Parallel batteries discussion

[BrianG]

I've read a number of threads talking about paralleling batteries (especially lipos) and have some questions - mainly curiosity. I've tried the search function, but turn up too many results to sort through.

Anyway, how do you keep parallel batteries from trying to charge the other (which in effect would discharge one while doing so)? One parallel "leg" will most certainly have slightly different parameters and will discharge faster than the other which will cause the other to try to charge the other one. The net effect of course is that the output is constant, but the charge/discharge effect could be happening "behind the scenes". I suppose a precisely matched set would be better, but they would have to be matched across the whole discharge curve.

You could use diodes to isolate them from each other, but the resulting voltage drop (.7v) times the current draw requires high power diodes - not to mention you don't get full battery power to the load. At 50 amp draw, a diode would need to dissipate 35 watts, which is power I'd rather have at the motor.

Has anyone analyzed the effects of paralleling batteries in this regard?

I would think with the relative complexity of charging/discharging lipos that this would be a greater concern.

I've never, ever, seen a real-world application where batteries are hooked in parallel - whether in R/C, UPS battery back-up systems, vehicles, etc. If larger capacity is required, you simply increase the size of the battery cells. Of course I realize this sometimes isn't feasible for R/C.

Anyway, just curious. :)

[MetalMan]

Lipos are much more simple than it seems you are thinking. Take two Lipo cells (at a similar voltage, + or - 0.05v), and stick them in parallel, and their voltage will equalize. In effect, one cell will be charged, and the other discharged. The reason for having such a small difference in voltage between the two cells is so that the lower cell will not be charged by the higher one at such a high rate.
Notice that I did not mention anything about the capacities of the cells. One could connect two cells of completely different capacities (like 1000mah and 4000mah) in parallel to give 5000mah and the sum of the amp rates. During a discharge, both cells will remain at the same voltage (theoretically, but the resistance of wire could affect it). The draw from each cell is a function of the resistance of each cell. For example, a 1000mah 10C cell(10amps) will have a resistance roughly 4 times greater than a 4000mah 10C cell (40amps). If both cells were to contain an identical composition (excluding size), then at a 50amp discharge the 1000mah will take 10amps of the load and the 4000mah will take 40amps. Obviously, no two cells will be some much alike as to allow such theoretical-ness, but I am just throwing the idea out there.
This same process goes for charging. Take both of the previous cells as examples. At a 5amp charge (1C for the combined pack), the 1000mah will take in 1amp and the 4000mah 4 amps.

The important thing to remember is that the cells will always be at the same voltage (neglecting wire resistance), and the internal resistance of the cells will determine the actual number of amps being pulled from (or put in to) each cell.

P.S., if you haven't seen a real-world application for parallel batteries, you need to get out more, my friend :).

[BrianG]

I realize two paralleled batteries (lipos or otherwise) will have the same voltage at the terminal, but that is only because one is equalizing the other. The reason for my argument is that if one is (for the sake of argument) 1v higher than the other, say 5v and 6v, the 6v battery will charge the 5v one with as much current it can muster only limited by the wire resistance and the charging battery's internal resistance. This is because the difference in potential between the two (1v) divided by the current limiting factor, about 0 ohms (wire and internal resistance aside), equals theoretically infinite current. By the way, internal resistance is not the only limiting factor in capacity - it is also the chemical makeup/density; not to mention the internal resistance vs load is probably not a linear curve. I can see how it would be easier to view it that way though.

So, what application does utilize a parallel battery arrangement then (non-lipos)? I do "get out", but any application I can think of either uses larger capacity batteries in lieu of paralleling them, or there is some sort of isolation between them. Please enlighten. ;)

Thanks for the input though. And I don't mean to be argumentative, but like the thread title says; it's a discussion. :)

[JThiessen]

Most diesel pickups & large trucks, RV's, camp trailers, use parallel systems.

Remember that electricity needs a recipient before current can flow. The main reason for a diesel pickup to run two 12V batteries is because it requires higher current capacity, not voltage, to operate effieicinetly.

15 years ago while I was in college, I probably could have answered your question, but my selective memory (powered by beer) has taken over and destroyed those evil invasive theorems on electrical engineering!

[BrianG]

Yes, but don't they use a battery isolator? It would just look like a device with three large terminals (two "in"s and one "out") which is just two diodes on a heatsink casing. Usually, each one powers specific parts of the vehicle, but are charged at the same time through the isolator. We have a 200kVA diesel generator at work and it uses only one battery (but what a large one it is!). I'll have to check out a dumptruck sometime.

Electricity needs a difference of potential and a connection to flow and both criteria are met if you have two batteries in parallel and one has a higher voltage than the other. Don't believe me? Find two batteries with different voltages and put an ammeter between the two + terminals (or between the two - terminals, just not between the + and - terminals) and measure the current.

