Another LiFePO4 Question or Two

[CoreyTrevor]

Hi. I have two Dakota Lithium 54AH batteries.

I emailed the company to ask about the BMS, and they replied that the BMS has high and low voltage cutoff, and high but not low temp cutoff. It seems absurd to me to put in a temp sensor and not also use it to keep the battery from being destroyed by charging at too low a temperature. So I'm wondering if maybe the guy I talked to has no clue and the low temp cutoff really is there. I want to find out.

What I would like to know is, if I freeze the battery really well and then connect a low amp (like 2 amp) charger through an ammeter for just a few seconds, will this damage the battery? I'm ass-uming that if the cutoff is actually there I should see no current going to the battery and if there is any current the cutoff is not there. How am I doing so far?

Next question, I searched but got lots of conflicting information. With my Victron MPPT 100/30, if the PV is putting out power and the Victron is charging the batteries, and the BMS disconnects the battery or I mistakenly flip the breaker between the Victron and batteries, will this cause harm to the Victron. I didn't see anything in the manual other than to connect the batteries first and the PV second.

Thanks!

 

[Vic Harder]

Hi. I have two Dakota Lithium 54AH batteries.


Next question, I searched but got lots of conflicting information. With my Victron MPPT 100/30, if the PV is putting out power and the Victron is charging the batteries, and the BMS disconnects the battery or I mistakenly flip the breaker between the Victron and batteries, will this cause harm to the Victron. I didn't see anything in the manual other than to connect the batteries first and the PV second.

Thanks!

I see no reason why it should. In fact, I have a breaker between my panel and the Victron so I can disconnect the solar panel if I want to. AND, I have done so a few times with no ill effect. YMMV

 

[CoreyTrevor]

Ok, but how about the other way around? The panels are in the sun and batteries are charging and I disconnect the batteries from the controller while it's all powered up?

Some of what I found online says that's bad and will damage the controller.

 

[Happyjax]

Yes I have instructions to always have the battery on 1st and then the charge controller. I guess pumping 5-10 amps into electronics which cannot dissipate them would not be good. Some of the more expensive controllers may have electronics to handle that but most of the cheap ones don't....YMMV

 

[ckent323]

There was a reply to that question in the Victron forum and the Victron rep said that disconnecting the Controller from panels or the batteries while current is flowing will not harm it, but I am having trouble locating that thread again.

I did find the following:

"The whole "to disconnect a battery while the controller is charging" question gets a bit circular, really... Victron says don't do it, but then again that's exactly what a BMS will do if it sees over-voltage (shut down the allow-to-charge contactor), and that's a Victron-supported function.
I asked Victron level 3 support about the issue and their response was, in summary, that cutting out the battery while the controller is charging full-bore could conceivably damage the controller, which is why their manuals always say to disconnect PV first, but then again they couldn't say that it's ever actually happened.
I've personally done extensive testing on my bench, trying to force an MPPT into fault by repeatedly cutting the battery connections in and out while the controller was charging full-bore, and failed to cause any faults after several hours of various methods of removing the battery connection... up to and including literally yanking the wires out of the "battery" terminals on the MPPT, trying to replicate a FUBAR fault condition. In no case was I able to cause a fault in the MPPT, so my bottom-line conclusion is that although it's recommended to cut out PV first, the likelihood of causing damage by turning off the battery connection first seems so slim as to be negligible."

https://diysolarforum.com/threads/use-a-victron-smart-battery-protect-bp-65-to-disconnect-solar-panel-controller-from-solar-panel.1868/

Here is a thread with Victron engineer response:

https://community.victronenergy.com/questions/28201/controller-damage-if-the-battery-is-disconnected-b.html



See also

https://community.victronenergy.com/questions/42014/disconnect-solar-when-no-battery.html

 

[CoreyTrevor]

Thanks for the replies and the links.

I did see some of that online, and the Victron manual doesn't seem to forbid disconnecting the battery while charging. There were a few posts online that said it would harm the CC. I also figured that the CC should be able to survive the BMS disconnecting the battery. That's probably unlikely to happen, but I could see myself (or my wife) flipping the wrong breaker, since they are side by side. I won't make a habit of it.

I'm very proactive on keeping houses and vehicles from catching fire, and I'm thinking that if I'm going out on a hike or something and the batteries are mostly topped off, I will just flip both breakers and cut the panels and batteries from the CC. The fewer things that are doing something electrical, the fewer things that can fail and burst into flames when I'm not around to deal with it.

Thanks again for the help!

 

[rando]

To answer your two questions separately:

1. Your idea to freeze your battery then try to charge it is a good one. The 'don't charge below freezing' rule is like the 'don't use below 50% rule' for lead acid batteries - they are both blown way out of proportion on the internet. Yes charging at low temperatures is not good for the battery, particularly at high rates of charge (like > 0.5C), charging at low rates of charge (< 0.1C), even below freezing will not immediately damage your battery, so there is no issue with the test you propose. Also note that there is a low temperature cutoff in your Victron charge controller - set that to 0C and it will prevent your battery charging below freezing.

2. I very much doubt any actual damage will occur if you were to disconnect the battery from the solar charge controller. I have done this on numerous occasions and not seen a problem. There could be an issue with this when you first power a system on, as it cannot auto-detect the battery voltage with no battery, but I think the controller remembers your settings, so it should be fine.

I am not sure disconnect the solar is a great idea. There is really very little risk here, and I foresee forgetting to turn it back on and running your battery way down by mistake as a larger risk.

One other thing to note - the BMS should almost never disconnect the batteries if everything in your system is configured properly. The BMS is a last line of defense, not a routinely used device.

 

[CoreyTrevor]

rando,

Thanks for the advice.

I will freeze the battery when I get a chance and do my test, and report back on the results.

There's one thing I don't get about the Victron and the low temp protection. I even bought the accessory temp/volt sensor as further protection. Even with the temp sensor stuck to the top of the battery, or the CC sitting an inch away from the battery, it seems like the huge thermal mass of the battery will take much longer to warm from a cold temperature than the controller or even the sensor. I'm concerned that the sensor will be reporting a nice acceptable temperature for charging, while the center of the battery is still many degrees cooler than 32*F. I could put some insulation over the sensor to keep it's temp closer to the battery temp.

Maybe I'm just being an idiot, but I have almost a grand in batteries that I'm hoping will outlive me.

I have LiFePO4 paranoia and I need help!

Seriously though, I appreciate the feedback.