[SOLVED] current rating for our robotics project

[venktech]

we are doing a project on robotics wherein we need to power a GSM module rated as 12 V / 1A . We also have 4 dc motors - 12V 1A, the microcontroller, a servo and a sensor.. we calculated the current requirement approximately as around 5- 6 A and we bought a 12 V 7 amps lead acid battery.

My question is can we use a battery that delivers 7 A for a GSM module that is rated as 12 V / 1A.. What would happen if the bot is stationary? would 7A flow throw the other circuits and cause any problem ? should i introduce a fuse or something?

 

[alexan_e]

Batteries are rated in Ah (Ampere-hour) not Amperes Ampere-hour - Wikipedia, the free encyclopedia
the higher the capacity of the battery the higher the rate.

for example a 7Ah battery can provide 1A for 7 hours or 3.5A for 2 hours or 7A for 1 hour or 10A for 42 min etc

In addition the power source will only provide the current that is needed for each load depending on the resistance it has, the power source can't force more current that the device needs.

 

[poorchava]

True that, but batteries also have current rating. Based on their internal resistance, heat capacitance, heat dissipation and so on, they have some maximum current you can draw for them. If the current is exceeded then the battery can catch of fire or even explode. It is important, that long term (sustained) current is much lower than short term (or impulse) current.

Good example is the car battery. It can source as high as above 800 AMPERES (i.e. when cold starting a large displacement diesel engine). On the other hand trying to draw that kind of current for a longer period would result in complete disaster.

As for the question, you need to make sure wheter 7A you quoted is the current CAPABILITY in Amperes , not CURRENT*TIME capacity (in Ampere-Hours). These are two completly different parameters. Either way you can run your robot stationary without any problems. 7A of current capability means that the battery CAN deliver that much current, not that it will always try to achieve that current. If only the voltage rating matches, then you'll be fine.

 

[venktech]

yeah...it is a 7 Ah battery... Thanks a lot for your responses... and one more question.. can i charge this battery (lead acid 12v 7 Ah) using a 12 V 1 A adapter and passing the current in reverse direction?..

 

[poorchava]

No, at least you should not. Rechargable batteries need their charging current and voltage to be controlled to lenghten their lifetime. I don;t know what type of battery do you have, but you should get a datasheet for it. It most certainly will contain information about chargin process.

There are many integrated circuits intended for battery charging control. I would check Microchip, NSc, Maxim-Dallas. Maybe AD and IR also have something like that. Generally portable devices are kind of hot topic, so most semidonductor companies have some offer concerning handling of rechargable batteries

 

[enjunear]

yeah...it is a 7 Ah battery... Thanks a lot for your responses... and one more question.. can i charge this battery (lead acid 12v 7 Ah) using a 12 V 1 A adapter and passing the current in reverse direction?..

As you previously stated, this is a lead-acid battery. While you can build a rudimentary charger using a wall-wart transformer and a current-limiting resistor, the math is a little tricky. I'd suggest finding $15-20 and buying a (simple) manufactured 12V lead-acid battery charger from your local hardware or electronics store.
Google shopping results - 12V 1A charger

 

[prassanna]

Thanks for your nice information.

 

[venktech]

Thank you