LiFePO4 charging - the overcharging question

[ptourin]

It hits me that it's not entirely a no-brainer. It sounds from your description like the Blue Sea is a latching relay with a momentary switch to change its state - I'll have to look - is it one of those where a positive pulse turns the relay on and a negative pulse turns it off?

If that's so, I'm going to have to figure out a way to pulse it correctly if I'm going to automate it.

[booster]

Originally Posted by ptourin

It hits me that it's not entirely a no-brainer. It sounds from your description like the Blue Sea is a latching relay with a momentary switch to change its state - I'll have to look - is it one of those where a positive pulse turns the relay on and a negative pulse turns it off?

If that's so, I'm going to have to figure out a way to pulse it correctly if I'm going to automate it.

Yep, that is how they work. They also make the normal actuating battery relays, but they consume quite a bit of current when they are active. On alternator or shore power, not a big deal, usually, but with solar it can kill your output a bunch.

[booster]

Blue Sea does have another product that might work well for you. They call the automatic charge relays, but they work just like a separator in that the are normally open to isolate battery banks, and then close when they see charging level voltage to tie the banks together for common charging. They make both one way and both ways versions. What makes them so they might work for you is that the have what the call ignition interrupt on them as a separate input. What it really is an input that if you put 152v on it, it opens the ACR. When I called, I seem to remember them saying it only takes like 50 milliamps, to do the interrupt, but not positive on that. Might be just what you need to do what you want, when triggered off the monitor contacts.

[ptourin]

Which of their models do you mean? I've looked at their site several times, but I have a lot of trouble figuring out what might be useful - their descriptions are more application-based rather than describing what each item actually does, and I find it really hard to read their descriptions and "translate". Are you thinking that I could come off the monitor relay and build a relay contact closure that would energize the Blue Sea relay to connect alternator line to battery? Are they open to non-boat people calling up and asking for info and advice?

[booster]

I think this would be the one you would want, but I surely would call them and explain what you want to do to see what they say. They are really good folks there and I have talked to them many times.

https://www.bluesea.com/products/762..._-_12V_DC_500A

This is a bistable relay, that runs on pulsed from the manual switch if you use it, but it is also a battery separator so if you left the switch in the auto position, the relay would be open until it saw charging level voltage on either side (alternator or shore). At that point it would close, tying the coach and engine together. It has ignition interrupt and engine isolation, so I think one or both of those inputs would make the unit open regardless of charging voltages being present, and those are not pulsed as far as I know. I have a couple of the 120 amp versions that we had in our old system, and Blue Sea would put 12v on the ignition interrupt on them from the 3 bank battery charger to isolate the multibank setup out of the charger, and I think they all do the same thing.

If you took the signal from you monitor for fully charged and had it put 12v on the ignition interrupt of the ACR, I think it would disconnect the engine from the coach, until the signal was no longer there, so you could never overcharge the batteries off the engine. If the signal was not there, the relay would close when you start the engine. If I understand correctly, that is what you are looking for, and it makes good sense to me.

Hope that makes sense, and that is also works!

[booster]

Actually, if it would work for the engine, it would also work for a shore charger, I think. You might even be able to kill two birds with one ACR, if the shore charger can stand to have just the 12v shut off without the AC side. I think it can because the PD chargers are really power supplies and shouldn't need a battery on them to regulate voltage.

[ptourin]

Yes, I could of course separate the battery from both the PD charger and the alternator via the same relay. I hadn't thought of that - I started by thinking of how to separate the charger before I started to worry about the alterator - and figured it'd be easier to break the AC cord than to break the 12V output. I don't think I'd ever be breaking more than around 60A - I can't see when I'd ever have shore power connected and the engine running when there was suddenly a demand for charging power...

[booster]

It sounds like you may be on your way to a very good solution to your problem! The Blue Sea will switch up to 500 amps, but if you are only looking at 60, you could probably get by with the smaller ACR.

[Jostalli]

Has anyone successfully used an ACR with LiFePO4 batteries? Charging begins at set voltages and combines both your LiFePO4 battery and cab battery. I don't see it working as expected due to the way voltage changes occur with LiFePO4 batteries.

I'm planning on a manual switch for alternator charging of my lithium batts: http://www.amazon.com/gp/product/B00..._dp_o_pC_S_ttl

[booster]

Originally Posted by Jostalli

Has anyone successfully used an ACR with LiFePO4 batteries? Charging begins at set voltages and combines both your LiFePO4 battery and cab battery. I don't see it working as expected due to the way voltage changes occur with LiFePO4 batteries.

I'm planning on a manual switch for alternator charging of my lithium batts: http://www.amazon.com/gp/product/B00..._dp_o_pC_S_ttl

Interesting question, but I can't think of a reason that they wouldn't work unless the voltage of the lithium was able to hold up high enough to keep the ACR closed when charging source went away. Most actuate in the low 13 volts area, so I suppose that is a possibility. In a case like that, you might need to try to find one that was one sided on the closing charge voltage if you were going to isolate the engine charging as then it would only see a lead acid battery and its charging voltage, not the lithium side.

