Controlling charge from multiple sources to keep LiFePo4 batteries from continual 100% charge

[Backdoctor]

I have installed 2x280AH lithium batteries in my motorhome. They have a max charge rate of 100 amps each, which I’m guessing would make for a total of 200 amp max as they’re paralleled. In addition I added small (25w) thermostatically controlled heatpads as I live in the Great White North! I have a victron 712 Battery monitor, a 225 Li-BIM, a Xantrex 3000 Freedom Pro Inverter Charger (that will charge at 150 amps) and 580 watts of solar on the roof with a Jaboni (POC) MPPT solar charge controller (programmable only for algorithm, not set points). The alternator capacity is 185 amps.

From what I’ve been reading, it would be best to limit the SOC of the batteries to 80-90% much of the time and only allow them to charge to 100% occasionally, like once per 2-4 weeks to balance them. My problem is multiple charge sources that each have different algorithms. The Xantrex is programmable, and I have the float set to 13.6v (perhaps that should be lower?). But the solar has the charge programme set to lithium and will charge to 14.4v whenever it can, and the alternator (185 amp capacity) will charge to 14v anytime the engine is running In 15-20 minute spurts.

Is there a way of controlling the incoming charge so that it only starts after the batteries fall to say 13.1v so I can get down into the 40% SOC? I’m thinking the alternator and solar are the main concerns.

Thanks for any help you can offer. I realize this is a bit unusual, as most times you want to maximize charge, but I have an 8kw generator for times when I’ve let the charge fall too far and need to recover rapidly!

 

[sunshine_eggo]

80-90%: You can't do this based on voltage. Even at 3.4V/cell, they can get to 95%+.

If you're looking to maximize cycle life, charge to 3.45V/cell and allow for a 2 hour absorption. You'll get to 98%+ SoC, and you'll reduce charging stress by hitting a lower voltage at lower current.

 

[John Frum]

Its best not to charge higher than .5c for longevity.
In your case that is 100 amps.
The other good thing about charging at 100 amps it is within spec for 1 battery for the case where 1 battery goes offline.
For alternator charging the best known method is a dc2dc charger which should give you fine grained control of the charge profile.
The alternative(see what I did there) method is to use a fancy alternator with regulator.

Charging into the high knee is a necessary stressor, especially if you have b grade cells(which you probably do).
Its necessary in order to maintain top balance.
Discharging into the low knee is typically an un-necessary stressor.

 

[Backdoctor]

Its best not to charge higher than .5c for longevity.
In your case that is 100 amps.
The other good thing about charging at 100 amps it is within spec for 1 battery for the case where 1 battery goes offline.

Charging into the high knee is a necessary stressor, especially if you have b grade cells(which you probably do).
Its necessary in order to maintain top balance.
Discharging into the low knee is typically an un-necessary stressor.

Thanks John. I’m not sure I know what the high knee and low knee are exactly. I suspect it is describing the charging profile (curve) of the battery as it charges and discharges? Would that be correct?

 

[hwse]

The high/low knees are where the voltage/current curves go from flat to steep and relates to the cells. 955 of the capacity of an LFP is between 13.4v and 12.6v. Once your batteries get above 13.4v you are almost full and below 12.6v almost empty.
I have 920Ah of LFP in my class A. I suspect that most of your loads are actually AC rather than DC like my coach. Given that, when you are in a full hookup with that much solar, I would turn off the charging from the Xantrex and let your solar charge the batteries. I will assume that they have Bluetooth so you can check them from time to time to make sure the solar is keeping up. Your 50A shore connection will still power all of the AC loads like refer, heating and such. I would also change the solar profile to absorb at 13.8v and float at 13.4v to keep from pushing to the limits. As you said, LFP is happier in the middle rather than at the top and bottom of its range.

Rather than turning off the Xantrex charging, you could also set your charge voltage to 13.0v so that it only comes on when the solar cannot keep up.

I found that I did not need the LiBim. My batteries are in the compartment in front of the front axle and that means that there is about 90' of wire between the battery and the alternator round trip. Even with the 2/0 cable that Tiffin uses I never see any more than 60A going to the battery so that keeps the alternator from working too hard. It also has very good air flow, so I am not too worried about it over heating.

