Questions about AGM -> LFP

[GallenH]

Originally Posted by MsNomer

I compare 3 to 2 because 3 lithium fit in the same (actually a bit less) space as 2 AGM, so maybe I should say that in the same space I got more than 5x…

And I understand that perfectly. Sorry I was confused.

[Urlauber]

Here's another question I haven't seen discussed much, so maybe I am overthinking 🙃 this...

In a camper with LFP batteries and solar charging, what happens if the BMS shuts down the battery while the sun is out?

My understanding is that without the battery to balance things, the voltage can go up beyond normal levels.

Can that damage the solar controller, or worse, connected appliances? Are there special SCCs which can handle such situations.

[booster]

Originally Posted by Urlauber

Here's another question I haven't seen discussed much, so maybe I am overthinking 🙃 this...

In a camper with LFP batteries and solar charging, what happens if the BMS shuts down the battery while the sun is out?

My understanding is that without the battery to balance things, the voltage can go up beyond normal levels.

Can that damage the solar controller, or worse, connected appliances? Are there special SCCs which can handle such situations.

That certainly can be an issue and is why the better control systems shut of the charging sources rather than shut down battery itself. This has been discussed here before in relation to control systems at it can also be a problem if you have the solar turn off with all the other stuff in the van through a master battery switch. If the switch is off and you drive out of the garage, you have the same issue.


It is probably a good time to bring it all up because the SOK thread is running also and they want you to charge to full every cycle so the batteries balance the cells and to do that they shut off the battery charging. If you have solar it might be an issue at that point.

[Urlauber]

Thank you booster! May be just one of those "things to keep in mind" issues.

After reading some more on various solar panel sites it seems like this only becomes a problem if there is no other load in the system.

Which, of course, happens frequently on cool, sunny days without any fans running and the fridge being in off-cycle. Not sure if something like a CO alarm would qualify as "load".

I am kinda surprised that this is not better explained in solar controller manuals, or maybe I have missed it. IMO either the SCC should protect itself and connected loads if the battery disappears suddenly, or, the looming catastrophy should be pointed out in BIG letters. Maybe overall it's a non-issue.

Of course, the BMS shutting the battery off is something that should not really happen often unless something is wrong, or it is about to freeze over, which is when the solar panels should be disconnected anyway, unless the LFP battery has heating pads.

[booster]

Originally Posted by Urlauber

Thank you booster! May be just one of those "things to keep in mind" issues.

After reading some more on various solar panel sites it seems like this only becomes a problem if there is no other load in the system.

Which, of course, happens frequently on cool, sunny days without any fans running and the fridge being in off-cycle. Not sure if something like a CO alarm would qualify as "load".

I am kinda surprised that this is not better explained in solar controller manuals, or maybe I have missed it. IMO either the SCC should protect itself and connected loads if the battery disappears suddenly, or, the looming catastrophy should be pointed out in BIG letters. Maybe overall it's a non-issue.

Of course, the BMS shutting the battery off is something that should not really happen often unless something is wrong, or it is about to freeze over, which is when the solar panels should be disconnected anyway, unless the LFP battery has heating pads.

I assume you are referring to the solar potentially going to high voltage if the batteries disconnect.


The deal is, if the batteries go off line the solar controller can't do anything because it doesn't have any power to do it. Most 12v panels in use now have a max open circuit voltage of over 21 volts which is plenty enough to damage the solar controller and possible find it's way through the unit from the failure or a slow to close relay, etc.


It is true that the solar will not connect to a battery that is offline, but the panels will still put that open circuit voltage on the controller.


I have no idea how somebody would say if there are no loads, nothing can happen as that makes no sense to me at all. Your compressor frig is not really off in the non compressor running state and all the electronics are still hooked up to the 12v system. If the system spikes, it will see it, as will most all the other electronics. The controller itself is the most vulnerable though.


If you don't have a B to B charger, the alternator is an even bigger risk if the batteries go offline unless you are parallel with the starting battery to take up the surge. Even shore chargers probably could also spike before they went off, but they usually have shore power available so may still be able to shut down.



