# Measuring the battery voltage of a Li-iOn and NiCd battery while charging

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
Good day!

I see these circuits with transistors or opamps which are suppose to measure the battery voltage while charging. I think I have some wholes in my knowledge concerning that, so I ask:

0. While charging will the transistor or opamp detect the battery or source voltage?

Joined Jan 15, 2015
7,135
Do you have a schematic of exactly what you are getting at? When I measure the battery voltage of a battery charging the voltage at the battery terminals is what it is. Before I start my truck the battery terminal voltage is about 12.7 volts or about 2.1 volts per cell. That increases to about 13.6 volts and higher depending on engine speed, so it is what it is. When measuring a voltage between two points it is what it is. Different battery types charge at different voltages so what are you asking exactly?

Ron

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
I am asking exactly if the battery is on "12VDC" at the moment, but the charger is charging at "14.4VDC" will I detect the voltage of the charger or battery? If you tell me I will detect the charger, how do these circuits detect the battery voltage?

For this circuit with LM358, will it detect the true battery voltage or the voltage of the charger?

#### crutschow

Joined Mar 14, 2008
31,493
If you tell me I will detect the charger, how do these circuits detect the battery voltage?
They are one and the same.
How could they be different?

#### spinnaker

Joined Oct 29, 2009
7,830
They are one and the same.
How could they be different?
I think TS wants to measure the battery voltage separate from the charger. I.E. being able to see the charge level. As I recall, I did this with a blocking diode from the charger.

#### crutschow

Joined Mar 14, 2008
31,493
I think TS wants to measure the battery voltage separate from the charger. I.E. being able to see the charge level.
Likely.
But monitoring the charge/battery voltage is how it's determined when the battery is charged, and the charging is terminated.
As I recall, I did this with a blocking diode from the charger.
I don't see how that can help.
If you block the charging voltage then it's not being charged.

#### spinnaker

Joined Oct 29, 2009
7,830
I don't see how that can help.
If you block the charging voltage then it's not being charged.
Now that I think of it, I must have been reading the solar panel voltage on the other side of the blocking diode. At night I was reading just the battery voltage since there was no charge.

#### mvas

Joined Jun 19, 2017
539
Good day!
I see these circuits with transistors or opamps which are suppose to measure the battery voltage while charging.
I think I have some wholes in my knowledge concerning that, so I ask:
0. While charging will the transistor or opamp detect the battery or source voltage?
When the battery is being charged, the voltage on the battery will increase and it will always be equal to the charging voltage.
When the charging stops, the voltage on the battery will decrease.

#### sghioto

Joined Dec 31, 2017
3,935
Adding a current limiting resistor to the solar charger will give you some isolation from the panel.
SG

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
In that case why do these circuits say they measure the true battery voltage? They say its a charging/discharging monitor. Also I see that the commercial off the shelf chargers do not stop the battery, how is that implemented? Also I was told that a capacitor will keep the voltage to its true level, regardless of the charger, that should be so for a battery also. I know I can do it with a microcontroller by stopping the battery, but the idea was to see the analog way and understand it.

Joined Jan 15, 2015
7,135
Your first circuit using the LM358:
For this circuit with LM358, will it detect the true battery voltage or the voltage of the charger?
No, it will detect a range, just like the other circuits. They are not a volt meter in that they do not indicate a voltage but merely use a LED or LEDs to indicate a range of voltages. The LM358 is a dual operational amplifier being used as a comparator circuit. The first drawing looks to have come from here, Battery Voltage monitor. The link explains part values and adjustment of VR1. They are pretty clear about and they reference using a IC LM709 configured as comparator. There are better choices. So as long as you are happy with a "range" of voltage a comparator will work but is not quite a voltmeter.

"When voltage level rise above 13.5 volts, the output from IC1 goes low as a result LED begins to emit RED light. Similarly, when the voltage fall below a preset level (10 Volts) the output goes high and the LED start to emit GREEN light. Resistors R3 and R4 is used as current limiter of LED".

