Batteries in parallel

[akis]

I think connecting batteries in parallel is a bad idea because they will invariably have different voltages and the higher voltage battery will drain into the lower voltage which has very low resistance so it will be like a short. I understand that soon equilibrium should be reached, but wanted to ask opinions. I could place Schottky diodes with every battery to prevent one discharging into another maybe, although then I'd be losing 0.5 precious Volts.

[retrolefty]

In general one should be very careful about using batteries in series or parallel. But it certainly is done by manufactures. In the R/C hobby world the battery manufactures offer Li-Po battery packs that can be both a combination of series and parallel cells to obtains a desired pack terminal voltage and amp hour capacity. They give descriptions like 3S1P for 3 series connected Li-Po cells, or 3SP2 for a six cells wired as two parallel connected, of 3 series cells. However for this to work without serious consequences the cells must be very carefully 'matched' by the battery manufacture and the user needing to use a 'balanced charger' to keep them matched over their lifetime.

[KedasProbe]

With not too much current and the right diode you should be able to keep the drop below 0.25v
http://www.comchiptech.com/cms/UserFiles/CDBA320L-G%20Thru042439.%20CDBA340L-G%20RevA.pdf

[amyk]

When paralleling batteries, you ensure that they are of the same voltage before connecting them together. This means charging/discharging as appropriate until they are. Then they will stay balanced.

[Rick Law]

When paralleling batteries, you ensure that they are of the same voltage before connecting them together. This means charging/discharging as appropriate until they are. Then they will stay balanced.

If they are in parallel, I think it would be easier to connect them via a resistor first.  The resistor acts as a current limiter.

Say if one is to parallel a pair of 12V SLA's, hook them together first via a 12volt-24W bulb, (ground to ground direct, but positive on battery 1 connected via the bulb to positive of battery 2).  That 24W bulb will limit current to 2A, so the stronger battery is charging the weaker.

After a while, when they are both around 12.6v or below, one can remove the bulb and connect the positive terminals together to charge then use.

I did exactly that with SLA's a number of times, but they are a good bit safer than LiPo.

[vk6zgo]

I think connecting batteries in parallel is a bad idea because they will invariably have different voltages and the higher voltage battery will drain into the lower voltage which has very low resistance so it will be like a short. I understand that soon equilibrium should be reached, but wanted to ask opinions. I could place Schottky diodes with every battery to prevent one discharging into another maybe, although then I'd be losing 0.5 precious Volts.

Why would the lower voltage battery look like a very low resistance?
The difference is going to be minimal unless one battery is flat & the other fully charged.

In any case,it will be nothing like a "short"----cars are "jump started" every day  somewhere in the world,with no damage to either battery.

The simplest way to obtain higher current than is available with one battery is to parallel it with another,which has been done safely for over a century.

[Whales]

Indeed placing batteries in parallel with different voltages will cause them to drive one-another.  As others have stated, it really depends on what batteries in what situation: the effects could be harmless or may cause things to start melting.

For consumer products using AA batteries: warnings are generally put on to avoid people mixing batteries.  As long as they don't do that (we trust them, right?) then everything will be fine: the small differences between fresh primary cells from the same batch are unimportant.

For anything using rechargeable cells: people will be tempted to mix cells of different charge level.  Then you need to seriously consider better design (or training).

[Rick Law]

I think connecting batteries in parallel is a bad idea because they will invariably have different voltages and the higher voltage battery will drain into the lower voltage which has very low resistance so it will be like a short. I understand that soon equilibrium should be reached, but wanted to ask opinions. I could place Schottky diodes with every battery to prevent one discharging into another maybe, although then I'd be losing 0.5 precious Volts.

Why would the lower voltage battery look like a very low resistance?
The difference is going to be minimal unless one battery is flat & the other fully charged.

In any case,it will be nothing like a "short"----cars are "jump started" every day  somewhere in the world,with no damage to either battery.

