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Questions about LiPo's in parallel

Questions about LiPo's in parallel

Old 07-23-2013, 08:19 PM
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Default Questions about LiPo's in parallel

Ok, I am mainly trying to make sure of a couple things here, I have a slight electrical engineering background (worked with electrical engineers on robotics and e-bike) so I beleive it should work well but want to make sure with a few guys.

I know you can charge LiPo's in parallel, and although I haven't seen it, would it be wise to run LiPo's in parallel in an RC? Makes since to to me since it'll keep high loads off the batteries and keep them balanced. Correct?

The big question though is how wise would it be to wire the balance plugs between the two packs is parallel as well? To me, it seems like it would prevent one cell from dropping too low since it would start pulling more load off the other pack's cell that it's in parallel with. To me this would wire it identically to how a 4s2p battery would be correct?
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Old 07-23-2013, 08:29 PM
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Makes sense to me . But I'm subscribing.
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Old 07-23-2013, 08:49 PM
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n/m

Last edited by Dave_S; 08-10-2013 at 12:44 PM.
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Old 07-24-2013, 03:45 AM
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Originally Posted by rc_dude
The big question though is how wise would it be to wire the balance plugs between the two packs is parallel as well? To me, it seems like it would prevent one cell from dropping too low since it would start pulling more load off the other pack's cell that it's in parallel with. To me this would wire it identically to how a 4s2p battery would be correct?
That's correct. Wire the cells in parallel. If the cells are significantly different in capacity or internal resistance, you may need to increase the gauge of wire and connector current rating over that used for the balance connector.

You are probably aware of this, but make sure that the cells to be connected in parallel are within a few mV of each other when you make the connection, to avoid large currents.
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Old 07-24-2013, 04:01 AM
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Yep, conceptually the idea makes sense, good question, but the balance wires may not be large enough to handle a significant imbalance with large loads. Full scale lithium E cars are typically connected this way, but the relative load is much smaller (Range is an issue, but they do go longer than 10 minutes or so)

In any event it’s best to somewhat regularly check pack balance when starting charging. Lipos are relatively cheap and expendable, shouldn't be using packs with balance issues anyway. Especially in a high draw use.
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Old 07-24-2013, 06:29 AM
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Thanks for the input guys!

hotrod: haha exactly what I would have done

Dave S: Thanks for the sight, I've actually done a few of those connectors for charging multiple packs myself. Nice to have graphics EDIT: BTW, are you the same Dave_S as at URC? just noticed from the parallel balancing board thread

Originally Posted by howardcano
If the cells are significantly different in capacity or internal resistance, you may need to increase the gauge of wire and connector current rating over that used for the balance connector.

You are probably aware of this, but make sure that the cells to be connected in parallel are within a few mV of each other when you make the connection, to avoid large currents.
Right, I know if one power source (lipo cell in this case) gets say .3 volts lower than the other in series, then the higher voltage source will pull almost all the current. It's ridiculous how a very small voltage difference means a huge load difference in parallel. I would only be connecting them at storage or a full charge though so the surge current shouldn't be an issue.

Originally Posted by Dave H
Yep, conceptually the idea makes sense, good question, but the balance wires may not be large enough to handle a significant imbalance with large loads. Full scale lithium E cars are typically connected this way, but the relative load is much smaller (Range is an issue, but they do go longer than 10 minutes or so)

In any event it’s best to somewhat regularly check pack balance when starting charging. Lipos are relatively cheap and expendable, shouldn't be using packs with balance issues anyway. Especially in a high draw use.
The Tesla, leaf, ect probably huge huge wire/tabs together to wire in parallel too lol Impressive that almost all the technology used in those cars is less than 10 years into development too. The packs I would be doing this to though are 4s 5amp packs though, so they were $50 lol

To both Howard, Dave H, and everyone else: I just thought of this.

While in use, say two cells get .05v apart in parallel, then by that point, the lower cell is constantly being charged by the one it's wired to. Say the internal resistence of both cells is 5 Milli-ohms (.005ohms) so with V/R=I (derived from V=IR) in the circuit around just those two cells, the voltage difference is .05 and total resistance is .01ohms. .05/.01=5amps charge for the lower cell, 5amps discharge from the higher cell. At this rate the cells would equal off almost immediately. Then the only concern is the voltage drop being different between the two.

I'll probably try it on my mini-revo first since those packs were $8 and already have seen some use...lol If I start running into warm balance wires, I can always order some .1 or .01ohm resistors to put in between them to limit the current lol
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Old 07-24-2013, 07:02 AM
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If it helps, I run 2 3000mah 30c 4s blue sky lipos in parallel all the time. I charge them with a parallel balance plug and always leave them connected in parallel. I've never had a problem with them, the cells stay balanced pretty close between each pack.

It probably helped that these packs were purchased at the same time and likely came from the same batch, and have never been apart. The cells have always been pretty much a mirror copy of each other.

I've read that running packs in parallel that have different amounts of usage is not always the best way to do it. I would suggest buying 2 cheap batteries and try it out. Leave them connected all the time. Just turn them into a 4s2p suggested earlier.
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Old 07-24-2013, 08:21 AM
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Originally Posted by rc_dude
I would only be connecting them at storage or a full charge though so the surge current shouldn't be an issue...
I can always order some .1 or .01ohm resistors to put in between them to limit the current
Keep them connected at all times. Don't use resistors.

Originally Posted by Darkgenerals
I charge them with a parallel balance plug and always leave them connected in parallel...
It probably helped that these packs were purchased at the same time and likely came from the same batch, and have never been apart. The cells have always been pretty much a mirror copy of each other.
Yes, this is excellent advice. Obey the Darkgenerals!

