Battery testing: CCA 161% of capacity?

[jeremyc]

I've had problems with starting: battery charged and within a few days and a small number of starts it won't turn over (at all, not even slowly).

I've had the battery tested on three occasions over the last few weeks, and all three times been told it's OK. The first couple of times the CCA came back as 64% of capacity. Today when tested it reported 161% of capacity (so CCA of 1449A against a nominal capacity of 900A), apparently still OK.

How can this be possible? I understand that CCA is a temperature related spec., but surely there wouldn't be such a dramatic change in the battery capacity compared to a few weeks ago (and certainly not an improvement from 64% to 161%).

Is there a failure mode of the battery that could cause this reading by an automatic battery tester?

Needless to say the battery is still under guarantee and the supplying retailer (who tested it) say it doesn't qualify for a replacement since it is fine.

[normalbloke]

It’s not in their interest to fail it. Get it tested elsewhere?

[LuS1fer]

What car is it? It can often be a corroded earth strap between engine and body or a duff alternator.

[jeremyc]

Battery was tested out of the car.

I'm aware that there could be other causes of the poor starting, but wanted to better understand what looks to me like confirmation of a failed battery.

I have bought a replacement battery anyway; time will tell whether then problems persist regardless.

[jeremyc]

Battery University has some good articles on this stuff.

How to measure capacity

Tracing battery capacity and resistance and part of aging

How to measure CCA

Battery University said:

Most rapid-testers look at the internal resistance and do a CCA approximation. Reading battery resistance is relatively simple, but this alone cannot predict capacity, nor can it tell when to replace a battery as the end-of-life characteristic is primarily capacity related. Most starter batteries crank the engine with very little capacity; a sudden failure might occur when the capacity drops below 30 percent.

So even though the tester device said my battery was "good", it may be that the capacity has dropped significantly.

[jeremyc]

normalbloke said:

It’s not in their interest to fail it. Get it tested elsewhere?

There is if they want to sell a new battery, but yes - little incentive to show a failure for a replacement under guarantee.

Testing elsewhere isn't going to help me get a replacement under the guarantee - they presumably need their machine to tell them that it has failed.

[E-bmw]

If there is some evidence that the battery may be healthy, the next step would be to (heavy handedly) check all main battery/earth/alternator/fuse box/starter connections.

If just one of these isn't quite right you will have the same symptoms.

I would normally suggest removing/cleaning/re-fitting/checking tightness of all.

[Jakg]

jeremyc said:

Needless to say the battery is still under guarantee and the supplying retailer (who tested it) say it doesn't qualify for a replacement since it is fine.

Is it a store with an orange logo? Have you tried a different branch?

I had a battery that tested ok but the car wasn't happy and they agreed to swap it (under warranty) there and then...