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capacitors in series

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kinarfi

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I have a GPS which will charge lithium battery batteries, only, when plugged in, but the lithiums I've had seem to have a short lifetime. The GPS has an open circuit charging voltage of 4.23v, so, I recently purchased 2 2.5v 15F Super capacitors to replace the batteries. I connected them in series to give me the voltage I needed, SOOOO, did I end up with a capacitor rated at 5v and 15F or 7.5F?
Jeff
 

Tony Stewart

Well-Known Member
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Lithium batteries with say a 2Ah life dropping 15% or about 1/4 of a RC=T cap time constant in voltage means since C=I*dt/dV

thus C=2Ah*3600s/h * 1/4 = 1800 Farads equivalent charge. so 7.5F will last even less time dropping the same 15% in voltage

obviously your W-h or VA-h load is greater than the W-h capacity you are providing, meaning your batteries may have expired.
 

audioguru

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The voltage of a discharging capacitor is the opposite to a Lithium rechargeable battery, it drops fast at first then is useless sooner than you think. The voltage of a discharging Lithium battery stays fairly high until it is nearly dead.
If the Lithium batteries were cheap Chinese ones from ebay then they are probably fakes. Buy Name-Brand replacement batteries from an honest local distributor.
 

spec

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Afraid you cannot connect capacitors in series and expect them to voltage share.

The actual value of super caps varies greatly, from -50% to +100% (I think).

With capacitors in series across a voltage, each capacitor will acquire the same charge, so that the voltage across a capacitor is inversely proportional to the actual capacitance.

For example, if you had a 1F and 2F capacitor in series across 3V, the 1F capacitor would have 2V across it and the 2F capacitor would have 1V across it.

Then there is leakage current to consider.

spec
 

kinarfi

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Thank you!!!!
 

kinarfi

Well-Known Member
No probs kinarfi.:)

spec
Just a little FYI, I have a DeLorme PN60 GPS that's about 5 years old, DeLorme is now owned by Garmin, and while it still works well, I keep having battery problems. I had MiMH batteries in it, but always forgot to put freshly charged ones in and it would drop out, probably due to momentary power connection loss, and I wouldn't notice for a while, So I would lose that part of the track. Because of my handicap, I never take it anywhere by foot, just in my Trooper. It will run for about a (minute)(to be edited) on the super caps now. The connection to USB cord has been modified also because the unique original became expensive to replace. Seems to be doing well now!
Spec, I think it was you that said something about having a resistor in the gate for FETS , on my power steering project's H bridge, I PWM the NFET (IRF2805) via a NE556 and 100Ω in parallel with a diode and just turn on it's complement PFET (IRF4905) via a 510Ω resistor, would you care to make any comments or suggestions?
Thanks,
Jeff
 

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spec

Well-Known Member
Most Helpful Member
Just a little FYI, I have a DeLorme PN60 GPS that's about 5 years old, DeLorme is now owned by Garmin, and while it still works well, I keep having battery problems. I had MiMH batteries in it, but always forgot to put freshly charged ones in and it would drop out, probably due to momentary power connection loss, and I wouldn't notice for a while, So I would lose that part of the track. Because of my handicap, I never take it anywhere by foot, just in my Trooper. It will run for about a (minute)(to be edited) on the super caps now. The connection to USB cord has been modified also because the unique original became expensive to replace. Seems to be doing well now!
Spec, I think it was you that said something about having a resistor in the gate for FETS , on my power steering project's H bridge, I PWM the NFET (IRF2805) via a NE556 and 100Ω in parallel with a diode and just turn on it's complement PFET (IRF4905) via a 510Ω resistor, would you care to make any comments or suggestions?
Thanks,
Jeff
Hi kinarfi,

Thanks for the information.

