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Old 11-05-08, 06:16 PM   #1
joshandlauri
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Battery Pack Question

For the life of me I can't figure out if this is right, 7 years out of electronics school and I can't remember anything.

Anyway I have a 10aa battery pack for my 20 watt halogen light (little short on 2500 energizers so I improvised)
If I use
8 2500mah
1 700mah and
1 1800mah
what is the MAH rating @ 12volts.
I think I am suppose to use the average of all batteries and it makes it about 2200mah @ 12 volts, sound right?
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Old 11-05-08, 07:58 PM   #2
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I can't remember the math for the milliamps either - however, just a note, the standard 20 watt halogen (MR16) bulb is rated at 12 volts, and I made the same assumption you did that 12 volts would be enough. However, you get a significant light output boost if you over volt it to 14 volts. The bulb will die slightly sooner, but we're talking in the hundreds of hours of usage still so it's not a significant drop in how long it lasts.
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Old 11-05-08, 08:37 PM   #3
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i have a charger that charges it to 14.3 volts, I just didn't want to be more confusing.
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Old 11-05-08, 09:11 PM   #4
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I don't think it's a good plan to mix capacities like that, if you're running them in series.
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Old 11-05-08, 09:21 PM   #5
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Quote:
Originally Posted by joshandlauri View Post
For the life of me I can't figure out if this is right, 7 years out of electronics school and I can't remember anything.

Anyway I have a 10aa battery pack for my 20 watt halogen light (little short on 2500 energizers so I improvised)
If I use
8 2500mah
1 700mah and
1 1800mah
what is the MAH rating @ 12volts.
I think I am suppose to use the average of all batteries and it makes it about 2200mah @ 12 volts, sound right?
if I remember my playing with electric vehicles well enough a series string is only as strong as it's weakest link so it would be 700mah.. If you run it after the 700mah hour is dead you risk reversing it heating it up and pop goes the battery. It is only 700mah instead of 34ah of liquid NICD or 220ah of PBA so I'm not sure if it'll be more or less dangerous.

You can theoretically average capacities that are in parallel not in series.
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Old 11-06-08, 06:28 AM   #6
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You don't want to have different capacities in series. Take the 700 and 1800 and put them in parallel and call it one battery, then you have 9-2500mAh to work with. You'd get 10.8V, 2500mAh which may be too low of a voltage for you.
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Old 11-06-08, 02:12 PM   #7
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The correct answer is 700mAh, the capacity of the lowest capacity cell . Since that cell is of lower capacity than the others, it will discharge more quickly, roughly 1/3 the time of the 2500 mAh cells. Once that cell is discharged, the series pack effectively becomes a 10.8V 1700mAh pack. But since the 700mAh cell is still wired in series, the current flowing through it start "reverse charging" it. This is very unhealthy for NiMH cells, and will essentially destroy it if this goes on for very long. If you continue drawing power the process repeats with the next weakest link, the 1700mAh cell.

As another poster said, it's not a good idea to run mismatched cells in a series pack , but if you do you have to be careful about not fully discharging any of the cells. Of course that's not easy to do, and especially so if the there's a big mismatch as in your example. Some light systems that are designed to use series NiMH packs, like the Dinotte AA series, have a low voltage cutoff that helps prevent this from happening, but even then they assume that your cells are fairly closely matched. It's unlikely that your halogen light has any cutoff circuit, so if you just ran it until it was dim, you'd likely destroy one cell, possibly two or more depending on how far you pushed it.

I hope that helps, I'm not sure that I explained it clearly, just ask if not.
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Old 11-06-08, 07:37 PM   #8
joshandlauri
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Originally Posted by bbrase View Post
The correct answer is 700mAh, the capacity of the lowest capacity cell . Since that cell is of lower capacity than the others, it will discharge more quickly, roughly 1/3 the time of the 2500 mAh cells. Once that cell is discharged, the series pack effectively becomes a 10.8V 1700mAh pack. But since the 700mAh cell is still wired in series, the current flowing through it start "reverse charging" it. This is very unhealthy for NiMH cells, and will essentially destroy it if this goes on for very long. If you continue drawing power the process repeats with the next weakest link, the 1700mAh cell.

As another poster said, it's not a good idea to run mismatched cells in a series pack , but if you do you have to be careful about not fully discharging any of the cells. Of course that's not easy to do, and especially so if the there's a big mismatch as in your example. Some light systems that are designed to use series NiMH packs, like the Dinotte AA series, have a low voltage cutoff that helps prevent this from happening, but even then they assume that your cells are fairly closely matched. It's unlikely that your halogen light has any cutoff circuit, so if you just ran it until it was dim, you'd likely destroy one cell, possibly two or more depending on how far you pushed it.

I hope that helps, I'm not sure that I explained it clearly, just ask if not.

thank you everyone for responding, I ran it today and started with almost 14 volts, after getting home (35 minutes) the 700mah was super hot and deader than dead, I installd another 1800mah in its place had to steal from the remote (don't tell the wife), this weekend I'll get more energizers.

Thanks again.
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