I have an ACR and two identical FLA's. If the SOC is lower on one battery, can it make the charging circuit in the alternator think that the batteries are charged even if one is not? I'm guessing the total voltage would be equal to the battery with the higher voltage unless left for long enough for them to equalize?
ACR's and SOC
- Thread starter Project_Mayhem
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The ACR is normally open, meaning the batteries are not connected. When the voltage on either side of the ACR reaches a predetermined value, the ACR closes and the batteries are connected. This will occur when one battery or the other is being charged. Once the voltage drops below that value the ACR opens again and isolates the battery, because charging has stopped.
Note: I can never keep the language straight on relays, where I said open, the proper word might be be closed and vice versa. Nonetheless, voltage above resting voltage will connect the 2 batteries. This is why ACRs won't work on LFP and LA batteries, the resting voltage on a LFP is too high leaving the batteries connected, that and the charging profiles are quite different. There is a chart on the Blue Sea website showing the voltages for combining and opening.
www.bluesea.com
Note: I can never keep the language straight on relays, where I said open, the proper word might be be closed and vice versa. Nonetheless, voltage above resting voltage will connect the 2 batteries. This is why ACRs won't work on LFP and LA batteries, the resting voltage on a LFP is too high leaving the batteries connected, that and the charging profiles are quite different. There is a chart on the Blue Sea website showing the voltages for combining and opening.
SI-ACR Automatic Charging Relay - 12/24V DC 120A - Blue Sea Systems
Automatically combines batteries during charging, isolates batteries when discharging and when starting engines.
www.bluesea.com
I'm guessing that's unlikely, I think that it 'could' happen if:
-the fully charged battery is the one connected to the alternator,
-the smart alternator has a lower voltage when it would switch into charging mode than the float voltage,
-the ACR doesn't kick in at float voltage, and has a delay before it turns on.
To anthropomorphize, the alternator would start charging and say, "hey we are charged up, back to float voltage" before the ACR recognized a charging voltage.
It is unlikely since you probably just started your engine, so the fully charged battery is no longer fully charged, and the alternator and ACR start charging.
lets look at some hypothetical numbers.
#1 battery is charged [12.6.v] and #2 is discharged [11.0v].
Scenario #1. Alternator connected to #1.
When you start the engine and the alternator starts generating current, the voltage on the fully charged battery #1 will quickly go to more than 13.0v and the ACR will close, and the batteries will be paralleled. At this time the voltage of both batteries will come close being the same with #1 a bit higher than #2. The result is that some of the charged energy in #1 and all of the current from the alternator will go to #2 until #2 gets charged enough to match the voltage of #1. After that, the current output from the alternator will be divided between #1 & #2 with most of the current going to #2. As #2 begins to fill up, the current it will accept reduces until both batteries are fully charged. The alternator will be suppling as much current as it can until its output voltage hits the set point and after that, it will maintain the set voltages and reduce the current to hold that voltage.
Scenario #2. Alternator connected to #2.
When you start the engine and the alternator starts generating current, the voltage on the discharged battery #2 will not get to more than 13.0v for a while and the ACR will remain open until it does. The batteries will be isolated until #2 gets to 13.0v which they will be paralleled. At this time both batteries will have approximately the same voltage but most of the current will still be going to #2 because it is still at a lover SOC. After that, the current output from the alternator will be divided between #1 & #2 with most of the current going to #2. As #2 begins to fill up, the current it will accept reduces until both batteries are fully charged. The alternator will be suppling as much current as it can until its output voltage hits the set point and after that, it will maintain the set voltages and reduce the current to hold that voltage.
Both scenarios work fine and will put most of the charge to whichever battery is at the lower SOC.
Note that this is for FLA batteries, but an ARC is completely inadequate for charging LFP batteries because they would be parallel 90% of the time even when there was no charge current. The resting voltage of an LFP even at lower SOC is higher than the 13.0v "close" voltage and they deeply discharged before they go below the 12.75v "open" voltage.
#1 battery is charged [12.6.v] and #2 is discharged [11.0v].
Scenario #1. Alternator connected to #1.
