multi cell battery charge limit circuit ideas

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hello guys ;)

i have a 4 cell battery , 3.6V each , connected in serial .
max charging voltage is aprox 15V
i need to put a voltage limiter circuit on each cell , so the cels will be balanced and not overcharged .i have a few ideas how to do this , but if any of u got some experience with batteries and knows how to do it "simple" , i would be happy to know .
:)
 
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Charging and just as importantly discharging series connected cells is always a problem.
A lot depends on the currents involved. For low currents (charge and discharge) even a simple zener across each may be the answer. For safety with a circuit protector or fuse in series with each zener. The zener would "limit" reverse polarity on discharge too for the first cell to discharge.
No easy answer... you could sense temperature rise of each cell... messy... monitor each cell voltage... extremely difficult for a series connected chain for it to be reliable, or just don't charge at currents that will damage the cells if left on for a long period.
The dischargings the real problem to me when polarity reversal occurs on one cell.
 
For each cell:

A micropower op-amp comparing a fraction of cell voltage against a micropower voltage reference and feding a power transistor/resistor to bypass as much current as required to keep cell voltage clamped. With modern ICs this can be done drawing only a few dozen microamperes from each cell.
 
hello eva , how are u? :)
its good to see u still post here ;)
thats an interesting idea with the op amp + power circuit , but i think it would be too complicated to make this type of circuit for each cell , as i wond be able to use a single op amp for all cells.

my charging current is too high for zener diodes ,and measuring cell temp also wont help in this case , as those cell i'm using ,dont get hot .
btw , i'm using A123 batteries , the're pretty amazing with their charge / discharge capabilities 70A discharge current !:hot:

i was thinking of using 4 small voltage references (some micropower type) and 4 discharge power circuits , made out of 4 transistors and resistors .
need the circuit to be as small and light as possible :) .
 
Finally a topic that I might be smart enough to offer some contribution to. I am a battery engineer for a laptop manufacturer, and prior to that worked in teh same capacity for 2 cell phone manufacturers. Over the past 10 years I've probably tested 10-20000 batteries of various chemistries. I'm not an expert on the A123 cells because that chemistry isn't that applicable to what I do.

I'm a little confused on why you say the max charging voltage is 15V, I don't think a 4 series stack of those cells should be charged that high. I don't have a detailed spec from A123 so I'm not sure if I'm just missing something.

The LiFePO4 cells like A123 are more tolerant of conditions than the regular Li-Ion. If you start with all 4 cells balanced, you may find it's not necessary to constantly balance them. You can set the charge voltage to 3.6V *4, with a 10A current limit (if you are looking at the 26650 cell from them) and monitor the voltage on each cell to make certain you don't go too high on any one cell. With a CCCV charging scheme, unless there is one battery that is severely out of balance the cells will stay within a few mV of each other. Although the higher your charge current the more likely you are to see a voltage imbalance on the cells, simply due to internal impedance

That is how most laptop batteries operate, they don't use balancing very often. When cells are of good quality, start balanced, and subjected to the same conditions they stay in balance very well through most of their useful life.

If you do want to balance the cells, and again start with balanced cells, you shouldn't need much current to keep them balanced. You can do a bleed balancing scheme that will use low currents only during charge to keep everything at the same voltage. Again the higher your charge current the higher you might need to bleed off. But you shouldn't need to bleed off more than a few percent of the total charge current unless you have a serious fault on one cell. In that case, you probably don't want to be using it since it will seriously affect the usuable capacity of your pack.

I know their are solutions for regular Li-Ion that are pretty easy to implement, there are probably some for phosphate cells too. TI has chips that have integrated bleed balancing, or there is the PowerPrecise technology that TI now owns that is a little more involved, but more effic
 
Eva said:
For each cell:

A micropower op-amp comparing a fraction of cell voltage against a micropower voltage reference and feding a power transistor/resistor to bypass as much current as required to keep cell voltage clamped. With modern ICs this can be done drawing only a few dozen microamperes from each cell.

