Discussion About parallel charging of Lixx / PB packs.

[GreenBar0n]

mrforsyth,

I was wondering if the length of the balance cables would matter, I should have asked first. If it helps any, the balance taps are all the exact same length with no cold joints or solder beads, I used flux and got the contacts nice and shiny, I used heatshrink tubing on all exposed areas. The entire balance cable itself though, is 4 feet long from charger to balance board to end taps. Is that asking for problems?




What is the farthest away I can have the balance taps without the IR drop, or at an acceptable loss?


EDIT: To make matters worse, I had to switch from the 26AWG tap leads, to 24AWG ribbon for about 2 1/2 feet in the middle, .to make up the distance I was going for.

[mrforsyth]

I doubt that it'll create any problems, per se, but long balance leads may very likely increase the time it takes to achieve perfect balance.

Quickie math -

Assuming 4 feet of 22 gauge wire, you'll have approximately .06 ohms of wire resistance. If the balancer balances at 500 milliamps current draw, you'll have .03V of drop in the wire (not including connector losses). Under no load, the balancer circuitry will sense a voltage differential and commence balancing. If it draws .5 amps, it'll stop balancing when the imbalance is at .03V, theoretically. It'll likely then start balancing again when current drops off and once again 'sees' the voltage differential between cells.

Once again, this is all 'in theory'. You'll have to monitor to make sure. My personal practice is to keep balance leads as short as practical and use fatter wire if I need them to be long. This way I have no worries and my packs stay in perfect balance.

Mark

[Screamin Demon]

Originally Posted by mrforsyth

A charger that charges through the balance taps is not necessary. You simply have to parallel the balance taps in addition to the main charge leads as shown in Julez' first post in this thread. All cells will be in balance upon charge termination.

Attached is a photo of one of my parallel cable sets in action.

Mark

Mark or anyone
How the the electronic in the chargers balance circuit discern the voltage differences from one cell to the other when all four packs are wired to the balancing circuit? perhaps an explanation on how the balance circuits works would be helpful.

[rotortuner]

Originally Posted by Screamin Demon

Mark or anyone
How the the electronic in the chargers balance circuit discern the voltage differences from one cell to the other when all four packs are wired to the balancing circuit? perhaps an explanation on how the balance circuits works would be helpful.

Someone should add to what i say on this, but i have been thinking about this also. Since you are paralleling each cell, the balancer will balance that PAIR of cells. example: if you parallel 2 3S packs, each of the 3 cells will immediately balance with whatever cell it is paralleled with when you hook the balance leads up together in parallel; this is because current will flow from the higher voltage cell into the cell with lower voltage and equalize. This is the reason why you should only parallel up packs that are close the same voltage. The charger will then balance each set of two cells with the other two sets of two cells(remember we are charging 2 3s packs together). hahah i know that sounds confusing. So when your charging in parallel each cell isn't really getting balanced on its own, rather, the cells are getting balanced in pairs, triples or quadruples against each other, depending on how many packs you have in parallel with the balance leads. I don't think that is necessarily a bad thing at all, but i think it could take the charger longer to balance because of the increased capacities being dealt with, but no more than if you were balancing one single BIG pack equal to the sum of the packs in parallel. Any one else add to this?

CJG

[mrforsyth]

You are absolutely correct. When the packs are connected in parallel, current will flow from the higher voltage cells to the lower voltage cells until they are at the exact same voltage. Cell #1 of each pack will 'self balance' to the same voltage within a few minutes. Same for additional cells (#2, #3, etc.) within the paralleled packs.

In order to limit the amount of current flowing through the balance taps, my standard practice is to connect packs via the main discharge connectors first, followed by the balance connectors. I also have a self-imposed voltage differential of .3V between packs to be paralleled in order to limit current flow between packs. Since my 3S packs are typically between 11.2 and 11.5 volts after flying, this is easy for me. Note that you can charge packs of different capacity, usage history, and C rating in parallel without issue. The only important considerations are to ensure that all packs to be paralleled are of the same cell count and are within a few tenths of a volt before connecting.

With my 106B, all packs are in perfect balance and at the exact same voltage upon charge termination.

Mark

p.s. Here's what my setup looks like.

[not.likely]

This is very good. Can someone tell me how the paralleling up of same voltage packs but of different capacities and C ratings may be affected during the discharge stage EG: actually in use?
For instance, lets say I parallel up a 3s 2000m/a 20C pack with a 3s 1000m/a 10C pack. I know I have created a 3s2p battery of 11.1V 3A capacity, and will charge at 3A but should the maximum discharge rate be 20C / 10C / or maybe even 30C ?
My feeling is that it should be limited to the lower pack ( 10C ) but then I may be wrong as I often am
PS yes I know the maximum C rate is somewhat hypothetical, and I normallly aim for around 50% of claimed C rates, but still ponder the above.

