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chmee
super member
Reged: 05/11/05
Posts: 158
Loc: Takoma Park, MD
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I'm currently using a big jump starter battery to power dew heaters and fan on my home-built dob, but I'd much prefer to use an 8-pack of rechargeable AA's in a holder mounted on the mirror cell.
However, I'm having problems finding out how long such a pack would last. My batteries are 1.2V 2500 mA-hours, so 8 of these would seem to be equivalent to a big honking jump starter battery that has 18 AH, right?
I'm pretty sure that the dew heaters will cope w/ less voltage, but what about the current draw?
I'm driving a protostar spider dew heater, plus a kendrick controller w/ 2-3 heating pads (2 eyepieces, 1 telrad).
Thanks,
pete
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16" dob
ED80sf
15x50 IS binocs
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2456
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all you have to do is measure the resistance of the heater and use ohms law.
amps = voltage/resistance
so if your heater measures 20 ohms:
you take 12(your volts) and divide it by 20 (your resistance) and that gives you .6 (your draw in amps)
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2456
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dont forget to move the decimal over to convert mA (milliamps) to amps.
2500 mA = 2.5 amps
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2456
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an 18 amp jump starter battery will usually deliver 9 amp hours in actual use
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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chmee
super member
Reged: 05/11/05
Posts: 158
Loc: Takoma Park, MD
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The spider heater measures 75 ohms, so 160mA.
How about for the other side, how to find out much current draw is safe for the AA's? I don't need them exploding or bursting into flames near my precious glass.
Conversely, I'd hope the 8 would last 3-4 hours, anyway. The batteries are nominally 2500 mAH. I can easily run one down in an hour (handheld fan). If we assume your 1/2theory = practice rule, this would seem to mean it's delivering near an amp, but that sounds high.
It sounds like a pack of 8 of these guys would last many, many sessions.
Thanks
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16" dob
ED80sf
15x50 IS binocs
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Ben Mullin
member
Reged: 08/26/05
Posts: 97
Loc: Lindstrom, MN
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Battery lifetime is a tricky thing. That 2500 mAH is rated at some predefined current draw. The actual life time could be considerably different depending on the actual current draw. You will have to find a datasheet on the battery to have any hopes of calculating it rather than finding out empirically.
Additionally, you will have to account for all of the heaters, not just one. Find out what the other heaters are as well. You can do the calculations individually and sum the currents or you can find the equivalent resistance of the heaters in parallel: 1/Rtot = 1/R1 + 1/R2 + ... and use the new Rtot in Ohm's law.
This assumes that the controller has no current draw of its own, likely not true, but possibly minimal compared to the current draw of the heaters. And also that the heaters are on fully.
Less voltage will mean less current at full on which will mean less heat. So with less voltage your heaters will possibly not be as warm as they were before.
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randtek
professor emeritus
Reged: 06/18/05
Posts: 694
Loc: Central Indiana
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Quote:
I'm currently using a big jump starter battery to power dew heaters and fan on my home-built dob, but I'd much prefer to use an 8-pack of rechargeable AA's in a holder mounted on the mirror cell.
However, I'm having problems finding out how long such a pack would last. My batteries are 1.2V 2500 mA-hours, so 8 of these would seem to be equivalent to a big honking jump starter battery that has 18 AH, right?
I'm pretty sure that the dew heaters will cope w/ less voltage, but what about the current draw?
I'm driving a protostar spider dew heater, plus a kendrick controller w/ 2-3 heating pads (2 eyepieces, 1 telrad).
Thanks,
pete
I used to have to test maH on camcorder batteries while servicing the camcorders. I designed and built a tester that would discharge a battery at a controlled rate, usually 1 amp, but this was adjustable. The circuit would keep the current draw constant until the battery tested a 10% decrease in voltage, which was then considered discharged. I would time the discharge. It was then easy to calculate maH rating.
If you are trying to run a 12 volt dew heater with 1.2v AA batteries, you will need 10 of them, not 8. 8 AA Batteries will only give you 9.6v. And the batteries will need to be wired in series, not parallel. Wiring 10 1.2v 2500mah batteries in a series configuration would give you a 12v pack, with approximately 2500mah. If wired in Parallel, you would have a 1.2 volt pack, with 25000mah (approx). These values are not definite, as was stated in a previous post, the maH (Milli-Amp Hour) rating is tested under specific controlled conditions that would likely not be replicated in your dew heater. In short, no, 8 AA batteries cannot be used as a direct replacement for a 12x 18AH battery. That would require about 80 AA batteries, wired in a combination series/paralell configuration. (probably not a practical sloution  )
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
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chmee
super member
Reged: 05/11/05
Posts: 158
Loc: Takoma Park, MD
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Quote:
Wiring 10 1.2v 2500mah batteries in a series configuration would give you a 12v pack, with approximately 2500mah.
Capacities (the 2500 mah of the individual batteries) should just add. Do you mean milli-amps? If so, how are you arriving at current from capacity, is there some rule of thumb here that I'm missing?
The 1.2V actually seems to be a minimum; the batteries are just under 1.4V when freshly charged, giving me ~ll volts w/ the 8. This seems close enough. I'm only powering heaters, here, so having undervoltage shouldn't cause any problems other than slightly less heat?
It's the current, and battery life that most concerns me. I've measured the telrad, 1/4" eyepiece heater, 2" eyepiece heater and secondary heater at 30, 93, 44, and 75 ohm. This should give me a max current draw of .96 amps at 12volts, or .77 down at 9.6 volts.
