I had recently replaced a failed car batter in my older BMW (10 years old). The battery was only 5 years old but it had failed none the less. Before replacing the battery it wasn't able to light the dome lights let alone start the car.
In the interest of "science" :) I decided to try to recover the battery. First I attached it to a typical 12V battery charger. I left it connected for a long time but the battery would not take a charge (the current meter of the charger showed 0 Amps). I plan on using the battery in my camping trailing to operate the lights and charge our cell phones and that sort of thing, so if I can get the battery in some sort of working order it will be useful to me.
My back of the envelope calculations estimate the average car battery should have an internal resistance in the order of 0.02ohms; my initial measurement (using a 1.3 ohm high power resistor as a test load) showed an internal resistance of over 30 ohms.
Reasons Car Batteries Fail:
Some online reseach indicated that often the reason for non working lead acid battery is sulfation (lead sulfate crystal build up on the battery plates). It is especially a problem if the battery gets deeply discharged; or isn't brought up to full charge regularly. This was likely the case with my battery. The car wasn't winter driven before I bought it so it is likely it sat discharged. Also I have a very short commute; so in the winter I start the car; run the defrosters, seat heaters and blower motor for a short time (7 minutes to work) then do it again on the way home.
There are lots of conflicting reports on battery desufators, I figured I would give it a try and puchased a cheap battery desulfator off aliexpress.com; as far as I know most battery desulfators use the same principle; pulse a high voltage high frequency across the battery to dissolve the sufates back into the electrolyte.
I bought this one (I'm not affiated with the brand or seller)
its a "Clen" brand; pretty much the cheapest available. It makes a fair amount of buzzing I assume from the DC-DC converter inside. The device specifies a pulse frequency of 10kHz with a peak voltage amplitude between 60 and 100V. It works with 12V to 48V batteries so I'm assuming that is the reasoning for a range of peak amplitues.
I followed the following procedure when checking the progress.
(1) Disconnect the charger and desuphator
(2) Attach a 1.3 Ohm resistor to get about a 10 Amp load on the battery.
(3) Measure the voltage with the load attached (V_load - voltage under load
(4) Disconnect the load and check the voltage again (V_oc (voltage open circuit)
(5) Calculate the internal resistance of the battery:
(i) V_drop = V_oc - V_load
(ii) I _load = V_load * R_load (1.3)
(iii) R_battery = V_drop / I_load
I took some initial measurement of the battery after it had been sitting for about a month and things didn't look good:
So the battery wasn't taking a charge (likely due to the high internal resistance). The high internal resistance is a sign of sulfate buildup on the plates which minimize the plates effective area.
So I hooked up my $25 desulfator (pulse conditioner) and let it run. I did the above measurement criteria quite often (as indicated on the graphs)
As you can see in the above plot the desulphator worked quite well for my battery. Within 24 hours the batteries internal resistance was within the right order of magnitude (yay)! Within about 4 days the battery was pretty close to as good as it was going to get.
I should have measured the resistance of the load after each measurement; as the resistor heats up the resistance can change significantly.
I should have had a set amount of time of applying the load, as voltages varied depending on how long the load was applied.
Final Capacity Test:
So it was great to see the battery's interal resistance returned to normal but how about the overall battery capacity? This battery specifications indicate an initial capacity of 80AH; some research showed this is measured by applying a load until the battery voltage reaches 10.5V. There are many unknown variables (temperature; load current, etc); but I figured I would wing it and atleast get an estimate of the battery capacity mostly to get an idea if the battery will be usable to me or not.
This is a little tricky to do manually; so I decided to use an microcontroller (an Arduino in this case) to take the measurements periodically.
I just hacked something together quick; it measures the voltage and the current (using a 1 ohm shunt resistor) every 30 seconds and if the voltage falls below 10.5V disconnect the battery (using a relay).
I used a load which would give me around a 3A draw (a 4 ohm dummy load used for audio amplifier testing). If the capacity is near 80AH I would expect my test to take at most 26 hours.
So twice per minute I log the current and voltage; and calculate the energy drawn in the 30 second period (1/120 * A); to calculate the total capacity used I simply summed up all the measurements.