# I need a voltage follower with >= 1Amp output for experiments

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#### Gazza_AU

##### Member
I want to do some experimentation with a 60v charger connected to 17x Lifepo4 cells in series.

The idea is the 2 resistor voltage divider will be connected across Battery Bank.
The voltage between resistors is Bank Voltage divided by number of cells.

Shown are (16KΩ + 1KΩ) resistor values for 17 cell Bank
So the voltage between resistors is the equivalent of a single cells charge voltage.
The 1 17th voltage will follow voltage changes across the bank.

What would be the best IC or circuit for at least 1/1.5 Amp output.

#### ronsimpson

##### Well-Known Member
You could have 17 (1k) resistors. The voltage at each junction should match the voltage of each battery. (did not say that clearly)
I see 16 amplifiers. Is that what your are thinking?

Problem: If the bottom battery needs 1A, it will get the voltage from 60 volts. That puts 56.5V and 1A and 56 watts on the poor little amp.
There must be another way.

#### Gazza_AU

##### Member
You could have 17 (1k) resistors. The voltage at each junction should match the voltage of each battery. (did not say that clearly)
I see 16 amplifiers. Is that what your are thinking?

Problem: If the bottom battery needs 1A, it will get the voltage from 60 volts. That puts 56.5V and 1A and 56 watts on the poor little amp.
There must be another way.
You have read a lot into this, so much so I have no idea what you are talking about.
It is all cool. LOL.

I am after a single voltage follower with at least 1 Amp output!

Thanks

#### ronsimpson

##### Well-Known Member
What would be the best IC or circuit for at least 1/1.5 Amp output.
What is the output of the amp going to? Does it really need to output 1.5A?

You are right: the output will be 1/17 of the total. Normally you stop charging at 3.6V/cell. So stop charging when the voltage reaches 3.6V or 61.2V total.

#### audioguru

##### Well-Known Member
You do not stop charging when a Li-Po battery reaches its maximum safe voltage. Then it is about 70% fully charged. Instead you limit the voltage to the maximum safe voltage then monitor the charging current and when the current drops to about 4% of the mAh rating then the battery is fully charged.

Usually a multi-cell Li-PO battery uses a balanced charger that monitors and charges each cell separately (even when they are still connected in series) to prevent a cell or more from being over-charged.

#### Gazza_AU

##### Member
Usually a multi-cell Li-PO battery uses a balanced charger that monitors and charges each cell separately (even when they are still connected in series) to prevent a cell or more from being over-charged.
That is what this experiment is about, just evaluating an idea to see if it has legs?

This is a Suitable Voltage Follower curtsy of MrChips on AllAboutCurcuits Forum.

volts in = volts out.

To elaborate further on part of experiment:
Diagram showing 2 of 17 Lifepo4 Cells between DPST relays.

Important: The voltage divider is just to show logic without the Voltage Follower.

A 555 is connected to counter connecting 1 of 17 relays for a few minutes each in a cycle.
The 1 17th bank voltage is connected across each cell one after the other in sequence.
The cell is either higher, lower or equal to the 1 17th bank voltage.

The 17 cells will be connected in series to bank charger whilst this is happening.

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#### audioguru

##### Well-Known Member
Are you trying to re-invent a balanced charger by charging each cell in a sequence? Your idea will take 17 times longer than a normal balanced charger that charges all the cells at the same time.
Relays were used in low current circuits 60 years ago. A modern balanced charger detects that a cell's voltage is higher than the others then it bypasses some of the charging current past that cell with a transistor or Mosfet.

#### alec_t

##### Well-Known Member
This is a Suitable Voltage Follower curtsy of MrChips on AllAboutCurcuits Forum.
Not strictly a voltage follower. It's a voltage-controlled voltage source, because it has gain.

#### Gazza_AU

##### Member
Are you trying to re-invent a balanced charger by charging each cell in a sequence?
Yes and No, just looking at the subject out of interest!

Your idea will take 17 times longer than a normal balanced charger that charges all the cells at the same time.
Relays were used in low current circuits 60 years ago.
Not my idea, but the statement is incorrect. The Balancer and Charger are working separately.

The idea of using relays is in fact another persons idea.
The relays are a schematic representation just like the resistors.
Most anything that can be demonstrated with relay, can be replaced by solid state.

A modern balanced charger detects that a cell's voltage is higher than the others then it bypasses some of the charging current past that cell with a transistor or Mosfet.
Yes, all I am interested in is the logic used.

I have a few commercial options for suitable balance circuitry, the plan is to buy a commercial "Active Non-dissipative balance" system.

I am here to discuss a concept, that I do not believe will work.
My contribution was to suggest using a Voltage divider across bank, bank voltage divided by Cell Number.
The logic is it would nudge cells towards balance, whilst the main charging is provided across the series string.

The original idea proclaimed to use a capacitor, that was shared between cells to either source or sink depending on level.
I think this is rubbish, the sharing would most likely result in net loss. The poster wanted others to guess how it worked before giving any explanation. Ha.

I am looking at the general mythology in Charging Lifepo4 cells because I am very interested.
Most of my research has been in the characteristics of cells and Partial State of Charge.
After about 6months research I understand the cells, and charge requirements very well.
The reason why it takes so long, is there is no one answer, it starts with the application.

Are you interested in discussing the logic side of the concept?
If the basic concept as is, can be demonstrated!
Then I have a lot more to discuss about the logic or intelligent design that might be applied.
---

Not strictly a voltage follower. It's a voltage-controlled voltage source, because it has gain.
Yes, so we remove the resistors. Use two transistors? Are you making a clarifying point or do you have circuit suggestion?
----

So stop charging when the voltage reaches 3.6V or 61.2V total.

