Questions on batteries

Sir_Pyro · 03-18-2003, 10:19 PM

Ok, I have a few questions about batteries....

First questions, are Lead Acid batteries the ONLY ones that you can trickle charge for a long period of time?
The reason I ask this is that I have a laptop I'm using in my project that use a 12v 2600mAH Ni-MH battery pack. To power it I just want to wire it up directly to my cars electrical system with a diode inline on the power wire. Will this kill the laptop battery? Out of 5 batteries I only have one good remaining and I don’t want to kill it.

Second question: I have some 6v sealed lead acid batteries I plan on using as a buffer in my trunk. On the side of it is "10Ah/20HR". What exactly does this mean?

Thanks.

digitallexus · 03-18-2003, 11:51 PM

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.

abz123 · 03-19-2003, 01:05 AM

"10Ah/20HR"

10 amps per hour for 20 hours.

digitallexus · 03-19-2003, 01:26 AM

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.

frodobaggins · 03-19-2003, 02:41 AM

your laptop battery only has 2800mah ?

Hell, I have a cell phone battery that will do 3800mah

I charge it once a month, or less

Electronicsuk · 03-19-2003, 02:53 AM

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

digitallexus · 03-19-2003, 02:56 AM

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!

digitallexus · 03-19-2003, 03:01 AM

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.

Sir_Pyro · 03-19-2003, 07:22 AM

Ok, thanks for your help.

About the laptop's battery, the reason I wanted to know bout trickle charging the Ni-Mh battery is I just want to tap the contacts for the battery in the laptop to power the thing. It has a 120v input on the back of it so I wanted to avoid using another inverter in the system. Looks like I'm going to end up building a new charging circuit into it....

Oh, yeah. I checked how much current the laptop pulled from the battery. It pulses between 0.5 amps and 2 amps.

Electronicsuk · 03-19-2003, 01:10 PM

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

Sir_Pyro · 03-19-2003, 05:11 PM

Ok, I suppose I should have told you WHY I'm using that laptop. To convince my dad to even let me do this to my car, I had to make the system 100% removable with out a trace that it was there. The only exception to this was a single power wire through the firewall and a few grounding points that are hidden. That means I cant have a dash mounted LCD

.

With out a LCD I could not do thing like edit playlist or transfer files over the wireless LAN while in my car. I happened to find these POS laptops at the school my mom works at and I swiped a few (they think I'm working on them). So, now I have a way to control the computer in the trunk through a in car LAN. I can’t use it while driving or any thing but that is what my palm is for.

Now why I want to keep the battery in it. First off, reliability. If the power cable comes undone or voltage drops to low when I start my car it will stay on. Second, I'm paranoid about my car's main battery. If my car is not on, I don't want more then the microprocessor in my shutdown controller drawing power from it.
Finally, I want to be able to take the laptop out of the car and use it with out ever having to charge it with 120Vac.

frodobaggins · 03-21-2003, 12:54 AM

Quote:

Originally posted by digitallexus
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!

duh...(I'm dumb)
It was late, was coding some VB, ran out of cigarettes and chocolate milk...point being, sometimes I forget to think before I post

Aaron Cake · 03-22-2003, 11:25 AM

There's a lot of BS in this thread.

digitallexus: I battery cannot supply power at 100% efficiency, period. The AH rating on lead acid batteries is calculated by the applied load until the battery is dead (below 1 volt per cell, if I remember correctly).

Electronicsuk: Ni-MH batteries have very specific charging requirements. A "dumb" trickle carger will kill the battery very quickly. In order to properly charge a Ni-MH battery, current and temperature must be constantly monitored, and charge adjusted to match. Most consumber devices don't do this, which is why battery life is so short.

Electronicsuk · 03-22-2003, 12:55 PM

The circuit is described is not really a 'dumb' charging system. The system would detect the current draw from the batteries during charging, and then they are almost fully charged the current would be reduced to something around the batteries self discharge rate. This would keep it topped off and hopefully in good condition. I have never had any problems with chargers that work in this way.

Still, I will take on board your comments.

Best regards,
Matthew

Aaron Cake · 03-23-2003, 10:25 AM

These chargers are fine lead acid batteries but quite wrong for Ni-Cad, Ni-MH, Li, etc.