Calculating resistor value for adding ESR to capacitor bank - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: May 23, 2013, 08:03:47 am
content: I'm working on some upgrades to my cars super capacitor bank (replacing the original car battery.)Currently i have to give the engine some gas and keep the RPM high for 5 seconds after starting on a cold day to avoid it stalling.This is because the ESR of the super capacitors is an order of magnitude smaller than a typical car battery. (Cap bank 3.6mR, Typical car battery 58mR)As soon as the engine starts the alternator tries to push 40+ amps into the capacitors from idle rpm and this engine load is too much.There's probably a better way to fix this by hacking into the voltage reg inside the alternator but for now i just want to put a large wattage resistor into the alternator feed so the ESR is more normal (from the alternators perspective).The thing i'm not sure about is calculating the wattage that this resistor should be. 20W, 50W, 100W etc..The alternator is going to try and pull one side of the resistor up to 13.8V while the other side is fixed at capacitor bank voltage.However, the voltage across the resistor is governed by the current going through it and this current will drop to zero once the alternator senses the output is at 13.8V.It makes my head hurt and i suspect involves some sort of differential math.(or maybe it requires knowing the internal resistance of the alternator).Anyone have any comments/ideas?

author: G7PSK
author_rank: Super Contributor
timestamp: May 23, 2013, 09:07:23 am
content: Just make the resistor wattage greater than the alternators, you should be able to look up the details of the alternator if you know the make, quite often they are 40, 60 or 80 amps but some cars now have greater than that due to things like electric power steering.

author: Psi
author_rank: Super Contributor
timestamp: May 23, 2013, 09:10:44 am
content: Quote from: G7PSK on May 23, 2013, 09:07:23 amThe alternator is 70A 70A x 12V = 840WThe resistor wattage doesn't need to be anywhere near that large.

author: mzzj
author_rank: Super Contributor
timestamp: May 23, 2013, 11:09:26 am
content: Quote from: Psi on May 23, 2013, 08:03:47 amCheck your numbers, Typical car battery is more like 10mOhmHow low does the supercapacitor voltage go during start?

author: Psi
author_rank: Super Contributor
timestamp: May 23, 2013, 11:58:31 am
content: Quote from: mzzj on May 23, 2013, 11:09:26 amI got that 58mR value from a quick google search.It could be around 10mR per lead acid cell, that would be 6x10mR = 60mR for the entire battery.I have no doubt that the cap ESR is much lower. The sound of the engine cranking is crazy fast compared to a regular car battery. Probably because a regular car battery voltage drops under load far more than the capacitor bank.Less cap vdrop = more volts to starter motor.200A/433F = ~0.46 volt drop per second (rough cranking time)In reality it will be less, the current drops well below 200A once the engine is turning over.

author: G7PSK
author_rank: Super Contributor
timestamp: May 23, 2013, 12:22:55 pm
content: The resistance does need to be rated for the full output of the alternator, in a car the power goes from the alternator to the battery then all the power is taken off at the battery terminals therefore the resistor is going to carry the full load of the car it will also make things like your headlights dim as well unless you alter the wiring harness of the car.

author: Psi
author_rank: Super Contributor
timestamp: May 23, 2013, 12:26:11 pm
content: I'm planing on adding the resistor into the alternator feed. There's a nice fusible link where i can add it in.For the car to be able to start the cap bank would have to be at least 8V.So Vmax on the resistor is 13.8V - 8V = 5.8VThere for Imax of 5.8V * 70A = 406WBut im sure that's far more than needed.

author: G7PSK
author_rank: Super Contributor
timestamp: May 23, 2013, 12:51:06 pm
content: Thats the problem as I see it the alternator feed comes before the power take offs, when the car is running the alternator supplies most of the electrical load.

author: Psi
author_rank: Super Contributor
timestamp: May 23, 2013, 01:10:34 pm
content: I'm only thinking of adding maybe 40mR of extra resistanceThe wiring that's already running between the battery and alternator is probably about 5mR in itself.

