how to measure host/switch resistance?

[psychbeat]

OK so I tested with the ohm setting but just got the
resistance of my test leads which are cheapo harbor frieght ;)

I tested a drop-in in my custom p60 HL host which is
suppsed to be 4090ma

when I tested from the 4 X18650 (parallel) battery carier through the
switch and cable into the drop I got 2.7a

when I tested straight to the drop from the carrier I saw 3.6a on the meter

and when I put the drop into my bored C2 and used a single 18650 it was
3.6a as well

so I assume Im losing voltage/current in the HL host --either in the switch
or the cable itself?

its a 10a mechanical switch and the cable is fairly beefy and high quallity

I guess my ? is how to else to measure the resistance of this host or do I need
an expensive Fluke meter?

I know the low voltage can make resistance a real issue with these HI amp LOW volt
modules ;)

thanks for any feedback!

 

[psychbeat]

Here's what I'm doing ;)

 

[moderator007]

The length of wire and size could definitely play a part in the resistance. I cant really tell what wire size you are using. The switch will also add some resistance. But shouldn't make the difference your seeing. My guess would be wire size is to small for that much distance. Just a guess. Here's wire size for current chart. Here's a wire size calculator that you can put in your length and amps and will give you the voltage drop but only goes down to 20awg.

 

[psychbeat]

^^ wow great chart !!

thanks,

I doubt the cable is lowering voltage enough to affect my results this extremely

I was trying to get a more solid connection with my leads and ended up getting
4.5a straight to the drop!! - which is only rated @ 4090ma but must have some
+/- tolerance with the mountains 7XXX resistors that are all stacked up on it ;)

so I ended up with 4.5a through the small gauge wire to ground and my crummy
test leads directly on the springs of the module
3a is the best Ive gotten going through the host and switch.

I know the driver resistors need a bit of overhead voltage from the cells but Im not
getting much sag using 4X18650 hot off the charger @ 4.3v
Im also still seeing full voltage through the host when not under load.

I dont have an elaborate enough setup to do voltage under load right now ;)

 

[psychbeat]

with 2p2s 123primaries Im getting 3.56a through the host which is pointing towards
voltage lost somewhere as Im sure 4px18650 can dish out the current

 

[Mr Happy]

The best way to measure low resistances is by passing a known current through them like 1 A and measuring the voltage drop in mV. Usually this requires the use of two meters, one to measure current and one to measure voltage both at the same time. But you can also use a laboratory power supply with regulated output current if you have one.

 

[HKJ]

In my DMM guide I show one way to measure low ohms.

 

[psychbeat]

Thanks HKJ!!

I learned how to do tailcap current measurements from your post!

SUPER helpful!!

I'll need to get a resistor and power supply at some point ;)


Ok so the thing is - I'm getting 4.6a
Draw direct to the drop from the male connector on my host.

I'm only getting ~3.1 After the switch and cord.
Maybe a bad solder on the switch?

I tried a 6v max drop I have with 2s2p cr123 and noticed less current drop.

I feel like these low voltage drops are very sensitive to voltage loss.

 

[HKJ]

I'm only getting ~3.1 After the switch and cord.
Maybe a bad solder on the switch?

Or maybe wire resistance.

I feel like these low voltage drops are very sensitive to voltage loss.

Low voltage and high current will give voltage loss.

Try connecting all the stuff without the ammeter, the use the volt setting and measure the loss over different part of the circuit (Wires, switch, etc.)

 

[psychbeat]

Omg- that brilliant and I can't believe I didn't try it already - duh!

I'll post back w results.

 

[psychbeat]

Ok so I had to actually place the module in the host as I don't have any magnets right now.

I took readings using the positive side of the battery carrier and check the neg voltage before and after the switch under load.
The positive lead was on the carrier for all tests.

W a 4amp module as a load here are the results:

NO LOAD:
4.16v @the carrier and both before and after the switch.

UNDER LOAD:

4v @ carrier

3.75 after the switch
3.75-3.76 before the switch

So I'm either losing the .25v from a bad solder @the positive connection to the board under the module.

Or the female side of the connector to from the carrier to the cable (I tested the male)

Finally (most likely IMO) the cable is causing the resistance under load.
It's really nice cable tho with 18ga wires I think or maybe 20ga.

Weird a drop would do 4.61a @4v & only 3a@ 3.75v if that's truly what's happening.

I only have one meter ATM so who knows what's exactly happening.

Thanks for the help guys!

 

[HKJ]

I only have one meter ATM so who knows what's exactly happening.

The is no reason to measure between plus and minus, just set the meter on the 200mV or 2 volt range and start measuring voltage from battery plus pole to connector or over any other piece of wire or switch.

1m of 18 awg wire is about 0.02 ohm and will drop 0.1 volt at 5 ampere.

 

[psychbeat]

Ahh-
The cable says 2x 0.5mm on it
Which I figured is around 18 or 20ga

So I guess I shouldn't be losing .25v @4a then.

Must be either the connector or the solder to the pos on the board the drop presses against.

 

[Mr Happy]

Contact resistances can be surprisingly higher than you might think in some cases, especially if there is an oxide layer on one or both of the surfaces.

Remember the old days with simple torches (flashlights) that used to flicker and go dim after sitting in a drawer for a while? It's not so much that the batteries were dead, often it was due to poor contact on some of the battery springs.

 

[psychbeat]

I've cleaned and applied ProGold to every possible contact!!
;) I'm losing my mind on this one!

Might have to use massive speaker cable ....

 

[HKJ]

I've cleaned and applied ProGold to every possible contact!!
;) I'm losing my mind on this one!

Might have to use massive speaker cable ....

With the DMM you can measure the voltage drop in each piece of wire and for each connection, it cannot be that hard to find where the voltage disappears.

 

[psychbeat]

Yah I'll need to desolder the positive connection to the board I think.

If it's still dropping then I'll remove the 2pin connector and try to go directly from the carrier to the cable to the drop.

Was trying to avoid taking it all apart ;(
I'll post my results and thanks again for the help!

 

[moderator007]

Ahh-
The cable says 2x 0.5mm on it
Which I figured is around 18 or 20ga

Coverting mm to inches. .5mm = .019685 inches Looking at the wire size to current handling link I give you above your wire would be equivalent to less than AWG24. If the .5mm is the diameter of the wire. According to the link the wire would only be good for 3.5A at short distances. Says .577A Maximum amps for power transmissions. I also read that .5mm is closer to 21AWG. So I am not sure but it appears to be right on the edge of being to small.

 

[kevinm]

Thanks for all the help guys; psychbeat and I have been trying to figure this out all day!:ohgeez:

The cable is Helukabel oz-500 1001, which is 20 gauge. They assured me when I bought it that it is rated for 9A with a healthy safety margin (more than 10%, so 10A would be possible).

The calculator says .166V drop. So we need to find the other .09V...

If I recall correctly, the AMC7135's require .12V + Vf, so that could be all the missing voltage...

 

[HKJ]

The cable is Helukabel oz-500 1001, which is 20 gauge. They assured me when I bought it that it is rated for 9A with a healthy safety margin (more than 10%, so 10A would be possible).

Current rating of cables is a strange thing, it is calculated around heat. I.e. a single wire in free air is rated considerable higher than a cable designed to be mounted in a wall (for the same gauge).
The actual voltage loss is not included in the current rating!