The simplest way to obtain higher current than is available with one battery is to parallel it with another,which has been done safely for over a century.

Car batteries are "high discharge" instead of "deep discharge" (aka marine batteries in the USA) - high discharge meaning they can discharge high current but usually doesn't discharge a lot (ie: not discharging deep) - once the car got started, the battery stop discharging and begin charging.

So, when two car batteries get together, current from the one with more charge "rush" to the one with lower charge.  The bigger the difference in SOC (state of charge), the bigger the rush.  Since even a very small car would need 200+ amps to start, so car battery design is designed to deal with large currents.  Between your contact resistance and internal resistance, I doubt most car battery can deliver 200A to the other battery for long.  So risk of damage there is minimal.  Now if you are dealing with the "deep discharge" kind, their design is not to handle large rush of current but instead trying to get as much capacity out of the battery as possible.

In fact, all batteries has an upper limit of charge current.  When you exceed that by significant amount, damage is not just possible but probable.

SLA's and lead acid in general is a lot more forgiving.  If you try LiIon, LiPo type and exceed the charge current limit by 5x or 10x, trouble is in the horizon and perhaps imminent.

[vk6zgo]

Batteries don't work like that!

If you parallel a charged & uncharged capacitor in parallel,what you described does happen,because:-

(1)Capacitors actually directly store an electric charge.

(2)Capacitors have an extremely low internal resistance.


Batteries don't store an electric charge directly---they convert the charge into a chemical relationship between the internals of the battery.

If you draw the equivalent diagram of a battery,it consists of an ideal battery,in series with its internal resistance,drawn as a resistor.
When the battery is flat,we can draw it with a greatly increased value of internal resistance.

OK,it's not a real resistor,but an external circuit sees the energy used in converting the battery's internal chemical state from discharged to charged (or vice versa),as power dissipated,implying the presence of resistance.
This "resistance" limits the current flow into the battery when you are trying to charge it.

If this was not the case,when you applied the jumper leads,there would be a blinding flash,it would instantly charge the flat battery,they could disconnect,& the "Donor" car could drive away.

The person you helped could then start his car,drawing start current with impunity.

This doesn't happen,-----you need to use the "Donor" car's battery to supply start current.

[Whales]

When the battery is flat,we can draw it with a greatly increased value of internal resistance.

Again that's a simplification.  The voltage/current source in your equivalent diagram also changes in value as the chemistry changes.

[vk6zgo]

It doesn't matter,the point is that there is not a massive current flow  between the charged & uncharged battery.

[jmaja]

Car batteries are "high discharge" instead of "deep discharge" (aka marine batteries in the USA) - high discharge meaning they can discharge high current but usually doesn't discharge a lot (ie: not discharging deep) - once the car got started, the battery stop discharging and begin charging.

So, when two car batteries get together, current from the one with more charge "rush" to the one with lower charge.  The bigger the difference in SOC (state of charge), the bigger the rush.  Since even a very small car would need 200+ amps to start, so car battery design is designed to deal with large currents.  Between your contact resistance and internal resistance, I doubt most car battery can deliver 200A to the other battery for long. 

You won't be seeing big currents from one lead acid battery to another, if they have equal amount of cells and no cells are shorted due to aging. 100% SOC battery may have 12.7 V and 0% 11 V (you shouldn't discharge most batteries that much). But when you put some load to that 12.7 V battery, the voltage drops very easily a few tenths. E.g. putting lights on on a car (~15 A) would drop 12.7 -> 12.1-12.4 V depending on the battery. Cranking at 100 A will drop to about 11 V and the voltage drops maybe to 10 V at 200 A.

And the same happens even more to the other direction. Charging close to 100% SOC battery just with 1% of C (less than 1 A for a car battery) rises the voltage to above 14 V. Charging 0% SOC battery with 30% of C current (20 A for a car battery) will rise the voltage to above 12 V. At 50% SOC you only need a few % of C to rise the voltage above 12.7 V.