author: G7PSK
author_rank: Super Contributor
timestamp: May 23, 2013, 01:16:41 pm
content: The resistor will still have to carry the full 70 amps. The rsistor will need to be rated at around 1000 watts as the alternator will always try to float the battery.

author: Rufus
author_rank: Super Contributor
timestamp: May 23, 2013, 01:17:13 pm
content: Quote from: Psi on May 23, 2013, 12:26:11 pmIf the maximum alternator output is 70A the maximum dissipation in a series resistance is 70^2 * R. Voltage doesn't come into it.

author: Psi
author_rank: Super Contributor
timestamp: May 23, 2013, 01:36:07 pm
content: Voltage only comes into it because if the car wont start then the alternator cant output anything.So we can assume the capacitor charge is at 8V or higher and so the resistor voltage is 5V or lower.Let me come at the problem from another angle.Lets say someone was building a car and had to decide on the wire thickness for the alternator feed.They wouldn't pick a wire gauge based on having to safely dissipate the full output of the alternator as heat.So it makes no sense to spec this added resistor to handle 840W either. (70*12)

author: Rufus
author_rank: Super Contributor
timestamp: May 23, 2013, 02:04:29 pm
content: Quote from: Psi on May 23, 2013, 01:36:07 pmThis is so simple I don't know how to make it any clearer.If the alternator is rated at 70A output and correctly sized for the car under maximum load it will be outputting 70A. That 70A flows through the alternator cable and will cause 70^2 * R dissipation in the cable where R is the cable resistance.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 23, 2013, 02:05:12 pm
content: I think concentrating on the 70A rating of the alternator is wrong - that's the continuous rating and I suspect that it's not only capable of higher currents short term but that the transient currentsaremuch higher, especially trying to charge the capacitor bank!Fortunately the currentswillbe transient so you won't need to rate the resistor fully.Psi didn't say what capacitance the system had and I don't know how much current his starter motor draws, nor how long the starter ran for so it's not possible to calculate what the voltage will be but let's guess 8V - it ought to be more than that otherwise he isn't going to get many attempts at starting but it's at the right end of the ballpark.Assuming that the alternator is putting out 13.8V and the bank has a 3.6mR ESR then the alternator is going to try to charge initially at (13.8-8)/0.0036 = 1611AIn practice the wiring will have at least as much if not more resistance so I doubt that charging will be at anything like that current - maybe a couple of hundred amps. Interestingly the alternator does appear to be able to keep up if it is run at full speed.Sticking an extra 60mR will reduce the current to 91A which I suspect the alternator will be more than able to deliver for a few seconds without the output voltage dropping too low for ignition, that would require a 527W resistor for continuous operation but it won't be, the power dissipated in the resistor will fall off quickly (with the square of the current, obviously). Farnell have some 50mR 100W chasis mount resistors that look plausible (if expensive).

author: Rufus
author_rank: Super Contributor
timestamp: May 23, 2013, 02:36:29 pm
content: Quote from: grumpydoc on May 23, 2013, 02:05:12 pmWhile I suspect the alternator won't be capable of much higher currents it being limited by the maximum field current the controller will provide. The continuous rating is worth concentrating on because driving along with lights on, demister, and other accessories it will be outputting 70A continuously.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: May 23, 2013, 02:44:50 pm
content: Alternator output, as well as input power demand from the engine is related to RPM as well. I find it strange that the alternator is loading the engine to the extent that it is stalling. I've seen a too small alternator used on a bus that ran at low speed. The alternator was operating at maximum output for that RPM (quite lower than it's "rated" output) yet it wasn't enough current to keep the batteries from draining. The engine was never stalled. Of course, this was a diesel engine with much higher torque at lower RPM. That may explain it.