[rotortuner]

I am a mechanical engineer, so this stuff isnt really my strong suit, however I think I can work through this.
Here is the example, we are using 2 3s packs one is a 10C 2000Mah pack amd a 20C 1000 mah pack. first i would find the max amps each one can put out at their C rating:

2000 mah at 10C would be 20amps
1000 mah at 20c would be 20 amps

Total discharge current therefor shouldnt be more than 40 amps.

Total pack capacity is 3000 mah since we are parallel, 1C at 3000 mah is 3amps, now we divide 40amps/3amps to get our C rating, i get 13.3C for this combined pack. Keep in mind this assumes packs of same manufacturer, cells are like the same age, etc etc.

Now that we worked this one out, i see the C rating ended up being a 1/3 of the way off of the smaller pack since 1 and 1/3 of 10c is roughly 13.33333. As we will see in the next example, this rule does NOT hold true.

lets try one more but change things up a little:

4000mah rated at 30C
1000mah rated at 10c

this is the case where one would think you could damage the 10c small pack.

ok, calc max amps
4000mah at 30c is 120 amps
1000mah at 10c is 10 amps

total amp draw limit is 120 + 10 = 130amps

we have a 5000mah pack so 1c is 5amps

130/5 = 26C limit, so yes this set of calculations, assuming i am thinking of this right should be done to find what you should limit your amps at. Someone that is REALLY good at this stuff, does this sound good?

CJG

[mrforsyth]

I've always been tremendously skeptical about 'C' ratings as there is no standard and is therefore only marginally useful in my estimation. As an example, I have some old 16C rated packs that hold higher voltage under load and run cooler than some newer 22C rated packs. As I don't like to purchase new lipolys on a regular basis, I monitor the temperature of my packs after use with an IR Thermometer and make every effort to keep pack temps under 125F post flight. So far, it's working for me.

That said, I would personally limit the current draw of the paralleled pack to stay within the limits of the lower 'C' rated pack. e.g. - If you hook a 2000mAh 8C pack in parallel with a 1000mAh 30C pack, limit current draw to 8C (3000 x 8 = 24 amps in this case). There certainly is some wiggle room here and it's recommended that you listen to your packs. If they're running cool, it's typically safe to increase current draw. If they're running hot, time to back off.

Mark

[Screamin Demon]

Thank everyone! Talk about teamwork!

so if i understand everything correctly, i could say,

true or false question #1:
three cells charging in parallel and connected to the same balance circuit would charge at the rate for the cell with the highest voltage.

true or false question #2
if i had a cell that was dangerously low in volts, it should not be parallel charged with a cell of a much higher voltage.

What effects could be possible for example #2 if it were true?


I have always been very cautous when handling lipos. I made one mistake one time and lost a workshop and 60K worth properity to a lipo fire. lipos are exteramly volitol when charged are very high currents and short circuits and can burst and throw flames of chemicals 5 ft. away.

For the sake of everyones families, person and properity, make sure you fully understand this material before attempting.

[GreenBar0n]

Originally Posted by mrforsyth

I doubt that it'll create any problems, per se, but long balance leads may very likely increase the time it takes to achieve perfect balance.

Quickie math -

Assuming 4 feet of 22 gauge wire, you'll have approximately .06 ohms of wire resistance. If the balancer balances at 500 milliamps current draw, you'll have .03V of drop in the wire (not including connector losses). Under no load, the balancer circuitry will sense a voltage differential and commence balancing. If it draws .5 amps, it'll stop balancing when the imbalance is at .03V, theoretically. It'll likely then start balancing again when current drops off and once again 'sees' the voltage differential between cells.

Once again, this is all 'in theory'. You'll have to monitor to make sure. My personal practice is to keep balance leads as short as practical and use fatter wire if I need them to be long. This way I have no worries and my packs stay in perfect balance.

Mark

Thanks for the breakdown, I will try to get some tests run over the weekend, with and without the balance cable extensions. I'm curious to see how much of a difference in accuracy or time the extensions would make.

Do you use a current limiting balance board, or do your balance taps connect straight through to the charger? I couldn't tell from the pic.

Thanks again mrforsyth

[mrforsyth]

Originally Posted by GreenBar0n

Do you use a current limiting balance board, or do your balance taps connect straight through to the charger? I couldn't tell from the pic.

