2P MOSFET build-batteries discharging unevenly?

[David Siddle]

I've just completed my first mod, a 2P MOSFET build with lock-out and a voltmeter. I'm using LG HG6's; the mod fires as it should and everything seems to be OK, but after a little use i put the batteries back into my charger to look at how much they had drained. One was reading 4.14v, the other 4.07v I have no idea why; does anyone else have any suggestions? i'm new to this and baffled.

Cheers guys.

 

[Project]

Lithium batteries are fickle things, they are all created equal but some are more equal than others .

The best way to determine this is with two steps:

Check actual capacity, a lot of RC hobby lithium chargers will have a few features, such as controlled discharge as well as a mah (milli-amp-hour) measurement. discharge the cells (individually) to a set safe minimum voltage (say 3.4v). then fully charge the cells to 4.2v and measure the amount of mah each battery consumed. That will give you an accurate measurement of actual capacity.

Both cells may say they're 3000mah capacity, but one could be 2850 mah, the other could be 3030 mah. A difference as big as that could explain your voltage difference.

The other thing to measure, that would give similar results to what you have got, is each batteries internal resistance. The higher this figure, the less amps you can safely discharge from a cell (as higher internal resistance causes the battery to heat up faster under heavy load, batteries that get too hot break down into lithium gas, puff up and potentially breach their package in a worst case scenario). Having one cell with slightly lower internal resistance than the other also means that cell will be more susceptible to bear more of the load when connected in parallel. Its the "path of least resistance" explanation.

measuring a batteries internal resistance isn't as straight forward as using a multimeter in resistance mode (NEVER DO THIS). Quickest explanation I found for figuring this out is here:

http://www.instructables.com/id/How-to-measure-the-internal-resistance-of-a-batter/

basically, measure the batteries voltage under no load, measure it under a known load, measure the voltage across the load as well as the cell. from that you can compute the batteries internal voltage.

Sorry for the rant, but those are the two best causes of your differential voltages.

A third, much less likely explanation, is that somewhere in your rig, one batteries path to the load and slightly higher resistance than the other path (weak solder joint, poor battery terminal contact, weakened connection wire or different wire lengths from the battery to the load). That you can test with a multimeter (without the batteries installed. Though unlikely, its easy as pie to test.

CJ

 

[David Siddle]

Lithium batteries are fickle things, they are all created equal but some are more equal than others .

The best way to determine this is with two steps:

Check actual capacity, a lot of RC hobby lithium chargers will have a few features, such as controlled discharge as well as a mah (milli-amp-hour) measurement. discharge the cells (individually) to a set safe minimum voltage (say 3.4v). then fully charge the cells to 4.2v and measure the amount of mah each battery consumed. That will give you an accurate measurement of actual capacity.

Both cells may say they're 3000mah capacity, but one could be 2850 mah, the other could be 3030 mah. A difference as big as that could explain your voltage difference.

The other thing to measure, that would give similar results to what you have got, is each batteries internal resistance. The higher this figure, the less amps you can safely discharge from a cell (as higher internal resistance causes the battery to heat up faster under heavy load, batteries that get too hot break down into lithium gas, puff up and potentially breach their package in a worst case scenario). Having one cell with slightly lower internal resistance than the other also means that cell will be more susceptible to bear more of the load when connected in parallel. Its the "path of least resistance" explanation.

measuring a batteries internal resistance isn't as straight forward as using a multimeter in resistance mode (NEVER DO THIS). Quickest explanation I found for figuring this out is here:

http://www.instructables.com/id/How-to-measure-the-internal-resistance-of-a-batter/

basically, measure the batteries voltage under no load, measure it under a known load, measure the voltage across the load as well as the cell. from that you can compute the batteries internal voltage.

Sorry for the rant, but those are the two best causes of your differential voltages.

A third, much less likely explanation, is that somewhere in your rig, one batteries path to the load and slightly higher resistance than the other path (weak solder joint, poor battery terminal contact, weakened connection wire or different wire lengths from the battery to the load). That you can test with a multimeter (without the batteries installed. Though unlikely, its easy as pie to test.

CJ

Brilliant! that's exactly the reason i like this site, there's always someone ready to help with the knowledge. Much appreciated. Bit late to start checking things out tonight, but i'll have a faff around after work tomorrow.

Thanks again.

 

[Project]

Brilliant! that's exactly the reason i like this site, there's always someone ready to help with the knowledge. Much appreciated. Bit late to start checking things out tonight, but i'll have a faff around after work tomorrow.

