Float current limit

Deveak_Kaizen · May 2020

Not sure how to phrase this properly for the google so i came to ask here.
I have a 24 volt roughly 800 AH battery bank of trojan l16s. I have a 300 amp alternator I want to turn into a alternator charger for my bank and generally use to keep them topped up. Fully charged and in float, how many amps can I use from the battery? If i was to use say the full 300 almost 8400 watts that would be in far excess than the max charge rate of somewhere around 80 amps. Would a battery in float basically be bypassed and the current go directly from the alternator to the inverter or would this heat my batteries up, discharging and recharging at the upper end at a high rate?

mike95490 · May 2020

The Charge Controller is what limits the voltage/current to the battery. The voltage regulator for your 24V alternator should be able to handle that.

The other side of the coin is, as the battery charges up and the regulator starts limiting the charge to the battery, the wind turbine becomes less loaded (less battery charge) and so it's RPM will start to increase and the voltage output from the alternator can increase.

Something has to slow the turbine down, or it will start loosing pieces from centrifugal force overspeed.

Deveak_Kaizen · May 2020

mike95490 said:

The Charge Controller is what limits the voltage/current to the battery. The voltage regulator for your 24V alternator should be able to handle that.

The other side of the coin is, as the battery charges up and the regulator starts limiting the charge to the battery, the wind turbine becomes less loaded (less battery charge) and so it's RPM will start to increase and the voltage output from the alternator can increase.

Something has to slow the turbine down, or it will start loosing pieces from centrifugal force overspeed.

Sorry I did a poor job of explaining. The alternator will go on an engine. I am worried that when i am in float and using things via my inverter I will cook the battery by over using. Such as using my central air. I don't know what the max discharge rate for flooded lead acid is but going by 10% charge rates I'm looking at 80 amps is the max rate it can be charged. I'm asking if while in float with an alternator supplying up to 300 amps of power via an engine, I use say 250 amps continuously, will it cook the battery since that is a very high discharge rate or does the alternators power just bypass the battery and go straight to the loads?

BB. · May 2020

Charging current for flooded cell lead acid deep cycle batteries is generally in the range of 10-20% of 20 Hour AH rating. And as long as the charging voltage setpoint is not too high, generally should not cause the batteies greef (once the battery reach 80% or so state of charge, should hit set point voltage / absorb and naturally reduce charging current as the battery bank goes >~90% State of Charge.

For float voltage--There really is no current limit because the battery bank should not be accepting any more current for charging--Other than self discharge amounts (1% or much less less float rate of charge). The high current draws would because of other loads on the DC Bus (AC inverter, water pumping, etc.).

Roughly, if you see "float current" exceeding 2% float charging--Either your float voltage is too high, or the battery bank is pretty much needing to be replaced. At 2% rate of charge continuous (hours/days/etc.), that is a problem. Battery bank will overheat and worse ("boil" electrolyte dry, could catch fire, etc.).

-Bill

Deveak_Kaizen · May 2020

I see, so me using up all that power won't hurt it than since the batteries are no longer taking it? so its just passed along to the loads?

mcgivor · May 2020

There has to be something to regulate the voltage, if the alternator has an internal regulator it will not provide a float, it is in essence a single stage charger.

Once the battery reaches the regulation voltage say 28.8 V the current will begin to drop but not necessarily be fully charged, depending on the size of load diversion it may prevent the battery from ever being fully charged. The best method would be to use loads only after the battery reaches fully charged state then transitions to float. Since there is no multi stage charging this would not be practical, furthermore keeping the voltage at 28.8V for extended periods of time will be damaging to the battery itself. Some sort of charge controller is what's needed to make your proposed system work correctly.

BB. · May 2020

The float voltage does not charge the battery bank, and if you have stable DC current, the battery does act a little like a buffer/capacitor to even out small spikes.

Where you get a little more of an issue... Is the ripple current from a single phase AC inverter. They typically draw a 120 Hz Sine Squared current waveform. Peaks when the inverter is outputting current at the 120 VAC sine wave peak. And near zero current when the AC waveform voltage crosses zero volts.

The theory is that you want the float voltage to hold the average battery voltage high enough that the "current peaks" don't take the battery below ~12.7/25.4 volts and cause "micro discharge" or "mico cycling" of the battery bank (ripple voltage/ripple current).

-Bill

Deveak_Kaizen · May 2020

Well My alternator has a adjustable regulator. 27/28/29 volts. Would a higher voltage help avoid the micro cycle? Its kind of going to work like a car I guess. I might run it 12-14 hours tops and shut it off for the night. I know car alts are 14/28 volts on average.

Photowhit · May 2020

I'd guess your battery bank is way undersized for a continuous 250 amp load, Batteries are rated based on a 20 hour discharge, when discharged quicker the available capacity becomes less. Trojan publishes these numbers to some extent.

Ampacity Discharge over time for L16H-AC, they make a bunch, easy to look up the specs for yours;

Image: https://us.v-cdn.net/6024911/uploads/editor/8t/3zrjq927o7c7.jpg

Assuming you won't run major loads, Central Air conditioning, until you are running the motor I Would worry about your inverter and it's ability to make a quick jump drawing from your battery bank, before the voltage regulator on the motor sense the draw and provides more power. In addition, I'd be curious what inverter you intend to use. You would want something at least able to cover the largest load you've mentioned 8400 watts, plus the inverters efficiency. Drawing just 300 amps You will want some stout cables. Something like kcmil 300-350...

Is there some method to your madness? It will be very expensive electric. Guess you will be rural and away from the grid?

Dave Angelini · May 2020

One can do a scheme like this for some time as as long as there is a plan to correct the design. Micro cycle charge/ discharge is a known way to shorten battery life. Being able to see a system over 24 hours really makes it alot easier. The graph below is only 12 hours but it can be 24 or longer.

Image: https://us.v-cdn.net/6024911/uploads/editor/hh/skcpp5oa1gfd.png

Deveak_Kaizen · May 2020

Photowhit said:

I'd guess your battery bank is way undersized for a continuous 250 amp load, Batteries are rated based on a 20 hour discharge, when discharged quicker the available capacity becomes less. Trojan publishes these numbers to some extent.

Ampacity Discharge over time for L16H-AC, they make a bunch, easy to look up the specs for yours;

Assuming you won't run major loads, Central Air conditioning, until you are running the motor I Would worry about your inverter and it's ability to make a quick jump drawing from your battery bank, before the voltage regulator on the motor sense the draw and provides more power. In addition, I'd be curious what inverter you intend to use. You would want something at least able to cover the largest load you've mentioned 8400 watts, plus the inverters efficiency. Drawing just 300 amps You will want some stout cables. Something like kcmil 300-350...

Is there some method to your madness? It will be very expensive electric. Guess you will be rural and away from the grid?

I already am off grid. I have a 4000 watt magnum inverter. I put a soft start on my 3 ton ac. I have free natural gas. The reason I would like to do this is because i can use any motor i want and run it at any speed i want. I'd like to run a 8-13 hp engine at 1800 rpm and just turn the speed up when i need it.

Float current limit

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