PL8 IR Parallell Weirdness

[scallybert]

I use a PL8 to parallel charge up to 8 3S 2200mAh packs at 2A. This works fine.

Well, except that the internal resistance readings are strange. The reading on the 3rd cell is always much higher. Maybe 3 or 4 times that of the other cells. [This is when parallel charging.]

I charged a single pack to look at the IR, and all 3 cells were comparable. ie looked normal.

First thoughts are that one (or more) of the 8 packs has an issue (however, I'm sceptical of this); or that there's some kind of problem with the stacked connectors (eg a dirty contact).

Any thoughts ?

[Mark_T]

Are you using the CellPro 40A connectors that have a fuse on both leads, or the older 30A ones that only have a fuse on one lead?

[scallybert]

Are you using the CellPro 40A connectors that have a fuse on both leads, or the older 30A ones that only have a fuse on one lead?

A mix. About 4 new-style; 2 original (really early); 2 that seem to be sort of half-way.

[Guest]

THis is probably the issue then. The IR is the whole resistance including connectors leads etc. The other thng to bare in mind is that the IR when parallel charging is the IR of all the packs but you cannot, in your case, divide by 8 and assume thats the IR of each cell. The only true IR measurment is when you charge a single pack. It is the case however if there is a high IR on a prallel charge that one of the packs probably does have a high IR cell or perhaps a dodgy coonnection. THis is one of the reasons when testing we do use connectors!

Andy

[scallybert]

I think if anything, I'd have loosely multiplied by 8 to guesstimate the IR of one cell... However, I would loosely guess that one dodgy pack might mess up the IR by ~12.5% - which is why I'm sceptical about a dodgy pack.

As you say though, attempting to measure a few milli-ohms through a series of stacked/daisy-chained connectors is asking for trouble. I'd just seen the IR values when scrolling through dislays, and thought the disparity across cells was strange. I think I've seen this with various other packs when parallel charging. I wondered if I had a charger issue, so tried with a single pack - which seemed fine (vindicating the charger.)

I'd guess something in the stack of balance connectors is slightly flawed or such. If I want to measure IR, I'll do one pack at a time.

Cool. 8)

One thing I will add. Often when I first plug a pack in, there's a spark. Presumably as a capacitor is charged up (like on many ESCs). IMO it's worth thinking which connectors you want to experience this spark. I go for the discharge lead / pack connectors. I certainly don't want it to be a balance connector, or one on the front of the PL8.

[Mark_T]

Just use one of the 40A cables sets with fuses on both wires if you want to check the IR on a single pack, FMA commented a while ago that using the original 30A cables could result in inaccurate IR readings for the first/last cells in a pack.

I don't recall that I've ever seen a spark when connecting to the charger though. I always power up the charger before making any connections to it, not sure if you do it differently?

[cjcj1949]

I would suspect a faulty solder joint, dirty connector or connector that is not as springy as it used ot be. When parallel charging you are relying on all things being equal to get equal currents. They won't be with stacked connectors. Cells and batteries that are charged slower will eventually catch up, as I'm sure you know. The IR readings will be telling you what is really going on second by second. I would simplify the setup to one battery to begin with and use that in all positions. I think and hope you'll find one position with a problem. As its a daisy chain system a faulty high current connector would affect the others downstream.

Or it could be one of your batteries that is faulty. Same methods apply though.

You can find the fault quicker by dividing the system in half each time. Try four batteries at a time and find out if the fault is one of the batteries or one half of the system. It's probably not worth doing for 8, but if you had lots it would be. Its the high low game principal.

[scallybert]

Just use one of the 40A cables sets with fuses on both wires if you want to check the IR on a single pack, FMA commented a while ago that using the original 30A cables could result in inaccurate IR readings for the first/last cells in a pack.

Cheers - I should read some of these things... I guess the fuse on just one lead makes a difference.

I don't recall that I've ever seen a spark when connecting to the charger though. I always power up the charger before making any connections to it, not sure if you do it differently?

Yep, charger powered-up. If I connect a 6S pack to the discharge leads first, I definitely get a spark (like with an ESC). I'm pretty sure I've seen this on the balance leads if I connect them first. (Low ambient lighting may have made this mor enoticable.)

I figure I'd rather have a spark on the pack & discharge leads connector, rather than somewhere more fragile and expensive (little balance lead connectors, PL8 front panel, etc).

[jamiem]

Mine spark when connecting from time to time, not always though. Always connect the main discharge leads first, then the balance taps once all packs are connected. If for instance you connect a pack with 50% left in it along with 3 others with only 20%, there will be a large rush of current initially to even themselves out, and there's a good chance you will trip one of the poly fuses. I did this by mistake not long after getting it! The balance wires and plugs/pins are not rated to carry this current, so in theory you could damage a pin on the board/pack this way.

[scallybert]

Mine spark when connecting from time to time, not always though. Always connect the main discharge leads first, then the balance taps once all packs are connected. If for instance you connect a pack with 50% left in it along with 3 others with only 20%, there will be a large rush of current initially to even themselves out, and there's a good chance you will trip one of the poly fuses. I did this by mistake not long after getting it! The balance wires and plugs/pins are not rated to carry this current, so in theory you could damage a pin on the board/pack this way.

I think I'd rather trip a polyfuse, rather than have the packs rapidly equalise through the discharge leads. [Or blow a balance lead fuse.]

I connect the first pack to the charger via discharge leads (possible spark); then connect the balance leads (PL8, then pack); then plug the other packs into the balance board 'chain' - while watching the polyfuses; then do the remaining discharge leads.