I have seen several manuals for controllers that warn of this issue.

[Urlauber]

Originally Posted by booster

I have no idea how somebody would say if there are no loads, nothing can happen as that makes no sense to me at all. Your compressor frig is not really off in the non compressor running state and all the electronics are still hooked up to the 12v system. If the system spikes, it will see it, as will most all the other electronics. The controller itself is the most vulnerable though.

No I think you read that backwards - I read if there are NO loads, bad stuff happens. If there are loads, voltages should not spike (as much) as the controller sees a battery just as another load.

I am just regurgitating what I saw, so it's not like I am an expert or even have an independent opinion. Just here to learn and speculate.

[booster]

Originally Posted by Urlauber

No I think you read that backwards - I read if there are NO loads, bad stuff happens. If there are loads, voltages should not spike (as much) as the controller sees a battery just as another load.

I am just regurgitating what I saw, so it's not like I am an expert or even have an independent opinion. Just here to learn and speculate.

Spikes are transient, AFAIK and can be very short. Loads will dissipate a surge but I just can't see any fragile electronics surviving it sometimes. Without a battery to provide a voltage reference the controller can't do anything either. I have never run across that claim it would be nice to know where they got that information.

[Urlauber]

Originally Posted by booster

Spikes are transient, AFAIK and can be very short. Loads will dissipate a surge but I just can't see any fragile electronics surviving it sometimes. Without a battery to provide a voltage reference the controller can't do anything either. I have never run across that claim it would be nice to know where they got that information.

Not sure where "they" got the info, apparently Victron did not provide an official comment. Like I said, different experiences and opinions.
https://community.victronenergy.com/...ge-if-the.html

[booster]

Originally Posted by Urlauber

Not sure where "they" got the info, apparently Victron did not provide an official comment. Like I said, different experiences and opinions.
https://community.victronenergy.com/...ge-if-the.html

I didn't really see much on there about spikes not happening, just that loads will prevent damage and no load it won't matter. In particular the no loads shows a complete lack of knowledge about how electronics work. Shore chargers, inverters, battery monitors, automatic charge relays, frigs, detectors, tank sensors are all active and connected to the 12v power even when not "on". Often to monitor conditions and be ready to do their job, or just to keep the touch pad or switches usable.


If you have a 48v nominal solar array running an MPPT controller to 12v batteries it could see a spike of nearly 90 volts if it goes to max open circuit voltage. If you have a charge relay that is closed so you are charging the starting battery also, the van electronics could also see any spike that happens.


Everyone is free to do whatever they want, but it is too much of risk for me and should be avoided at all cost if possible.

[Urlauber]

Well, yes, that would/could certainly fry 12V electronics. In my case, my panel outputs 24V open circuit which isn't that high, but may still damage stuff.

How do folks prevent that?

[booster]

Originally Posted by Urlauber

Well, yes, that would/could certainly fry 12V electronics. In my case, my panel outputs 24V open circuit which isn't that high, but may still damage stuff.

How do folks prevent that?

Probably the best way is for you to have a BMS that directly controls the charging sources and instead of just disconnecting the batteries, it would stop the charging and/discharging by signalling the controls first to shut stuff off like the solar panels before taking the battery offline.


The other way would be to have a small AGM battery in the system that would hold the voltage up enough to prevent the sudden shut off.


My guess is that the high end integrated Victron systems have it all programmed into the central controller which controls everything anyway so quite easy to do once you have it all there anyway. I assume other integrated systems do similar.

[Winston]

Originally Posted by booster

Probably the best way is for you to have a BMS that directly controls the charging sources and instead of just disconnecting the batteries, it would stop the charging and/discharging by signalling the controls first to shut stuff off like the solar panels before taking the battery offline.

Our philosophy is to design the various charging sources to control charge rates and automatically turn themselves off at the appropriate time. The BMS never intervenes unless one of the charging sources malfunctions at which point the BMS "externally" disconnects the charging source. How can we be certain that a malfunctioning charging source - - that appears to be "stuck" in a charge mode - - will respond to a BMS signal and shut itself off?

