Charging & Powering at the same time.

[Lower232]

I am new to this forum and require some help. I am just starting out with electronics and have placed a £20 wager with my son that i can build the ultimate camping friend. This is a box that can charge 2 mobile phones, power 12 led lights a small computer speaker amp and have a built in speaker. It is to be run from 2 packs of 10 aa rechargeable batteries. Both giving out 12 volts. thus 1 being used and the other as the back up. I would like to be able to plug this unit into the mains when the batteries are flat not only to charge them but to use at the same time. The charger will be a 12volt input one. I was going to dismantle a battery charger and use the circuit board from that but i am not sure of the wiring or is there a better way using diodes?

Please help me win my bet....... he is 14 and i would hate to think he beat me !!!

 

[BobK]

Without knowing the current draw of the chargers, the LEDs and the speaker amp, I cannot say whether you would win or not, but my gut feel would be to bet on your son. 10 AA cells is barely enough to give a full charge to two mobile phones.

Bob

 

[brevor]

Get out your 20.

 

[BobK]

Tell me the current draw of the LEDs and the amplifier, then we can talk

Edit: Oh, and how long they must be powered. If you say a few seconds, you probably win.

Bob

 

[Lower232]

Hi

The amp is a small one running current is 2amp. The led lights state operating current 200ma? I have now changed my idea with the 10 aa batteries and have decided to go with a 12V Sealed Lead Acid Battery 7.2Ah. The box is to be built to take to a festival. It will require to charge 2 mobile phones both days (4 charges total). The 12 small LED lights are just to give a bit of light in the tent during the night for getting up (nature calls). The amp will be used for about 1 hr per night. With the sealed lead acid battery, i also have a charger. I just need to know how to wire this so when the battery needs charged, the charger will power the others and charge battery at same time. Hope this helps.

 

[BobK]

So you are admitting you lost the bet?

Your new battery has 12 * 7.2 = 86.4 Watt Hours of energy.

Calculate the Watt hours needed by each device by multiplying the current by the voltage by the hours of operation. For example, say the phones charge at 5V and 1A for 3 Hours, which I think is pretty typical. That would be 15 Watt Hours. So the 4 charges are using up 60 Watt Hours or about 70% of the battery's energy. Actually, that is optimistic, because the conversion from 12V to 5V will be only about 80% effiecient, so the real energy use might be more like 75 Watt Hours.

You don't say what voltage the LEDs and the amplifier run off. If 12V, then the amp, used for 1 Hour takes another 24 Watt Hours --- ding ding ding, you are now at 84 out of the 86.4 Watt Hours of energy. If the LEDs take 12V as well, 12 * 12 * 200mA = 28.8 Watts, so your remaining 2.4Watt hours will run the LEDs for a grand total of 5 minutes.

And lest you think you have won, the capacty of a battery is rated for use at a constant current level for 20 Hours. Pretty much any other usage profile will result is less energy available, often as little as 50% of the rating.

So, though you are close, I still think you lose.

Bob

 

[Lower232]

Thank you, I think i will need to take 2 batteries with me. I have admitted to him that he was correct..... big smile on his face and a bigger one when i handed the money over !!!. I will be using the 12v batter. I will be installing a power jack at the back of the box and wiring this to the battery for charging. I want to be able to use this in the house also. What would be the best way of wiring the unit so it can be plugged in for charging or plugged in to use or mobile (just using the battery) is there a schematic i can look at?