Thus connecting a 100% and 0% SOC batteries in parellel you may see up to 30% of C current for a while. Thus nothing more than having lights on. The current will lower to a few A level in a few minutes. If the difference in SOC is less than 50%, you will not see more than a few % of C current.

[Rick Law]

Batteries don't work like that!

If you parallel a charged & uncharged capacitor in parallel,what you described does happen,because:-

(1)Capacitors actually directly store an electric charge.

(2)Capacitors have an extremely low internal resistance.


Batteries don't store an electric charge directly---they convert the charge into a chemical relationship between the internals of the battery.

If you draw the equivalent diagram of a battery,it consists of an ideal battery,in series with its internal resistance,drawn as a resistor.
When the battery is flat,we can draw it with a greatly increased value of internal resistance.

OK,it's not a real resistor,but an external circuit sees the energy used in converting the battery's internal chemical state from discharged to charged (or vice versa),as power dissipated,implying the presence of resistance.
This "resistance" limits the current flow into the battery when you are trying to charge it.

If this was not the case,when you applied the jumper leads,there would be a blinding flash,it would instantly charge the flat battery,they could disconnect,& the "Donor" car could drive away.

The person you helped could then start his car,drawing start current with impunity.

This doesn't happen,-----you need to use the "Donor" car's battery to supply start current.

Of course a battery doesn't store the electrical energy directly, that is a matter of semantics.  When you connect one with higher SOC to a lower one, of course the energy does not flow instantaneously - it flows as fast as it can.

Capacitor does not store charge either - ie: it doesn't store up the electrons for use.  For every electron, there is an equal positive on the other plate so as a whole the capacitor is neutral.  It stores energy by separating the + and - into onto two plates (areas) separated by non-conducting material thus creating a potential difference between the two.  Off-hand I can think of a couple of examples of storing charges directly: storm clouds and Van De Graaff generators.

But those distinction makes no difference in real life.  In practical terms, it doesn't matter whether it was the chemical conversion taking time or electrons cannot move faster than the speed of light.  The current rush over as fast as it can, and in practical terms faster than the plate can affect the chemical conversion or safely dissipate it in some way - that is faster than the plates can handle.  The exact reason doesn't matter.  That it is faster than it is designed to handle is what is important and is when problems can occur.

Ideal batteries to me is a battery that lasts forever and stays 100% capacity for ever.  Well, ideally, it should cost nothing also...

This reminds me of a joke:
A balloonist landed on a farm.  He has no idea where he is.
He saw a guy there and he ask: "Hey, where am I?"
The guy replied: you are at longitude xxx.xxx, latitude xxx.xxx, and elevation of zzz.
The balloonist replied: you must be a technology guy.  I asked a simple question to help me get out of here and you tell me something completely useless.
The guy reply back: you must be a marketing guy.  I gave you the exact answer, you don't even know me and you are blaming me for your getting lost.

[vk6zgo]

The "exact answer" is that a damaging current flow doesn't happen!

Both myself & jmaja have told you why,in different ways.

OK,there is no such thing as an ideal battery,but a real battery acts like an ideal voltage source in series with an internal resistance,just as any other real voltage source does.

The external circuit sees a flat battery not as a source of ZERO volts, but as a relatively high voltage in series with a high internal resistance.
As jmaja pointed out,the voltage difference is quite small.

Have you ever actually "jump- started" a car?

If you have,you will have noticed there is no huge current inflow to the flat battery,and you cannot charge the flat battery enough to draw starting current,unless you sit there for about 20 minutes.

[Rick Law]

The "exact answer" is that a damaging current flow doesn't happen!

Both myself & jmaja have told you why,in different ways.

OK,there is no such thing as an ideal battery,but a real battery acts like an ideal voltage source in series with an internal resistance,just as any other real voltage source does.

The external circuit sees a flat battery not as a source of ZERO volts, but as a relatively high voltage in series with a high internal resistance.
As jmaja pointed out,the voltage difference is quite small.

