Elementary Question on basic charging - Page 3 - Fiberglass RV


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Old 07-27-2016, 03:04 PM   #29
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10 volts?The tow battery (alternator) must be greater that the trailer battery for charging to occur. ...
Oh... I guess we are assuming that the alternator (or other charging source) voltage is higher than, 1. the voltage of the battery when fully charged PLUS, 2. any voltage drop in the system. Otherwise you end up with incomplete charging.
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Old 07-27-2016, 03:17 PM   #30
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Oh... I guess we are assuming that the alternator (or other charging source) voltage is higher than, 1. the voltage of the battery when fully charged PLUS, 2. any voltage drop in the system. Otherwise you end up with incomplete charging.

I believe you mentioned that you're a ham radio operator. Look at your basic electronic stuff. A battery charging profile looks kind of like a capacitor charging. Then look at ohm's law. The difference between the tow battery voltage (13.8 is norm for vehicle systems) and house battery. So your house battery is about 12 volts and you start charging it from the tow it's a 1.8 volt difference you can calculate the current without the losses. As the house battery gets closer to full charge the differential voltage gets less. The IR (wire loss) is so small that even with 12 to 14 gauge wire the loss is so small you won't be able to detect the difference.
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Old 07-27-2016, 03:32 PM   #31
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... The IR (wire loss) is so small that even with 12 to 14 gauge wire the loss is so small you won't be able to detect the difference.
But the effect DOES remain. By now you have probably figured out that I was playing devil's advocate to point out that whether or not voltage drop has any noticeable effect, or even is measurable within reason, it still exists. I think some people may have been left with the impression that line loss does not exists at all with two batteries in parallel being charged.
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Old 07-27-2016, 03:40 PM   #32
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There is no IR loss. It's a two source network. The voltage drop across the charge line, connections, what ever is between the battery terminals is always the difference between the sources. KVL
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Old 07-27-2016, 03:50 PM   #33
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Wire has resistance.. unless at absolute zero. Im done.
True. And that resistance determines the current. Replace the trailer battery with a light bulb and now you have voltage loss across the wires. A one source network which results in a voltage divider.
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Old 07-27-2016, 04:12 PM   #34
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The mistake folks make is that they wish to multiply the current by the wire resistance and subtract it from the tow battery to get the trailer battery voltage. What they forget is that the trailer battery is also a power source and the chemical reaction in the battery determines it's voltage. Don't treat the trailer battery like a light bulb.
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