Question about FLA battery charging.

[paulcheung]

Hi Guys,

I have a question about charge the FLA battery bank. Say I have a 48 volt 400AH FLA batteries. When the charge current reported by the Amps meter flow go into the battery is 40 amps at 50 volts, is the energy that will be store into the battery bank equal to same 40 amps at 57 volts?
For example If I charge the battery bank 1 hour with 40 amps when the battery system volts at 50 volts. is the same amount energy saved to the battery for later use compare to when I charge 1 hour with 40 amps at 57 volts?

Thank you.

[thastinger]

The Watts work out the same on my smaller battery bank. The CC will put 20A in when the batteries are at 50V but by the time the battery voltage is up to 57.2V the initial abrord current is down to 15ish Amps then falls on down to 3 or 4A toward the end of the absorb cycle.

[Sunking]

Hi Guys,

I have a question about charge the FLA battery bank. Say I have a 48 volt 400AH FLA batteries. When the charge current reported by the Amps meter flow go into the battery is 40 amps at 50 volts, is the energy that will be store into the battery bank equal to same 40 amps at 57 volts?
For example If I charge the battery bank 1 hour with 40 amps when the battery system volts at 50 volts. is the same amount energy saved to the battery for later use compare to when I charge 1 hour with 40 amps at 57 volts?

Absolutely NOT. Charge efficiency of a FLA battery is 80 to 85% efficient, but Charge Efficiency is only specifying Amp Hours, NOT Energy Efficiency which is measured in Watt Hours. Amp Hours go in at higher voltage than they come out for every battery type. If you look at Watt Hours going In vs Watt Hours going Out Energy Efficiency is very poor in the 70 percentile range.

When we calculate panel wattage required to generate X amount of Watt Hours required is where the Correction Factor comes into play as we are takeing into account system losses. The panel has to be able to generate 1.5 watt hours for every 1 watt hour used in a MPPT system and 2 watt hours using PWM.

Another example Lithium Ion batteries have 100% Charge Efficiency. For every electron you put in you can get out. That makes Coulomb Counters (gas gauge so to speak) very accurate. However because voltage is higher pushing Amp Hours In than Out Energy Efficiency is in the mid 90 percentile range.

[Willy T]

I'd say that as long as your below gassing ( 75-85% ) soc, the voltage has very little to do with efficiency. Once you reach gassing voltage the difference between MPPT and PWM is erased, as both are using PWM and the batteries are gassing and a percentage of the current is being converted to heat and less current is being absorbed. In a 48 V system the gassing voltage is around 55-57 volts temperature corrected.

In this case 57 v is on the edge of is it gassing or not. If it's gassing and it is probably less efficient. Obviously your never going to fully charge the system with 50 v.

[paulcheung]

You guys are misunderstand my question. what I am try to ask is, when I charge the battery bank with 40 amps at 50 volts for one hour. is it equal to when I charge the battery bank with 40 amps at 57 volts for one hour. does the battery bank get the same amount of watts or amp hours in storage for the two different charges?

Dereck, I know the inefficiency of FLA battery system, that is why I ask the question above. if the energy get stored is the same for the two different voltage charge, that maybe one of the reason for inefficiency? higher voltage same result as the resistance increase?

[Sunking]

You guys are misunderstand my question. what I am try to ask is, when I charge the battery bank with 40 amps at 50 volts for one hour. is it equal to when I charge the battery bank with 40 amps at 57 volts for one hour. does the battery bank get the same amount of watts or amp hours in storage for the two different charges?

Dereck, I know the inefficiency of FLA battery system, that is why I ask the question above. if the energy get stored is the same for the two different voltage charge, that maybe one of the reason for inefficiency? higher voltage same result as the resistance increase?

What you are asking Paul, there is no answer without know a few parameters. You can set your controller Set Point Voltages as high as you want, makes no difference. When any charger is in Constant Current Mode aka Bulk the voltage is a product of the Battery Open Circuit plus the voltage drop of the current x the Internal Resistance of the battery.

Example lets say we use a 100 volt supply voltage to charge a 12 volt battery. The 100 volt supply is current limited to 40 amps. Resting battery voltage is 12 volts or fully discharged, and internal resistance = .001 Ohms. When we apply 40 amps of charge current the voltage at the battery is 12 volts + (40 amps x .001 Ohms) = 12.04 volts. That would be 482 watts.

But simple math would answer your Question. Does (50 volts x 40 amps) equal (57 volts x 40 amps). Simplified does 2000 watts = 2280 watts?

Even in Float Chargers which is a Constant Voltage algorithm like Adsorb they are still Current Limited. Using the same example except this time We set Float or Absorb Voltage to 13.6 volts and the charger has a current limit of 40 amps. Same 12 volt discharge battery with open circuit voltage of 12 volts and .001 Oms internal resistance. Initially we charge at constant current of 40 amps. At start the battery voltage is the same 12.04 volts as before. As the battery charges, the battery voltage starts to rise and still receiving 40 amps of current until the battery voltage reaches 13.56 volts. At that point the current will start to Taper Down to 0 amps as the battery voltage approaches the set point voltage of 13.6 volts. When the battery finally reaches 13.6 volts, current stops flowing 0 amps. In practice the current never stops flowing because all FLA batteries have self discharge. In reality depending on age and condition of the battery self discharge current will be roughly .5% of C. Take that fully charged 12 volt battery off the charger and let it rest to bleed off the surface charge and you will see it come to rest at 12.6 volts.

