Thread: Questions on batteries

Ah = Amp-Hour.

1 Ah means that the battery can supply 1A of continious current for 1 hour. If you are powering something that consumes 2A at any instant, then you will only run for 30 minutes. If it's only 500mA (1/2A), then it would run for 2 hours.

It's not an exact science since these numbers are usually based on particular consumption characteristics. The battery may change it's ability to output a particular amount of current or voltage depending on where it is at in its discharge cycle.

"10Ah/20HR"

10 amps per hour for 20 hours.

No, I don't believe that is correct.


The 10Ah specifies the total amount of power in the battery. So in theory it can provide 1A for 10 hours, 10A for 1 hour, 500mA for 20 hours, etc.

The Ah rating is calculated by the manufacture over a 20 hour discharge period (apparently an industry standard). The battery is only designed to provide that amount of power if you draw it at 500mA/ hour. If you load it with a device that draws 10A, it will only last 1 hour at 100% effeciency. The problem is the battery can't sustain that large amount of current for an extended period of time. You will likely get much less than 1 hours of use if you are drawiing 10A.

So the way to look at it is, divide the Ah rating bv 20 (unless specified otherwise) to get the max number of continious current you can draw for it to last the longest. Anything less is probably fine. Anything more will show diminished returns.

Digitallexus is 100% correct about the Ah rating. As for trickle charging, there is no reason you can't do it on a Ni-Mh battery. Just remember that trickle charge should have a charge rate of C/100, which basically means the batteries capacity divided by 100. With a 2800mAh battery you would need a trickle charge current of 28mA.

Do not wire the battery straight into the cars electrical system. You have A LOT of current there, and you will almost certainly damage the battery by pumping it all in at once. You want a charge current of about 280mA to be on the safe side. You can use a resistor to limit this current, but you still need a trickle charge circuit.

I have never attempted a trickle charger before, but it shouldn't be too hard. You can probably get away with putting a number of diodes in series before the charging resistor, and then put an optoisolator and resistor in parallel with this set of diodes. When the battery is discharged it will draw a large amount of current, most of this passes through the diodes but a small amount through the optoisolator. You could use the isolator with a transistor and relay to keep the trickle circuit turned off. When the battery becomes charged it draws less current, and the isolator will turn off. This could then activate your trickle charge circuit to keep the battery topped up.

This is just an idea, I haven't tested it, and you may not understand what i'm talking about anyway.

Best regards,
Matthew

Frodo - Keep in mind the voltage of the battery. Laptop batteries are usually 14.8v or more. Digital cell phones probably 3.6v.

So assuming your cell phone battery is 3.6v, and rated for 3.8Ah. That's 13.7W of power.

His 2800mAh laptop battery provides (2.8*14.8) or 41 watts!

Nearly 3 times more power even though it appears 1000mA less!

When calculating the number of hours that your carputer will run, you need to find the comparitive multiplier.

In the laptop case (14.8 vs 12v), you'll get 1.23x the stated rating. A computer using 1A at 12v will last 3.444 hours on his 2800mAh 14.8v battery rather than 2.8 hours.

Of course you need to account for power supply effenciency to get a true runtime.

Don't see that many of them with internal PSU's any more. Why exactly do you need the laptop battery in there when you are tapping into the contacts? I've probably missed exactly what you are trying to do, but not using the battery at all would be a lot less hassle.

Best regards,
Matthew