 

[12VoltInstalls]

Even with the 2/0 cable that Tiffin uses I never see any more than 60A going to the battery so that keeps the alternator from working too hard

The length between alternator and battery is resistance, resistance is a loss due to converting electricity to heat. I’m thinking your alternator is seeing a lot more amps than you imagine. 60A of charging along with the voltage drop losses; they are losses because they are…lost.

 

[hwse]

The length between alternator and battery is resistance, resistance is a loss due to converting electricity to heat. I’m thinking your alternator is seeing a lot more amps than you imagine. 60A of charging along with the voltage drop losses; they are losses because they are…lost.

The current is not lost along the path, the voltage is. So, on a long wire round trip, the current is the same all locations along the wire but at the source the voltage is much higher than at the load. What the long length does is makes for a large voltage drop between the alternator and the battery. High loss means that the even though the voltage at the battery is reading, 12.3v, the alternator is actually reading 14.2V. At that voltage, the internal regulator will be throttling the output down to keep the output to its set point of 14.2 which is why it is only sending 60A to the battery.

 

[12VoltInstalls]

battery is reading, 12.3v, the alternator is actually reading 14.2V. At that voltage, the internal regulator will be throttling the output down to keep the output to its set point of 14.2 which is why it is only sending 60A to the battery.

Ok. Most 12V alternators will do into 15.x+ Volts in my experience with winches/off-roading. In my own jeep, the ~85A alternator could be ~16V+ with heavy loading. Whatever the voltage no heat happens without current.
Is the alternator only 60A? Or 140A or…

I’m just saying that the alternator may be seeing a much larger load. The alternator only sees a load; the 60A limit suggests in my small mind that the sum of the house battery charging load and cable losses summed would be the actual load on the alternator.

I’ve installed a number of high-amp cabling and modified charging systems for trailers and off-road vehicles. A 140A alternator after a long winching pull that draws the battery way down will fairly predictably trip a HD 150A DC circuit breaker. (Winch cables not in line with alternator but parallel to protect alternator battery cables)
Whatever the cable voltage drop from battery to alternator is, the alternator is maxed out amps and volts- just pulled down voltage from the insane load. And even 2/0 cables get hot; not dangerously hot if crimps and connections are done well.

Fwiw I’m not arguing with you. I’m learning if possible. Two things crossed my mind when I read your post.

1) In my experiences (like long cable runs for trailer winches, recovery trucks) the alternator is often still maxed with a surprising attenuation of the amps at the battery.
2) a breaker close to the alternator/vehicle charging trips more often than a breaker located at the winch; after a long cable run I’d ‘expect’ the opposite to be true (amps higher, voltage lower at the winch proportionately).

So that suggests to me that the cable losses can be a significantly larger load to the alternator than what happens at the final point of the load.

I guess this is a bit of a thread departure- sorry- but my past observations suggest different results than what you mentioned so I was hoping for a little science on that.

 

[hwse]

There is a lot to unpack here but I will give it a try.

[A] Ok. Most 12V alternators will do into 15.x+ Volts in my experience with winches/off-roading. In my own jeep, the ~85A alternator could be ~16V+ with heavy loading. Whatever the voltage no heat happens without current. (true. Power is what creates heat and the power equation is P=Iv)
Is the alternator only 60A? Or 140A or…

[A] An alternator should never go to a higher voltage than what it is set to no matter how big the load. If the SOC (state of charge) of the battery, the alternator will put out its maximum power in Watts.

• For simple numbers let's say we have a hypothetical alternator rated at 100A @ 12.0v at a rated 2000-RPM which equals 1200W which is the max power that it can produce. Let's also say that the regulator is set to 12.0v. 1200W is the power produced when the field current is at 12v. If the voltage at the regulator is less than 12v, the power produced will be less than the rated 1200W @ 2000rpm because the field current is lower than the rating.
• If the regulator (internal or external) sees any voltage less than its set point of 14.0v it will send full voltage to the field windings of the alternator to produce as many watts as it can. Per Ohms law, V-Ir so if the alternator is putting out its full power of 1200w and the battery is at 12.0v you should see 100A of output at the alternator output post. The battery will see less than that due to the other loads on the system.
• Once the regulator sees 14.0v it will reduce the current going to the field windings which will reduce the power from 1200W down to whatever is needed to prevent the voltage from going any higher than the set point of 14.0v. If the regulator is doing its job, you should never see more volts than that. If on your system with an internal regulator, you get more than the set point which is ussually around 14.2v then something is wrong with it.
• With a nonadjustable internal regulator, you can find the set point by running the engine at cruise rpms when the battery is full and there are no big loads on the system and the voltage in those conditions is the set point.

