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Measuring battery voltage while charging it

Hi all,

I have a tricky problem on hand, but one that I am sure has been solved by several of you out there.

I am building a battery charger cum load controller that uses a solar panel for charging the battery. I use the PWM output of a PIC microcontroller for charging the battery. However, the problem is generic for any other microcontroller types.

If I measure the battery voltage while it is getting charge current from the solar panel, the measured voltage tends to be more than the actual battery voltage. If I give a small delay after switching off the charging current(turning off a MOSFET controller by a microcontroller pin), the voltage seems to be closer to the true battery voltage.

What is the method used commonly for measuring battery voltage while it is getting charged? Is a delay as indicated above a good way of doing it? If so what should be the ideal delay between turning off the charging current and measuring the battery voltage?

I wonder how the analog PWM controllers measure the voltage?

thanks for any suggestions, Sreenath

Reply to
sreenath
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Are you making this too difficult? There is a correlation between the charging voltage and the true battery voltage. Maybe the charging voltage is all you need. Or, if you really want to get more useful data, you could program a discharge cycle every 10 minutes or so and measure the battery voltage as it supplies current.

Reply to
Charles Schuler

Thanks for the response.

It would be really great if I can get the battery voltage from charging voltage.

there a fixed voltage to be subtracted to arrive at this?

In the current project, introducing discharge cycles (to measure battery voltage is not possible) since there are strict rules as to when the load is to be switched on.

thanks, Sreenath

Charles Schuler wrote:

Reply to
sreenath

Lead Acid, NiCad and NiMH battery chargers typically measure the voltage on charge...but it can be a problem if the charger doesn't supply a constant current or is noisy (eg from PWM) and you are looking for the small delta V peak on an NIMH cell.

Reply to
CWatters

How about a dummy load?

Reply to
Charles Schuler

Voltage is measured while charging but voltage alone is a very unreliable charge state indicator. Manufacture data sheets give the charging voltage-current-temperature-time curves for each cell chemistry and size and charging methods. Keep in mind the curves are for new cells but cell impedance increases and charge acceptance and retention changes as they age.

Reply to
H. Dziardziel

Hi, thanks for all the useful information.

I think I will stick to measuring the battery voltage after connecting the solar panel to it, and see how it goes. While it is true that measuring the battery voltage while discharging to a dummy load/actual load gives the TRUE indication of battery condition, it does not seem to be practical in this case.

thanks, Sreenath

H. Dziardziel wrote:

Reply to
sreenath

snip

Please note unless the cell is nearly completely discharged, the only thing meaningfully measured with a momentary load is cell impedance. If the initial charge state and ensuing charging current-time is unknown the state of charge can be measured only by discharging the cell to a standard cell voltage and integrating that current -time.curve area.

Smart charge controllers use cell chemistry parameters to determine when a cell reaches its fully charged state from a given initially charged state. E.g., voltage slope changes, temperature, constant voltage with current level termination etc. but none are as accurate as starting with fully discharged cells and then charging fully per the manufacture recommended method/s.

Solar panels are constant current sources so the method may actually be quite simple if say, NiMh are being used and the charging current is around C/10.. Please do look over the cell literarure.

Reply to
H. Dziardziel

This design is for VRLA and VMLA types lead acid batteries, the battery type IDP switch selectable. I am provided with peak voltage each cell can be allowed to reach on "bulk charge" and the float level etc.

A couple of years ago I had made a similar charger using another PIC device. With the 8 bit ADC of the device, it was quite an exercise to get the voltages correct.

The customer used a digital multimeter to verify our design and the voltage indicated by the digital multimeter was different from the values we used in the design.

Of course the customer insisted that we change the code to match readings from his multimeter.

-Sreenath

H. Dziardziel wrote:

Reply to
sreenath

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