Battery maintenance and use.

[Redoverfarm]

So if you have followed my build  @ http://countryplans.com/smf/index.php?topic=3613.msg142984#msg142984  you will see that I now have a electrical system in place which is  48 volt battery bank via 4400 watt Magnum inverter.  In addition I have a Automatic Generator Start feature which works in conjunction with a Battery Monitor.  In principle I preset the draw down volts of the battery bank.  Once the battery bank is depleted to that point the Battery Monitor signals the AGS and inturn starts the generator.  Once the battery bank is charged back up the Battery Monitor again signals the AGS which turns off the generator.  It is still in the testing stage as it has only been operational for about 2-3 weeks but has worked great so far.  I have only allowed it to deplete from 52 volts to 46 volts.  I intend to have a deeper draw in the furture and monitor it's effectiveness.

Just how far should a bank system be drawn down? Some say no more than 50% in this case would be 24 volts.  Is it better to keep a narrower voltage gap than a  larger one?  Meaning should it be less in the neighborhood of 30-36 volts or greater.  What is better for the battery life? Less charging time which would mean higher voltage draw down or greater charging time which would mean a lower draw down voltage?  

About the most exposure that I haved had with batteries is when my car,truck or mower wouldn't start.  

[Rob_O]

I have only allowed it to deplete from 52 volts to 46 volts.  I intend to have a deeper draw in the furture and monitor it's effectiveness.

Just how far should a bank system be drawn down? Some say no more than 50% in this case would be 24 volts.  Is it better to keep a narrower voltage gap than a  larger one?

It doesn't work like that. 46 volts on your system is almost completely dead, the genny should be starting at about 49V (~40% DOD)

[MountainDon]

You are correct in that lead acid batteries should not be drawn down more than 50%. That 50% is based on the SOC (state of charge) reading that comes from a hydrometer, Not a percentage of voltage reading.

This is why I always say that a good hydrometer should be included with the purchase of any flooded lead acid battery (one that has removable battery caps and the ability to have water added.... AGM's don't count). A temperature corrected hydrometer is the only reliable SOC indicator.

EDIT: removed first chargt I posted. See next post for more reliable temperature compensated chart. Sorry, only a 12 volt and a 24 volt chart available. I could run off a 48 volt chart if you would like or send you the spreadsheet (runs in Excel).

[MountainDon]

Temperature compensated battery charge charts are just a hop, skip and a jump.... HERE.



The less deep the discharge the better for longer battery life.

[MountainDon]

The following is copied from Northern AZ Wind-Sun

"Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film. The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary."

Read their complete Deep Cycle Battery FAQ for lots of good info.


Temperature affects how much charge is available to do work; colder temps = less available electrical power. hence the temperature compensated battery charts above. Take that into account when calculating/estimating battery capacity and depth of discharge.

[Rob_O]

Don:Thanks for posting that chart, I'm having issues with the interweb tonight

Red: you should really consider adding some solar to your system. Keeping the average charge higher will extend the life of your batteries enough to more than pay for the solar in the long term

[glenn kangiser]

Long term with low batteries will cause sulfation lowering battery capacity or destroying them.  As stated above it is the amp capacity that gets drawn down.  Nominal voltage should still be around 48v plus when charged - lower when a charge is needed.