batterries

Hi Peter,

What is the technical benefit of a "good discharge" every now and then on an AGM battery?

Cheers

Captain

Hi Captain,

all lead acid batteries sulphate up gradually if left sitting there for some time.
Sulphation changes the structure of the electrolyte/lead interface with an increasing amount of larger lead sulphate crystals acting like a barrier preventing electron/ion exchange in the deeper regions of the electrode.
By partially discharging the battery, fresh lead sulphate is formed and deposited around the larger/older/harder crystals. The fresh lead sulphate is in amorphous form thus can easily be reclaimed in the following charging process (in contrast to most of the hard crystals).
Because under constant voltage charging, the SOC determines the amount of charging current. This 'inrush' current at the begin of the charging process re-arranges the landscape in the electron/ion boundary layer. During this, amorphous lead sulphate can cause partial disintegration of the (otherwise electrically inactive) crystallised stuff which then becomes useable active material again.

So there you have it.

cheers, Peter

Hi Peter,

I agree totally with the above, its why a battery should be left in a charged state and also a good explanation of the "battery reconditioning" priniciple. But it is also why leaving a battery on a good multi stage charger with an approriate float voltage is perhaps better than only recharging every 2 weeks.

With a fully charged new battery, there should be no lead sulphate present, only a "pure" lead anode and a lead oxide cathode. By applying a small float voltage constantly, the internal discharge is constantly being overcome. By waiting two weeks between re-charges, there would now be some measure of lead sulphate due to internal discharge (worse on a wet cell, minimal on an AGM).

Additionally, a battery only has so many recharge cycles, though this number is exponentially dependant on how low the SoC is before recharging. While pulsating a battery helps recover sulphanation, minimising the number and depth of discharge cycles also helps battery life. By keeping the battery on a float charge and not needing a "good discharge", it is an alternative path to help improve battery life while in storage.

I prefer to keep a battery on a float charge than to cycle it periodically when in storage. But it does require a good quality charger as many of the cheap ones do not have a proper float voltage and simply boil away the electrolyte. In that case, its better to recharge every 2 weeks :)

Cheers

Captain

Yes Captain,

it appears to be a good idea to leave it on float 24/7.....
But with the VRLA types you have this oxygen recombination on the negative electrode. This effectively reduces the negative electrode's overpotential, so that sulphation can still occur at the negative electrode. You're basically discharging the negative electrode while float charging the battery.
That's why VRLA batteries always need to be charged with a voltage high enough to allow some overpotential to develop on the negative, i.e. by charging slightly in the gassing region.
This is especially true for ageing VRLA batteries which have lost part of their initial electrolyte saturation of the separator, therefore facilitating a more complete gas recombination.
So to overcome negative plate sulphation during float charging, it is necessary to increase the float charging voltage gradually as the battery ages, which leads to more loss of electrolyte again - so you're entering a race which you can never win.

Of course, charging also causes postive grid growth and embrittlement which are other non desirable side effects which you can alleviate by not charging it constantly.
In my opinion, it's better to have the VRLA battery resting there, followed by some weekly, or fortnightly discharging/charging sessions, than constantly leaving it on float.

cheers, Peter

Hi Peter,

OK, that makes good sense for a VRLA - neither way is perfect but I can see the advantage of periodic charge over float charge.

But on a wet cell, there is no recombination issue, coupled with the higher internal discharge and the ability to top up, so perhaps better to leave a wet cell on float?

Cheers

Captain

yes Captain, but only if it was designed for that.

Examples are 2V monoblocks for standby apps, and traction batteries.

12V wet cell batteries on the other hand, with their small footprint usually only allow for a limited amount of grid growth, and the positive plates aren't thick enough to stand extended periods of float charging during their designed life.
These batteries give maximum service life, if never discharged much below 10% DOD, and as long as they receive their daily/weekly dose of float charging at 13.6~13.8V in combination with occasional watering in hot ambients.
They also benefit from occasional equalisation charging.

cheers, Peter