Join Date: Apr 2006
Location: Spokane, WA
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Oh good, a challenge
Hellosailor, I knew I could count on you!
All battery manufactures of lead-acid batteries recommend a constant voltage, or float voltage, method for maintaining the charge level of their lead-acid batteries. This is what the voltage regulator of any engine alternator does; maintain the output level at a constant voltage. Most battery manufactures also recommend limiting the charge current into the batteries to about 20% of the rated 8 hour amp hour capacity, for a 200 AH bank this equates to 40 amps. Charging the batteries with much more then this current would promote self heating, electrolysis and constitutes charging abuse. Generally this current is limited by reducing the voltage output once the charging has reached this current limit and the output is regulated by current until the float voltage level is reached. Some class of chargers will do a secondary charge level, termed boost, and allow the current to be maintained until a higher voltage is reached, on the order of 14.4 to 14.8 V, and then the float or maintenance level is resumed, somewhere between 13.3to 13.8V. Equalize charging should be done only when recommended by the battery manufacturer and using the manufactures specific procedure. This helps overcome plate sulfation and can reestablish correct specific gravity levels in all of the cells. Other special charge cycles are possible but not recommended unless you can monitor temperature, outgassing, current draw, etc.
Practically one limits (or increases) the size of the alternator to limit the current flow to the battery. In the case stated above, a 60A alternator would be about the maximum size suggested.
As the batteries charge, the electrochemical state within the battery changes and the equilibrium cell voltage rises. For a smaller capacity battery, the “bad” battery in our example, this equilibrium voltage will increase faster then the larger capacity battery for a given current. If the two are connected in parallel, the smaller capacity battery will require less current to charge to the same voltage level as the larger. This is the mechanism for the current being “shared” by the two batteries. The larger one will use twice the current as the smaller one to get to the same equilibrium voltage level.
Using our example of one “bad” battery and one “good” with a 40 amp charge current, the “bad’ battery will charge at 13.3 amps, or 26% of the rated 8 hour capacity, and the good battery will charge at 26.6 amps, also 26% of the rated amp hour capacity. This is a little high but not catastrophic. We can also see in this mechanism that the paralleled battery system is “self healing”, one battery will not be overcharged and one will not undercharge.
This is from both theoretical and empirical data. I have measured currents under similar conditions and the only time I did not get these results some other factor was present.
The discharge sharing is similar to the charging. The same reasoning applies.
There are two types of fools...
One says this is old, and therefore good..
The other says this is new, and therefore better...