But wouldn't a power resistor discharge the cell thats being bypassed?

I assume that mult-cell balanced charges used a totem pole between each cell to isolate charging to the cells that are under charged. I would imagine the power resistor used to clamp the voltage could result in the clamped battery from becoming excessively hot resulting in reduced battery life.
 
jc2 said:
Finally a topic that I might be smart enough to offer some contribution to. I am a battery engineer for a laptop manufacturer, and prior to that worked in teh same capacity for 2 cell phone manufacturers. Over the past 10 years I've probably tested 10-20000 batteries of various chemistries. I'm not an expert on the A123 cells because that chemistry isn't that applicable to what I do.

I'm a little confused on why you say the max charging voltage is 15V, I don't think a 4 series stack of those cells should be charged that high. I don't have a detailed spec from A123 so I'm not sure if I'm just missing something.

The LiFePO4 cells like A123 are more tolerant of conditions than the regular Li-Ion. If you start with all 4 cells balanced, you may find it's not necessary to constantly balance them. You can set the charge voltage to 3.6V *4, with a 10A current limit (if you are looking at the 26650 cell from them) and monitor the voltage on each cell to make certain you don't go too high on any one cell. With a CCCV charging scheme, unless there is one battery that is severely out of balance the cells will stay within a few mV of each other. Although the higher your charge current the more likely you are to see a voltage imbalance on the cells, simply due to internal impedance

That is how most laptop batteries operate, they don't use balancing very often. When cells are of good quality, start balanced, and subjected to the same conditions they stay in balance very well through most of their useful life.

If you do want to balance the cells, and again start with balanced cells, you shouldn't need much current to keep them balanced. You can do a bleed balancing scheme that will use low currents only during charge to keep everything at the same voltage. Again the higher your charge current the higher you might need to bleed off. But you shouldn't need to bleed off more than a few percent of the total charge current unless you have a serious fault on one cell. In that case, you probably don't want to be using it since it will seriously affect the usuable capacity of your pack.

I know their are solutions for regular Li-Ion that are pretty easy to implement, there are probably some for phosphate cells too. TI has chips that have integrated bleed balancing, or there is the PowerPrecise technology that TI now owns that is a little more involved, but more effic
i dont care if the balancineg will be done after the charge , for example , when the battery is "idle" .
so i dont think that the charging current will be a problem , because i wont need to heavy balance it while charging.
 
I guess my point is that you shouldn't really need to balanced them much at all. If your charge voltage is correct, and the cells start in balance they will stay in balance very well. I can run some tests pretty easily and show that cells have to get pretty out of balance before the voltage rises significantly on one cell vs the others during charge. I don't have the A123 cells, but I have a few other LiFePO4 cells in my lab.

Do you have any circuitry in the battery pack at all, or are you just planning on using only the voltage limiter you are designing?.
 
You can use some common charge pump chips to slowly balance the cells. The design is extremely scalable (use N-1 circuits for a N cell pack, each one balancing overlapped pairs) and has been used successfully in homemade EVs.

For higher balancing currents, there's a circuit based around a multi winding flyback transformer. The idea is that the primary side runs from the entire pack voltage, and there's a secondary circuit for each cell. The lowest cell charges up first and the highest cell ideally doesn't charge at all. (If one cell is high, the circuit will discharge the pack while recharging the lower cells, thereby discharging the high cell until it's equal to the others. A good amount of energy is recovered.) For the circuit to work, the transformer must be wound such that the secondaries are as identical as possible and the rectifiers must be well matched. (The circuit is obviously more forgiving when used with lead acid batteries as is often the case in a homemade EV.)
 
i will be charing the batteries from a motorcycle charging system.
using any type of chargers will only complicate things up.
so , my best solution is to discharge the overcharged batteries .
i think Eva`s solution is the way to go .
the only problem i see , is i need to compare a voltage reference to the cell voltage.i'll need to use a ref voltage lower then the battery voltage (because the ref will be powered by the battery) .so i will need a voltage divider on the battery voltage which i'm going to compare the ref voltage to.
i think the voltage divider will draw too much current :cannotbe:
 
