Batteries(Again)

[roger dolan]

  I replaced my 4--6 volt golf cart batteries (Flooded cell, 215 AH) with 8 of the same batteries.  My question is :  At what maximum rate can they be recharged?  Starting at 1125 SG and 11.5 volts when I recharge with the bus alternator it will show 200 amps.  I am afraid of damaging the batteries at this rate.  When I use the Inverter ( Dimensions 12V-2000 Watt Winnebago Surplus)  it starts at 140 amps charge for 5 min, 110 amps for about 2-3 hours then to 60 amps then stops at 12 hours with the batteries only charged to 1175 SG which is not full charge and will damage the batteries if used this way.  I sent the inverter back to the factory to be reset for the 4 batteries.  Now I would like some suggestions for a reset for 8 batteries instead of the 4.  P.S.  What is considered 50% discharge volts and SG?  Inverter disconnects at 10 volts.  Thanks All,  Roger 4106

[JackConrad]

   I think I read somewhere that charge rate should be limited to 20% of total capacity to prevent overheating the batteries, but I might be mistaken.  The 140 amp maximum charge rate with your inverter is probably the maximum output of the charge portion of your inverter. Our inverter has a temperature sensor that attaches to the battery bank to prevent overheating the batteries.
Not sure about SG at 50%   discharge, maybe Sean will post with a much more comprehensive answer.  Jack

[Sean]

Rather than reinvent the wheel, I will refer you to Bill Darden's web site that I have mentioned here many times:
http://www.batteryfaq.org/

Bill has done an excellent job in putting together and organizing what is probably the most comprehensive single web site on starting and deep cycle batteries.

To answer your specific questions, the maximum charge rate for flooded batteries, in amps, should not exceed 20%-25% of the 20-hour capacity rating (often called C/20) in amp-hours.  So for your 215 amp-hour batteries, if they are arranged in two "strings" of four batteries each (for a 24-volt system), your total capacity is 430 amp-hours.  20% of that is 86 amps, 25% would be 107 amps, so I would set the maximum charge rate to 100 amps.  For batteries so rated, you should also not exceed 10% of the "reserve capacity" (a rating usually not found on true deep cycle batteries).

For state-of-charge (SoC), have a look at the tables on this page, from that same site:
http://jgdarden.com/batteryfaq/carfaq4.htm

There you will find tables for SoC based on specific gravity (by temperature) as well as by voltage.

IMPORTANT NOTE:  You absolutely, positively can not tell the state of charge of a battery by its voltage alone if there is anything at all connected to it (besides the voltmeter).  The battery must be "out of circuit" and it must "rest" for several hours this way before the voltage is a useful number.  This makes it of very limited use for bus conversions, where we can seldom remove the batteries from service just to check resting voltage.

This is why it is critically important to have an SoC meter, sometimes called a "battery gauge" or "battery gas gauge" such as the Tri-Metric or Link-10.  These work by monitoring all the energy going into the batteries during charge, and coming out during discharge, and calculates the SoC or "depth of discharge" (DoD) based on information you give it about bank capacity, chemistry, and performance.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com