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Measuring battery voltage with an MCU while charging and discharging

YoTech

Mar 21, 2018
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Ok. The problem that I meet is that I have to measure the battery voltage while the battery is charging also or at least when its plugged and when the charger stops (about 6h). It will be charged from an ABC-1220D FST charger, rated for 20 ampers. The load will consume 24V, 25A. And the shit of having to charge separately and discharge toghether while measuring. That is the reason behind the analog components (the comparator circuit) which are then checked by the MCU. Because I dont see another way to measure the battery while its charging except to measure charger voltage until the charger stops and the battery voltage starts dropping from 14.7 to 13.7V. At 13.7 the charger will start maintain mode and the battery will not fall more.
 

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YoTech

Mar 21, 2018
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Also will this transistor be good enoug? Given that I have 24 volts from 2 batteries, this transistor is 20V, but I dont think the whole voltage will be applied to it?

MOS FET: SI9926CDY
 

kellys_eye

Jun 25, 2010
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What's wrong with using the MCU (via an ADC port) to measure the battery voltage directly? Equally you can fit a sense resistor and measure the charge current.

Using the data from the battery manufacturer you can find out the ideal charge/discharge rates for the system and monitor and/or charge appropriately.
 

YoTech

Mar 21, 2018
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The wrong is that the voltage measured will not be of the battery, but of the charger, or at least it will not be accurate, because the battery needs to be stopped until the voltage settles.

Can you give a circuit for the sense resistor please?

This is the proper circuit for my idea, if I have 3 grounds like on this circuit, not connected to Batt1- and Batt2-, will it work?:
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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During charging the battery voltage will be the charger voltage.

If you want to measure the battery voltage whilst it is not under load (the charger represents a type of load -- albeit one that has a negative power) then you need to disconnect or otherwise disable the load.
 

YoTech

Mar 21, 2018
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So this should work?
 

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YoTech

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How to use the comparator circuit from above with an MCU?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Using n channel MOSFETs like that will require a gate voltage of about 30V to turn them on.

I would recommend you use a p channel MOSFET which can be turned on by pulling the gate toward ground.

You will have to make sure you don't exceed Vgs(max), and that you don't try to source current into the microcontroller. The easiest way is to use the microcontroller to turn on a bjt which pulls the gate low against a resistor from gate to source. You probably also want a 10V zener diode between gate and source, and a resistor in series with the collector (or gate) to limit the current.

I may be able to draw you a circuit later.
 

YoTech

Mar 21, 2018
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I think a logic NPN MOS FET will work at 3,3V. It has Rds on at 2 to 4V, so its ok?
A P MOS might be good, but I cant find 1 at 3.3V, there I will have to use the suggestion you made.
 

(*steve*)

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The way you have that mosfet, the gate will have to be raised to 24V + the Vgs, so 28 volts or so.
 

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This is a suitable high side switch for a microcontroller:

upload_2018-3-29_20-5-55.png

R1 limits the current into the base of Q2

Q1 is a P channel mosfet used to switch the load (whatever it is)

D1 prevents the gate from being destroyed by too high a Vgs. This is important if the supply voltage is above 20V (the typical Vgs(max)). This limits Vgs to 10V which should be sufficient.

R2 holds the mosfet off by pulling the gate high through R3 when Q2 is off.

R3 limits the current through D1 when Q2 is turned on.

Placing a logic high on the input turns the load on. Leaving it open or pulling it low will turn the load off.
 

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Another problem your circuit has is that the resistors at the bottom of all of your voltage dividers are in parallel. If all the voltage dividers are the same, then either pick a value 3 times as large as you need, or eliminate all but one of them. This assumes that you'll only turn on one mosfet at a time (which seems sensible).
 

YoTech

Mar 21, 2018
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The MCU will be 3.3V. I meet 2 more problems with the practical realisation of this project. I have to be done by Tuesday, so thank you very much for you help!

First: The batteries will be connected in series while discharging and separated when charging. What will happen with the upper voltage divider if its still connected to the lower battery's minus, while they are both charging separately with 2 chargers?

Second: I intend to put a copper fill on the bottom copper, which will act as ground and be connected to the lower battery's minus. Do I need a second copper fill on the internal layers to use as a second ground and should the 2 ground layers be connected? Can these 2 grounds for the upper and lower battery be replaced with 1 external ground?

On the other hand if the circuit is connected not to ground, but only to each battery's plus and minus, will that not feed some electricity from the lower battery into the ADC pin?

I attach 2 block diagrams.
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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If you connect all the batteries negative terminals together while charging, you had better disconnect them before you connect them in series -- unless you like pyrotechnics in your batteries.

I'll try to address the other questions later.
 

YoTech

Mar 21, 2018
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I edited the post:
They will be disconnected before they are connected in series. But the PCB will stay mounted on a battery bank of
2 batteries 12V, 120A/h. The idea is that there is no real ground. Instead everything has to go to the upper battery's "-" or the lower battery's "-". And if I connect the upper battery's voltage divider to the lower battery's "-", while they are in series it will be ok, but when I flip the switch and disconnect the series connection separating the 2 batteries and connect them separately to the charger what will happen with the PCB, which has a voltage divider that is still connected to the upper battery's "+" and the lower battery's "-"? There will be a PCB layer (bottom copper) which will act as ground, but it still goes to the lower battery's "-".
 
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YoTech

Mar 21, 2018
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I will post the diagram for the other connection when we review this one if possible.
 

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(*steve*)

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You can't use a simple single switch like that to swap between a series config and one where the negatives are connected.
 

YoTech

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I am not switching between a series and parralel connection. All I do is disconnected the series connection or reconnect it, thats why its a problem what will happen if after I disconnect the series connectin there is still a voltage divier between the upper battery's "+" and the lower battery's "-".
 

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But in this post you show the negative ends of the batteries connected together
 

(*steve*)

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At this point, all I can tell you is that unless you have a common ground, your voltage divider scheme won't work. And if you have a common ground the series connection won't work.

Perhaps you need to describe exactly what you want to do so we can suggest how you can do it rather than you chase two mutually incompatible ideas in separate threads.

Why do you need to separate the batteries?
 
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