Last edited by howardcano; 07-24-2013 at 09:02 AM. Reason: Added "this is excellent advice".
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Old 07-24-2013, 08:34 AM
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Originally Posted by howardcano
Keep them connected at all times. Don't use resistors.



Yes. Obey the Darkgenerals!
lol, not sure of sarcasm or serious.
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Old 07-24-2013, 09:57 AM
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Darkgenerals: That is exactly what I would be doing with these. They are new, identical, and i don't even have them in my possession yet lol I will be trying them on my mini-revo's $8 battery's first though.

Howardcone: The resistor would be between the pack ONLY while in the car, not on the charger were it would cause a false voltage reading. I would be putting them between two female connectors that then plug into the battery's on the car only. Them I'd just use a parallel balance board on the charger.
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Old 07-24-2013, 11:09 AM
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Originally Posted by rc_dude
Darkgenerals: That is exactly what I would be doing with these. They are new, identical, and i don't even have them in my possession yet lol I will be trying them on my mini-revo's $8 battery's first though.

Howardcone: The resistor would be between the pack ONLY while in the car, not on the charger were it would cause a false voltage reading. I would be putting them between two female connectors that then plug into the battery's on the car only. Them I'd just use a parallel balance board on the charger.
I would still not suggest using a resistor, seems like it is too complicated and over thinking this. KISS is a very good rule to do this by, also suggest looking up construction of LIPO packs. I bet 10 bucks not one single lipo manufacture that builds good batteries does this. 2p2s batteries are becoming very common and it if lipo manf were adding this alot of people would suggest it. it just adds an extra failure point.

You would also have to worry about a resistor burning out or something weird happening and damaging the bats in some way.
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Old 07-24-2013, 12:46 PM
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Originally Posted by Darkgenerals
I would still not suggest using a resistor, seems like it is too complicated and over thinking this. KISS is a very good rule to do this by, also suggest looking up construction of LIPO packs. I bet 10 bucks not one single lipo manufacture that builds good batteries does this. 2p2s batteries are becoming very common and it if lipo manf were adding this alot of people would suggest it. it just adds an extra failure point.

You would also have to worry about a resistor burning out or something weird happening and damaging the bats in some way.
Good point.

Well, thanks for everything guys!
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Old 07-24-2013, 04:44 PM
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Originally Posted by rc_dude
........

The Tesla, leaf, ect probably huge huge wire/tabs together to wire in parallel too lol Impressive that almost all the technology used in those cars is less than 10 years into development too. The packs I would be doing this to though are 4s 5amp packs though, so they were $50 lol

.....
The last few years have been interesting no doubt and the future needs sunglasses, but there is a little more than 10 years behind the electrics. Lithiums started in the '70s, with significant milestones in the '90s. The '90s GM Impact - EV1 had a lot of technology in it that has carried over, regenerative braking (just like some RC controllers of the era!), composite and aluminum suspension components, low resistance tires, etc. A lot of the control experience comes from forklifts, golfcarts, and other industrial drive tech. Brushless motors are near ancient. Go really old school, electric cars outsold gas cars in the early 1900s. Sorry for going off topic, sort of an interest of mine.
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Old 07-24-2013, 05:45 PM
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Originally Posted by Dave H
The last few years have been interesting no doubt and the future needs sunglasses, but there is a little more than 10 years behind the electrics. Lithiums started in the '70s, with significant milestones in the '90s. The '90s GM Impact - EV1 had a lot of technology in it that has carried over, regenerative braking (just like some RC controllers of the era!), composite and aluminum suspension components, low resistance tires, etc. A lot of the control experience comes from forklifts, golfcarts, and other industrial drive tech. Brushless motors are near ancient. Go really old school, electric cars outsold gas cars in the early 1900s. Sorry for going off topic, sort of an interest of mine.
I keep thinking were in the 2000's when i spit out a number for age...I think my mind got so used to it growing up it assumes it still is sometimes lol

I don't mind going off topic for this at all, the thread has really stopped anyway.

When I said that, i meant it as the technology wasn't really usable until then. The EV-1 should have never been discontinued (ever seen "Who Killed the Electric car?" documentary?) Yes the actual physics and concepts have been around a very long time, not much of it was very practical till the 90's, and the EV-1 didn't even use lithium batteries. Brushless motors are actually old design wise, and FET's were invented in the 20's i believe, but weren't realistic to use until for large amounts of power till fairly recently.

The electric car gave way to a gas engine around 1900ish and remained fairly untouched (couple little exceptions) till GM came along. Comparing that to the IC engine that powered most inventions with the push for horsepower pushing them to develop them as fast as possible for over a century is apples and oranges. The IC engine was around back in the 1870-1880's (maybe before) and widely used for transport in the 20's. So respectively, they are 100 years ahead in development.

Not intending to rant or anything, just kinda kept talking...lol
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Old 07-24-2013, 07:12 PM
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It’s rather odd for me to be defending GM, but it seems to me they might be treated a touch unfairly on this topic. California had a mandate that a specified percentage of sales had to be non-polluting by a certain date for the majors (5% by mid ’90 something? Can’t remember for sure). GM is the only automaker that did much of anything about it, everybody else lobbied up and fought the mandate.

GM spent a ton of money and did some rather interesting things with the Impact, far more than just the powertrain. Found themselves standing all alone as everybody else in line stepped back a few and left them wasting in the wind. California backed off on the mandate, GM decided to cut the financial bleeding until conditions improved.

Seems odd that the only company who actually did something substantive gets all the blame in the view of some, while the ones who did nothing much but lobby against the initiative escape blame free it seems.

Eh, such is life I guess. Sometimes you’re the bug on the windshield.
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