When you start the engine and the alternator starts generating current, the voltage on the fully charged battery #1 will quickly go to more than 13.0v and the ACR will close, and the batteries will be paralleled. At this time the voltage of both batteries will come close being the same with #1 a bit higher than #2. The result is that some of the charged energy in #1 and all of the current from the alternator will go to #2 until #2 gets charged enough to match the voltage of #1. After that, the current output from the alternator will be divided between #1 & #2 with most of the current going to #2. As #2 begins to fill up, the current it will accept reduces until both batteries are fully charged. The alternator will be suppling as much current as it can until its output voltage hits the set point and after that, it will maintain the set voltages and reduce the current to hold that voltage.
Scenario #2. Alternator connected to #2.
When you start the engine and the alternator starts generating current, the voltage on the discharged battery #2 will not get to more than 13.0v for a while and the ACR will remain open until it does. The batteries will be isolated until #2 gets to 13.0v which they will be paralleled. At this time both batteries will have approximately the same voltage but most of the current will still be going to #2 because it is still at a lover SOC. After that, the current output from the alternator will be divided between #1 & #2 with most of the current going to #2. As #2 begins to fill up, the current it will accept reduces until both batteries are fully charged. The alternator will be suppling as much current as it can until its output voltage hits the set point and after that, it will maintain the set voltages and reduce the current to hold that voltage.
Both scenarios work fine and will put most of the charge to whichever battery is at the lower SOC.
Note that this is for FLA batteries, but an ARC is completely inadequate for charging LFP batteries because they would be parallel 90% of the time even when there was no charge current. The resting voltage of an LFP even at lower SOC is higher than the 13.0v "close" voltage and they deeply discharged before they go below the 12.75v "open" voltage.
This won't happen because of the way smart charging works. First, this only applies to LA batteries not LFP. Second, once combined the batteries are electrically one battery as they are paralleled. Third, this is for the Blue Seas ACR
When a smart charging source (charger or regulator) starts, it starts with the Constant Current (CC) stage. In this stage it produces as much current as possible initially at a low voltage and then as the battery becomes more charged the voltage increases. When the voltage reaches 13.0v for 90 seconds the ACR or 13.6v for 10 seconds, the ACR closes and combines the two batteries. Once combined more current will flow to the battery with the least resistance which is the more deeply discharged battery while some small amount of current flows to the other battery. Once the voltage in the circuit reaches the Absorption voltage, the charging source will shift to the Constant Voltage (CV) stage during which the voltage remains constant and the current drops. The ACR remains combined until the voltage is 12.75v for 30 seconds or 12.35v for 10 seconds. These voltages could only be attained if the both batteries are fully charged, the charging source is no longer charging, or there is a large electrical load placed on the system so that the battery voltage drops.
Also bear in mind, when charging the batteries are simply a load on the electrical system, no different than any other device that might be operating at the time. Thus, if both batteries are fully charged, the devices will continue to draw current from the charging source and keep the charge voltage above the cutoff voltages.
This really isn't the problem everyone gets excited about. Using a fixed voltage ACR like described, the lead battery will be paralleled to the LFP and held at 13.2-13.4V most of the time. However, this range is the voltages that most lead batteries are designed to float at for longer term storage, so all that is happening is the lead batteries are being held at float voltage most of the time - just like they would if they were on a separate lead charger. This is particularly true for AGM batteries. FLA's have a slightly higher float than that, but being a bit low won't be any problem.
From the LFP point of view, they don't even see the lead battery until the LFP SOC gets into very low discharge territory.
So the lead thinks "I'm on float charge most of the time" and the LFP thinks "I'm here all alone".
Mark
Mostly true except that it takes energy to hold the FLA at 13.2v to 13.4v range. As you said that FLA will be happy getting the float charge, but that charge is coming from the LFP. If you are constantly on shore charge, this might not be a problem. If you are living off grid, you will have a continuous load on your LFP bank which increases the charging requirement.
I do not plan my system for when I am at the dock. I plan it for when I am at anchor.
Since the resting voltage of LFP is within the range of the ACR combining, the batteries will be combined even when there is no charging source. The LA battery will be a drain on the LFP battery when it is not necessary due to the self-discharge rate.
Thank you for a thorough and straightforward answer
This defies logic - The device with the lowest resistance should draw the most current. The behavior of a capacitor comes to mind. One with no charge, is close to a dead short in many scenarios
What's the advantage of slowly increasing the voltage?