You make no mention of battery type (chemistry) which on some batteries is very important. Just using SLA (Sealed Lead Acid) type as an example I suggest you read this: BU-403: Charging Lead Acid.
They say its a charging/discharging monitor. Also I see that the commercial off the shelf chargers do not stop the battery, how is that implemented?
In your first circuit B1, the Battery is said to be 12 volts. If we assume a 12 Volt SLA battery we know the normal charged voltage will be about 12.7 volts and we know we charge it at about 13.6 volts so below 12.7 volts the battery is discharging and above 12.7 the battery is charging. Those are approximate and if you read the link for SLA batteries, temperature figures into things.
A Google of for example LM3914 battery circuits will get up dozens of circuits using much better LED ranges and colors.

Most chargers I see on the market today have auto shutoff or reduce to maintain a "float" charge.

Ron

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
Thank you for explaining, I already know these things. The battery chemistry is mentioned in the topic title.

The question stands, does the comparator or transistor regardless of the model measure the true battery voltage or the applied source(charger) voltage? How is it done without stopping the battery?

#### MrChips

Joined Oct 2, 2009
28,109
You are measuring the voltage of the charger, not the battery.

All batteries (and chargers too) have internal resistance. Being able to measure the internal resistance of the battery will give you a good indication of the usefulness of the battery to supply the nominal battery voltage.

When you disconnect the charger the measured voltage of the battery will be lower than the charger voltage.
Smart chargers will disconnect the charging voltage to measure the battery voltage. Some chargers will even supply a load and measure the battery voltage. This will give a measure of the battery internal resistance and hence will be able to indicate a failing battery.

#### Picbuster

Joined Dec 2, 2013
1,041
MrChips is correct.
To add more trouble the internal battery remains constant (when not defect) the internal resistor is changing getting larger when the power is going down.
Resulting in a low voltage at the connection lead when a load is placed. U-leads= R extern x vbat / (Rintern + R extern)
if U-leads= U extern then Rintern = R extern
This mechanism will give you a tool as described by MrChips.
Disconnect charger connect resistor and measure voltage over resistor.
This will allow you to calculate internal resistance.
Compare the calculated resistance with the battery spec's as specified at floating voltage.
This information is an indication of the battery status and quality. ( defect when it fails to reach the desired resistance in specified time and current)

Picbuster

#### mvas

Joined Jun 19, 2017
539
The typical Retail ( off-the-shelf ) Smart Charger does NOT measure the Internal Resistance of the battery.
That is because your typical Retail Smart Charger has no clue what size ( amp-hr ) battery is attached.
And without knowing the exact Model # of the battery, the Charger has no clue what size load to apply,
nor what value of Internal Resistance is valid, or not valid.

Typically, this feature of "Disconnecting and Load Testing the Battery" is reserved for only higher priced devices, like an APC Server UPS.
An APC Server UPS knows exactly what Model # battery pack is attached and
it can intelligently perform a "Disconnect & Load Test on the battery pack"

Using a simple voltage comparator ...
A Li-Ion battery must be higher than their "Minimum" voltage, otherwise the BAD Battery LED will be ON.
A Li-Ion battery must be lower than their "Maximum" voltage, otherwise the BAD Battery LED will be ON.

The steps for recharging a Li-Ion battery are not compatible with the steps for recharging a NiCd battery.
Li-Ion vs NiCd - Two completely different chemistries, that require two completely different charging procedures.

Last edited:

#### mvas

Joined Jun 19, 2017
539
Your first circuit using the LM358:
You make no mention of battery type (chemistry) which on some batteries is very important. Just using SLA (Sealed Lead Acid) type as an example I suggest you read this:
This thread is regarding: "... Measuring the battery voltage of a Li-iOn and NiCd battery while charging ..."

Joined Jan 15, 2015
7,135
This thread is regarding: "... Measuring the battery voltage of a Li-iOn and NiCd battery while charging ..."
Yes, I know that. The idea was to provide an example. I also gave a link to Battery University and as to Li Ion Batteries here is a more focused link on battery type. Regardless and back to the initial post:
I see these circuits with transistors or opamps which are suppose to measure the battery voltage while charging. I think I have some wholes in my knowledge concerning that, so I ask:

0. While charging will the transistor or opamp detect the battery or source voltage?
Battery terminal voltage is what you see is what you get. Whatever the voltage at the terminals are is what it is. Matters not if the battery is charging, discharging or just sitting around waiting for lunch.

Ron

#### MrChips

Joined Oct 2, 2009
28,109
Thats controversial, some said its the source voltage, some said its the true battery voltage. Which one is it?