Have you ever actually "jump- started" a car?

If you have,you will have noticed there is no huge current inflow to the flat battery,and you cannot charge the flat battery enough to draw starting current,unless you sit there for about 20 minutes.

Either I did not express myself well or you misread.

In my original reply, I never said jumping a car will damage the battery.  I said car batteries are designed to deal with large current and I doubt you will push beyond the limit.

Further, when I was talking about pushing large current, I said if you push more current than a battery is designed to handle, it will likely damage a battery - that should be obvious.  The final comment I made was if you are pushing 5x to 10x max charge current into a LiPo and likely you will have trouble.  That doesn't have much to do with jumping a car or car batteries.

[sleemanj]

The "exact answer" is that a damaging current flow doesn't happen!

I think that you really should be qualifying this with stating that you are talking about Lead Acid in case somebody misreads this thread.

ESR in a (healthy) lithium polymer cell for example is very low (3 mR) and flat over it's charge state.

Connecting a fully charged LiPo in parallel to a dead flat LiPo, could make for an exciting day.

[vk6zgo]

The "exact answer" is that a damaging current flow doesn't happen!

Both myself & jmaja have told you why,in different ways.

OK,there is no such thing as an ideal battery,but a real battery acts like an ideal voltage source in series with an internal resistance,just as any other real voltage source does.

The external circuit sees a flat battery not as a source of ZERO volts, but as a relatively high voltage in series with a high internal resistance.
As jmaja pointed out,the voltage difference is quite small.

Have you ever actually "jump- started" a car?

If you have,you will have noticed there is no huge current inflow to the flat battery,and you cannot charge the flat battery enough to draw starting current,unless you sit there for about 20 minutes.

Either I did not express myself well or you misread.

In my original reply, I never said jumping a car will damage the battery.  I said car batteries are designed to deal with large current and I doubt you will push beyond the limit.

Further, when I was talking about pushing large current, I said if you push more current than a battery is designed to handle, it will likely damage a battery - that should be obvious.  The final comment I made was if you are pushing 5x to 10x max charge current into a LiPo and likely you will have trouble.  That doesn't have much to do with jumping a car or car batteries.

This is the part I was disagreeing with"
So, when two car batteries get together, current from the one with more charge "rush" to the one with lower charge"

I made the point that there is no "rush"----the current flow is relatively low,for the reasons already discussed

Lithium batteries are not the only,or even the most common types that are likely to be connected in parallel,so the OP's original statement as it stands,is incorrect.

If he,(or you) had said:"It is a bad idea to parallel Lithium batteries,as they have low ESR & you may cause damage to the one with a lower charge",everybody would have "gone about their business------nothing to see here".

I will give you the Lithium case,& say:

"When paralleling Lead Acid,Nicad,Nimh ,Carbon Zinc,Alkaline,& most other batteries,there is a vanishingly small possibility of damage.
Note ,however,the special case of lithium batteries which have a very low ESR,& may be damaged"

[jmaja]

How often is there a problem even with lithium batteries? If we take a look at this datasheet: http://www.panasonic.com/industrial/includes/pdf/Panasonic_LiIon_CGR18650DA.pdf

It's a 2.45 Ah cell and there are curves for discharging at 0.466, 2.33 and 4.66 A and charging at 1.63 A. A 100% SOC battery drops to 4.05 V at 2.33 A and 3.85 V at 4.66 A discharge. A 0% SOC battery voltage is about 3.55 V at 1.63 A charging. Thus connecting 100 and 0% cells in parallel would likely produce a current between 2.33 and 4.66 A, which would be well withing what the cell can handle. After 10-20% of the charge has transferred from cell to cell the current is below 1.63 A, which is the recommended charging curent.

Same applies to this one: www.saftbatteries.com/force_download/MP144350.pdf
The maximum discharge currect (2C for this one) is certainly not exceeded by connecting 0 and 100% SOC cells together, but the maximum charging current (1C) may be exceeded for a short while.

