12V starter Battery

[rick]

I have a 12V battery, not used on a car but is connected to a 12V recovery winch in a boat shed. This is kept between use connected to a CETEK MXS 5 - an 8 step charger with correct Float charge once up to full charge.

Over winter I remove and put indoors, and leave it on the charger. The same charger wired in parallel across a pair of batteries (worked fine on previous years) Fitted it on w/end and it was dead ..... So to be sure put it on the charger again for 24 Hrs Now it appears to be working fine.

Checking voltage ..... it measures 13.5V after charge - which I can only think must be a surface charge. (i.e greater than ~12.6) I used the winch to pull in a 2.5T load, and seemed OK, Voltage after the recovery was 12.75V

Would this suggest battery is probably OK ?

Perhaps having the charger across 2 batteries, when one of them was new and higher capacity, may have provided imbalance and it shut down charging due to back emf of the larger battery and then failed to charge the winch battery ?

Next year I'll use separate chargers.

[newshound]

Or just one charger, swapped between batteries once a week.

[Fredxx]

Sorry I don't follow. What was dead, the charger, battery or winch.

Flattening a battery will always damage it to some extent.

I see no issue with using one charger for 2 batteries. A charged battery on a float charger should take very little current.

[T i m]

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That can be ok and especially if the batteries are of the same voltage / charge status (when you hook them together), because ...

You should also connect them via fuses (or at least one fuse in the case of just two batteries, between the batteries) in case one battery goes short and tries to discharge the other one ... very quickly. ;-(

Cheers, T i m

[GB]

I think that one issue could be that the two batteries were NOT exactly the same voltage to begin with. They have low internal resistance, so even a small voltage difference between them could drive rather large currents through them.

I think it would be better to charge them separately in future.

[Dave Plowman (News]

If the battery is good and not connected to anything, no need to leave it on float, as they keep their charge for a long time. Float is useful when installed in a car etc where there is a quiescent load.

But if using the float feature, I'd make sure both batteries were fully charged first.

[newshound]

Definitely.

[rick]

The battery ....

On previous years charged each fully individually then put them in parallel and hooked up charger and left it on over winter and all was fine ... this year the one battery was flat.

Maybe I had a bad connection but all were in place OK

[rick]

That is my thought - Voltages were not going to match One was a new 170 Ah The other a 9 yr old 74 Ah

[Fredxx]

I would say you had a bad connection or the battery failed o/c. I guess a bad connection is more likely when you string two batteries in parallel.

[Fredxx]

A float charge voltage from such a charger should be independent of battery capacity or age, unless one of the batteries is taking current, usually evident by gassing.

It is a good idea to check battery voltages periodically. Float charge should be ~13.7V Anything over 13V will maintain charge.

[Dave Plowman (News]

The voltage on a lead acid isn't dependant on the capacity. But rather obviously different sizes are going to take different times to charge from low.

[rick]

I realise its Volts per cell that makes up the 12V and number of cells is the same.... but with massively bigger cells thought there may be other factor, differnet back emf, resitance, etc.

[Vir Campestris]

The bigger cells will give you more current for a longer time.

Andy

[Dave Plowman (News]

Yes, but in terms of charging none of that makes much difference. It's the charger which determines the charge rate.

[Paul]

The voltage is "electrochemistry". There's a professor in the chemistry building at your local university, that's all he does :-)

It's an electrochemical potential. If you sit still long enough, the electrochemistry prof will teach you the Nernst equation.

See the third page - the battery acid is roughly 12 molar when the battery is fully charged. That is giving slightly over 2 volts per cell. There's also a temperature term in the equation, and battery chargers should really know how warm the battery is.

It is modified by the boundary layer (the concentration of materials there can be different than the bulk material). There is a layer of material next to the plate, and after the battery is charged, that's a bit "disturbed". After a few hours, the cell settles to its final (real) voltage.

A cell has resistance, but that only affects the voltage under load. If the cell has a resistance of one ohm, one ampere of current is drawn, then the cell will appear to be delivering one volt less voltage than before the load was applied. A car battery will have resistance values, well under an ohm. For example, by measuring my own car, one operating point is "9V @ 150A". When the starter starts cranking, the battery voltage drops due to its internal resistance, and those two values are what I measured with two meters set to "peak hold" mode.

The behaviors of the battery are also measured by various pieces of equipment. At the Battery University website, they talk of "impedance spectrum", which is related to the transient behavior during charging. It is possible to collect health information about a battery, by studying the impedance. And doing it with pulses of current.

There is no back-EMF as such. Back-EMF is a property of motors (and by analogy, likely generators or alternators as well).

If you change the chemical composition of the lead paste used in the plates, that's more likely to alter the potential. Just as lithium iron and lithium cobalt have radically different voltages.

Paul