I am running LTSPICE IV and it is objecting to your .asc file: could you post an image of your schematic?

spec
 

kinarfi

Well-Known Member
do a screen shot of what spice is say it's missing and I'll make a zip file and make sure I have my special little additions in it, please & thankls
 

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Also, no matter how good your Li-ion batteries are, unless there is a good battery management system in your charge/discharge system, they won't last nearly as long as they could. Temperature control is really important as well. Heat kills Li-ion batteries.
 

spec

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do a screen shot of what spice is say it's missing and I'll make a zip file and make sure I have my special little additions in it, please & thankls
Got it:)

Can you explain what M1 and M2 (BS250) are doing?

How much current does 'MOTOR' take?

spec
 

kinarfi

Well-Known Member
motor pulls around 47.5, measured, when stalled, 15 has been the best I could measure running, I'll work on getting some more accurate numbers,
M1 & 2 are start up delay, they come on when you start the system and pull the gates off via the 556
also, M 1& 2 are used to turn off the side not running, that is, if turning right, the feed back turn the left side off.
 
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spec

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Hi kinarfi,

I have had a look at the way that you are driving MOSFETs: in one case via 100R resistors and diodes and in the other case 510R resistors. If the circuit is working without problems that will be OK because it will shape the drive voltage on the MOSFETs and also protect the 555 outputs.

510R is a bit high so it will slow the voltage edges and cause the MOSFETs to dissipate more power during transitions. But I can see that the high value is to allow the gate drive to be inhibited by the two lower PMOSFETs.

Normally, as a defense against MOSFETs oscillating, you can put a 10R to 100R resistor physically on the gate terminal to act as a base stopper: but if there is no oscillations the gate stoppers are not needed.

Your circuit looks very sophisticated: I would be intrigued to know exactly how it works.:)

spec
 
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kinarfi

Well-Known Member
Hi kinarfi,

Your circuit looks very sophisticated: I would be intrigued to know exactly how it works.:)

spec
The simple answer would be " It appears to work quite well on the bench,:happy: but i'm thinking you want a more detailed theory of operation, I'm putting together a zip file with all you'll need.
Jeff
 

kinarfi

Well-Known Member
Hi kinarfi,


Your circuit looks very sophisticated: I would be intrigued to know exactly how it works.:)

spec
This should take care of what you asked for, unzip it and look over the files, then click on either 20160830 STEERING-2016-10-06P.asc or 20160830 STEERING-2016-10-07.asc, there are a lot of comments that explain what's happening on the schematics, the main difference is 20160830 STEERING-2016-10-06P.asc uses a PFET, the other uses an NFET to turn the circuit on and off, earlier attempts at other methods would not do a total zero current shut down, simulation doesn't seem to like the NFET design. They should run from the unzipped folder.
I wasn't able to upload the .zip file, so I put it in google Drive. https://drive.google.com/file/d/0ByQAhs0e-yF9d21sTlJ5NkNLOWs/view?usp=sharing
enjoy,
Jeff
 

kinarfi

Well-Known Member
Hooked it up today and started tuning it up, but after a while, it just seemed to lose any authority to drive the motor. After returning the bench, I finally found that the "Delay " 555 was oscillating after a while. I removed the 555 chip and had not further problems on the bench, tomorrow, I'll try it back in the vehicle.
 

MrAl

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I have a GPS which will charge lithium battery batteries, only, when plugged in, but the lithiums I've had seem to have a short lifetime. The GPS has an open circuit charging voltage of 4.23v, so, I recently purchased 2 2.5v 15F Super capacitors to replace the batteries. I connected them in series to give me the voltage I needed, SOOOO, did I end up with a capacitor rated at 5v and 15F or 7.5F?
Jeff
Hi,

As spec pointed out you may have a problem running two caps in series for this purpose. If you still really way to do that, you have to employ a charge balancing scheme similar to what is used for charging two Li-ion cells in series. That basically allows both 'cells' to charge until one reaches the max voltage and then it only allows the other cell to charge, until it too reaches max voltage.
 

kinarfi

Well-Known Member
I let the GPS charge up the caps and then unplugged the GPS and timed it until it shut down, about 2.5 minutes, nothing great, but enough to for momentary power loss and I'll never have to change batteries again. If I have to go away from the vehicle, I have one of those USB battery back ups.
 
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