I’m just saying that the alternator may be seeing a much larger load. The alternator only sees a load; the 60A limit suggests in my small mind that the sum of the house battery charging load and cable losses summed would be the actual load on the alternator. (true)


I’ve installed a number of high-amp cabling and modified charging systems for trailers and off-road vehicles. A 140A alternator after a long winching pull that draws the battery way down will fairly predictably trip a HD 150A DC circuit breaker. (Winch cables not in line with alternator but parallel to protect alternator battery cables)

• The alternator does not "see" load. It only monitors voltage and adjusts the power that it produces to get as close to the set point as possible without going over. The winch in your system does not care where the power comes from, but it will take all that it is built for. As it nears the top of its capacity, the motor will begin to stall and that increases the power that it consumes. The highest draw it will ever pull is when connected to a load under tension and you hit the power button. This is called a locked armature state produces an inrush current that is far greater than consumed when running at full rated load. When you power the winch from the battery only the battery supplies all of the power. When the engine is also running, the alternator will add as much additional power to supply the load of the winch to reduce how much is pulled from the battery.
  

Whatever the cable voltage drop from battery to alternator is, the alternator is maxed out amps and volts- just pulled down voltage from the insane load. And even 2/0 cables get hot; not dangerously hot if crimps and connections are done well.

Any time there is a current flowing through a conductor it will produce heat which is equal to P=Iv or to write it in another form by converting volts to IR per the Ohms law and substituting, P=I^2 x r. If resistance is very low from a gigantic conductor, the very little heat will be produced but it is still there but likely cannot be measured outside of a laboratory. On the other hand, if you connect a small 14-guage wire which has high resistance across the + & - terminals of a big battery, will flash white-hot and vaporize to plasma.

Fwiw I’m not arguing with you. I’m learning if possible. Two things crossed my mind when I read your post.

1) In my experiences (like long cable runs for trailer winches, recovery trucks) the alternator is often still maxed with a surprising attenuation of the amps at the battery.

• I would suggest that you use a DC clamp ammeter to check the current in your wires.
• Set your winch up with a vertical pull on a given load that that you can duplicate the load from one test to the next.
• Check it at the point closest to the battery / alternator that is beyond the point where the last load branches off and you only have the winch downstream.
• Then check it again as close to the winch as possible.
• What you should find (as long as there are no stray current losses between the two test points) is that the meter will read the same number of amps at both places.
• Then check the voltage across the the + & - at the same two points. What you should find is that there is a mearuable lower voltage at the winch than at the battery end. This difference is due to the loss caused by the resistance of the total differnece in the length of the wires to the motor. The difference will equal v = I x r and you can calculate the resistance of the wiring by dividing the difference in voltage on the two tests by the amperage.

2) a breaker close to the alternator/vehicle charging trips more often than a breaker located at the winch; after a long cable run I’d ‘expect’ the opposite to be true (amps higher, voltage lower at the winch proportionately).

This makes sense. breakers trip from heat. Even though you have the same amps a the winch and the battery end breakers, they are not seeing the same power so the one with lower power will be cooler and trip later. P= Iv so for the same current and a lower voltage due to the voltage drop in the long run of wires, that breaker will have less heat building up inside of it so it will trip later.

So that suggests to me that the cable losses can be a significantly larger load to the alternator than what happens at the final point of the load.

The alternator will always have a higher load than is seen at the end of a long run that has a very large power draw. In your case, that load is the winch. In the case of my motorhome that load is the battery that is discharged to a voltage lower than the set point of the regulator. In my case, I do not see anything more that 60A being passed into the 920Ah LFP pack even when the pack voltage is very low in the 12.0v range.

