Any electronic gurus about? Battery questions inside!
13 June 2008, 10:59 AM
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Any electronic gurus about? Battery questions inside!
I'm quite a novice when it comes to electronics, so I'll probably sound silly but hey, it might stop me blowing something up 
If I have a requirement to provide a minimum input voltage of 11.6V, is there any benefit of going for say a 14.8V battery against a 12V battery (assuming similar mAh ratings for both batteries)? Would the 12V drop off before the 14.8V? Would there be a big difference in run time? Am I talking nonsense?
If it helps, the battery will be driving a buck converter and 3 LEDs. At full power, the LEDs should take 1000mA.
I'm currently trying to understand discharge curves, "C" ratings and how safe LiPo battery packs are...
Thanks!
If I have a requirement to provide a minimum input voltage of 11.6V, is there any benefit of going for say a 14.8V battery against a 12V battery (assuming similar mAh ratings for both batteries)? Would the 12V drop off before the 14.8V? Would there be a big difference in run time? Am I talking nonsense?
If it helps, the battery will be driving a buck converter and 3 LEDs. At full power, the LEDs should take 1000mA.
I'm currently trying to understand discharge curves, "C" ratings and how safe LiPo battery packs are...
Thanks!
13 June 2008, 11:49 AM
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and how safe LiPo battery packs are...
YouTube - LiPo Explosion: The Glorious Destruction
NiMh seems to be the way to go!
13 June 2008, 02:26 PM
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A buck converter shouldn't care too much what the input voltage is. The higher the voltage the less current it'll draw, keeping the total power about the same. (Power = volts * amps).
The total run time will depend on the capacity of the battery times the voltage. So, for example, a 12V battery with a capacity of 1000 mAh will last the same time as a 6V battery with a capacity of 2000 mAh. If the mAh rating is the same for both a 12V and a 14.8V battery then the 14.8V will last longer, because the buck converter draws less current from it.
LiPo batteries are quite safe provided you use the correct charger and don't physically mistreat them. I wouldn't keep one in my phone 6 inches from my ********* otherwise!
The total run time will depend on the capacity of the battery times the voltage. So, for example, a 12V battery with a capacity of 1000 mAh will last the same time as a 6V battery with a capacity of 2000 mAh. If the mAh rating is the same for both a 12V and a 14.8V battery then the 14.8V will last longer, because the buck converter draws less current from it.
LiPo batteries are quite safe provided you use the correct charger and don't physically mistreat them. I wouldn't keep one in my phone 6 inches from my ********* otherwise!
13 June 2008, 02:53 PM
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Mobile phones ( and laptops) usually use Lithium Ion batteries which are more stable than Lithium polymer for general use.
13 June 2008, 08:18 PM
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A buck converter shouldn't care too much what the input voltage is. The higher the voltage the less current it'll draw, keeping the total power about the same. (Power = volts * amps).
The total run time will depend on the capacity of the battery times the voltage. So, for example, a 12V battery with a capacity of 1000 mAh will last the same time as a 6V battery with a capacity of 2000 mAh. If the mAh rating is the same for both a 12V and a 14.8V battery then the 14.8V will last longer, because the buck converter draws less current from it.
LiPo batteries are quite safe provided you use the correct charger and don't physically mistreat them. I wouldn't keep one in my phone 6 inches from my ********* otherwise!
The total run time will depend on the capacity of the battery times the voltage. So, for example, a 12V battery with a capacity of 1000 mAh will last the same time as a 6V battery with a capacity of 2000 mAh. If the mAh rating is the same for both a 12V and a 14.8V battery then the 14.8V will last longer, because the buck converter draws less current from it.
LiPo batteries are quite safe provided you use the correct charger and don't physically mistreat them. I wouldn't keep one in my phone 6 inches from my ********* otherwise!
The guy who makes the bucks (I've got a bFlexV2b coming from TaskLED) has said:
With 3 LEDs and 10 nimh cells you will be out of regulation pretty quickly.
NiMH cells range from 1V (fairly flat) to 1.2V (charged and under load). So you have 10V to 12V as you battery range.
You really need to be using 12 NiMH cells to get sufficient 'headroom'. The higher the input voltage the lower the current draw from the batteries (since bFlex is 'converting' input voltage to output voltage sort of/kind of like a transformer does for AC). So, with lower input current you get longer runtime.
The calculations are pretty simple.
1) Take output voltage x output current and that gives output power in watts.