There are multiple ways to handle the balancing in smps fashion.
Quite simple, but IMHO not bad is to use a flyback.
The primary supplied from the entire series connection.
One isolated output for each cell.
You can settle such a flyback balancer even in a lossless snubbered way, like I showed in my 'lossless snubbered flyback' thread.
Please note: To use the natural balancing capability of the flyback has been filed for patent, but still not granted. So for business use you might run into headache.

Funny thing, two people at different places of the world are doing the same thing...
I am also planning to change my motorbike battery to LiFfePo, but struggling with the unsaint current consumption of my bike.

For starting at 20°C it draws 280A - and unbelievable... the 2-3 years old 12V/12Ah lead battery (AGM type) still remains at 9V during this load.
I was also considering 16 cells from A123 in size 26650, but with this it will just work so so, when the cells are still new. The Ri of these cells at DC load is around 14mOhm. No headroom for battery aging or cold ambient temperatures.
So for my bike I have to search cells which have approx half of the Ri at same size...
How much current does bike draw duriing starting?

If your bike really pushes the battery to 15V, then your LiFePo cells might run into issues with the life time.
My bike does limit it at 14.4V...14.47V, which is fitting to 4x3.6V - if they are very well balanced.

Good luck
Markus
 
Some additional infos:

If you want to balance it with an flyback you must go for a snychronous rectifying flyback. Otherwise you not get a natural low balancing impedance and the balancing currents at small unbalances will be small.
Eva's proposal is not bad, but may cause quite some heat.
Another option would be to use a DC/DC converter or just some sort of smps- bypass for each cell instead of a linear bypass.

If I run 8 cells ( very similar capacity) in 2p4s connection and simply charge unbalanced, then I am seeing a voltage distribution of approx.
3.51V+ 3.53V + 3.51V + 3.79V.
Not good for long cell life time - a balancer/limiter is a must.

I measured the starter current with a current probe and a 1mOhm shunt as well and got very good results. The current shunt can be a simple copper foil, tapped in a 1mOhm distance for the scope.

Screen shot:
White trace: Battery voltage 5V/grid
Red trace: Load current 100A/grid
Time base: 2.5s/grid
 

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sss said:
i will be charing the batteries from a motorcycle charging system.
using any type of chargers will only complicate things up.
so , my best solution is to discharge the overcharged batteries .
i think Eva`s solution is the way to go .
the only problem i see , is i need to compare a voltage reference to the cell voltage.i'll need to use a ref voltage lower then the battery voltage (because the ref will be powered by the battery) .so i will need a voltage divider on the battery voltage which i'm going to compare the ref voltage to.
i think the voltage divider will draw too much current :cannotbe:


Suitable references and OP amps:
http://focus.ti.com/docs/prod/folders/print/ref3020.html
http://focus.ti.com/docs/prod/folders/print/tlc1078.html
From component side it is absolutely no problem to use high impedance dividers. The issue is more due to humidity and pollution, which do limit reasonable values for the circuit design - especially in a motorbike application.
 
I'm not sure, but you might be able to use 4 boost converters that overlap. The first one pumps up itself in series with 2 and 3. The interleaving would be as follows.

1 into 1, 2, 3
2 into 2, 3, 4
3 into 3, 2, 1
4 into 4, 3, 2

A potential concern is that the ends cells 1 and 4 get less charging. Yet the feedback might overcome that.

It might be simpler than flyback if it works.

?
 
ChocoHolic said:
If we count cell 1 the cell at the minus of the pack, then the first two boost clusters should be OK.

But I guess you did not try to draw the schematic for your 3rd and 4th boost clusters...
:tilt:

I was thinking that they could be voltage inverters and flyback in the negative direction. Could that work? I still use simple lead acid for now on my power assisted bike.
 
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