Parallel Charging Lipo question

Hi I found a thread on here a while back where someone built a 18S2P pack consisting three 6S bricks of Lipo twice. They built a harness so that they could charge all bricks in one go by altering so that the pack broke down into 6S6P (I think).

My question is, I am thinking about building a 12S3P pack and using the same method to convertor into a 6S6P do parallel charging using a balancing charger, but what happens when you join the balance leads if each parallel string if the corresponding cell is out of balance - surely the inrush of current would fry the balance lead?

Does anyone get what I mean? I can't find the thread to ask the guy the question.

Thanks
Jon

They would have to be REALLY badly out of balance, and at the bottom soc it makes less difference too. I think if they'd be that far out of balance you'd have bigger issues?

And why would anyone want to go thru all the trouble/hassle/risk of breaking/building a pack for every charge/discharge? The battery should keep balance if healthy and treated right- so just bulk charge! Get the packs with bullet connectors for series, and then you'd just need two 3<1 parallel connections for 3p.

sounds like tench's bighit middrive hub build with parallel series charging loom. That was an awesome build making good use of the icharger high amps but low series 6s or 10s for lipo. I thought about the same thing. I think the trick is check the 18s or 12s in your case before you disconnect and reconnect in parallel. One thing Tench's system had was an automatic disconnect in the plug. he made it impossible for the user to plug both series and parallel at the same time which is a short circuit or sparky sparky time potentially leading to some nice KFF .

Was this it by any chance.
https://endless-sphere.com/forums/viewtopic.php?f=14&t=39666
You can charge a 12s3p made from 4s packs many safe ways. That's only 9 4s packs. I've parallel charged as many as 12 4s packs in parallel. There's really no need to build a harness to turn the 12s into 2 6s plugs to parallel charge. Only if you use a 12s charger to charge the whole pack at once is that needed. And the easiest way to charge it would be using 3 battery medics and a bulk charger so you never have to reconfigure to charge.

It was an orange bike. Very well put together. Will have to search to see if it was Tench.

And yes it was a custom harness that simply needed one plug to charge and then to discharge simply unplug the harness and in its place goes a wiring harness to reconfigure the parallel strings to series.

I bulk charge at the minute with my 14S system but the blinky balancers just don't have enough balancing power to even the cells out. They multistar 16ah so it takes forever.

nutspecial said:

And why anyone would anyone want to go thru all the trouble/hassle/risk of breaking/building a pack for every charge/discharge? The battery should keep balance if healthy and treated right- so just bulk charge!

Click to expand...

And that's how you successfully blow up your bike.

If you don't have the patience to balance charge EVERY time, then you need a BMS. Bulk charging bare LiPo's is never, ever, safe. The RC crowd learned this the hard way many years ago. It doesn't much matter if the "battery should keep balance"... it's the one time you think it should and it doesn't that burns your house down.

-Jim

How about those big multistar packs big as ah.

I have that same setup. But I don't have an orange bike.

I use Anderson power poles keyed in such a way as to have a charge plug and a discharge plug that both snap to the same Andersons on the pack depending on the activity. The charge harness parallels the cells down to 12S for charging and the discharge harness (mounted to bike) runs it to 18S (I have another setup that I do this from 12S and 24S).

Bulk charging bare lipos can be safer than bulk charging with a BMS. Without BMS, the human is expected to be the charge monitor and I trust myself much more than I trust a BMS. (I use cell monitors and charge from 3.3V to 4.0 VPC as part of my comprehensive cell longevity and safety plan and don't charge unattended). Balance charging every time is certainly not necessary, especially if you care for and vet your Lipos properly when building (cycling and eliminating outliers). I have only had to balance my lipos ("had to" is rather strong language since it was still much below 50mV delta) 2 times in as many years.

cal3thousand said:

Bulk charging bare lipos can be safer than bulk charging with a BMS. Without BMS, the human is expected to be the charge monitor and I trust myself much more than I trust a BMS. (I use cell monitors and charge from 3.3V to 4.0 VPC as part of my comprehensive cell longevity and safety plan and don't charge unattended). Balance charging every time is certainly not necessary, especially if you care for and vet your Lipos properly when building (cycling and eliminating outliers). I have only had to balance my lipos ("had to" is rather strong language since it was still much below 50mV delta) 2 times in as many years.

Click to expand...

We clearly have different definitions of balance charging and bulk charging. When I say "bulk charging bare lipos" I mean charging cells in series without monitoring individual cell voltages, thus the "bare". When I say "balance charging" I mean charging cells in series and monitoring each individual cell. In balance charging, cells are either charged or discharged to make them equal the voltage of the other series cells. Additionally, and most importantly, if a cell goes over a certain voltage the charging is stopped.