[MetalMan]

Assuming resistance is very low (0.001ohms for instance), then current can flow at a high rate. For Lipos, this can be unsafe, which is why the voltage difference should be no greater than about 0.05v. With 0.05v and 0.001ohms, the current flow could reach 50amps! So with a voltage difference of 1v and a low resistance like 0.001v, an amp spike could hit 1000amps! A more realistic Lipo resistance would be 20milliohms (depending on size), and with a 0.05v difference, amps could reach 2.5.

My thinking may be off a bit, but I blame it on never having a class that teaches electricity! (But, there is the internet. This year in AP Physics we will be studying electricity, so I should know a little more then :).)

P.S., I was being a little sarcastic about getting out. You probably aren't going on a forum everyday where Lipos are discussed, like I do. There parallel packs are seen at least 60% of the time! All of the Lipo packs I own are parallel packs (but there are groups of cells in parallel, and then those groups are put in series).

[standard_63]

I'm also wondering:
If I want 5s Lipo on my Emaxx, can I run one 2s lipo on one side and a 3s lipo on the other? Connected parallel with a y connector the same way I currently run 2 x 2s lipos for 4s lipo?

[BrianG]

So, are you saying lipos maintain a relatively constant voltage (+/- 0.5v) regardless of charge state?

Yeah, I have an electronics/electricity background, but haven't read enough about lipolys, except all the bad stuff. I know the lipos are paralleled very often, that was what really prompted the thread. Even though my main thoughts were about lipolys, the discussion can also be about other battery types.

EDIT:
After submitting this reply, I got message saying my post count reached 50 and I have a prize! Wow, that's cool!

[mwe-maxxowner]

Originally Posted by BrianG

EDIT:
After submitting this reply, I got message saying my post count reached 50 and I have a prize! Wow, that's cool!

Wow, this thread must be old! lol

[coolhandcountry]

Originally posted by standard_63
I'm also wondering:
If I want 5s Lipo on my Emaxx, can I run one 2s lipo on one side and a 3s lipo on the other? Connected parallel with a y connector the same way I currently run 2 x 2s lipos for 4s lipo?

It is not a good idea to hook up 3s and 2s in parellal. You can hook in series and get the 5s.

[Dafni]

Originally posted by standard_63
I'm also wondering:
If I want 5s Lipo on my Emaxx, can I run one 2s lipo on one side and a 3s lipo on the other? Connected parallel with a y connector the same way I currently run 2 x 2s lipos for 4s lipo?

You're talking about series there. I guess you confused S with P.

[standard_63]

Originally posted by Dafni
You're talking about series there. I guess you confused S with P.

Yes! --> I get them confused all the time!! Series increases power, parallel increases capacity, is that right?

[BrianG]

Series increases voltage. The product of current and voltage is power. And capacity is current, which increases when batteries are in parallel.

Series = To increase voltage, but keep the current the same. Actually, the capacity of the smallest capacity cell will determine the max current.
Parallel = To increase current/capacity, but keep the voltage the same.
Series and Parallel = Increases both.

[MetalMan]

Originally posted by BrianG
So, are you saying lipos maintain a relatively constant voltage (+/- 0.5v) regardless of charge state?

Yeah, I have an electronics/electricity background, but haven't read enough about lipolys, except all the bad stuff. I know the lipos are paralleled very often, that was what really prompted the thread. Even though my main thoughts were about lipolys, the discussion can also be about other battery types.

EDIT:
After submitting this reply, I got message saying my post count reached 50 and I have a prize! Wow, that's cool!

Lipos don't maintain a certain constant voltage, unless you consider + or - 1.2v to be constant (I wouldn't). But when paralleling 2 cells (or packs), they should be within 0.05v of eachother to prevent a large charge/discharge condition between them.

Originally posted by BrianG
Series increases voltage. The product of current and voltage is power. And capacity is current, which increases when batteries are in parallel.

Right, but cells of different capacities should not be put in series with eachother. The lower capacity cells will reach 3v before the higher capacity cells, and their voltage could continue to drop to unsafe levels. I realize that no two cells will be able to maintain the exact same capacity, but at least if the the cells are stated to be the same (by the factory), then the tolerances should be low enough as to not cause any problems with overdischarge.

P.S., congratulations on your 50th post! The "prize" is a 5% discount on your next order through RC-Monster. You have to contact Mike to get this discount when ordering, though.

[BrianG]

OK, so as long as the packs have the same voltages (or very close) when they start, they will discharge within .05v of each other so any current flow between the paralleled batteries is negligible.

I know cells of different capacities shouldn't be used together, just like cells at different charge levels shouldn't either - I was just pointing out the fact that if you have several large capacity cells in series and one low capacity cell, the whole chain will only have the capacity of the smallest cell. I think everyone knows that for best/safest results, all cells in series or parallel should be of the same type, voltage rating, capacity, and charge level.

Cool about the 5% discount! That will come in handy when I get the rest of my BL stuff!!! $25 off a $500 order should cover shipping (or close to it). Is there an expiration date in which to use this discount?

I still haven't read of any examples (other than R/C use) of paralleled batteries. I don't really think dumptrucks, RVs, campers, etc use paralleled batteries without an isolator, so that doesn't count. :)