The engine is a diesel with a seperate start battery consisting of two Group 31 starting LFA batteries. Because it is a diesel, it uses very little 12v power to stay running consisting only of the ECM and driving lights which are all LED. Given that, except for the few minutes that the alternator needs to fully replace the power consumed by actaully starting the engine, almost everything that the alternator produces will go to the battery to be distributed to the rest of the coach. Since I am only seeing 60A (at the BMK battery monitor shunt on the house primary ground cable, that is the maximum current that the wires can supply with 14.2v at the engine. I have seperate monitors for engine and start batteries, and within minutes of starting the start battery is showing 14.2v which means that the regulator has reduced the power that the alternator is producing to maintain that voltage. At the same time, I see voltages from 12.5v which increase fairly rapidly to about 13.2v and receiving the 60A current. This means that I have a resistance in all wires and connections from the coach transfer solenoid to the house monitor which is equal to r = v/I
(14.2v - 13.0v) / 60A = 0.2phms of total resistance. This is a lot of resistance in a wire but remember this total run is about 90'. The total heat in watts put into the 90' run of wires is 1.2v * 60A = 72watts and you will never be able to feel that.

I guess this is a bit of a thread departure- sorry- but my past observations suggest different results than what you mentioned so I was hoping for a little science on that.

I think that your observations are confusing power (W), current (I), voltage (v) and resistance (r).

 

[hwse]

To get back to each of your questions. And FYI, I do not have any Xantrex products so do not know that they can and cannot do so I will be talking about my Magnum system for names. I believe that the Xantrex 3000 Freedom Pro Inverter Charger allows you to create custom charge profiles so most of this should apply but might have different names.
Disclaimer: These are only my opinion and YMMW. ;>)

I have installed 2x280AH lithium batteries in my motorhome. They have a max charge rate of 100 amps each, which I’m guessing would make for a total of 200 amp max as they’re paralleled.

With your system, the max current from all charging sources combined should not exceed 200A and I would probably drop that to not more than 90% of that for some headroom due to differences between them.

In addition I added small (25w) thermostatically controlled heatpads as I live in the Great White North!

This does not really play from the charging side as long as your BMS's have low temperature cutoff. I prefer to pull the heating load from the house side which is where it will be if you have drop-in batteries. I would set the heating pads to turn on several ºC so that before the battery hit OTC you begin heating them and hopefully keep charging on.

I have a victron 712 Battery monitor, a 225 Li-BIM, a Xantrex 3000 Freedom Pro Inverter Charger (that will charge at 150 amps) and 580 watts of solar on the roof with a Jaboni (POC) MPPT solar charge controller (programmable only for algorithm, not set points). The alternator capacity is 185 amps.

with 150A on the Xantrex, and another 46A from solar, you are pushing your charge limit without anything additional from the alternator. You can control that by never starting the engine when plugged into a 120v source of power.

From what I’ve been reading, it would be best to limit the SOC of the batteries to 80-90% much of the time and only allow them to charge to 100% occasionally, like once per 2-4 weeks to balance them.

The following is assuming that you are in full hookup rather than boondocking.

On my Magnum, I set it to charge on "silent mode" which charges up to the absorption stage and the produces no power after than until the battery drops to my re-bulk voltage which I have set at 12.5v and the charger has not produced any power for over 5-days, and I have consumed 425Ah and the voltage is at 13.15v. I have no solar so my all-electric 2016 Tiffin Phaeton has used about 85Ah per day of DC and the rest is AC. with your 580w of solar you should get at least 480w / 3.5 = 137Ah per day so will be topped up all the time if you only use DC for direct DC loads and all others are supplied from shore power.
If you want to prevent the batteries from hitting too high of a SOC you can set your MPPT to 13.4v for bulk, absorp, and float so that it will never go above that.

When I go to boondocking or dry camping, I would change my charge parameters to let solar go up to 13.8v to capture as much as possible then.

My problem is multiple charge sources that each have different algorithms. The Xantrex is programmable, and I have the float set to 13.6v (perhaps that should be lower?). But the solar has the charge program set to lithium and will charge to 14.4v whenever it can, and the alternator (185-amp capacity) will charge to 14v anytime the engine is running In 15–20-minute spurts.