What you describe above (you manually monitoring the cells) is for all intents and purposes, balance charging. Even more so with well matched cells that don't drift in voltage.... until they do.

The problem is that to do due diligence with your method is extremely inconvenient. So much so, that even the best of us will eventually "trust" our pack to not go out of balance during charge and we'll hook the pack up to the bulk 2C charger and let it do it's thing. Eventually, this will lead to a fire.

You don't trust a BMS to do it, but durn it I don't trust me to do it! BMS's don't get lazy and if I want I can verify that it's working by manually checking cell voltages. Plus you are probably using a cheap chinese cell monitor to check your batteries... you trust that $8.00 gadget to accurately display the voltage but not a quality BMS?

I've had LiPo fires due to bulk charging. I've learned my lesson. Since swapping over to ONLY balance charging or using a BMS I've never had any issues.

To the OP:

Back many years ago when the idea of balance charging first propagated through the RC community, your question of connecting imbalanced cells in parallel came up many (many) times. Many tests were run, many experiments made. The result of all of that was that small differentials in cell voltages do not produce enough current to do damage to the low, or high cells. My rule of thumb is 0.1V/cell difference is ok. If it's more than that, then you need to think about there being something wrong with the low pack.

For your application, you probably will never have any problem balance charging as you describe since your packs should be uniformly discharged. For RC folks who use packs seperately then charge together (unlike what we do on a bike) they end up with packs at different voltages regularly. Even in extreme circumstances, it's surprisingly not a big deal. The concept in the video has been proven true for a good 8-9 years now in the RC community.

[youtube]b5p-xUtWM1w[/youtube]

-Jim

Jim, looking at that video he doesn't actually say whether the three individual cells from each of the three packs are balanced already. If they were already well balanced, and the three packs are connected using his parallel charging harness then each pack would equalise using the discharge leads. Now that would be fine as they're capable of carrying much higher current than the balance leads.

But what if one of the cells is, for whatever reason, out of balance, but all the rest are okay? When you connect the three packs in parallel they'll equalise, but that one low cell will still be out of balance. When you plug the balance leads into the parallel balance board the higher cells will try and equalise with that single out-of-balance cell through the much smaller balance leads and the balance board.

Surely they are not made to take high currents?

Jon

The main leads will equalize pack levels, not cell levels. Like Jim said .1v is a reasonable limit to parallel the balance taps, and as I mentioned SOC likely affects this too (4.1 -4.0 VS 3.1 -3.0), probably fairly inconsequentially @ those variances.

I've had LiPo fires due to bulk charging

Click to expand...

We may disagree, but imo the simplest methods are best. In this unfortunate case (btw- pictures or it didn't happen - don't see much in your history) you had ONE thing to do/monitor and failed? Using bms or breaking down and using hobby chargers adds complexity and unfortunately more failure modes, which require monitoring too, unless you trust hinky electonics and extra harness, with a volatile chemistry. I opt for the simpler.

My pack charges @ 1.5c and I typically am charging for 30min or less, 'attended', and it's easy quick and dependable. It takes seconds each time to verify balance, and I think that's simply good practice added to typical regular visual checks like tires brakes susp, and not crossing the street in front of a bus etc. If stuff goes out of balance (>.05v over several cycles) I limit charge/discharge appropriately. If lipo cells start self-discharging and/or needing regular heavy balance they're not worth having around imo. Healthy well treated packs keep balance naturally.

To each their own of course, and the car manu's are pretty successful with their systems I guess- when they're new at least, for non-lipo.

The custom harness sounds pretty cool for using a balancer, but imo balancers seem like a pricey/ undependable/ junky/ unnecessary way to charge. Something more like adapto seems better esp for bigger packs, but that's a form of bms with good user control/integration.

nutspecial said:

The main leads will equalize pack levels, not cell levels. Like Jim said .1v is a reasonable limit to parallel the balance taps, and as I mentioned SOC likely affects this too (4.1 -4.0 VS 3.1 -3.0), probably fairly inconsequentially @ those variances.

I've had LiPo fires due to bulk charging

Click to expand...

We may disagree, but imo the simplest methods are best. In this unfortunate case (btw- pictures or it didn't happen - don't see much in your history) you had ONE thing to do/monitor and failed?

Click to expand...

My fire was in the days before lithium packs had balance leads it was in 2004 so it well precedes my time here on ES. It was using (at the time) the best LiPo charger available (Astro Flight 109), it had the best voltage monitoring system, but was bulk charging only. An imbalanced (bad probably, but it all went up in smoke so I don't know) cell caused the "good" cells to puff then explode while in my hands trying to get the pack outside the house. I was at the bleeding edge of LiPo's use in the hobby RC market in the late 90's and early 2000's.