I couldn't find any datasheets for lithium batteries for very high currents (e.g. used in RC devives). This article shows a car battery rated for 12C discharging and 3C charging. It doesn't show full curves, but certainly 12C current is not exceeded by connecting 0 and 100% cells in parallel. http://www.evs24.org/wevajournal/php/download.php?f=vol3/WEVJ3-5340444.pdf

[akis]

The OP was me, and yes I was talking about LiOn/LiPo batteries mostly. I have jump started cars and yes there is a spark, and that is how they blow up in your face and that is why there are rules as to which leads to connect first and which to disconnect first in order to avoid sparks that could lead to an explosion. For lead acids and jump starting a car there will be a rush of current into the weaker battery and there is no current limiter in place other than the cables, the contacts and the chemistry. Obviously when we jump start a car we fully intend to use one battery plus the alternator of the running car to charge the battery of the other. Again, because we have placed the leads directly over the battery terminals, there are no fuses, no current limiting of any type, and we venture into the unknown, because we have no idea what kind of load the weak battery + dead car are going to present to our good car's battery. If all is well it will be a couple of volts difference and in many cases the engine of the car has to be running else the battery by itself and the connections cannot provide the power to charge the weak battery and crank the engine at the same time. Hence the 20 minute wait, as someone mentioned above.

When we take this analogy into battery powered consumer electronics my main question was that while the electronics have been designed with specific parameters around the power consumption, and with fuses, protection diodes and so on - there is nothing in place to protect one battery discharging into another other than the chemistry. There are no fuses, no diodes, no current limiting. I found this to be a bad idea because there are all sorts of batteries and simply placing them in parallel would lead to currents, maybe heavy currents, flowing in directions not shown or expected on the schematic. And when we talk about many amps from LiOn batteries it starts becoming scary.

Someone mentioned above that the ESR on most batteries is so great that there will be a natural current limiter in place, but I am not so sure about that. I think the ESR will be on the donor side (the more charged one), whereas the depleted or faulty battery will have very little ESR. How many battery chargers exist with no current limiting? In my opinion none. That means the ESR of the battery being charged cannot be depended upon and should be considered a short.

[Rick Law]

Whenever I jump my car, a spark almost always go with the the initial jumper contact.  Whether you call that a "rush of current" or not, in my view,when it is powerful enough to create a spark, it is powerful enough to demand respect.

Back to LiPo/LiIon...  Some of the "protected battery" circuit do have current protection.  But from what I have read, most don't.

Since when a balanced parallel pack stays in balance as long as they remain a pack.  So balancing is a one time thing.  For a one time thing, there is no sense taking a risk.  Current limit it with a good size resistor and "balance" it overnight - then you don't have to worry about it anymore as long as they stay in parallel.  "Good size" resistor in my book is to limit the current to at least below 1/2C perhaps as low as 1/5C.

[jmaja]

I have jump started cars and yes there is a spark, and that is how they blow up in your face and that is why there are rules as to which leads to connect first and which to disconnect first in order to avoid sparks that could lead to an explosion.

The rules of connecting the leads are for avoiding short cut not for avoiding rush currents between batteries. E.g. if you would connect both leads to working battery first, you might accidently short circuit it with the free ends of the leads.

You don't need big currents to get sparks. If you do the connection with the working motor running, the voltage is over 14 V and there will be a high current (as much as the alternator can provide, 100+ A with modern cars). That is not recommended!

Someone mentioned above that the ESR on most batteries is so great that there will be a natural current limiter in place, but I am not so sure about that. I think the ESR will be on the donor side (the more charged one), whereas the depleted or faulty battery will have very little ESR. How many battery chargers exist with no current limiting? In my opinion none. That means the ESR of the battery being charged cannot be depended upon and should be considered a short.

