How to Measure the Internal Resistance of a Dry or Flooded Battery?

[Wafeeq]

Hellooo All

i am working on a project in which i want to monitor the health of the battery. i am measuring the volts of the battery as well as current flowing out of the battery or into the battery. now i want to measure the internal resistance of the battery. Please help me how to find the internal resistance of the battery. and if you have any suggestion how to measure the health of the battery?

 

[FvM]

Internal resistance is basically a differential quantity: observed voltage change with a defined load current change:
Rd = ΔV/ΔI
Unfortunately, ΔV is also time varying. So you have perform the measurement under defined timing conditions. Or perform an AC impedance measurement at specfied frequencies.

 

[Wafeeq]

Actually i have a bank of 6 batteries connected as making 24 volts total. each battery is of 12V. i want to check the health of the battery continuously. now please tell me what method should i adopt to measure the health.

 

[Kripton2035]

here you have a good example with source code for measuring internal resistance of a bank of 4 nimh batteries
it should be easy to adapt to your configuration
regards,
https://www.edaboard.com/threads/83469/#post365543

 

[KerimF]

It is indeed an interesting project. I planned since long to do it but I had always other designs more urgent to work on.

Let us start with a single 12V battery.
The simplest way it to measure both its voltage and current in two states;

(1) when the battery current (I1) is zero or constant during the test time.
(2) when the battery current (I2) is increased by adding a dummy external load.

Then:
R_bat = (V2 - V1) / (I2 - I1)

The dummy load could be a heater wire though it doesn't need to heat up too much during the rather short testing time (during the dummy load is applied). I guess in your case, measuring the battery health is actually a relative matter if the same test is repeated once a while. I mean the exact value of the internal resistance is not as important as the change with time relative to its measured value when the battery was fresh (this initial value may need to be saved as a reference).

The next step is to design a controller to monitor the 6 batteries. It needs to be electrically isolated from the individual testers (which are also isolated from each other). The testers will exchange data with the main controller via optocouplers.

Kerim

Off topic:
Building an automatic charger for each battery (not just a big one for all) helps in extending the battery life to its expected limit.

 

[FvM]

(1) when the battery current (I1) is zero or constant during the test time.
(2) when the battery current (I2) is increased by adding a dummy external load.

Then:
R_bat = (V2 - V1) / (I2 - I1)

As already suggested in post #2. Unfortunately you'll face three problems:
- voltage is time-varying, already mentioned
- internal resistance changes with temperature
- internal resistance changes with charge state (in other words, the battery model is actually non-linear)

Furthermore, it's said in literature, that it would be difficult to determine the health state when the battery is kept almost fully charged and one should better trace the V/I characteristic over charge/discharge cycles. To determine parameters of a linear battery model in a particularly state (charge and temperature), impedance spectroscopy can be applied. It's tried to relate change in impedance characteristic to battery health state.

 

[KerimF]

Hi FvM,

On one side, you added valuable points for the poster to consider if he looks for a precise monitoring.

On my side, I added on my previous post the hidden keywords; "simplest way", "relative" and "same test" ;-)

Actually, "same test" means same conditions, including temperature

May I add that since my projects are not made for NASA :grin: , I need always to trade off between realizable and accuracy. And sometimes I have no choice but to build my own algorithm based on repeated observations of a specific application.

For instance, I use this inaccurate (and fast) method to discover old (small rechargeable) batteries at the retailers :-|

Kerim