I'd modify your statement to say "The simplest method that is safe is the best", but otherwise agree with your post / methods. The key here is that individual cells must must be monitored at every charge. There are varying ways of doing that: Checking each cell before bulk charging, checking each cell manually while charging, having a balancing device on the pack while charging, or having the charger monitor each cell. Which method you choose is up to you, however IMO what should not be up to you is whether each cell is somehow monitored EVERY charge.

To the OP:
Nutspecial is right. SOC matters a lot. At low SOC (something like 3.1V) differential voltages don't matter as much. At low SOC you are likely to have the greatest differential and one that will most quickly equalize. Additionally lower voltage means lower current flow while the two cells equalize. It's also a good time to talk about temperature. LiPo's have lower internal resistance at higher temperatures. So if you parallel packs of different voltages immediately after discharging them when they are hot, the current flow will be higher. Best to leave them to rest for a bit after using them to let them cool down.

Let's do some quick math to look at the problem you are describing.

A quality LiPo cell might have 10mOhm internal resistance. If you connect to cells in parallel, one with a voltage of 3.3 volts and another with 3.2 volts. Ohms law V=IR. V in this case is the differential voltage so 0.2V. R is the internal resistance of the cells 0.010 ohms. So the amperage at the very beginning of the equalizing will be 10amps... pretty high but it doesn't last long at all because the higher pack gets pulled down in voltage by discharging (it is after all almost empty), and the low voltage cell starts coming up very very quickly at low SOC so the 10 amps lasts for a very short period of time. The cells very quickly equalize at low SOC and then are ready to start the balance charging step.

-Jim

Naked bulk charging RC Lipo can be a real crapshoot. I know folks do it, I still do it from time to time as long as I’m very confident the pack is in good health. But for day in day out commuting, I pretty much use a small BMS for bulk charging only.

I take power directly from the battery leads/tabs. My logic is that the controller has pack voltage LVC and I’m operating the bike outdoors so if an exothermal event occurs, it’s a much more manageable situation than charging in the living room of my apt. This keeps BMS cost down and provides the major assurance I want/need.

However, the one thing to be aware of and check constantly with balancing BMS boards? Balance circuit transistors sometimes fail and short circuit. This flows power across the bleed balance resistor and will eventually drain the cell group to critical levels. Ugh!

My advice, use a BMS but check it often. About 50% of the time my commercial BMS’ develop a shorted bleed circuit. When that happens I generally lift the resistor for that channel and continue to use the pack/BMS anyway. It no longer balances that cell group (crappy balance scheme anyway) but I still get cell level LVC/HVC protection via the power port used for bulk charging.

Balance leads serve two functions: The balance leads are used both to sense balance and to discharge/bypass the higher voltage cells.

If the balance leads to each tap (in each series string of cells) had a small series resistor...perhaps .1 ohm or so, then the max current that could flow through the balance leads would be deltaV/.2 ohms...so a whopping .5V difference would only give 2.5A of current. Of course this would require using 1 (or 2, just to be sure) Watt resistors, which are not so small, but tolerance is not much of an issue. Such resistors could be part of the parallel balancing harness, so needn't be part of the battery pack, and can be open to the air to avoid heat problems.

Voltage sensing solutions are going to have input impedance in the Kohms, if not megohms, so a fraction of an ohm won't disturb measurements by even a millivolt, and all taps will be roughly impedance matched anyhow, so sensing should not be a problem.

The resistors of various packs would be effectively in parallel for the balancing discharge function, but will still lower the balancing current slightly, but will also absorb some of the power dissipation that the balancer would otherwise see.

Ideally, the balancing circuitry would be designed with this in mind, and these resistors could be larger and serve as the balancing load resistors...then they could be a bit higher resistance, making the current between packs very small indeed.

It would even be possible to use stainless steel cable for the balancing leads, and let the high resistance of the wire serve the function, but attaching to batteries might be an issue, but it should work in a paralleling harness. Also, need to get the teflon coated stuff, as PVC won't tolerate much dissipation...otherwise fishing leader would work. Stainless has a pretty high positive temperature coefficient of resistance, so will tend to reduce the current as it heats up...good for the safety function, but might be problematic for discharge/balancing.

individual cells must must be monitored

Click to expand...

Of course! Not just willy nilly either imo, it's kinda important knowing the little bastards can spark a mf fireball. I'm sorry if my earier post/history/etcetc gave impression otherwise and sorry about your accident- 'bulk charging' (imo) never means absense of cell monitoring and I've never heard anyone suggest such a thing or use the term in that way.