer.
 
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