2) multiply the output watts x 1.1 to get input watts (assuming 90% efficiency in bFlex)
3) take the input watts / input voltage and that gives you the input current
4) take you amp.hour battery rating / input current and that gives you runtime in hours
NiMH cells range from 1V (fairly flat) to 1.2V (charged and under load). So you have 10V to 12V as you battery range.
You really need to be using 12 NiMH cells to get sufficient 'headroom'. The higher the input voltage the lower the current draw from the batteries (since bFlex is 'converting' input voltage to output voltage sort of/kind of like a transformer does for AC). So, with lower input current you get longer runtime.
The calculations are pretty simple.
1) Take output voltage x output current and that gives output power in watts.
2) multiply the output watts x 1.1 to get input watts (assuming 90% efficiency in bFlex)
3) take the input watts / input voltage and that gives you the input current
4) take you amp.hour battery rating / input current and that gives you runtime in hours
14v 3700mAh battery, 3 LEDs with a Vf of 3.3v each, run at 1000ma.
1) 9.9v (output voltage ie sum of Vf?) * 1000ma = 9900 output watts
2) 9900 * 1.1 = 10890 input watts
3) 10890 / 14 = 777.85 input current
4) Should this be 3.7 / 777.85 or 3700 / 777.85? 3700/777.85 gives a run time of 4.75hrs which seems too good to be true!
Last edited by ChrisB; 13 June 2008 at 08:21 PM.
13 June 2008, 09:08 PM
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It'll probably be less that that, as when the cells discharge, the voltage drops, so the "actual" run-time will be the length of time it takes discharge to the minimum acceptable voltage.
And thats a bit more complicated to do accurately, but I seem to recall an arbtitary figure was 60% of the cell's rating for Ni-Cd cells for r/c use (I read it somewhere many moons agao, hell I could have it completely wrong!
).
And thats a bit more complicated to do accurately, but I seem to recall an arbtitary figure was 60% of the cell's rating for Ni-Cd cells for r/c use (I read it somewhere many moons agao, hell I could have it completely wrong!
).
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13 June 2008, 09:09 PM
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Output power = 3.3 * 1000mA * 3 = 9.9W
Input power = 9.9 * 1.1 = 10.9 W
Input current = 10.9 / 14 = 778 mA
Therefore, run time is approx. 3700/778 = 4.75 hrs, just as you calculated.
Looks like a good little device, it should work fine.
Input power = 9.9 * 1.1 = 10.9 W
Input current = 10.9 / 14 = 778 mA
Therefore, run time is approx. 3700/778 = 4.75 hrs, just as you calculated.
Looks like a good little device, it should work fine.
13 June 2008, 09:12 PM
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As lipo'd discharge, they generate heat, overheat them and they die.
Also if you keep using them once the current starts to drop off, you will perminantly damage them, or cause them to swell (risk of explosion)
Mart
Also if you keep using them once the current starts to drop off, you will perminantly damage them, or cause them to swell (risk of explosion)
Mart
13 June 2008, 10:15 PM
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Thanks guys - your help is much appreciated. I'd be happy with 3 hours run time @ 3 x 1000ma (which would be more by using 50 or 75% power on easy sections).
I might look at LiPo or LiIon at a later date - the cost on the higher voltage / mAh is a bit steep (plus the correct LiPo charger on top).
Could you connect two smaller individual packs (eg 8.4v 3700mAh) in series, would it make a feasible 16.8v? Would you run into problems with having two packs?
I might look at LiPo or LiIon at a later date - the cost on the higher voltage / mAh is a bit steep (plus the correct LiPo charger on top).
Could you connect two smaller individual packs (eg 8.4v 3700mAh) in series, would it make a feasible 16.8v? Would you run into problems with having two packs?
Last edited by ChrisB; 13 June 2008 at 10:18 PM.
14 June 2008, 10:47 AM
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Shouldn't be an issue:
An 8.4v battery pack is just seven 1.2v cells soldered together in series, so doubling it up is possible. Putting another pack in series is effectively connecting fourteen cells in series.
However charging them maybe an issue, or at least finding a suitable charger. Could always wire it with quick connectors so the two packs could be split and charged individually (or in parallel).
An 8.4v battery pack is just seven 1.2v cells soldered together in series, so doubling it up is possible. Putting another pack in series is effectively connecting fourteen cells in series.
However charging them maybe an issue, or at least finding a suitable charger. Could always wire it with quick connectors so the two packs could be split and charged individually (or in parallel).
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