ARC-LS & KL1 circuit issues - findings and questions

Klaus

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I have been trying to runtime-test my KL1 on UBH with 2700ma NiMh A cells and in an E2e body with 700ma NiCd 2/3 A cells and had some interesting results I want to share and possibly start some brainstorming on the issue.

I had been doing those tests and started to measure the run-down voltage of the used cells as CM came up with interesting findings about the KL1´s used IC and its cut-off voltage of 2.5V which IMO seemed to explain the shorter-than-can-be calculated runtime of KL1s on 2 NiMh/NiCd cells or one 123 cell.

(You might try to look up CM´s threads about modding the KL1 to 5W - there is some talk about the KL1 circuit and the baseline is that the circuit is using an IC which operating voltage is 2.5V - which means Chris (CM) found the reason for the KL1 not operating as well with just 1 123 cell or two NiNMh/NiCds as the IC won´t properly regulate anymore when voltage drops too much (I think to around 2.25V or so) - if you compare the 1 x 123 runtime and Brocks data on 2 Alkalines or 2 NiMhs all of those are well below what one would expect given the current draw and 2 x 123 runtime - considering the 2.5V operating voltage everything starts to make sense - as the IC doesn´t regulate anymore a lot of juice is left in the single 123 cell of the NiMh/NiCd pair)

But actually what I found now is that ALWYAYS just one cell is run down while the other still shows plenty of life (like 1.05V to 1.25V) when the circuit starts to flicker - I´m still doing some tests but so far it seems that always the FIRST cell is run-down while the one at the back is still doing perfectly fine. I didn´t do large scale scientific testing and my sample rate is still below a hand-full but the results so far are very straightforward - the KL1 circuit runs the nearest cell down and keeps plenty of juice in the back one.

I would like to do more testing and if other CPF-ers would join in we might get faster and better results - how is it behaving with 2 or 3 123´s or 2 Alkalines´s or 2 Lithiums AA in an UBH - same behaviour with different discharge characteristics depending on cell placement or will the primary cells in contrast to rechargeables be depleted equally ?

To me this is of some interest as if my findings hold true one might pretty much be able to lengthen the run-time of this circuit (others maybe too) in swapping the cells half-way through the discharge cycle - swapping between the front and back cell.

I did saw some discussions lately between lemlux and other members discussing the theoretical and/or practical difference in cell placements while centering around different capacity and chemistry cells and this might be a similar topic where in theory what I am saying might make no sense but real life testing shows it to be true.

I challenge the theoretical camp to try on their own though
grin.gif


Klaus
 

Josh

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I was going to say switch the cell too
wink.gif
kinda strange that it only depletes one at a time.Good luck!
 

Klaus

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Josh - not really one at a time but the front one much much more than the back one - maybe related to the switching circuit ? Will do some more testing and maybe one of the local electronic gurus will chime in.

Klaus
 

Alan

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Klaus, Interesting finding! If my memory serves me correctly, another member also found the same phenomenon on BB500 (Madmax?).

Alan
 

Doug S

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Klaus, I am assuming that you have switched the position of the two cells to confirm that you are not just seeing that the two cells differ a bit in capacity. If it is truly dependent on position as you say, then I would suggest that the forward cell is exposed to higher temperatures during discharge which accelerates the rate of self-discharge. NiMh cells have a high rate of self-discharge and this rate increases rapidly with rising temperatures.
 

Klaus

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Alan - might be the same issue with switching circuits which are a little nasty in regards to wire lengths as we have seen - so maybe also the distance to the second cell makes a difference.

Doug - yes - I tried different cells and I´m pretty shure I´m not fooling myself - I also swapped cells after 1 hour just to see the higher discharge move over again to the front cell after another hour - you might be right with your high temperature attempt for an explanation - the front cell does get warmer than the back cell. But I wouldn´t assume self discharge to take place in hours ? And I pretty much can rule out the different capacity cell issue as I have been testing with different cells and swapped those too - and unless I have totally screwed up and by accident always used a bad cell in the front row my finding seem to be accurate. Also after one hour even with some capacity difference the voltage delta shoudln´t be that large.