[tlillard23]

Originally Posted by tlillard23

I had one battleborn 100ah battery, started the genny, onan 2800 just fine. alternator charging just fine. solar charging just fine. I let the BMS worry about the cells, I just use it.
.....

There is that one little tiny small issue with lithium, thermal runaway. So get a good brand with a good BMS.

I have an update here, I was troubleshooting some long standing alternator issues on the roadtrek, found out the previous owner (or roadtrek?) is using the sense line on the alternator. I can see the line is long enough to go down to the engine compartmen fuse box main terminal, but it is (was) attached to the output of the alternator.... Long story long, looks like the PO was trying to avoid high current into the lithium batteries. I moved the sense and now understand the NEED for a B2B or some other way to charge without getting excessive currents.

what's that mAy-kuLpA thing... It applies here

[Urlauber]

Just had a new thought (or a couple of them) after reading Interblogs concern about the longevity of LFP batteries.

Here are the weak points related to the proposed switch to LFP:

• Unknown quality/lifespan/cycles of cheap LFP batteries
• Exposure to heat and cold if exposed in a battery carrier below van
• Danger of frying electronics if BMS disconnects LFP while being solar charged
• Questionable if generator can be started with single 100Ah LFP
• Expenses related to Lithium-specific upgrades of electronic components (converter, SCC, BIM, etc)

As I mentioned here and in other threads, I purchased a 100Ah LFP "Power Station" to try out both for RV applications and power backup in case of outages at home.

So here is the idea:

1) Replace the two 100Ah Trojan Grp31 AGM batteries with a single lower capacity (maybe 30Ah) AGM battery. No other wiring changes, it will be charged and used like the two batteries it replaces.

2) Use the 100Ah Power Station as main energy source for the "house". Charge it via solar and/or from the AGM battery (which is charged from the alternator).

Advantages:

• No need for large expenses, just a single small AGM battery (whenever the existing Trojans approach EOL).
• Keeping an AGM battery in the system means no solar spikes in case the LFP battery goes offline (or is removed)
• No need for wiring changes - I already added a connection between the Power Station and the fuse box.
• The Power Station is just a fancy LFP battery with some added electronics built in: MPPT charge controller, SOC and Current (in and out) displays, 1000W inverter, etc
• This makes the LFP battery easily removable to place it in a cool environment in between trips, extending the life-span which is otherwise affected by heat (or cold).
• Likewise, the Power Station can be easily used for other purposes, like backup in case of power outages
• Emergency start procedures (start generator from chassis battery, or start engine from house battery) are maintained

Drawbacks (for the particular Power Station I purchased):

• Slow charge rate - the maximum in my case is 180W, meaning it takes 7 hours to fully charge from 0. Not a big deal for me...
• Low 12V output - maximum is 120W, which may be a deal breaker for many. Works for me as my compressor fridge takes 55W, and fans/lights/water pump (in Summer) and Truma (in Winter) still keep it close to 100W total.
• This means I cannot start the generator nor extend the awning or similar high current operations, which is where the AGM battery comes in (which can be connected to the high CCA chassis battery via the emergency switch).

Connections/switches:

• The Power Station is plugged into the 12V system via the Fuse panel. Via a switch, I can either power the camper, or charge the Power Station from the house battery/batteries.
• I added a switch to redirect the Solar Panel output to the power station, which accepts 120W charging power.
• I added a switch to the BIM160 which allows me to override its function and connect or disconnect chassis and house batteries via a manual switch. This won't be needed much, it was to be used to prevent the alternator burning up while charging regular LFP batteries. Not an issue with the Power Station.

(Keep in mind that much of the above refers to the particular (cheap) power station I acquired - there are many out there, some accepting much higher charge rates, and perhaps output higher 12V wattage. Obviously, one cannot run the AC or induction stoves off a Power Station like I have).

[Urlauber]

If the trend continues, these will be free pretty soon!