I believe that you are confusing effect with cause. Your charge source does not increase the voltage. It only controls the current, and the voltage is the result of the internal resistance of the battery and the current being sent to it. The slow increase in the voltage is due to the fact that the internal resistance of a battery increases as the battery gets to a higher state of charge. The battery starts out being charged in the constant current mode where all of the current that the charger can supply is not sufficient to raise the voltage to the set point of the regulator. As the SOC increases, so does the internal resistance. At some point, the max current will push the voltage to the set point. Then the charger changes to constant voltage and gradually reduces the current to prevent the voltage from going over the set point.
That is exactly what I said, the battery with the least resistance will get more of the current. Simple Ohm's Law. I don't see any distinction between least and lowest in this case.
What Hayden said. Again, it is Ohm's Law in action.
This is where the practical gets over-shadowed by the theoretical, and people go down rabbit holes. This seems most unique to LFP discussions for some reason.
Here is how this works in a practical way in the real world. Our two FLA start batteries are currently paralleled with the LFP house bank. I just measured the current passing from the LFP to the FLA. One of them is taking 3mA, and the other 1mA to stay at the 13.2V the LFP is currently at.
But let's consider this throughout a LFP charge cycle. My FLA batteries apparently require 0.4W to stay on float (I'm using the 3mA one as worst case). So as the LFP discharges down to 12.75V, the FLA will draw 3.2mA from it, while as the LFP charges to 14V, the FLA will only take 2.9mA.
Basically, 3mA no matter what is happening with the LFP. I stand by my statement that the LFP doesn't even know the FLA is there. Well, until the LFP has been drawn down below 12.7V, but then the FLA is a bonus, not a hindrance.
3mA is the amount of draw a single tiny indicator LED on a panel takes, but nobody gets twisted about that, or even considers it in a power budget or system design.
Our system is planned for at anchor and underway, since that is the majority of our time. I am concerned about a lot of other power draws, but not this one.
Before staying with your hypothesis, I urge you to actually do the experiment to support it.
Mark
The current drain is a little higher than 3ma if the ACR's 175 ma current draw is included. Over a 24 hour period that's about 4ah, for a large battery that is frequently recharged, it is fairly negligible. On the other hand for weekend warrior sailors with smaller banks that draw of 40+ ah is more substantial.
The bigger issue with using an ACR is the mismatch between charging profiles for LFP and LA. The ACR will leave the LA battery chronically undercharged. While the absorption voltage for some AGMs comes close to matching the absorption voltage for LFPs, LFPs spend little time at absorption voltage. This can lead to LA batteries sulfating and living a short life.
The better solution is a DC-DC charger that can match the LA battery's charging profile. A small 18a Victron DC-DC charger does not cost much more than an ACR and will lead to longer battery life. The charger only draws 80 ma under no load, thus the parasitic load is lower than the ACRs.
That is a good point about the ACR draw. I just measured ours and it is 120mA. I suspect the 175mA spec is a maximum possible. So that is 2.8Ah over a 24h period. Compared to the example of the DC-DC parasitic load, the ACR is using <1Ah more per day.
If one is concerned over <1Ah per day, particularly with LFP batteries, then one has made a mistake in their design and outfitting.
Your bigger issue is again a theoretical one that appears to not be based on actual practical measurements. Let's examine it closer with practical experience.
LFP manufacturers give charging recommendations of 14.4-14.6V. Some of them recommend a short absorption period, while others do not. Most of them recommend a float voltage of 13.5-13.8V. (As an aside, I've never seen a manufacturer's recommended charge routine I agreed with).
So it is not true that there is a large mismatch between LA and LFP charging profiles. This is even more acutely so for start batteries, which are never run below 99.9% SOC when starting an engine. Even hard start engines will leave the start battery at 99% SOC.
Not to put too fine a point on this, but automobile alternators charge at a continuous 14V all day long. Car batteries seem to have no problem with this "chronically undercharged" profile.
Moving on to actual practice, our $50 Walmart start batteries are solely charged by whatever they take from the LFP. They never see more than 13.8V, and spend most of their time at 13.2V. They are now 6yrs old and still cranking hard. They show no sign of chronic undercharging or sulfating. If they crap the bed tomorrow, then I'd consider 6yrs a good lifespan.
I would also wager that they would not live any longer on a DC-DC charger.
Mark
Edit: I meant to make the point that we use programmable ACR's, so they can shut down and turn on at any arbitrary voltage chosen for them. I do have them shutting down at 13.0V, but if I made that higher, then the parasitic draw would be almost nothing whenever an active charge source wasn't active.
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