Battery Life

[twomules]

I spent eight years designing chargers and instrumentation for motive batteries, lift trucks, buses, wheelchairs and tanks and still find these vile boxes of noxious chemicals perplexing.

If you find ice in the battery it's not fully charged.  A fully charged battery freezes around -74F.  At 50% charge it freezes around 5F, at 0% charge it freezes around 18F.  Batteries have a lot of thermal mass if you keep them fully charged when idle they won't freeze.  At any reasonable temperarure.  I live in Wisconsin so reasonable is on the cold side.  If they must sit partially charged for long periods (a bad idea) then insulation or even a heater may be necessary.

The sulfuric acid is turning into water as the battery is discharged and the lead plates are converted into lead sulfate.  This is normal and not harmful, it's how the battery works.
Charging the battery converts the lead sulfate into lead and the acid increases in strength. 

The sulfate on the plates is at first porous and reversible but the crystals grow becoming a nonporous insulating mass which is difficult to reverse.  If the crystals grow too big they mechanically damage the plates and then the battery is not recoverable.  A second reason to keep the battery fully charged is corrosion.  Plate and inter-cell connection corrosion is increased when the battery is not fully charged.

I tested many types of desulfators in the laboratory.  The best desulfator is gently charging and discharging the battery.  Full charge, slowly then discharge to 20%
slowly, repeat as necessary.  At the point when the battery is looking better an equalization charge would probably be a good idea.

Large batteries benefit from having the electrolyte stirred.  Some batteries have an air stone like those used in fish tanks built in and compressed air stirs the electrolyte.  Pulse charging and pulsed discharge during charge sometimes claim the benifits of stirred electrolyte but I couldn't verify them in the laboratory.  In fact I couldn't veryify any benefit from them.  I tried pounding on the steel case of a 4000lb battery with a mallet to see if shaking bubbles off the plates had any effect.  Didn't seem to.  Maybe I didn't use a big enough mallet. 

Determining the depth of discharge by measuring the voltage under load depends on the discharge rate and the temperature.  If you measure open circuit voltage the battery has to rest first and it still depends on temperature.  If the battery management system doesn't have a temperature sensor so it can compensate the measurements it will be inaccurate.  Here is a good article on SOC measurement
http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

Even with temperature compensation, checking the computed state of charge by measuring the cells specific gravity (temperature compensated of course) is a very good idea.  These guys sell a nice hydrometer http://www.freasglass.com/battery.htm

I guess I'm rambling now, eh?  Keep an eye on the specific gravity, don't discharge the battery past 20%.  Charge it to 100% as soon as possible especially in cold weather.
Remember that 100% includes a "topping" charge.  How much is that?  Stop just short of when the battery boils as it does during equalization charging.

Good luck!

[Sprucegum]

Twomules;
          Thank you for sharing - that's good stuff to know.

[mad murdock]

Welcome to the Forum Twomules!  great information on a very overlooked item, the battery.  We deal with them constantly, as we depend on them to start expensive turbine engines, and not fail in the middle of a start.

[DouginUtah]

Is there some rule of thumb which says how fast a fully-charged 12 V. battery looses its charge when not being used?

The reason I ask is because I don't know how often I should put the charger on my teardrop battery during the winter.

Does it occur faster the colder it is?

[twomules]

It depends on the battery and the installation.  Even small loads connected to the battery like engine control modules and clocks make a huge difference.  It's best determined by experimentation.

Keeping the surface of the battery around the terminals clean and the battery disconnected from everything, I connect the charger every three months over winter.  That works for my old tractor but ymmv.  If it's difficult to reach the battery terminals a disconnect switch may be a good idea.

Cold slows self-discharge down because it depresses the voltage.  It also reduces capacity so it may appear that the cold is discharging the battery. 

Main thing, keep enough charge on the battery so it won't freeze.  Not only can it ruin the battery by buckling the plates but it can crack the case.   Then all the acid pours out when it thaws.  Yes, I've done this.  Didn't do the motorcycle much good.

[DouginUtah]

Thanks for the reply, twomules.

I've been charging it about every two months, and the coldest it got here this year was -5°F.

[maple flats]

Gary C, I do not use batteries off grid, but I do know a bit about battery construction, and what factors effect battery service life, etc.  As has been aptly stated, close monitoring of discharge state, and not letting batteries get below a certain level will have a great effect on what usable life one gets out of a battery.  Another thing is battery construction itself.  I am speaking from my experience with 25+ years of aviation maintenance and operation background here.  The best batteries (as far as capacity, discharge/recharge cycle life, and overall performance have plate material that is very pure, and the real expensive batteries will have plates made from solid lead plates, rather than the lead powder pressed plates, with the mesh composite construction.  The solid plates are less susceptible to sulfidation issues, and have a much higher cycle life, the downside is that these type of batteries typically are quite expensive to build, and to buy.  This may or may not be of help to you.  The heavy industrial type batteries make excellent choices for aux. or off grid power systems.  In my area, if a guy can find the telecom batteries that the phone companies discard usually every 2 years, are good to build a system around, as the cells are HUGE, and you can string whatever you want for your system in 1 cell (2 volt) increments.  One of those cells (2 volts) weighs about 40 lbs, they have a ton of capacity, and good cycle life.

mad murdock, how would these 2V cells be listed in Google. I tried every way I could think of and get nothing. Seems someone one the web will be selling them if they exist. Do you know if they are Rolls Surrette batteries? They are the only on I am familiar with that come in 2V. There may be many more out there but I have not seen them.

[maple flats]

In my area, if a guy can find the telecom batteries that the phone companies discard usually every 2 years, are good to build a system around, as the cells are HUGE, and you can string whatever you want for your system in 1 cell (2 volt) increments.  One of those cells (2 volts) weighs about 40 lbs, they have a ton of capacity, and good cycle life.

Mad Murdock, Do you have any idea how these might be listed on Google or other search engine. Or what excatly are the batteries removed from service by telcom companies?

[mad murdock]

I've not tried to google them, they are used by phone companies to supply system voltage at all times in case of power grid failure, (tht is why your phone works, if it is not a fancy wireless phone, when the power is out).  You could contact the local phone co, or someone that is a tech for them if you know one.  Trojan makes a 1110Ah battery that is 2V and weighs 116 lbs, along the lines of the ones I mentioned.  a bank of those would give a guy lots of power, and they have a 5 yr warranty.  Model # L16RE-2V .  That is the best I can do for you right now.  They have these on ebay and other sites as well.