DiploStrat
Expedition Leader
Sorry!
You're right; I was testy. My apologies.
Voltage and voltage drop are hotly debated subjects when it comes to battery charging, but grossly:
-- Your charge source must be at a higher voltage than the battery you are trying to charge to charge at all. The greater the difference, the faster the charge.
-- The resistance of a wire drops as the voltage rises and the amp flow drops. (That is why 12v wiring is so much larger than 115v wiring.)
Voltages you should know and love: (For a 12v lead acid battery.)
-- 14-15.5v: Modern charge voltages, varying with exact battery formulation and temperature. (lower temperature requires higher voltage)
-- 13v+: Float voltage, the voltage to which most chargers will drop once a battery is "fully" charged.
-- 12v: 40-50% charge. Point at which you normally want to stop using and start charging a 12v battery.
-- 10.5v: Point at which your 12v battery becomes a paper weight.
As a practical matter, you want to discharge a lead acid battery as little and as slowly as possible and recharge as soon as possible. Thus your refrigerator is a much easier load than my microwave. All of this gets messy as the voltages above are "resting" voltages, that is voltages where the battery is under neither charge nor discharge and a camper battery, unless disconnected, is always charging or discharging.
So how much voltage drop is too much? Ideally, you want zero, but there are practical limits. In my case, my batteries are some 20+ feet from the alternators, so I generally shoot for 0.5v or less - at full charge, at full voltage. Technically, this means that you start off charging at between 13 and 14v, but if the target battery has dropped to between 12 and 12.5v, you will still get a decent charge rate. And as soon as the battery starts to take a charge, the voltage will rise and the battery will take an ever lower charge rate. At this point your voltage drop will decrease. Playing with the calculator, you see things like this:
Start charge. Assume full 130A @ 14v: Drop = 0.38v Voltage = 13.6v
When the charge drops to 100A @ 14v, a more likely starting figure, you get: Drop = 0.29v Voltage = 13.7v
As the batteries take a charge, you will see the charge rate drop to about 50A @ 14v, so now you get: Drop = 0.15v Voltage = 13.8v
Finally, due to the perversity of lead acid batteries, you are going to spend a LOT of time around 25A @ 14v, and thus you will get: Drop = 0.07v Voltage = 13.9v, essentially negligible.
You can run the numbers with different wire sizes and compare the results. In your case, I would not go lower than a 1/0 AWG. Why? Because the alternator excels at putting out a lot of amps up front for the bulk stage of charging. Typically, however, you never run the engine long enough to achieve a really full charge, and that is where solar or shore power come in. They complete the long absorb stage where you need fewer amps but much more time.
If you look at my website, under "Documents" you will find all manner of geeky stuff on this subject.
Hope this helps and that I avoided any factual errors or typos. (And, as my Ozzie friend noted, get more solar!)
You're right; I was testy. My apologies.
Voltage and voltage drop are hotly debated subjects when it comes to battery charging, but grossly:
-- Your charge source must be at a higher voltage than the battery you are trying to charge to charge at all. The greater the difference, the faster the charge.
-- The resistance of a wire drops as the voltage rises and the amp flow drops. (That is why 12v wiring is so much larger than 115v wiring.)
Voltages you should know and love: (For a 12v lead acid battery.)
-- 14-15.5v: Modern charge voltages, varying with exact battery formulation and temperature. (lower temperature requires higher voltage)
-- 13v+: Float voltage, the voltage to which most chargers will drop once a battery is "fully" charged.
-- 12v: 40-50% charge. Point at which you normally want to stop using and start charging a 12v battery.
-- 10.5v: Point at which your 12v battery becomes a paper weight.
As a practical matter, you want to discharge a lead acid battery as little and as slowly as possible and recharge as soon as possible. Thus your refrigerator is a much easier load than my microwave. All of this gets messy as the voltages above are "resting" voltages, that is voltages where the battery is under neither charge nor discharge and a camper battery, unless disconnected, is always charging or discharging.
So how much voltage drop is too much? Ideally, you want zero, but there are practical limits. In my case, my batteries are some 20+ feet from the alternators, so I generally shoot for 0.5v or less - at full charge, at full voltage. Technically, this means that you start off charging at between 13 and 14v, but if the target battery has dropped to between 12 and 12.5v, you will still get a decent charge rate. And as soon as the battery starts to take a charge, the voltage will rise and the battery will take an ever lower charge rate. At this point your voltage drop will decrease. Playing with the calculator, you see things like this:
Start charge. Assume full 130A @ 14v: Drop = 0.38v Voltage = 13.6v
When the charge drops to 100A @ 14v, a more likely starting figure, you get: Drop = 0.29v Voltage = 13.7v
As the batteries take a charge, you will see the charge rate drop to about 50A @ 14v, so now you get: Drop = 0.15v Voltage = 13.8v
Finally, due to the perversity of lead acid batteries, you are going to spend a LOT of time around 25A @ 14v, and thus you will get: Drop = 0.07v Voltage = 13.9v, essentially negligible.
You can run the numbers with different wire sizes and compare the results. In your case, I would not go lower than a 1/0 AWG. Why? Because the alternator excels at putting out a lot of amps up front for the bulk stage of charging. Typically, however, you never run the engine long enough to achieve a really full charge, and that is where solar or shore power come in. They complete the long absorb stage where you need fewer amps but much more time.
If you look at my website, under "Documents" you will find all manner of geeky stuff on this subject.
Hope this helps and that I avoided any factual errors or typos. (And, as my Ozzie friend noted, get more solar!
)
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