sosidge

Yes they are.

blueboy

Hey Danny I just ordered 3 packs of your HVs! Picking them up Saturday, Ill let you know what I think of them.

The general impression I get from reading this thread is that these cells should be discharged at 30 amps. Is it OK to discharge at less than this because I only have a cheapo discharger, the LRP Nimh Discharger or something. Im not sure what it discharges at, but it takes a couple of hours to dump a freshly charged pack. I dont really want to spend another £90 - £100 on a decent discharger just yet, but do I really need one?

Thanks

Danny/SMC

iTrader: (2)

First of all thank you for choosing SMC and I hope they run to your satisfaction.

Just run the HV's on the track until your race/practice is done and then you should build your self a bulb discharger using 15 lights. Here in the US they are called 1157 automotive lights. When the lights start to dim you should remove the lights.

This should cost you 20-25 US dollrs to make.

Good Luck.

blueboy

Thanks Danny.
I had thought about making one of these bulb dischargers, but wasnt sure what bulbs to use. I dont think you can get 1157 bulbs over here, so do you (or anyone else!) know any specs of these bulbs; eg. voltage and wattage so I can find the closest thing in the UK?
Thanks all

Danny/SMC

iTrader: (2)

I believe those lights are used in the rear brake lights. I'm not sure if newer cars still use this light.

blueboy

OK i talked to my dad about making this 30 amp discharger out of 15 bulbs. I found out that the 1157 bulbs are 21 watt 12 volt bulbs. The problem is that my dad started working out numbers to check what I wanted to do. Here's what he found:

(From Ohm's law) current = power/voltage
current = 15 x 21 watts / 7.2 volts (average 6-cell pack)
current = 315 / 7.2
current = 43.75 amps
This means that when the pack is full it is being discharged at 43 amps, not 30. Also, as the pack voltage drops, the current increases:
current = 315 / 5.4 (where the cutoff switch should stop the discharge)
current = 58.33 amps
So as the pack gets close to being finished, the discharge rate is virtually twice what has been recomended to me!

Could anybody please explain this to me, whether I have the wrong specification on the bulbs, or the math is somehow wrong, or indeed if the discharger is suposed to work at twice it's stated amp rate.

Sorry for the long and techy post guys, but someone out there should be able to understand it (I hope!) and hopefully they will try to explain this to me.

Danny/SMC

iTrader: (2)

Typically an 1157 bulb pulls 2 amps. So 15 will be close to thirty. As your voltage decreases the amp rate will also decrease.

Amps = v/r

Years ago we use to put 10 lights and it came out near 20 amps so 15 should be thirty.

blueboy

Thanks for the reply Danny, I showed it my dad and he thought about what you said for a bit, then I pointed out that you make a living from discharging cells and he agreed to bow to your knowledge.

He says thanks for your help, so we're gonna go ahead with the 15 bulb discharger.

Danny/SMC

iTrader: (2)

An easy way to check the amp draw is to hook up a voltmeter to your pack while it's discharging and take a stopwatch and right down the time it took to discharge to 5.40 volts.

You must do this as soon as it peaks.

A typical HV should last 5 minutes 45 to 6 minutes at 30 amps.


Good luck,
Danny

blueboy

Thanks for the tip Danny, I'll give that a go just to show my dad.

Im gonna make a black box type cutoff switch too so that i don;t have to keep monitoring my cells as they discharge. The one on Bryan Balak's website (not sure if i can post direct link) has a voltmeter built in.

blueboy

Hey all
I just got my SMC HVs can I compare numbers with you guys? I wanna see how my cells measure up (not for bragging rights, just to see how good a deal I got )

heres my numbers:

Pack 1:
358 sec
1.158 volts
3.3 i-r

Pack 2:
361 sec
1.159 volts
3.3 i-r

Pack 3:
360 sec
1.16
3.3 i-r

sosidge

look virtually the same as mine - I reckon the manufacturing tolerances on these HV cells must be pretty good.

dw

Danny and blue boy neither of you are on quite the right track.
Danny- resistance in parrallel doesn't work like that. You can't just add currents.

Blueboy a 21 Watt bulb at 12volts will not put out 21 watt at 7.2V.
This is where you are going wrong.

First work out the resistance of the bulb from the 12 volt specs.

power = voltage * current
but as v= ir, i=v/r
so power = v*v/r

21 = 12*12/r

so r=6.9 ohms

If we wire 2 bulbs in parrallel.

Total resistance = R = 1 / (1/r+1/r)

so for 15 bulbs R = 1 / (15/r) = r/15

6.9/15 = 0.46 ohms

From i = v/R we can now calculate current at 7.2 volts.

i = 7.2/0.46 =15.75 amps for 15 bulbs.

To get 30 amps
If i =30 at 7.2V then R= 0.24

R=r/n where n is the number of bulbs.
n=28.75 so you need 29 bulbs.

If danny is correct about 10 bulbs drawing 20 amps at 7.2 volts then you must have the specs of the bulb wrong.


Remember that this doesn't take into account resistance of the cell so actual current will be less.

blueboy

Thanks for clearing that up dw. Heres a guy who knows his physics

dw

Ignore the comment about not being able to add currents, having brain fade when I wrote that. Rest should be correct though.