On edit---I just went to the Blue Sea site to get the part number for the single sensing ACR, that would probably take care of the higher voltage of the the lithium side issue.

What is odd is that I can't find any of the single sensing versions anywhere on their site any more. When I bought my smaller version ones a few years ago, you could get them either way, as dual or single sensing. I know that sometimes the website is kind of messed up and some products don't show up unless you already have the part number, the cool fuses Avanti found was one, and I think even the manual relay above is the same way.

It probably will take a phone call to find out if they are still available. If they aren't, things get more complicated.

[booster]

Still at it here--I looked at the current Blue Sea catalog, and they have a grid of their battery switches. It includes the 7700 mentioned above, and that I have in our system. It is a bistable pulsed activation.

In the catalog they describe things a bit better in the details, and there is another, very, very interesting option.

The model number 7713 is very similar, but not bistable. It looks to use a coil type relay, but it is somehow latched mechanically so the coil current is tiny when actuated.

The big thing is that the 7713 doesn't need the pulsed inputs. It wants low current CONTINUOUS power to activate, and releases when the current is removed.

https://www.bluesea.com/products/771...-Release_-_12V

I could see this working in the system ptourin is building, I think.

If power from the engine running circuit of the van went to this relay, through the normally closed contacts of the relay ptourin is going to put in that is controlled by the "full" battery signal from his monitor, the lithiums would charge as soon as the engine was started and put power to the 7713, and as soon as the batteries were full, the monitor relay would open the contacts and shut off the 7713.

Might just work

[ptourin]

That's a cool relay alright. I called them up and here's what I got...
It's a remote battery switch controlled by their ACR module - he said it's the same module, but they don't want to call it that lest customers get confused beween the 7713 and the ACR's.

When you ask it to close, the momentary ~7A is to latch the relay; once it's latched, I think the ~13Ma is current to the ACR module to keep it thinking and keep the relay closed - so it's an almost-latching relay - it has some current draw when latched, but less than 10Ah/month, which is negligable.

When you remove the 12V signal, the relay opens again. We talked a bit about the 7620/7622's, but when I described the application he said that the 7713 was specifically designed to get around the issue of how to trigger a bistable relay.

It has a remote switch, but I don't really see why you'd need one. Their LED indicator is a good idea - in addition to lighting when the contacts are closed and going out when they're not, it will give a double-flash error signal for a contact closure error (where you expect it to be closed but it's not).

That's about it for now - I haven't yet searched for their tech docs on the relay. I did find out that their 1830 battery monitor has an internal control relay similar to my Victron, though it may be more flexible - he said it can be programmed to look at SOC, voltage or current.

[booster]

Sounds like you may have found yourself a solution!

[ptourin]

Yeah, I like it. No problem with the relay being under-designed <g>... They're planning a smaller relay but it's a few years out.

[ptourin]

I've been thinking over what the easiest way to do charge control might be. Here's s simple relay circuit - I'd be interested to know if anybody has played with this sort of thing and has a comment. The trick is to convert the Victron internal control relay into something that can handle enough current to control the Blue Sea ML7713. I have a single external control relay to do that, but I also have 2 relays to check whether I either have shore power or the engine is running - if neither, I don't allow the external control relay to draw power. So it's 5 relays. If anybody has any comments or suggestions, I'd love to hear them.

[ptourin]

I've run into a couple of things, and I'm hoping for a bit of comment and advice. First, I realized that I can buy 12v high current solenoids from the automotive & marine world - I've already got a 200A solenoid in the RV as my isolation relay. They're cheap, around $40, and one that I saw has 12vdc @ 0.75A for the coil current - my battery monitor's internal control relay can handle 1 amp, so I can control the relay directly.

Second, I drew up the diagram at the top of the attachment and showed it to an engineer friend. It has 2 relays that only close when either the alternator is running or there's shore power for the charger - they keep me from closing a relay that draws current when there's nothing that'll charge my batteries.

My engineer friend said, "Why do you need those relays? You should be able to do the same thing much easier with blocking diodes." I drew the bottom diagram and asked him if that was what he meant - he answered, "not quite" - and I haven't had time to meet with him since.

Can anybody here tell me more about blocking diodes? I understand how they work, but I don't know anything about applying them - and he seemed to be talking about using several to separate the alternator output from the charger output. Any useful info for me?......

[ptourin]

What's the story with damaging alternators by lifting the load off them? I just read about this in a cruiser forum article and want to know more. Does this apply to opening the output and lifting all the load entirely? In my case I don't have a separate alternator - my Sprinter alternator charges both chassis and house batteries. So if I open the circuit to the house batteries, the alternator still has the RV load. I'm guessing this is OK, but want to make sure.