ESR works both ways. Chargers use current limiting, but they have a capability of producing much higher voltages than "charging" with another battery. E.g. a car alternator (and lead acid chargers) produces 14-14.5 V typically with the only current limitor being its winding, while another battery will never have even open circuit voltage of more than 12.8 V. The key to low currents between batteries just connected parallel is the low differences in open circuit voltage. For lead acid the difference of open circuit voltage is less than 10% from 0 to 100% SOC. For lithium batteries it is even more than 20%, which makes it a bit more problematic.

A few weeks ago I changed batteries for a remote controller of an audio system. It used two AAAA batteries (a bit thinner than AAA). Very strangly the battery holder had a spring on the "+" side for one battery and on the "-" side for the other. I didn't even check the "+" and "-" signs in the holder just put the "-" towards the spring as always. The remote didn't work, but instead it became very hot. Fortunately I noticed that and removed the batteries after just one minute or so. Then I noticed the strange location of the spring, put the batteries back the right way and the remote started working.

I didn't look closer how the batteries were connected internally, but maybe they were in parallel. Normally, when batteries are in series,  connecting one battery the wrong way does nothing dangerous.

I think that is the major concern of putting batteries in parallel. If the user can accidently connect one cell in wrong polarity very bad things can happen.

[akis]

If I understand this right : we take two LiOn/Po batteries of the same nominal voltage, measure them to make sure they are at same charge, connect them in parallel and never disconnect them, even while charging. From that point onwards there will be very little currents (and dangers) flowing between the two batteries *almost* regardless of their individual characteristics?

[Rick Law]

If I understand this right : we take two LiOn/Po batteries of the same nominal voltage, measure them to make sure they are at same charge, connect them in parallel and never disconnect them, even while charging. From that point onwards there will be very little currents (and dangers) flowing between the two batteries *almost* regardless of their individual characteristics?

Yes, assuming your connections between them are good (low contact resistance).  With current flowing freely (no or low resistance), the two will always be around the same voltage so the current between them will be next to nothing - until one of them dies or is damaged.

[vk6zgo]

"Someone mentioned above that the ESR on most batteries is so great that there will be a natural current limiter in place, but I am not so sure about that. I think the ESR will be on the donor side (the more charged one), whereas the depleted or faulty battery will have very little ESR."

You impression is exactly opposite to the facts----The charged battery will always have the lower internal resistance,otherwise it could not source useful current.

If you read the open circuit voltage of a "flat" car battery,with a DMM or similar,the reading will be less than the same reading of a charged battery,but only by a few volts--say,11.8 volts compared to around 13volts.
If you now turn on the car headlights,it will drop to around 9 volts or so.

If you try to operate the starter,just the solenoid operating will drop it still further.

The internal resistance of a battery is made up of real resistance,due to the mechanical construction of the device,which is the same no matter what the charge state is & the apparent resistance,which varies with the state of charge.

In Electronics,there are a number of cases of these "apparent" resistances:

In an AC motor,the mechanical load appears as a resistive component of the current drawn by the motor,moving the current phasor towards the resistive axis.

In an Antenna,when energy is radiated,the so-called "Radiation Resistance" also appears as a resistance.




"How many battery chargers exist with no current limiting? In my opinion none. That means the ESR of the battery being charged cannot be depended upon and should be considered a short."

In fact,there are millons of battery chargers worldwide,with no current limiting other than the internal resistance of the transformer secondary & rectifier.

These are used every day---if a flat battery looked like a short circuit,they would all be destroyed,the first time they were used.

This is fact---not opinion!

By the way,when you jump start a car,you are not trying to charge the battery --you are just providing a  high current source to turn the starter.
As soon as your engine is running,its alternator will take over charging of the battery.

I made the comment about "20minutes" because I recently had an alternator fail in the Outback.
Fortunately,someone came to my aid,& charged my battery about every 40km until I made it to my destination.
Even with that long a charge,we had to use the "Donor" to operate the starter.