author: SeanB
author_rank: Super Contributor
timestamp: May 23, 2013, 03:46:04 pm
content: If you want a simple cheap resistor take a 2.5mm welding rod and clean off the coating at the tip, then crimp and silver solder a lug on each end. Mount on a SRBP or GRP board after winding into a coil and place in series. Can handle the load, will self limit ( as it gets hotter it goes higher in resistance) and can handle the current. Place where it will not cook anything and where it is not going to touch. Paint it with 4-5 coats of road marking paint or another cement based paint.We used an old bedspread as a test load for the DC gensets, worked quite well up to full rated power of 8kA, though the springs did glow nearly white hot at that point.

author: G7PSK
author_rank: Super Contributor
timestamp: May 23, 2013, 04:17:04 pm
content: The other thing to consider is the stalling due to the alternator or is there a problem with the engine management or carb's.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 23, 2013, 04:44:44 pm
content: QuotePresumably, if it is (or was recently) OK with a battery but not OK with the ultra capacitors that would tend to point away from a straightforward problem with the car but it's a good point.You could take the simple approach - if the ultra capacitor ESR is 3.6mR and a battery is 60-70mR then add in 50mR series resistance to the "low current" connection to the battery (i.e not the starter motor wiring) - that should limit the peak charging current to the sort of amperage the alternator can supply.QuoteI must admit I found it strange enough that I didn't think of it. Car starter motors are usually a HP or two (say 750W to 1.5kW or 60-120A running at load) so only a few % of the engine power.I'd assumed it was voltage droop to the ignition.Although, thinking about it - it shouldn't drop less than the capacitor voltage so should stick above 8V or so.

author: G7PSK
author_rank: Super Contributor
timestamp: May 23, 2013, 06:03:52 pm
content: Having read the original post again I note the alternator is putting out around 40 amps on start up, this would also be the case for a battery. I have had a lot of cars that have had an amp meter and on start up the alternator will momentarily put the amp meter onto the stop at 50 0r 60 amps. So again I think other things need to be looked at for the reason of stalling. Most modern cars engine management systems run the car at a fast idle for 10 or 20 seconds or so from a cold start without any input from the throttle pedal, and cars with chokes run fast while on the choke.

author: Psi
author_rank: Super Contributor
timestamp: May 24, 2013, 12:20:23 am
content: Quote from: grumpydoc on May 23, 2013, 04:44:44 pmYep, that's exactly what i want to do, (i was going to add the resistor to the alternator charging feed).Everything else, ignition, starter etc. would be connected directly to the battery.The idea was to get the ESR back to "normal" range (from the alternators perspective) so the car behaves normally.QuoteYeah, it definitely started as soon as i changed from a battery to a capacitor and ESR is really the only thing i can think of to explain it.I now suspect the issue of resistor wattage is going to require some actual testing.Just to see how hot it gets in practice.There's to many unknowns to even work out a rough idea.

author: IanB
author_rank: Super Contributor
timestamp: May 24, 2013, 07:25:52 am
content: If you put a resistor in the alternator charging feed I think you need to consider the typical and maximum electrical load in your car--add up all the lights, fans, blower motors, alternator field coil and other electrical demands.For example, suppose you put a 50 milliohm resistor on the alternator output and the car is wanting 30 A, then this resistor will drop 1.5 V. This means that your car will have a 10.5 V electrical system instead of a 12 V system (or maybe 12.5 V instead of 14 V, which might not be so bad).Under this load, the resistor power dissipation will be 302x 0.05 = 45 W. This is a non-trivial amount of heat to dispose of. (If the electrical demand happened to be 50 A then it would be 125 W. This is starting to get into large metal-finned heat sink territory.)

author: Magicmushroom666
author_rank: Contributor
timestamp: May 24, 2013, 06:59:42 pm
content: I can vouch for an alternator stalling an engine, back when I used to dabble with large car stereos, I added a 100A alternator to a 1.3 engine, and that would stall it at tickover when drawing near full power. Dont forget the engine doesn't have much power at tickover, especially if there's no idle control valve to counteract something like that.

