Lead Acid Batteries: Mixing Old/New Cells- Damage?

[david90]

Can mixing (in series) old cells with new cells of the same voltage and rating damage the new cell? If so why? The cells are recharable.

 

[CharlesP]

It shouldn't hurt the new cells. They just won't get charged enough which is bad in the long run. But in a discharge the bad cells will easily be reverse charged which should be even more damaging. Generally folks keep all cells in a series string at the same ampere-hour rating and state of charge. That way the end points are reached by the whole string at the same time. That gives the most available ampere-hours with the least unused.
Also old cells tend to self discharge more.

 

[david90]

Can OCV test accurately tell the charge level of a cell?

I notice the voltage of my old cells increase slower than my new cells after a load is applied. What does this say about the old cells?

If I have to mix cells, what are some ways to bring them down to the same charge level or get the most life out of them?

 

[CharlesP]

Can OCV test accurately tell the charge level of a cell?

The only cell that works good on is lithium-ion. Lead acid is not too bad. Nicad and nickel-metal hydride are history dependent. That means either a charge or discharge perturbes the voltage which slowly returns to the normal curve. Heat can make these cells drop voltage which fools a charger into burning them up.

I notice the voltage of my old cells increase slower than my new cells after a load is applied. What does this say about the old cells?

I find that the voltage only decreases when a load is applied.

If I have to mix cells, what are some ways to bring them down to the same charge level or get the most life out of them?

When lead acid cells get old they fall apart. The best thing to do is keep a separate string and prepare to throw them away.