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Old 08-28-2001
Contributing Authors
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Charging Issues

This is a charging question. I have 800 amps in my house bank. This should allow me to replace 200 amps per hour if all was perfect. If I use 200 amps from the bank and starting with full batteries, I start to recharge using the engine and 150-amp altenator, but the digital amp meter says I am replacing only 50 to 60 per hour.

Why does it not place more amps per hour back into the bank? Does the quarter value of the bank charging input belong to the full amount of the bank or the amount used and being replaced? 50 to 60 amps would be one quarter of the 200 used. Also, when my voltmeter says 144 volts under charge, is the bank full? Please help me to understand this.

Sue & Larry respond: 
Thanks for your question. You are correct in your original assumption that an 800-amp-hour battery bank is capable of receiving one quarter of its capacity during charging, or 200 amps. This is only true, however, during the period of bulk charging. The bulk-charging phase of a multi-step regulator begins with a deeply discharged battery, and ends when the level of charge approaches 75 to 80 percent.

If you discharge your battery bank more greatly, say to the 50-percent level (-400 amp-hours), then by initiating the charging via your 150-amp alternator you should see a charging output from your alternator much greater than the previously reported 50 to 60 amps.

You really can't use your voltmeter to accurately determine a state of charge while your battery is still being charged. To determine your battery bank's state of charge with your voltmeter, you first need to make sure that your battery has not been charged or discharged for at least 15 minutes, and preferably several hours. To help, we've included a table below showing the correlation between state of charge and battery voltage.

Open Circuit Voltage Level of Charge

12.6 or more 100 percent
12.4 - 12.6  75 to 100 percent
12.2 - 12.4  50 to 75 percent
12.0 - 12.2  25 to 50 percent
11.7 - 12.0  0 to 25 percent
11.7 or less  dead battery

For more information see Kevin Jeffrey's Calculating Your Electrical Load

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