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sss 15th July 2009 10:55 AM

multi cell battery charge limit circuit ideas
 
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 .
:)

Mooly 15th July 2009 11:10 AM

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.

Eva 15th July 2009 12:14 PM

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.

sss 15th July 2009 01:48 PM

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 :) .

jc2 15th July 2009 03:47 PM

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

TechGuy 15th July 2009 09:56 PM

Quote:

Originally posted by Eva
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.

sss 15th July 2009 11:22 PM

Quote:

Originally posted by jc2
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.

jc2 16th July 2009 03:39 AM

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?.

star882 16th July 2009 03:46 AM

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.)

heinrichs 22nd July 2009 03:02 PM

hmm... maybe you could use one of these for each cell?

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4002

if you have individuel acces to each cell, the skill to solder them and the option to make pcb's with that small components.

not really a simple ideer... but... you can have it anyway..


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