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Old 12-19-2016, 12:19 PM   #11
Brandon1107
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Default Re: Low Voltage Alarm

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I think the opposite ought to be true (slow drop, near instant recovery). You would have to experiment with a resistor or a few tries, to find one that drains the capacitor of roughly 10v in say 5 seconds.
cgtech,

If you come up with a list of stuff I need to pick up to try it I will. I know it will be experimental but I'd like to learn.

Brandon
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Old 12-19-2016, 12:19 PM   #12
cgtech
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Default Re: Low Voltage Alarm

Well, when the battery voltage has had a little time to recover. The voltage creeps back up during the first half hour after the load is removed, the majority of that rise happens during the first minute or two.
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Old 12-19-2016, 12:21 PM   #13
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Default Re: Low Voltage Alarm

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cgtech,

If you come up with a list of stuff I need to pick up to try it I will. I know it will be experimental but I'd like to learn.

Brandon
"home game" it, find a old broken dvd player, or something, and you will have plenty of parts to work with. All you will need is a soldering iron & a screwdriver.
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Old 12-19-2016, 12:24 PM   #14
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Default Re: Low Voltage Alarm

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"home game" it, find a old broken dvd player, or something, and you will have plenty of parts to work with. All you will need is a soldering iron & a screwdriver.
Challenge accepted. If anyone has a ballpark range of diodes and capacitors to try let me know.
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Old 12-19-2016, 01:43 PM   #15
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Default Re: Low Voltage Alarm

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I think the opposite ought to be true (slow drop, near instant recovery). You would have to experiment with a resistor or a few tries, to find one that drains the capacitor of roughly 10v in say 5 seconds.
I was referring to the battery pack voltage sag rather than voltage on the capacitor. I clarified the statement in my post.

As for the TC, the shorter it is, the more the meter bounces.

Lets say your cart drew 200A while climbing a hill at full throttle and the pack voltage dropped from an at rest voltage on the meter of 37.5 (38.2V less a 0.7V diode drop) down to 35.5V (which is only 1.0V drop per 100A of current draw) Five seconds later, the charge on the cap would be 25.5V if the battery pack wasn't connected to, which is lower than the 35.5V that battery pack voltage would still be at 5 seconds after the load is removed. So the meter would read pack voltage (less a diode drop) just as if there wasn't a 5 second damper in to the circuit.

On the chart I posted, my 36V pack only recovered 320mV in the first 5 minutes, so the capacitor discharge rate would have to be less than that to avoid meter bounce. Since both the battery pack's recovery rate and the capacitor's discharge rate are non-linear, the math gets a little complex.

Basically, what we are trying to do is roughly the same as the analog respiration rate meters found on the physiological monitors I worked on when I started fixing medical equipment. The high and low alarms were triggered by where the needle was pointing. If the cap was drained too fast, the needle swung downscale a lot between breaths and caused confusion as to what the rate actually was. On the other hand, if the cap was drained too slow, it took too long for the needle to drop low enough for the meter to set off the low alarm when the patient stopped breathing. The analog heart rate meters of the era had a similar set of problems.

The conflict of choices was solved in later generations of physiological monitors by adding a timed dump circuit to a capacitor with a slow drain. If a breath (or beat) didn't occur within a specified time after the the last detected breath (or beat) than the charge of the capacitor was discharged rapidly. Of course, there is no need for anything as complex as a time delayed dump circuit for a low battery alarm in a cart.
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Old 12-19-2016, 01:50 PM   #16
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Default Re: Low Voltage Alarm

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Challenge accepted. If anyone has a ballpark range of diodes and capacitors to try let me know.
A 1N4004 or similar will work for the diode.

Here is a link to a RC time constant calculator: http://www.digikey.com/en/resources/...-time-constant

Have fun.
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Old 12-19-2016, 08:19 PM   #17
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Default Re: Low Voltage Alarm

I had a funny little "10-led" meter laying around that operated nothing like the typical "Curtis 10-led meter", it was very "bouncy". This led me to disassemble it to see what made it tick. What I found was delightful (to a nerd like me). Here's some fun stuff for a potential project that would be accurate enough, easy to compensate for the diode drop, and easy to integrate a warning beeper. And, with the "capacitor/resistor" idea, easy to control & set how sensitive it is to sudden voltage drops. Not quite as specific as a volt meter, but easy to use by the "electrically uninformed".

Here's a pic of the meter, and a link to a awesome project idea. If you search the LM3914 chip, there are plans for one with a warning buzzer.

https://learn.sparkfun.com/tutorials...r-hookup-guide
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