LFP batteries + BMS in RV - Q's? current draw + disconnect procedure for dormant system?

[DW SD]

I'm in the planning stages of adding an LFP battery bank to my Bus RV, mostly for fun and learning opportunity. I already have Magnum inverter MS-2812 and PT100 (MPPT) charge controller (Magnum BMK / shunt), 3 x 300W panels and 660Ah of Lifeline 12V batteries. Plus starting (2) AGMs and Generator start AGM.

I'm now assembling parts for a 12V / 280Ah LFP battery with this BMS to add capacity to the house bank, wired in parallel to the AGMs. The BMS will control a Gigavac GV200 relay for high current. I plan to use the mixed chemistries where the LFP bank does the heavy lifting, and increases charge efficiencies where the AGM will spend 99% of its time float in the background.

Eventually, I may have a pure LFP house bank, but these Lifelines have plenty of life left and I hate to sell for a tiny fraction of new price ... I've researched the pros and cons and am comfortable, (I actually like the AGMs in the system as they'll ensure alternator and SCC always sees a battery in the event of LFP failure). I will install a Blue Sea bank selector switch so I can select either bank or both. Mostly I boondock (no shore power) in California beach campgrounds, so freezing nor high temps is an issue. If either is an issue, I plan to just switch to AGM only, OR also if I had a long term hookup.

I use the RV maybe 30 to 40 days / year. So at times it sits for 60 days with the systems totally shut down and the RV parked in a barn w/o shore power and all batteries and systems disabled.

Sorry if I'm long-winded, but wanted to share the context. Here are my questions:
1. How much does the BMS (or similar) draw without any other loads on the batteries?
2. Does it draw enough that it could kill the LFP batteries? Or do these shut themselves down prior? I assume there must be logic in the BMS to avoid killing a dormant pack?
3. If they can kill the batteries:

3a. would I unplug the BMS when I store? Or just put single pole switch inline with the ground to the BMS? I think I read somewhere here BMS's weren't intended to be disconnected.
3b. is there a better BMS that my draw less current when my system is dormant? I looked at the Chargery. I decided on what I bought due to price and that I could manage from my phone via bluetooth (and Will's endorsement). These batts will be stored in a bay, so the remote Chargery screen won't be handy inside.

I don't have a schematic software to show my system diagram. Sorry. Hope what I've described makes sense.

Thank you in advance,

Doug

 

[Dzl]

Daly states self consumption for their 4s 150A common port BMS is <150 micro amps and <20 micro amps in 'sleepy mode.' Just to be clear micro not milli. If this is at 12v, then 1.8 milliwatts and 0.24 milliwatts respectively.

ElectroDacus SBMS0 has a self consumption of 300 mW (milliwatts) to 800 mW (milliwatts)

Couldn't find data for the Chargery but it uses substantially more if you include the external relays which are needed to connect and disconnect chargers and loads

I would reccomend you reach out to the company that makes your BMS directly or read the manual to find out the consumption.

 

[DW SD]

Thank you for your reply. I will try harder to find the BMS info. I’ve only seen the kanji (i think Chinese characters are kanji) version which didn’t do me any good.
Will a BMS kill its own battery if left connected without a charge for too long? Or do they auto shut off at some safe voltage threshold?

 

[Dzl]

I will try harder to find the BMS info. I’ve only seen the kanji (i think Chinese characters are kanji) version which didn’t do me any good.

Yeah, this is definitely one of the difficulties with some of the Chinese products on the market, not much English language info or support. It never hurts to reach out to the company directly, they are usually pretty responsive.

Will a BMS kill its own battery if left connected without a charge for too long? Or do they auto shut off at some safe voltage threshold?

I'm not positive, but I believe they can theoretically drain the battery they are protecting. Buuuuut if Daly's numbers are to be believed (at this point I don't believe them). It would take 222,222 days to drain a normal 12v 100ah battery if the BMS was in sleep mode, or 30,000 days if the BMS was in normal mode...