The balance parallel harness connector connects directly to the 106B to keep IR drop as low as possible. That's one thing that I like about the 106B over the 1010B - no need for the outboard balance board. Cleaner look and fewer connector interfaces to go through.

Mark

[mrforsyth]

Originally Posted by Screamin Demon

true or false question #1:
three cells charging in parallel and connected to the same balance circuit would charge at the rate for the cell with the highest voltage.

Charge rate is based on the resultant capacity of the packs in parallel. i.e. - Say you want to charge two 2100 mAh packs in parallel with two 1500 mAh packs. 1C charge current would be 2100 + 2100 + 1500 +1500 = 7200 mA or 7.2 Amps.

Remember that lipo packs of different cell counts cannot be charged in parallel. Attempting to do so will most assuredly result in a fire.

Originally Posted by Screamin Demon

true or false question #2
if i had a cell that was dangerously low in volts, it should not be parallel charged with a cell of a much higher voltage.

What effects could be possible for example #2 if it were true?

Packs of widely varying charge state should not be connected in parallel. I personally have a .3V voltage delta rule to limit current flow between packs and ensure maximum safety. Attempting to connect a pack that is 'dangerously low in volts' to a pack with significantly higher voltage would be unwise. The worst possible effects of doing such would be catastrophic failure of the lower voltage pack. Not saying that it will happen - only that it is a possible consequence depending on the quality and voltage difference of the packs in question.

Best to stick with matching to within a few tenths of a volt until you've established the maximum tolerable difference for the specific packs that you desire to charge in parallel.

Mark

[Ampdraw]

Firtly thankyou Julez and other participating members for this informative thread. I have read its entirity and just want to double check a few things if I may.

Currently I balance charge singular larger batteries and have made a lead for 3x single cell packs to charge serially.

I am going to make a multiple lipo balance charge cable for 2x 2s packs for a Bantum BC6. Want to charge 2x 800mah and also 2x 850mah for my Gaui

Should I make a parallel or serial cable?
(I know to be careful with polarity serially) Batteries will be charged at similar charge states after flying to close to cutoff.

Advantage of parallel cable: Can charge different mah packs. Faster on balance. Easy.

Advantage of serial: Get indervidual cell info rather than grouped info.

Leaning toward the parallel so i can mix up the packs:

1: Are there any safety concerns with the charger only being able to see the "grouped" cell voltage?

2: The charger will shut off just as efficiently if I were to have a bad cell either way correct?

3:Is there a need to indervidually balance the packs every now and then or will balance charging in parallel be fine for the long term?

Thanks for the help.

Amp

[mrforsyth]

My 2 cents...

Originally Posted by Ampdraw

Leaning toward the parallel so i can mix up the packs:

Wise choice. Charging two discreet packs serially is extremely risky if the packs are not very closely matched in capacity and IR, have the same usage history, and have been discharged as a single serial pack. Overcharging is a very distinct possibility. Parallel charging eliminates these concerns.

Originally Posted by Ampdraw

1: Are there any safety concerns with the charger only being able to see the "grouped" cell voltage?

No. It's no different than charging packs singly, assuming that all of the packs in parallel are in good health.

Originally Posted by Ampdraw

2: The charger will shut off just as efficiently if I were to have a bad cell either way correct?

Yes. However, packs should be removed from service if it's been established that they have a bad cell.

Originally Posted by Ampdraw

3:Is there a need to indervidually balance the packs every now and then or will balance charging in parallel be fine for the long term?

No need at all. Balance charging in parallel is identical to balance charging individually. I have packs that have never been charged individually, except for the break-in cycles and initial characterization discharges on my CBA. The only reason that you'd ever need to balance charge individually is if you suspect that you have a pack with a weak cell. Balance charging individually will allow you to isolate the suspect pack.

Mark

[Ampdraw]

Thankyou Mark,
Exactly what I wanted to know.
Appreciate your effort.

My charger allows me to program memory presets (5 in total)

When setting these up I am able to set the capacity mah as well as cell count and charge rate.

When I just enter non preset "lipo charging" mode I am unable to enter the capacity mah.

I was advised by the ever helpful Bantam rep (plug for great service) that the charger uses CC/CV charging and that either setting ends the same with the same safety level and the capacity mah was for convenience when selecting memory presets.

Just want to be sure after I make the parallel lead, simply connect discharge leads first (to equalise packs) > connect the balance taps > enter 2s 7.4v (for 2 packs) > enter charge rate (1.6amp for 1c) > hit charge.

Presume the CC/CV charge type doesn't look for final mah to decifer full pack. Am I correct to assume this?

Amp