More on Batteries

[Jonathan Hutchins]

Thanks for the responses on batteries, SAS might be the place to go.

What I'm really looking for is a replacement for the battery in my Ryobi 
Mulchinator mower.  It was originally a 24v unit, but later replacements used 
two twelve-volt batteries in series.  It requires gell-cell batteries because 
it's not as reliably upright as a car; it can be stored on end.

The batteries it takes are about the same as those common in UPSs - they use 
gell-cells so people can handle the unit casually instead of having to handle 
the batteries seperately and deal with spillage potential.  They're also very 
similar to the average Japanese motorcycle's battery, except the bikes use 
sealed liquid batteries rather than gell cells.

Regular lead-acid "car" batteries give the longest life if they are kept near 
full charge at all times.  Discharging them will shorten their life.  So will 
charging them too fast - I think the calculation is 1/10 the AH rating, so a 12 
AH battery should charge at no more than 1.2 Amps.  Faster charging warps the 
plates in the battery, misalignment reduces efficiency and life.  Reduced 
capacity requires slower recharge, so you have a snowball effect here. For a 
gell cell battery, too fast a charge will cook the gell, and cause it to 
seperate from the plates.  Any area no longer in contact isn't contributing to 
the battery any more.

Another big hit to battery life is time spent discharged.  When a battery is 
discharged, reactions take place in the cells that convert the lead on the 
plates to sulfites, which perform no useful work.  The sulfites insulate the 
plates and crud up the bottom of the cells, reducing efficiency.  The less time 
a battery spends discharged the healthier it will be.  

Stored batteries will loose up to 10% of their charge per day.  Obviously, any 
period of time when your battery isn't kept recharged will result in the 
battery spending time partially discharged, and hence deteriorating.  This is 
probably why UPS batteries appear to have such short lives - they've been 
sitting in the box deteriorating since they were packed.  

This is also why recreational vehicle batteries usually last only two or three 
years.  They are typically stored for several months starting with a full 
charge, but the charge deteriorates and they come out of storage fairly low.  
While reasonably new, they are capable of starting the vehicle, or someone 
slaps a 10Amp charger on them to get the thing going.  The heavy charging 
current, along with the heavy recharge once the vehicle is running, along with 
the sulfite formation during storage, and the deep discharge of starting the 
vehicle from a low charge all shorten the battery's life.

The battery in my BMW is currently eight years old, and works fine.  I use 
a "Battery Tender" brand float charger that monitors the state of charge and 
uses a very low current top-up charge to keep it almost full almost all of the 
time.  This is a "smart" charger that doesn't just constantly apply a low 
current, which would eventually boil off the electrolyte.

"Deep Cycle" batteries intended for marine and remote power applications are 
designed so that they can be fully discharged without shortening the battery 
life as much as a conventional battery, they also suffer less from time spent 
at partial charge.

Nickel-cadmium batteries were originally developed for use in jet aircraft, 
whose engines require significantly more power to start than internal 
combustion engines.  Ni-Cad's can provide more current than lead-acids, but 
have some quirks.  The old liquid-electrolyte Ni-Cads did have "memory" 
problems of not being able to deliver  a full charge if they had only been 
lightly discharged repeatedly.  A regular maintenance procedure was to 
completely drain the battery by using shorting plates connecting the terminals 
of each internal cell.  This restored the ability to deliver full capacity.

The modern Ni-Cads we use in electronics are a far remove from these old open-
container things, and studies have shown that capacity lost to "memory" is 
insignificant.  Still, high-tech chargers for Ni-Cads and other modern 
batteries often do a "conditioning" cycle, fully discharging the battery and 
recharging it in a carefully calculated pattern to give the battery new life.

So your best bet for a UPS is a good car battery, keep it clean and charged, 
don't deep cycle it if you don't have to.

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