If you were fortunate enough to have a total Victron system on their VE-net. you just set it and forget it and the network will manage everybody very nicely. You and I do not have that nice integrated system, so we need to trick the multiple charge sources into playing nice together.

Is there a way of controlling the incoming charge so that it only starts after the batteries fall to say 13.1v so I can get down into the 40% SOC? I’m thinking the alternator and solar are the main concerns.

I do not know your MPPT but it sounds like it is a set and live it without the ability to change the charge profiles. That means that you cannot reduce your bulk/absorption voltage below 14.2v. If that is true, I would upgrade to a Victron which is extremely programable. I would then choose my MPPT to be the primary charge profile for topping up the batteries. I would then set the next most important charge source to to be 0.1 volt lower so that it drops out before the primary charge controller.
FYI, in my reading of the charge profile on the LiBim 225 it stops charging the LFP bank when the coach bank resting voltage gets to 13.4v so it sounds like it with shut down significantly before the alternator hits its voltage set point of 14.2v.

Thanks for any help you can offer. I realize this is a bit unusual, as most times you want to maximize charge, but I have an 8kw generator for times when I’ve let the charge fall too far and need to recover rapidly!

To summarize I have different settings for when I am connected to 120v AC power which keep the voltage to 13.5v max to not stress that system more than needed. I then change the programing to 13.8v or 13.9v when I am without AC mains power using the genset or solar for all of my power needs.

 

[12VoltInstalls]

Thank for the reply.

If the regulator is doing its job, you should never see more volts than that. If on your system with an internal regulator, you get more than the set point which is ussually around 14.2v then something is wrong with it

Many older vehicles can exhibit nearly 15VDC and be normal. My jeep- can’t remember what I measured with a meter- gauge says around 14.8 with lights on and heater fan on high. Iirc I think it’s off about .2V high at idle but haven’t metered at 50 mph ?

I think that your observations are confusing power (W), current (I), voltage (v) and resistance (r).

No, I get that. Just seeing the results on various installations over the years including my own stuff.

Check it at the point closest to the battery / alternator that is beyond the point where the last load branches off and you only have the winch downstream

I will do that. New winch is not cabled yet. I had gutted all the mfgr cables as they were 2ga aluminum. Then winter cold hit and I haven’t finished but need to. Have 2/0 copper on hand and all the terminals.

alternator does not "see" load. It only monitors voltage and adjusts the power that it produces to get as close to the set point as possible without going over

Semantics or colloquial on my part. It still drags 1-2HP from the motor so it does ‘get’ loaded.

 

[hwse]

Thank for the reply.

Many older vehicles can exhibit nearly 15VDC and be normal. My jeep- can’t remember what I measured with a meter- gauge says around 14.8 with lights on and heater fan on high. Iirc I think it’s off about .2V high at idle but haven’t metered at 50 mph ?

If this is the case, I would suggest not charging LFP batteries directly from that alternator. Anything greater than about 14.2v will force the BMS to shut down charging due to OVP unless the pack is perfectly top balanced. My 460Ah drop-in packs could not be charged to anything greater than 13.8v when I first got it due to a large imbalance. I now have them balanced to the point where I can charge to 14.3v without charging cutoff. It is generally recommended to set your charge parameters to a level where the charging stops before the BMS cuts it off. This is especially true for charging with an alternator because the cutoff under full power can blow the diodes in the alternator's rectifier.

No, I get that. Just seeing the results on various installations over the years including my own stuff.

I will do that. New winch is not cabled yet. I had gutted all the mfgr cables as they were 2ga aluminum. Then winter cold hit and I haven’t finished but need to. Have 2/0 copper on hand and all the terminals.

That should greatly reduce your voltage drop and give you more pull on your winch.

Semantics or colloquial on my part. It still drags 1-2HP from the motor so it does ‘get’ loaded.

My point is that the alternator can care less about how much load is attached and has no way of knowing how great the load is. It only cares about the voltage at the regulator terminals. if it is below the set point, it will produce all of the power it possibly can at the given RPM's. If it is at the set point, it will reduce the power generated to the level that will not overshoot the set point voltage. The battery is what is supplying the power to the load and the alternator is trying to keep it at the set point voltage.

 

[audiobill5]

Thanks!!
Great info on resistance and losses through the wire.
Darn physics.