Connect Battery and PSU at same time...?

[grrr_arrghh]

Hi.

I'm sure this question was asked a while back, but I can't seem to find it.

I have a circuit that will run off anything from about 9v to 60v, and normally I would power it using a 9v PP3 battery. Sometimes, though, to conserve battery life, I would like to connect it to the mains, using a 'wall-wart' style psu. This may, however, be a different voltage to the battery, maybe 12v or 15v.
1) If I connect these (see diag below if I didn't make myself clear) at the same time, would it do any damage to the battery or the circuit? Also, would the circuit draw power entirely from the mains, and not take any power from the battery?

2) If I had a rechargable Ni-Cad battery (probably also 9v), and connected the circuit and battery to the mains supply (as already said, about 12v-15v), would it power the circuit, and charge the battery at the same time? What would happen if the battery became fully charged, and was still connect to the mains supply?

Thanks

Tim

 

[canadianpoet2012]

Well there are a number of questions there!

To answer 1; I think you would have to put a diode in series with the battery, to prevent any current from the PSU flowing into it and frying it! And yes, while you have the PSU connected, I think the circuit would pull its power from the PSU and not the battery. If you wanted it to take power from both, then you would need a potential divider to control the amount of power the circuit took from each source. Although I'm not entirely sure how you would do that!

As for question 2; it is very possible to use a re-chargeable battery that gets recharged when the PSU is hooked up. I'm sure if you search the net, you will find a circuit that does just that. However, you will need to make a re-charge circuit and a circuit to detect when the battery gets low, so it can switch to the PSU. Then detect when the battery is fully charged and switch back again! That would be the most economical way of doing it, and you could make just one circuit to do the switching and charging. Again, that circuit is out there somewhere on the net, I'd imagine!

I hope that helped in some way.

 

[panic mode]

Quick and dirty solution is to use two diodes one from battery and
one from PSU both powering your device. When PSU is connected
it's higher voltage will reverse bias diode that goes from battery
to load and battery will not run.
If the battery is rechargable, you might add resistor in parallel with the
diode that connects to battery so when PSU is connected, battery
would be tricklecharged.
This is not the best solution as charging process would not stop
when battery is full.
This could be resolved with zener diode in series with the resistor
so battery is charged only while sum of zener and battery voltage
is lower than that of PSU.

 

[grrr_arrghh]

Quick and dirty solution is to use two diodes one from battery and
one from PSU both powering your device. When PSU is connected it's higher voltage will reverse bias diode that goes from battery
to load and battery will not run.

ok, but what if the PSU was the same voltage as the battery? (I know I originally said it would be higher, but in the interest of covering all angles...)

If the battery is rechargable, you might add resistor in parallel with the
diode that connects to battery

what sort of value resistor? and would there still be a problem when the battery became fully charged?

Thanks

Tim