Lithium auxiliary battery and cold weather charging

[shade]

Does the Victron BMS support heater control?

I saw this enclosure calculator. Enter the enclosure size and insulation. It will tell you the passive cooling load, which is the heat transfer to the exterior. For a a 1 cubic foot box with 1" insulation, in 0F ambient and 50F internal temps, the heating load is only 20W.

Enclosure Cooling Calculator

www.lairdthermal.com

The BMS has a low temp charge cutoff that's integrated with a probe inside the battery.

The BMV-712 Smart monitor pulls temp data from the same sensor, and has a programmable relay. It can also use a second temp probe. A little searching resulted in finding the relay specs. I think it could be used to control a heater relay/circuit.

Otoh, I'd probably want a standalone option to just turn it on and let the heating pad's thermostat take care of itself. I'm not sure there's a need to rely that heavily on the BMV to get the job done. I could do both, in case I forgot to turn it on manually.

 

[luthj]

I personally think the other way around. Both the BMV and the thermostat need to supply signal for the heater to be on. I am considering the possible failure modes. If the heater fails off, the BMS disables charging. If the heater fails on, you could cook the battery.

 

[john61ct]

Yeah, but the Dutch haven't released that tech yet.

I wasn't talking Victron, no need to wait for them.

Any battery can be put in a warm box.

When unused for extended periods, the cells should be isolated from **everything**, ideally including any BMS-type circuitry.

In very cold weather if it's too long to keep warm, just let them freeze (down to a point, see specs). Then start warming the box, say 48 hrs before use, and make sure it's nice and toasty before charging.

Obviously portable modules you can bring inside would be even better, cheaper, easier, less risk.

Just not too warm for long term storage, cold in that context is **much** better for longevity.

 

[shade]

I wasn't talking Victron, no need to wait for them.

Any battery can be put in a warm box.

When unused for extended periods, the cells should be isolated from **everything**, ideally including any BMS-type circuitry.

In very cold weather if it's too long to keep warm, just let them freeze (down to a point, see specs). Then start warming the box, say 48 hrs before use, and make sure it's nice and toasty before charging.

Obviously portable modules you can bring inside would be even better, cheaper, easier, less risk.

Just not too warm for long term storage, cold in that context is **much** better for longevity.

I know Victron isn't the only game in town. My goal with a heating system will be to keep the battery from freezing in the first place. Thawing it out could take a long time, especially since I can't get any heat directly between the cells. Always having alternator & solar charging should make it easier to maintain a reasonable temperature without dipping the SOC too low.

When not needed for a significant period, I'll store my totally unplugged LFP battery in a cool, dry basement at 50% SOC. I can keep an eye on it via Bluetooth and charge it as needed. The Bluetooth function is built into the battery, and it doesn't consume much power. Iirc, Victron allows it to be turned off, but once that connection is terminated, there's no recovering it.

 

[john61ct]

Link to its manual?

 

[shade]

Link to its manual?

https://www.victronenergy.com/upload/documents/Manual-Lithium-iron-phosphate-batteries-Smart-EN-NL-FR-DE-ES-IT.pdf

 

[shade]

I personally think the other way around. Both the BMV and the thermostat need to supply signal for the heater to be on. I am considering the possible failure modes. If the heater fails off, the BMS disables charging. If the heater fails on, you could cook the battery.

I can see that. I was also thinking that the BMS would see an overtemp condition if the heater failed on, which would trip the load disconnect, powering down the heater.

 

[hour]

This is confusing. I put the box out Saturday night at 7:30pm, 33*F outside at the time and the box was reading a solid (even throughout) 75*F. The lid was uninsulated for this test, everything else insulated as it appeared in pics on pg2 of this thread.

The temp dropped pretty fast on the BMS which was to be expected since it's on a shelf above the cells, and closest to the uninsulated lid. Definitely doesn't respond quickly (possibly at all) to the warmth rising from the bottom.