Here is a video on battery balancing. This is close to what I have talked about before. Have a separate battery charger/battery.
http://www.linear.com/solutions/1128
Thanks ronsimpson, I have the option of pre-built PCB.
I am not interested in Hardware fixed charge voltages, it has not got relevance in PSOC applications, other then, perhaps the cells operating limitations!
I also contacted a Chinese PCB Manufacturer by mistake, the company asked if I have a picture or IC they could make it.
This is a little odd, but they claim to provide many BMS to suppliers and open to ideas.

In any case, I will study the LTC3300 data sheets,

Cheers

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#### alec_t

##### Well-Known Member
Are you making a clarifying point or do you have circuit suggestion?
Clarification. You may be aware of the distinction, but others reading this thread may not.

#### Gazza_AU

##### Member
Clarification. You may be aware of the distinction, but others reading this thread may not.
No worries, here to help. Or hinder as the case may be

This is a Voltage Follower suggestion curtsy of (*steve*) at ElectronicsPoint Forum.

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#### Mosaic

##### Well-Known Member
The OPA is a hi impedance device. You can assess that via the formula Impedance = Input V/input bias current. For this approach that impedance can help determine the maximum resistance you can use on the OPA input. This will protect the OPA from hi current transients. If your OPA uses FET inputs then a clamping Zener is a good idea, after the input resistor.

Thanks

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#### Gazza_AU

##### Member
Are you trying to re-invent a balanced charger by charging each cell in a sequence? Your idea will take 17 times longer than a normal balanced charger that charges all the cells at the same time.
Relays were used in low current circuits 60 years ago. A modern balanced charger detects that a cell's voltage is higher than the others then it bypasses some of the charging current past that cell with a transistor or Mosfet.
The individual cell sequence runs above the banks series voltage charger 49.5 to 60v.
The 1/17th voltage division is 2.9v to 3.529v.

The voltage divider connects to voltage follower.

The voltage follower drives a "Transistor?" that charges a Capacitor through to Time Constant.
http://www.mouser.com/ds/2/257/Maxwell_HCSeries_DS_1013793-9-341195.pdf
50F - 2x 100F Maxwell "BCAP0100 T01" Capacitors in Series:
Rated Current 5A
Max. Current 18A
Max. ESR 30 mΩ

Is it possible to automatically increase the follower output to compensate for a voltage drop across added diode to protect it?
With the gain control, or by adding another diode to cancel out the first?

Thanks,
Gary

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#### ronsimpson

##### Well-Known Member
I think you need a resistor from (-) input of the amp to ground. 10k ??
I want a small amount of current flowing to keep the diode turned on all the time.

#### Gazza_AU

##### Member
Decided to look at 10 to 15Amp range

For diodes, it was suggested I need something with low leakage and a repetitive reverse-voltage.
This is a little beyond my general knowledge of diodes, not up for turning every stone the following came up in Google search.

I found MUR1620CTRG ULTRAFAST RECTIFIER 16 AMPERES, 200 VOLTS
http://www.mouser.com/ds/2/308/MUR1620CTR-D-79742.pdf
Looking for a matching Darlington Driver.

I think you need a resistor from (-) input of the amp to ground. 10k ??
I want a small amount of current flowing to keep the diode turned on all the time.
Is this an issue related to minimum voltage at OP + in, if so, what is the result.
Or a characteristic of diode and the OP - input.

The 10k might need to be compensated for if there is not another way.

There will be a resistor after diode to limit current to the rating of Capacitor.
I would then put the same value resistor in the feedback loop after the other diode to compensate.

Ultimately, I want adjustment for a small amount of gain, starting from the 1/17th baseline!
Any changes to the baseline need to be compensated for if possible.

The operating voltage 2.9v to 3.529v.

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#### Gazza_AU

##### Member
The OPA is a hi impedance device. You can assess that via the formula Impedance = Input V/input bias current. For this approach that impedance can help determine the maximum resistance you can use on the OPA input. This will protect the OPA from hi current transients. If your OPA uses FET inputs then a clamping Zener is a good idea, after the input resistor.
I am interested in driving 17x FET (or alternative) from picaxe 28X2 outputs,
in replace of: Relay Board Module Optocoupler LED for Arduino PiC ARM AVR
SRD-05VDC-SL-C Rated: 7 Amp Resistive or 3 Amps Inductive at 28V DC.

Volts x Amps = Watts
28v x 7 Amp = 196W DC
3.529v x 55.5A = 196W DC

The relays are not easy to replace, as they are tapping into a series string of cells and are bidirectional.
That said, the aim was to top cells as the primary function, which means only one direction.
The bidirectional approach may be better, but simplicity for more reliability rules!

I suspect the same logic can be achieved with solid state if you are smart enough with electronics.
The current flow will be limited and peak as the capacitor charges, after which it is determined by voltage difference between 1/17th divider voltage of capacitor and the "actual individual cell being tapped" in the 17 series cells.
I am just plodding through and picking stuff up on the way.

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#### Mosaic

##### Well-Known Member
Simplicity:
1) Use an array of ULN2003 switches with a current limiting resistor to deliver the charge from a capacitor (supercap?) across each cell individually. Have the super cap charged to the peak desired cell voltage before applying to each battery.
2) Cycle this continually, the cells with lower charge will receive extra charge, and you will approach cell balance after sufficient iterations.

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