Also, does anyone have anything to say about using the alternator to charge 2 different battery types? Do I have an issue when I'm driving with the alternator connected to the chassis load and charging the chassis AGM battery, and then I connect it to the house side and connect it to the LiFePO4 house batteries? Again, I was reading about this issue, but in the small amount of driving around and monitoring the system, it doesn't seem to be an issue - when the house bank is down, I see about 45 amps or so at around 14.2 volts going to the house system, then the current drops as the batteries fill up.

Trying to get a handle on theoretical issues and thinking about whether they have any practical effect on me.

[booster]

Originally Posted by ptourin

What's the story with damaging alternators by lifting the load off them? I just read about this in a cruiser forum article and want to know more. Does this apply to opening the output and lifting all the load entirely? In my case I don't have a separate alternator - my Sprinter alternator charges both chassis and house batteries. So if I open the circuit to the house batteries, the alternator still has the RV load. I'm guessing this is OK, but want to make sure.

Also, does anyone have anything to say about using the alternator to charge 2 different battery types? Do I have an issue when I'm driving with the alternator connected to the chassis load and charging the chassis AGM battery, and then I connect it to the house side and connect it to the LiFePO4 house batteries? Again, I was reading about this issue, but in the small amount of driving around and monitoring the system, it doesn't seem to be an issue - when the house bank is down, I see about 45 amps or so at around 14.2 volts going to the house system, then the current drops as the batteries fill up.

Trying to get a handle on theoretical issues and thinking about whether they have any practical effect on me.

From what I have heard, just dumping a big load, like having coach batteries taking 100+ maps and disconnecting them can cause a voltage spike, but probably not one that is big enough to hurt things like electronics or the alternator itself. The biggest issue would be what we have discussed a few times here, and doesn't apply to you, and that is starting or stopping the output from an alternator that doesn't have a battery in the circuit for voltage reference and ballast. That can generate a very large voltage surge and be a problem. Only the stand alone engine generator systems would normally have the issue as the chassis battery is always in the one alternator systems. Avanti has such a system and has put a smart regulator on the separate alternator to take care of the problem. For reference, solar can have similar issues if you connect the solar and panels with no battery in the circuit, or the sun comes out with batteries off line.

Charging two different battery types is normally considered a bad thing, but in some cases I think it is not near the death wish it is made out to be. If the batteries have close enough to the same desired charging voltage, you are fine all the way to the point they get full. A mismatch then can be an issue if one fills early and sees charge voltage for long time after that. If you have a way to monitor each type to know when they are full, and a way to take them off charge voltage at that point, I really don't see a problem with it. I think lithium and AGM can be charged OK at the 14.6/14.7 voltage range, so may be just fine with proper controls. Being a bit lower on voltage off the alternator is probably not an issue, either, as it just should slow things down a bit and is on the "safe" side.

[ptourin]

Thanks - that's more or less the way I was thinking about it - but wanted to make sure I wasn't just being naive <g>... If the M-B alternator really is always going to put out ~14.2, I'm starting to wonder if it's going to be any real problem if I leave it in the circuit. Still planning to make it switchable, just wondering...

I'm also starting to think about simpler circuitry for handling the charge control. I'm wondering about simply using the Victron relay to feed a double pole relay - one circuit to open the isolator (and disconnect the alternator), the other circuit to open the AC feed to the charger. That's about as simple as it gets - it ain't a biiig boat, it's just a leetle RV <g>...

[ptourin]

I've been lurking silently for awhile, and I've come up with a question or two about M-B alternator charging vs charging with my CCCV charger. My questions are about alternator charging. Basic info: the battery supplier says that my full charge rest voltage is about 13.4, and that's what I observed. They recommend a 14.6v CCCV charger, and that's what I've installed.

My measurements on alternator charging show that with the batteries partially discharged, alternator charging started at around 14.2V and ~45A, and the current dropped down as the battery voltage approached 14.2 volts.

There are 2 things I want to understand.

First, my understanding is that once you remove the charger, the battery will have a surface charge that will drop over a period of time. I understand that the time is longer for the lith phosphate batteries than for flooded cells. Let's say that I use the charger to charge at the recommended 14.6 until my charge current drops below a set point, say 10A. I assume that if the batteries sit, the voltage will drop from ~14.6 down to 13.4.

Now let's say that I use the alternator to charge at 14.2 until the current drops below 10A. After the batteries drop down, will they be at 13.4 or will they be slightly lower? - that is, will the lower voltage bring the batteries up to full charge, or only near full charge? I know that this is a bit of hair-splitting practically speaking, as the batteries don't mind being left at full charge, near full charge or lower - this is my effort to understand how the system works.

Second, if the potential difference is less (13.4 - 14.2 for the alternator vs. 13.4 - 14.6 for the charger), will the CV part of the charging be slower? It seems that this should be true, but I keep getting fooled by not understanding things in depth. I suppose I should ask the same question about the CC charging part - suppose both the alternator and the charger can source 50A - then will each of them take about the same time for the CC charging part? Again, I'd assume this is more or less so, but I might be getting fooled again <g>...

Any thoughts?