[paulcheung]

What you are asking Paul, there is no answer without know a few parameters. You can set your controller Set Point Voltages as high as you want, makes no difference. When any charger is in Constant Current Mode aka Bulk the voltage is a product of the Battery Open Circuit plus the voltage drop of the current x the Internal Resistance of the battery.


But simple math would answer your Question. Does (50 volts x 40 amps) equal (57 volts x 40 amps). Simplified does 2000 watts = 2280 watts?

.

Is that the end result? in the battery? or just the amount needed to charge the 40 amps?

Thank you Dereck.

[Sunking]

Is that the end result? in the battery? or just the amount needed to charge the 40 amps?

Thank you Dereck.

You are welcome Paul but I am not sure I understand your question.

An AC charger uses a hard source of commercial power so when used as a Constant Current Source the current Stays Fixed like 40 amps until the battery Voltage plus IR voltage equalize. At that point both current and power taper off. With a hard source the metric work differently Say I have a 40 amp 48 volt Float battery charger. Well a 48 volt Float Charge output voltage is 52 volts and limited to 40 amps maximum. You connect a discharge battery say resting at 48 volts with .001 Ri and the charger voltage has to fall back to 48.04 volts. So initially that is only 40 amps x 48.04 volts = 1920 watts. As the battery chargers the battery voltage goes up but current remains fixed at 40 amps. Rising voltage x fixed current = rising wattage. That will continue until the battery voltage reaches 52.96 volts x 40 amps = 2120 watts. So you started at 1920, and finished at 2120 in a pure Constant Current mode form a hard commercial source.

Solar is not a hard source and hence in Bulk Mode technically is Constant Current but in reality Constant Power Source. The controller acts like an AC Transformer where Power In = Power Out. That is a Power Converter or transformer. For example let's say you have a 2000 watt panel system with enough sun to drive it to 200 watts with the same battery. We start charging at 48 volts. 2000 watts / 48.04 volts = 41.6 amps of charge current. Just before we reach the 52 volt set point the current actually drops because voltage went up. 2000 watts / 51.96 volts = 38.5 amps.

Understand the difference?



Hoped that helped.

[paulcheung]

Thank you Dereck,

Do you notice Trojan L16RE-B data sheet has the graph regarding the depth of discharge and the cycle live are all equal, from 20% to 80%, if you convert the cycle and the depth of discharge to watts they are all the same for all 20% to 80%?

http://www.trojanbattery.com/pdf/datasheets/L16REB_TrojanRE_Data_Sheets.pdf

[Sunking]

Thank you Dereck,

Do you notice Trojan L16RE-B data sheet has the graph regarding the depth of discharge and the cycle live are all equal, from 20% to 80%, if you convert the cycle and the depth of discharge to watts they are all the same for all 20% to 80%?

http://www.trojanbattery.com/pdf/dat...ata_Sheets.pdf

Not sure what you are saying Paul.

[paulcheung]

Not sure what you are saying Paul.

If you discharge the battery to 20% DOD and discharge to 80% DOD and get the same amount of watts in the battery life. For example the L16RE-B has 4000 cycle at 20% DOD and has 1000 cycle at 80% DOD, you get the same amount KWH for it's life no matter how deep you discharge as long you stay in 20 to 80%.

[SunEagle]

If you discharge the battery to 20% DOD and discharge to 80% DOD and get the same amount of watts in the battery life. For example the L16RE-B has 4000 cycle at 20% DOD and has 1000 cycle at 80% DOD, you get the same amount KWH for it's life no matter how deep you discharge as long you stay in 20 to 80%.

Ok. With an 80% DOD you can get more kwh output "per cycle" but have to replace the batteries in 1/4 the time then only going using a 20% DOD. That math works on the table but will it work in real life?

My fear is that going below 50% DOD puts more strain on the battery that could result with less then the listed cycle count.

Just about all types of electrical devices have efficiency curves that are not linear. The same goes for their lifespan based on usage. Harder usage usually means much shorter life where that "curve" is not a straight line.

Maybe there is a happy medium between 20 and 80% that gets you the best bang for your buck.

[Willy T]

Ok. With an 80% DOD you can get more kwh output "per cycle" but have to replace the batteries in 1/4 the time then only going using a 20% DOD. That math works on the table but will it work in real life?

My fear is that going below 50% DOD puts more strain on the battery that could result with less then the listed cycle count.

Just about all types of electrical devices have efficiency curves that are not linear. The same goes for their lifespan based on usage. Harder usage usually means much shorter life where that "curve" is not a straight line.

Maybe there is a happy medium between 20 and 80% that gets you the best bang for your buck.

I agree with you, that sweet spot is usually expressed somewhere between 50 - 60 %. The other charging scheme is to charge to 85 % daily and absorb once a week to 10 days to 100%. I haven't figured it out in 30 years of being part time off grid. The weather usually dictates what I do. Running the Generator once week sometimes works out just for fuel costs alone.

[SunEagle]

I agree with you, that sweet spot is usually expressed somewhere between 50 - 60 %. The other charging scheme is to charge to 85 % daily and absorb once a week to 10 days to 100%. I haven't figured it out in 30 years of being part time off grid. The weather usually dictates what I do. Running the Generator once week sometimes works out just for fuel costs alone.

There you go again blaming the weather on less solar capture.

My guess as battery technology improves then hopefully that sweet spot increases.

[Sunking]

I agree with you, that sweet spot is usually expressed somewhere between 50 - 60 %. The other charging scheme is to charge to 85 % daily and absorb once a week to 10 days to 100%. I haven't figured it out in 30 years of being part time off grid. The weather usually dictates what I do. Running the Generator once week sometimes works out just for fuel costs alone.

That would be a job for lithium.