drover avatar image
drover asked

Do I need a BMS for the Caravan when using a 75/15 MPPT Controller?

I am using a 75/15 MPPT Blue Solar controller (with BT VE Connect) to charge my 200AHr LiFePo4 Battery (unfortunately not a Victron) inside my Caravan that simply feeds into a 2000W 12V/240V Inverter (via a 150Amp Fuse Block). It is a stand-alone solar panel system and it is not connected to the main Caravan battery source at all. Does this system warrant any BMS. I did not think it would need one as the 75/15 Controller performs 'some' of the BMS functions but just curious as to what advice is out there please?

If it is considered overwhelmingly to need a BMS is there a recommendation available to suit this quite simplistic system?

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scubasail avatar image scubasail commented ·

Are you referring to the BMS specifically to protect the LFPs? IOW, if you had LA you wouldn't even think about a BMS?

What the BMS does is prevent the LFP from being overcharged or deep discharged as well as limiting the current. You have the current liiting covered, of the fuse is close to the batts.

You have the overcharge covered by using the correct profile for the LFPs on the MPPT. The deep discharge is covered if the inverter turns off at a voltage that's still safe for the LFPs, say 12.4V. (That's where the knee is for LFPs)

Most inverters have a fixed lower voltage limit of 10.5V or lower, which is not dangerously low for LFPs either. The manufacturers ususally quote 10V for LVC. This might be pushing the lifetime of LFPs though.

All in all, you should be safe without a BMS in this scenario, as long as you can make sure no one connects some other consumer (without an LVC) to that batt, ever.

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drover avatar image drover scubasail commented ·

Thanks for this info. I believe that I have it covered and will check it all out this weekend. I do not intend to run anything else off the Battery - if anything it may be the Diesel Heater igniter which also is fused and has a LVC.

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2 Answers
wkirby avatar image
wkirby answered ·

You need to use a BMS with lithium battery systems. The BMS' primary function is to balance the individual cells which make up the battery. It ensures that all of the cells are the same Voltage when they reach full charge (top balancing).

Commonly, when the celks' Voltage drift apart from each other the overall pack capacity drops. Worst case is that a really low cell gets reverse polarity and releases some flames.

You didn't say exactly which battery you have, some of them already have a BMS built in.

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thanar avatar image
thanar answered ·

Main purpose of the BMS is to balance and monitor the series of cell group, so as no cell group drops below the minimum or goes over the maximum threshold for the particular Lithium battery chemistry. At the same time, they also monitor the whole pack voltage and cut-off if necessary.

That said, there are a couple of chemistries (for example LiMn used by some Sony cells) that actually self-balance, by bleeding-off some energy at the top of their charge cycle, hence they tend to remain in-balance when bulk-charged fully and slowly.

There are even other batteries that already include a BMS. The only way to know that is from the manufacturer, unless you break the battery apart down to cell-level.

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drover avatar image drover commented ·

Thanks this advice everyone. I checked, and the battery has inbuilt cell balancing so that gives me some relief.

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