1) am I missing anything here?
2) assuming this is correct, the question is whether the AA pack can push an amp for four-five hours. Nominally, the 8 AAs should have an aggregate of 20 ah, so pushing one amp for four-five hours should be cake, right?
Quote:
If wired in Parallel, you would have a 1.2 volt pack, with 25000mah (approx). These values are not definite, as was stated in a previous post, the maH (Milli-Amp Hour) rating is tested under specific controlled conditions that would likely not be replicated in your dew heater. In short, no, 8 AA batteries cannot be used as a direct replacement for a 12x 18AH battery. That would require about 80 AA batteries, wired in a combination series/paralell configuration. (probably not a practical sloution )
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16" dob
ED80sf
15x50 IS binocs
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randtek
professor emeritus
Reged: 06/18/05
Posts: 694
Loc: Central Indiana
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Quote:
Capacities (the 2500 mah of the individual batteries) should just add. Do you mean milli-amps?If so, how are you arriving at current from capacity, is there some rule of thumb here that I'm missing?
The charge capacity of a battery is referred to as "ampacity" It is measured in aH (ampere Hours). Smaller batteries are listed in "milliamp hours". a 1.2v 2500 maH battery when NEW should be capable of delivering a theoretical 1 amp for 2.5 hours at 1.2v. That does not mean that you can safely discharge that particular rate. You may only be able to draw 500mA safely before heating of the cells becomes excessive.
And no, the capacites of batteries do not "just add". As I stated in my previous post, it depends on the configuration that the batteries are wired in. When wired in series, voltages add, current remains the same. When wired in parallel, voltage remains the same, currents add. Ampacity is a measure of current, not voltage, so it is calculated as current would be.
Quote:
The 1.2V actually seems to be a minimum; the batteries are just under 1.4V when freshly charged, giving me ~ll volts w/ the 8. This seems close enough. I'm only powering heaters, here, so having undervoltage shouldn't cause any problems other than slightly less heat?
You are measuring the batteries in a "no load" condition. When under load, they will be running at a nominal 1.2v, not 1.4. They will only test that high when fully charged and not under load, and will have decreased in voltage by the time you have discharged them. You will be starting with a nominal 9.6v, discharging to 8.6v when discharged.
Quote:
It's the current, and battery life that most concerns me. I've measured the telrad, 1/4" eyepiece heater, 2" eyepiece heater and secondary heater at 30, 93, 44, and 75 ohm. This should give me a max current draw of .96 amps at 12volts, or .77 down at 9.6 volts.
That's exactly the point. .96 amps at 12v is the figure you should probably be working toward. Your 9.6v AA battery array will be able to run for about 3 hours, with dew heater performance starting lower than rated, and ending even lower than that. These figures will only get worse as batteries age. The heaters may not operate warm enough at 8v instead of 12 v, as this is only 2/3 rated voltage. Heat output will probably not decrease linearly with voltage. When designing electronics it is usually wise to over rate, and unwise to under rate. Running you batteries so close to the limits may result in decreased battery lifetimes.
Quote:
2) assuming this is correct, the question is whether the AA pack can push an amp for four-five hours. Nominally, the 8 AAs should have an aggregate of 20 ah, so pushing one amp for four-five hours should be cake, right?
No, because the ampacities DO NOT add like that. You are beginning with an incorrect assumption. With 8 AA batteries, you would have a 9.6v 2500maH battery. Nothing more.
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
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chmee
super member
Reged: 05/11/05
Posts: 158
Loc: Takoma Park, MD
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Quote:
No, because the ampacities DO NOT add like that. You are beginning with an incorrect assumption. With 8 AA batteries, you would have a 9.6v 2500maH battery. Nothing more.
Okay, finally got it. I had shifted over into power, not current. Bummer. Need this.
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16" dob
ED80sf
15x50 IS binocs
Edited by chmee (08/04/08 03:49 PM)
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randtek
professor emeritus
Reged: 06/18/05
Posts: 694
Loc: Central Indiana
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Now you've got it mate!
Please accept my apologies if I came across a little grumpy in my previous post. That was not my intention. I just realized as I was looking over it again that it could be interpreted that way.
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
|
chmee
super member
Reged: 05/11/05
Posts: 158
Loc: Takoma Park, MD
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No worries, I appreciate the help.
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16" dob
ED80sf
15x50 IS binocs
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Gregg Lobdell
sage
Reged: 01/20/06
Posts: 373
Loc: Covington, WA
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chmee, randy has you straightened out. Basically you can add up the volts (series), or you can add up the amp-hours (parallel), but not both. The only way to get more of both is to get a bigger battery.
The heaters are probably simple resistive devices, however the telrad is not. On the other hand, it's the least of your worries. If you want to get a true reading for the telrad, you'll need to directly measure the current at the operating voltage.
With your heaters, the heating is proportional to the wattage, and wattage is V^2 / R, so your heating goes down as the square of the voltage. So at 8 volts, you'll get less than half the heat, compared to what you'll get at 12 volts. This may be a good thing or a bad thing.
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Clear skies,
Gregg
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chmee
super member
Reged: 05/11/05
Posts: 158
Loc: Takoma Park, MD
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Quote:
The heaters are probably simple resistive devices, however the telrad is not.
Actually, the telrad is powered by its own battery. What I meant was the telrad warmer, which is just a resistor pad like all the others.
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16" dob
ED80sf
15x50 IS binocs
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