Simple yet confused

[Jkham]

I need to charge two 9v cells and isolate whichever cell is NOT being charged while connecting it to load.

Basically, a charger, two batteries, and a load. The load is a preamp signal processor which feeds An amplifier, fwiw.

The rest of it : I would also like to disable load from everything I planned on a dpdt switch to leave +/- both "open when system is powered off(may charge some with system off, and preamp can be shut down by this"

Any suggestions?

 

[(*steve*)]

Can you indicate:

1) what type of battery
2) the reason to isolate the battery from the charger whilst in use

 

[Jkham]

Can you indicate:

1) what type of battery
2) the reason to isolate the battery from the charger whilst in use

Yes-battery type is a 9.6v nickel metal hydride. Capacity 2600 mah and the load will have anywhere from 300ma to 1200ma draw.

Isolation-audible noise produced from pulsing currrent. So far I am noise free manually swapping things. Noise is intermittent if charger is connected to battery feeding preamp.
I thought about toying with a few filter caps to see if ripple could be managed but would rather isolate them if possible.

Can I connect the grounds of the battery to the charger and load grounds and use a DPDT switch to flop the batteries back and forth? Using charger (+) for one pole and load+ for the other?

 

[KrisBlueNZ]

Yes-battery type is a 9.6v nickel metal hydride. Capacity 2600 mah and the load will have anywhere from 300ma to 1200ma draw.

Do you know how the charger will handle being disconnected from one battery and immediately connected to a different battery? NiMH batteries are charged through a series of phases - pre-charge, then main charge, then top-off charge, at least. You might have to switch the charger off and on, or at least disconnect the battery for a period of time, before it will realise that there's a new battery connected to it, and start from the beginning of the charge process.

Isolation-audible noise produced from pulsing currrent. So far I am noise free manually swapping things. Noise is intermittent if charger is connected to battery feeding preamp. I thought about toying with a few filter caps to see if ripple could be managed but would rather isolate them if possible.

I agree.

Can I connect the grounds of the battery to the charger and load grounds and use a DPDT switch to flop the batteries back and forth? Using charger (+) for one pole and load+ for the other?

Nice idea but I think you'll find that at least some of the interference is due to common-mode noise, and you'll need to isolate the load from the charger on both positive and negative sides. This means you would need a 4PDT switch.

Here's a suitable-looking 4PDT switch with two positions: http://www.digikey.com/product-detail/en/1004P1T1B1M1QEH/EG2428-ND

This version has a centre OFF position that you might be able to use to make the charger realise that its attached load has changed: http://www.digikey.com/product-detail/en/1004P3T1B1M1QEH/EG2429-ND

Toggle switches are a good choice because they're always non-shorting - they're designed so they can't short the two side contacts together, even briefly, during the changeover.

 

[shumifan50]

It sounds like it is not a portable solution with mains power for the charger available.
So why not get a good quality SMPS to power it directly and charge the batteries offline to be used when needed?

 

[KrisBlueNZ]

Another approach would be two separate DPDT switches, one to select which battery is connected to the charger (this one would have a centre off position: http://www.digikey.com/product-detail/en/100DP3T1B1M1QEH/EG2413-ND) and a second one to select which battery is connected to the load (either a 2-position switch or another one with a centre off position).

 

[Jkham]

I Prefer the simplicity of a single switch...actually considered this very idea.

Another approach would be two separate DPDT switches, one to select which battery is connected to the charger (this one would have a centre off position: http://www.digikey.com/product-detail/en/100DP3T1B1M1QEH/EG2413-ND) and a second one to select which battery is connected to the load (either a 2-position switch or another one with a centre off position).

 

[Jkham]

Nice input on the niMh charging, never considered that.

Battery type and charger is subject to change at some point but not now.

This is meant to be portable. Batteries are mounted in a concealed(vented) position and not easily accessed. Hence the need for charging. The pair is needed obviously for backup, one charges and one is used.

 

[KrisBlueNZ]

I think the reason I suggested separate switches is that if the charger needs to be disconnected for a short time during the changeover, so it will detect that a new battery has been connected, you could do that using a switch with a centre OFF position, but if you use the same switch to select the source for the load, the load will lose power during that time. I don't know whether that would be a problem or not.