Battery Charging: what limits the amperage to the house bank? - Fiberglass RV


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Old 01-18-2011, 03:00 PM   #1
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Name: Jon
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Question Battery Charging: what limits the amperage to the house bank?

I am new to RV's but have lived and cruised on a sailboat so have lived for a while with 12v. One question to the people here, what limits the amperage to the house bank? I know with my system on the boat if the batteries are discharged the alternator will put out upwards of 150 amps (large frame alt and large battery bank). If the TV battery was discharged along with a discharged house bank wouldn't the alt be putting out the amps and would the 10 gauge wire handle that, or would you expect the fuse to pop in this case? I would also assume the built in regulator in the TV alt would not handle the different voltages of the TV battery and the house bank very well at all.
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Old 01-18-2011, 11:27 PM   #2
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I'll try to answer your questions individually. Sorry for the lengthy dissertation.

Quote: I am new to RV's but have lived and cruised on a sailboat so have lived for a while with 12v. One question to the people here, what limits the amperage to the house bank?

Response: Charge current is limited by several things. First is the output capability if the alternator in the TV. Second is the total resistance in the circuit, and that includes voltage drops in all the wiring as well as connectors and the ground return. The ground return is frequently through the vehicle frame which is actually a rather poor conductor. In order to charge the house battery the voltage available at the battery must be higher than the battery voltage. A fully charged battery, when under a light to moderate load, has a terminal voltage of approximately 12.6 V. Normally 13.8 V is considered to be a good value on the charge line to force enough current through the battery to ensure a full charge. Voltage drops through associated wiring will drop the alternator output voltage significantly at even moderate current draws. So the short answer to your initial question is that the resistance of the wiring will limit the current supplied to the house battery. If you have an in-line fuse or circuit breaker the only time it will open is if you have a short circuit or near short circuit in the wiring.

Quote: I know with my system on the boat if the batteries are discharged the alternator will put out upwards of 150 amps (large frame alt and large battery bank). If the TV battery was discharged along with a discharged house bank wouldn't the alt be putting out the amps and would the 10 gauge wire handle that, or would you expect the fuse to pop in this case?

Response: If both the TV battery and house battery are discharged the TV battery draw most of the charge curent first since there is a much lower voltage drop between the alternator and the TV battery. Consider that the wiring to the TV battery is usually #8 or larger (in my truck it is #2) and a much shorter run than the line to the camper. Most factory charge line wiring in a TV is #14 which has a much higher resistance per unit length than #8 wire and is, of course, a much longer run. Some owners install a much larger charge line in their TV to reduce that voltage drop, like a #10, #8 or even #6. After the TV battery is brought up to 13.8 V and the current draw decreases then the house battery will recharge. Even then the voltage drop through the long run of undersized wire will drop the voltage at the house battery to the point where the charge current will be significantly reduced. Remember that the 13.8 V must be measured at the house battery (directly at the + and - terminals). Measurements at any other point do not give a true indication.

Quote: I would also assume the built in regulator in the TV alt would not handle the different voltages of the TV battery and the house bank very well at all.

Response: The alternator doesn't care. It just outputs whatever current is demanded at the set output voltage (within the limitations of the alternator of course). The alternator regulator is a voltage regulator, not a current regulator. It holds the alternator output at about 13.8 V at the alternator output terminal. Modern alternators also have temperature compensation so the output voltage also changes with temperature since batteries require a higher charge voltage in cold weather.

I hope this answers your questions.

73
Orlen
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Old 01-19-2011, 11:44 AM   #3
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Name: Jon
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Thanks Orlen, that makes sense. Looks like I will need to be wiring this up.
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Old 01-19-2011, 11:47 AM   #4
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Foot note: My battery charger shows a value in amps not volts while outputting voltage is above the battery voltage. Well explained; near full charge charger meter rate drops to about 2.
The explanation is great.
In a prior post I put water weight at 10 lbs per gallon; it is 8.3 lbs US per Gallon and 10 lbs. Canadian per gallon: My water tank is in Canadian Gallons but the water heater is in US Gallons. I was glad to have this pointed out. Have learned a lot reading the posts at fiberglassRv
Thanks for all the help.
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