The SBMS0 would consume a much more believable 7-20 watt hours per day, meaning a full 100ah battery would be completely drained in 64-177 days

These numbers should be taken with a grain of salt as its just some quick back of the envelope math, based on (in the case of Daly) some suspiciously low numbers..

 

[BiduleOhm]

Buuuuut if Daly's numbers are to be believed (at this point I don't believe them).

It's totally possible, today's MCUs can be put in sleep modes that are in the nA range... If all the BMS do is check the temperature and cells voltages every few seconds this current consumption can be real. Of course the more you do and the more often you do it, the more power you eat.

 

[Dzl]

It's totally possible, today's MCUs can be put in sleep modes that are in the nA range... If all the BMS do is check the temperature and cells voltages every few seconds this current consumption can be real. Of course the more you do and the more often you do it, the more power you eat.

Interesting, thanks for enlightening me. I suppose my skepticism stems from the Daly using orders of magnitude less than the other BMS' I looked at (Daly measures in microamps, TinyBMS measured in milliamps, SBMS0 measures in milliwatts, and Chargery (with relays included) measures in Watts

But admittedly all I did was quickly lookup the consumption numbers, so I can't really say how fair/comparable a comparison it is. TinyBMS has the best most comprehensive data on current and power consumption, and like you said in sleep mode it measures in uA, otherwise in single to low double digit mA.

 

[BiduleOhm]

It also depends a lot on what the BMS can do: if it has com (especially wifi/ethernet), if it has relays, additional sensors, ... all of this can grow the power consumption by multiple orders of magnitudes (just the ethernet com on my BMS is half the power consumed for example) so it's not surprising to see a lot of differences between BMSs.

In sleep mode most of the consumption is probably power supply stuff anyway (like 95 µA lost in the PSUs while the MCU sleeps at 5 µA...) so you can't really do better than that. Where you can save power is in active mode, but it's easier said than done (and it's more expensive to produce)...

 

[Dzl]

It also depends a lot on what the BMS can do: if it has com (especially wifi/ethernet), if it has relays, additional sensors, ... all of this can grow the power consumption by multiple orders of magnitudes (just the ethernet com on my BMS is half the power consumed for example) so it's not surprising to see a lot of differences between BMSs.

In sleep mode most of the consumption is probably power supply stuff anyway (like 95 µA lost in the PSUs while the MCU sleeps at 5 µA...) so you can't really do better than that. Where you can save power is in active mode, but it's easier said than done (and it's more expensive to produce)...

Good points, the other BMS' I looked at are much more feature rich and communicative than the Daly BMS.

 

[DW SD]

Thanks much for the info.
is there a standard and convenient and safe way to disconnect The BMS? Vs unplugging for example.
Could I just disconnect with a simple switch the negative number 1 terminal on the BMS (on the cell voltage taps)?

 

[DW SD]

I would plan to kill the relay manually. That would save a few watts. Which addd up fast in sixty days or so.
doug

 

[Dzl]

Thanks much for the info.
is there a standard and convenient and safe way to disconnect The BMS? Vs unplugging for example.
Could I just disconnect with a simple switch the negative number 1 terminal on the BMS (on the cell voltage taps)?

I'm not sure, I haven't come across anyone who disconnects their BMS. One other option is some BMS' like the charger can be powered by an external power source, so the BMS self consumption would never drain your primary battery bank.

 

[DW SD]

Yeah. My camper is in mothballs when i locked it in the barn. No power. I could maybe add a solar panel but the way it is all wired I don’t really want to power SCC.
I guess I could test the drain myself too. But am guessing that would take months.
thank you for the thoughts. Doug

 

[DW SD]

I can’t figure out who makes the BMS.
I sent an email to the batteryhookup folks (who sell it) to ask for a manual or if they could share consumption info.
since Friday when I sent they haven’t replied. So far, I’m not blown away with their service levels.