Lead acid battery tester

For your questions:

1. Better not to mix new and used batteries together, they have different inner resitances.

To test a battery, you could use battery testers, either analog or digital, both are ok. Digital testers will also calculate the inner resistance for you. Analog testers could not. 

2. A car battery has a life of 5 years or longer. A sealed lead acid battery has a life around 3 years. I just buy a new one if the old is bad. On youtube, someone used alkaline to wash the battery container and then refill it with sulfuric acid. I just feel it is not safe for driving.

My mower has a 36 v lead-acid battery (3x 12 v batteries). I replace it every three years. I notice that 3 batteris had different conditions. One on the left is pretty bad, even twisted. So I replace all three together. Not just replacing the bad one.

 

I have several testers:

1. Analog:

The meter panel:

The switch:

The 0.1 ohm resistor inside:

The tester basically is a Voltage meter and a Current meter. You put two alligator clips on your car battery, it is a volt meter (the resistance is 16.17 k ohm, in serial with the meter); when you turn the switch on (the one between the two wires on box), the meter is a current meter (0.1 ohm, in parallel with the meter). When you turn the switch on, you will see the resistor turning red with smoke (so be careful). And you could read the cold cranking amps (CCA) on the meter.

2. Digital:

Specifications:

Some features:

I checked on my battery:

 

 

 

There is a chart in the manual:

You could also buy a specific gravity tester for your car battery:

Be careful with sulfuric acid which is the liquid in your car battery, wear a goggle or face mask please.

 

3. Just a module (this one is for a battery with maximum 50 A), I think this one can also be used for 18650, but the resistor is too low (around 1- 2 ohm) so I did not try on 18650:

I don't have a lithium ion bettery analyzer, but I ordered one today:

There is more expensive one: