Systems+Engines

Surging Power

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"I have two 12-volt lead-acid batteries in my boat, a starting battery and a house battery. The positive leads from both batteries are connected to a conventional battery switch with the standard 1, 2, Both, and Off positions. My two negative battery terminals are connected, and the one closest to the battery switch is grounded to the engine. A Guest battery charger is connected to the number 1 and 2 terminals on the switch.

My question stems from something I observed while looking for stray currents I thought might be responsible for eroding my shaft zincs. When I connected an ammeter between a positive battery terminal and a disconnected positive cable and then put the switch in the appropriate position (either 1 or 2), I got a zero reading for each battery. However, checking both batteries with the switch in the Both position showed a draw of .2 amp. Does this reading mean the switch is defective, or is this a normal cross current between the two batteries?"

-- Alan Therrien , Boxford, Massachusetts

Nigel Calder replies : The cross-current reading of .2 amp with the switch in the Both position is normal. The voltage in one of your batteries is probably marginally higher than the other, and when you parallel them through the switch the batteries will seek to equalize the voltage. That’s what is causing the very small amount of current you are reading. If the voltage differential is significantly higher when you first take a reading—with one battery discharged and the other one recently recharged, for example—you will see a large surge in amps. Over time, the voltage of the two batteries is equalized and the reading gets smaller. I don’t believe this cross current has anything to do with your problems with galvanic corrosion.

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