author: Psi
author_rank: Super Contributor
timestamp: May 25, 2013, 01:24:31 am
content: Maybe i should try a few NTC inrush current limiters in parallel.That would provide maybe 0.3R at 25C but drop to near short once it heated up.But yeah, as IanB said, I would need to know the typical and max current draw from the car.I'd need some current readings for AC/Heater/Fans/Lights etc.Maybe the best solution is a resistor and NTC thermally bonded to it.That way if the resistor gets too hot the NTC will take over and everything should balance out.The problem with an NTC by itself is the R at 25C is pretty high so you need multiple ones in parallel, doing that reduces the heat and stops them getting to low R.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2013, 08:18:10 am
content: QuoteMost of the time, of course, the alternator will supply the load so normally little current will actually flow through the resistor (it will just be the charging current for the caps).While a 50mR resistor will drop 1.5V at 30A this would also be true of the battery if it were supplying that much current. The scenario that you describe (30A draw on the battery from electrical accessories in the car) is not sustainable for very long if you replace the battery with an ultra-capacitor bank so I don't think you need to design for sustained dissipation in the resistor.

author: Baliszoft
author_rank: Frequent Contributor
timestamp: May 25, 2013, 08:52:20 am
content: Cant you just use a longer piece of thick wire? :-)

author: Psi
author_rank: Super Contributor
timestamp: May 25, 2013, 09:14:52 am
content: Quote from: grumpydoc on May 25, 2013, 08:18:10 amThe resistor is in the alternator feed.So with a fully charged battery/capacitor all of the energy to run the lights/AC/ignition etc will come from the alternator through the resistor.Quote from: Baliszoft on May 25, 2013, 08:52:20 amYep but it would have to be quite long if it was the same thickness as the current alternator wiring.Making it thinner reduces the length and makes it more practical.3 meters of 14awg wrapped around a ceramic core for example.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2013, 11:15:47 am
content: QuoteWrong place then.There are usually two connections to the battery - a heavy duty one to supply the starter current and a lighter one to carry charging current (or current for accessories when the engine is off) - the resistor should be in that link to the battery (or rather capacitor bank).

author: Psi
author_rank: Super Contributor
timestamp: May 25, 2013, 12:44:13 pm
content: Quote from: grumpydoc on May 25, 2013, 11:15:47 amThat's where i said i would put it, in the alternator feed to the battery.But that feed also runs the car.There's only one link from the alternator to the battery and it provides both charging and power to the car accessories.There's nowhere you could put the resistor where it would only be used for charging and not running the car.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2013, 01:19:41 pm
content: QuoteAh, that's a pain.Classically there's a big fat cable from the battery to the starter, smaller but still substantial line from the alternator output to the main fuses/distribution bus and a separate link from the fuse box to battery - possibly lighter still cable wise but still usually good for at least 30A, possibly 50A.Is there a separate line from the alternator to battery and then to the fuse box or is there just one high current link between battery, starter, alternator and main fuses?Either way I think you're going to have to do some rewiring to introduce the resistor - you need a high current (few hundred amps) link from battery to starter motor, high current link (at least 100A depending on alternator output) from alternator to fuses, then a separate lighter (30-50A) link back to your cap bank from the alternator side of the fuses. That link then will just carry charging current and you can stick as much resistance as you like into it (but 50 or 100mR is probably where you need to be).I don't think you need to worry too much about running the car electrics from the capacitor bank without the engine running as would be possible with a battery as presumably the caps would discharge within only a few minutes in that scenario.

author: Psi
author_rank: Super Contributor
timestamp: May 25, 2013, 02:21:58 pm
content: yeah, the car isn't wired like that.The starter motor cable + alternator cable + ignition power cable all come together right at the battery terminal.I could change it i guess.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2013, 03:05:41 pm
content: QuoteOr live with the voltage drop and power dissipation from putting it in line with the alternator.