Klaus
 

Doug S

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Originally posted by Klaus:
Also after one hour even with some capacity difference the voltage delta shoudln´t be that large.

Klaus
<font size="2" face="Verdana, Arial">When measuring cell open circuit voltage, the capacity difference represented by cell voltage of a few tenths of a volt vs 1.10V is *very* small. To convince yourself, measure the voltage of the more discharged cell then put it in the charger for only 1 minute, take it out and remeasure.
 

Klaus

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Doug,

not shure what the effect of surface charge should tell me ?

Do you want to say that a cell measuring open-circuit-voltage of 1.05V and the other at 1.2V after being removed from the same device isn´t strange ?

Klaus
 

Doug S

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Originally posted by Klaus:
Doug,
Do you want to say that a cell measuring open-circuit-voltage of 1.05V and the other at 1.2V after being removed from the same device isn´t strange ?

Klaus
<font size="2" face="Verdana, Arial">Correct. Not strange.
 

CM

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Klaus,

Is there any way you can measure the voltage of each cell individually under load? The open circuit voltage does not tell the entire story but in my experience, every series NiMH I've used (on Arcs and various incandescents) showed similar open circuit voltage after depletion. Your finding is very interesting and puzzling and inquiring minds like mine want to understand what's going on with the circuit. The KL1 does have an input capacitor so that the two batteries should look like an almost ideal low impedance to the circuit so lead length from the power source to the circuit should not be an issue.

CM
 

Klaus

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Chris,
Doug,

I will try to do some voltage measurements under load - another thing is that the warmed up cell in the front might show lower open circuit voltage due to the higher temp - one test today seemed to confirm my suspicion - I got some longer runtime in swapping the cells avery hour.

Right now I have more cells charging and will do more testing & measuring.

Thx for comments so far

Klaus
 

evan9162

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The rate of internal discharge at 40C for NiMH is about 2-3 times worse than at 25C.

-Darin
 

Klaus

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Don´t think so actually - I would expect that the ICs minimum voltage limit CM (Chris) found would nullify any advantage - seems that with those circuits the more cells used the better - actually with more cells in series you get the minimum voltage of according to Chris 2.5V distributed over n cells - using 1 cells 2.5V, 2 cells 1.25V, 3 cells 0.83V, 4 cells 0.625V - you get the idea - this would nicely explain the higher relativ runtime of the KL1/KL3 when used with 1 vs 2 vs 3 123 cells as the cells can get drained much more befor the cells combined voltage drops below the IC´s minimum operating voltage and it falls out of regulation.

Klaus
 

jeff1500

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I have a two led table light that runs on two C nimh cells. It uses a max1674 step up circuit. I let it run all the way down last week. When it stopped I measured the battery voltage. The plus side cell was dead and the negative side cell measured 1.2 volts.

Sounds like the same thing you're talking about. It's the standard data sheet circuit but it does not have an input capacitor. I wondered if the lack of an input capacitor had anything to do with it, but from what you say, you saw the same thing with an input capacitor.

I wonder if the pulsing of the circuit favors the first cell?
 

CM

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Klaus, jeff1500,

You guys are really onto something interesting here. I'm beginning to suspect it may be temperature as Klaus said. Doing a runtime test means *continuous* on so heat build up is inevitable. I have an Arc SLS 5W (actually driven at 3.2W) with 2AA I run intermittently (few seconds to a minute ON and then long time off) and I don't see the batteries exhibit the same behavior. Klaus, can you do an intermittent test with your setup and see if you experience the same thing? Or at least discharge the cells in the light at short periods as to not let temperature rise become evident.

CM
 

Klaus

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CM,

I will do some more testing and include some non-continous tests and I´m also trying to figure a way to properly run the circuit without excessive wiring and still be able to measure voltage drop on the single cells under load.

Klaus
 

jeff1500

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Originally posted by Abe Furburger:
Jeff1500,

And when you swapped the cells - could it still work some more ?

Abe.
<font size="2" face="Verdana, Arial">I didn't try that. I'll check next time.
 
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