Low ICV reading

[nukeman]

Does anyone have experience recovering from a low ICV on a large lead acid station battery? One cell in a sixty cell string dropped to 2.13V from the nominal 2.25 reading. We just completed a capacity test a week before the low reading was obtained and the cell along with the bank performed well (>105%) during the discharge test. The battery bank is 5 years old and is a lead-calcium type.

 

[gezcooke]

You're just seeing ageing effects beginning to take place - i.e. the inevitably rise of internal cell impedances after a few years in service. Interestingly, however, it looks like the cell you picked up on is actually very slightly better than the rest!

Your readings show 98% of the cells with the same float voltage - indicating that they have the same internal impedance - and the one 'rogue' cell as being slightly less across it's terminals. This indicates that it's impedance is slighly lower than the rest. Just think of it as 59 identical resistors in series with 1 more thats slightly lower than the rest. The 59 drop the same voltage with the lower one reading less.

The internal impedance of a cell is an indication of the health of the battery. In an ideal world the lower the impedance the more current the cell can provide and consequently larger AH cells have a lower internal impedance.

Overall your set looks to be in pretty good shape - as proved by your testing. Continuous monitoring like you're doing is the best way to pick up early cell failure.

However, the best readings to take are the ones taken while the battery is actually under load. In this condition there's a lot more amps flying about and very small differences in internal impedance translate to much larger differences in individual cells voltage - hence making it much easier to pick up on defective cells.

Doing the cell voltage readings just with trickle charging currents flowing though doesn't show this up as clearly. Pulling load out of the cells is the only 'real' way to prove their capacity - but routine impedance testing is also a great tool. If you're worried repeat the cell voltage measurements under load and log these.

 

[nukeman]

gezcooke,

Thanks for your reply. The capacity test we performed utilized a resistive load bank (contact current to a specified end voltage) and the analysis software plots each cell's ICV during the entire discharge. The cell performed well during the discharge just didn't appear to recover during recharge. We ended up replacing the cell with a spare cell of the same lot number. Being in a nuclear facility I do not have the luxury of just monitoring low ICVs until the next discharge test.

I also thought about the internal cell resistance but we have not started recording this value yet on our batteries. We have placed an order for the instrument but have not yet received it.