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Old 13-05-2017, 08:35   #16
Marine Service Provider

Join Date: Dec 2003
Location: South Carolina
Boat: Philip Rhodes Custom
Posts: 409
Re: Yandina Trollbridge2400

Some nitty gritty details for those still following the thread.

Batteries are charged with current. The amount of charge injected is measured in amp-hours, not volt-hours or even watt-hours. They are not charged with volts, that is just a measure of the force pushing in the amps.

Alternators and chargers are current sources. If you connect an alternator to a battery at 10 volts with 2-0 cables the alternator voltage is 10 volts. If you connect it to a battery at 13 volts the alternator voltage is 13 volts, the voltage is not fixed, it is governed by the battery.

This is not exactly true because there is some resistance in the battery and some in the cables so as you put in more amps the alternator will be at a higher voltage than the battery equal to amps x resistance but the current still goes out and still charges the battery (nearly) at the same rate as it would if there was no resistance.

The regulator in an alternator or charger will decide (depending on smartness and settings) how hard to push out amps depending on battery voltage. This is done to protect the source from overload and to prevent damage to the battery due to high voltage. For lead-acid style batteries this voltage limit is about 14.2 volts. Once the limit is reached a smart charger will taper back to a maintenance current.

The resistance of the charging line will raise the "apparent" voltage at the alternator so assuming you can put out 200 amps into a battery at 13 volts the voltage drop should be limited to about 1 volt so the alternator is still putting out maximum current. As the battery voltage rises the voltage difference gets lower but so does the voltage drop along the cable and as the batteries reach full voltage the current drops to zero, the cable voltage loss drops to zero and the batteries reach full charge.

Based on this a 6 gauge cable would be suitable for a run up to 6 or 8 feet and you can do the calculations to get a gauge for longer runs if you really need to pump in 200 amps.

However under nearly all applications of a Trollbridge the duty cycle is fairly low and there is no need to charge at maximum possible rate. We typically under-size the charging cable specifications deliberately to limit alternator loads to a safer level and save installation cost. Your 200 Ah battery could be charged through an 8 gauge cable over 20 feet and still get fully charged, it would just take (?) minutes longer. For typical bow thruster, or winch type applications this is not a problem.

Resistance in the circuit is much larger than the internal resistance of a Lithium battery, in fact much larger than the internal resistance of a lead-acid battery, so it tends to be self regulating and a little resistance is much cheaper, safer and more reliable than sophisticated current/voltage controls, at the cost of extended charging times.
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Old 13-05-2017, 10:32   #17
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Re: Yandina Trollbridge2400

Please note my responses here are not meant to dispute anything; I just want to lay out my understanding and current approach, so any mistakes can be corrected.

I am also likely willing to change my design approach, if needed to fit within the TB's design, and appreciate any guidance in doing so.

Quote:
Originally Posted by Andina Marie View Post
as you put in more amps the alternator will be at a higher voltage than the battery equal to amps x resistance but the current still goes out and still charges the battery (nearly) at the same rate as it would if there was no resistance.

The regulator in an alternator or charger will decide (depending on smartness and settings) how hard to push out amps depending on battery voltage. This is done to protect the source from overload and to prevent damage to the battery due to high voltage. For lead-acid style batteries this voltage limit is about 14.2 volts. Once the limit is reached a smart charger will taper back to a maintenance current.
Some mfg these days recommend a much higher absorption voltage, some lower. Whenever possible my charge sources allow for custom programmed volt levels, absorption timing calculation, end-current setpoints, etc. For my LFP bank, I don't want Absorption any higher than 13.9V

All my sources also sense temp and voltage at the bank with dedicated wires, separate from power output.

My understanding is that the charge source sets the (max Abs) voltage/pressure, but the bank resistance in lead is usually what "regulates" amps/flow rate. So the bank "pulls" amps current, rather than the charge source trying to "push" it.

There are many reports of large depleted LFP banks killing normal, not-smart small-frame alternators, by causing them to run at max output for extended periods and overheating. Lead banks' resistance prevents this, as amps flow drops quickly as the SoC rises.

My large-frame 320A LN alt is designed to output well over 250A all day, and the Balmar MC-614 will derate as needed if things get hot.

Maybe I should think about putting its temp sensor on the TB2400? (just kidding but ?)

Quote:
Originally Posted by Andina Marie View Post
As the battery voltage rises the voltage difference gets lower but so does the voltage drop along the cable and as the batteries reach full voltage the current drops to zero, the cable voltage loss drops to zero and the batteries reach full charge.
LFP is damaged getting to that point. My usual endcurrent threshold is .025C, at max 13.9V.

Quote:
Originally Posted by Andina Marie View Post
Based on this a 6 gauge cable would be suitable for a run up to 6 or 8 feet and you can do the calculations to get a gauge for longer runs if you really need to pump in 200 amps.

However under nearly all applications of a Trollbridge the duty cycle is fairly low and there is no need to charge at maximum possible rate. We typically under-size the charging cable specifications deliberately to limit alternator loads to a safer level and save installation cost. Your 200 Ah battery could be charged through an 8 gauge cable over 20 feet and still get fully charged, it would just take (?) minutes longer. For typical bow thruster, or winch type applications this is not a problem.

Resistance in the circuit is much larger than the internal resistance of a Lithium battery, in fact much larger than the internal resistance of a lead-acid battery, so it tends to be self regulating and a little resistance is much cheaper, safer and more reliable than sophisticated current/voltage controls, at the cost of extended charging times.
Since my charge sources sense voltage at the bank, their output voltage will need to be a bit higher than 13.9 if a wire of lower than usual gauge is used. Not an issue though right?

LFP requires that I have already invested in these precise controls.

And there are times when I do want to minimize charging time, since LFP don't need to get to full, reducing the amps rate will increase engine runtime in pretty direct proportion, potentially incurring a significant cost over time.

But that isn't my main concern; I think the flexibility TB offers may be worth it, as long as I'm not creating undue risk of fire.

The Blue Sea / ABYC calculator says 200A over 30' (RT) requires minimum 1GA, ignoring voltage drop.

To keep voltage drop to 3% it goes up to 4/0.

What gauge do you think will be safe for both the wiring and the TB unit?
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