Thanks again.

No worries, Ive only been vaping for 3 months but I've been playing with lithium cells for 10 years faffing around with RC planes. Matching cells can be difficult. And as always when testing, especially with multimeters, always double check which mode the multimeter is in before hooking it up. Then check it again . if you check across a lithium cell in resistance mode instead of voltage theres three outcomes depending on how expensive your multimeter was. It'll either blow an internal fuse (best case), knacker the multimeter, or the lithium cell will puff and run away from you pretty quickly (worst case). Experience speaks volumes lol.

Lemmi know how you get on, I'd be keen to hear your results!

CJ

 

[David Siddle]

No worries, Ive only been vaping for 3 months but I've been playing with lithium cells for 10 years faffing around with RC planes. Matching cells can be difficult. And as always when testing, especially with multimeters, always double check which mode the multimeter is in before hooking it up. Then check it again . if you check across a lithium cell in resistance mode instead of voltage theres three outcomes depending on how expensive your multimeter was. It'll either blow an internal fuse (best case), knacker the multimeter, or the lithium cell will puff and run away from you pretty quickly (worst case). Experience speaks volumes lol.

Lemmi know how you get on, I'd be keen to hear your results!

CJ

Will do, though it might take a couple of days to get around to it.

 

[David Siddle]

"measure it under a known load, measure the voltage across the load as well as the cell."

Sorry if I appear daft, but what do you mean by "across the load", as opposed to "under load"?

Thanks.

 

[jock1092]

"measure it under a known load, measure the voltage across the load as well as the cell."

Sorry if I appear daft, but what do you mean by "across the load", as opposed to "under load"?

Thanks.

Across the load is on the rda, positive pin and negative, under load is the battery level while firing

If you check across the load, and under load quickly a few times you will get different numbers, the difference is the voltage drop of the mod, in a mosfet box this should be low (0.1-0.2 area not much more depending on things like what 510 you have /what gauge of wire used it may be a little higher)

So if you have your voltmeter to the sled, it's showing you voltage under load (if its wired to come on when firing)

I would start with a quick check of your contacts on the sled aswell, make sure they are both making firm contact with the batteries

I'd also double check the sled bridges ( + to + and - to - for the parallel set up) as if they have a poor solder join it could cause a little imbalance

With this I'm assuming this is paired batteries, as in new or used together from new, if they have been used separate it could be one has aged little more

All these are solutions to problems I've had at one point or another, making mods it's a bit of a learning curve at the start but a great thing to do

 

[Tubbyengineer]

Easiest way to check if the connections are at fault is to charge the batteries fully, put them in the mod and vape for a while, check the voltages. now recharge again and put the batteries back in the mod the other way so instead of 1-2 put them in 2-1, if the lower voltage moves with the battery then it's the battery at fault, if the lower voltage stays at say slot 1 then you have a connection issue...

 

[David Siddle]

Across the load is on the rda, positive pin and negative, under load is the battery level while firing

If you check across the load, and under load quickly a few times you will get different numbers, the difference is the voltage drop of the mod, in a mosfet box this should be low (0.1-0.2 area not much more depending on things like what 510 you have /what gauge of wire used it may be a little higher)

So if you have your voltmeter to the sled, it's showing you voltage under load (if its wired to come on when firing)

I would start with a quick check of your contacts on the sled aswell, make sure they are both making firm contact with the batteries

I'd also double check the sled bridges ( + to + and - to - for the parallel set up) as if they have a poor solder join it could cause a little imbalance

With this I'm assuming this is paired batteries, as in new or used together from new, if they have been used separate it could be one has aged little more

All these are solutions to problems I've had at one point or another, making mods it's a bit of a learning curve at the start but a great thing to do

Thanks again. Weirdly I took the mod out with me yesterday in order to use it for an extended period ; when I checked the batteries later, they both showed precisely the same voltages. Don't know whether this is good or bad. Don't you just hate intermittent faults and their like?

 

[jock1092]

Thanks again. Weirdly I took the mod out with me yesterday in order to use it for an extended period ; when I checked the batteries later, they both showed precisely the same voltages. Don't know whether this is good or bad. Don't you just hate intermittent faults and their like?

Yep
Might have been as simple as the battery just not sitting right, depending on the sled you have it can be easily done, I recently rebuilt my parallel box because of how often it wasn't making a solid connection ( usually just downright not working at all)