At 5am it was 28*F outside and the probe (wedged between cells, beneath the BMS' platform) was 64*F, the BMS was 56.3*F.

Thing is, for all I know, it could have been the same temperature in there had I applied no heating at all.

The plate has been 80*F +/- 0.2* the entire time, still is - the thing has been sitting outside since Saturday evening. Obviously soaking up some sun in the day time which isn't helping my testing. I don't own a chest freezer and this thing would shatter the glass shelf of my refrigerator, probably collapse it even, so I think I'll just have to do more testing in the wild. If I get an alert that the probe or the BMS fell below 45*F I'll make adjustments.

So I think I've been maintaining the plate at 80*F for 39 or 40 hours now and have used 32 amp hours powering it + router + microcontroller + a screen on a raspberry pi (didn't disconnect before throwing it all in the can for testing). It's pretty much used 4-10 watts the entire time. Even when the BMS was 92*F yesterday from the sun blasting the lid, the plate chugged on at 4-10 watts (probe was like 74*F at the time)

80*F at the plate is obviously not a temperature I'd maintain in real life, but I'm pretty sure if I set it to maintain 50*F the bms and probe will drop too low on cold nights. I'll see what difference lid insulation makes... probably somewhat significant. I just have to be certain whatever I put up there doesn't fall off, like adhesive failure, especially if I use metal lined stuff.

None of this would be an issue if I just made the plate heat based off the probe+bms average temperature, but that would end up with the plate getting far too warm. Tricky situation that I can't wrap my brain around programmatically to resolve. As in how to write sound logic that operates the plate on BMS+probe temps, but tapers off if the plate itself gets too warm.

Convinced this will work but I wish my BMS was enclosed in the battery area instead of on a shelf above it. And TL;DR because I'm at the office and don't want to start working

 

[john61ct]

A very well-insulated box with a tightly sealed envelope is the best starting point.

Check out those super-coolers that keep ice frozen for a week in Death Valley.

Plenty of DIY HowTo's rather than spending $500+

Thus the externally wired BMS and other gear **are** at the same temp as the cells, thermal stratification just isn't an issue.

And the Wh / day to maintain desired target temp is minimized, obviously A. Good. Thing.

3-4" Polyisocyanurate is my reco, edges & corners sealed with Great Stuff, lid / hatch design from marine fridge boxes, in fact can buy those ready-made.

 

[SBDuller]

I'm in agreement to the Polyisocyanurate ( hard word to spell) constructed battery box. When I have battery on hand, I'll start mock-up. Looking @hour container, needs a blanket at least

 

[hour]

Unfortunately I couldn't do any of the thick rigid insulation given the size of the box so I kept the 1/4" stuff (alum skin -> polyethylene backing -> closed-cell polyethylene foam -> polyethylene backing -> alum skin) which lines the entire can and is taped in all seams... better than reflectix having real foam in between (I think). The cells themselves have some additional thick rigid foam surrounding them, but not encapsulating.. so who knows how much that'll help.

I ended up taking a piece of the 1/4" thick stuff sized about 2 inches larger than the footprint of the can and set it on top, then shoved the lid on... very nice seal. I'll see if that makes the difference. Couldn't do anything thick on the lid or it'd touch components i mounted on the walls (BMV shunt mainly). So here's to hoping it passes the outdoor test tonight. I'm going to adjust my heater code to shut off at 51*F and turn on at 47*F - with those values being the average of the BMS and its probe's temp readings. Heat will still operate by PID and I guess maintain like 90*F at the plate when running

 

[john61ct]

The conditions + use case may require going to a different box design, or building an external box that the existing box fits within.

1/4" may have some impact, but hardly rates as significant for reducing the energy required for maintaining a decent temp delta.

And yes Reflectix would be 99.99% useless for that scenario, as with any mylar and/or bubble wrap "solution"

 

[luthj]

There isn't any reason you can't use a dual input control algorithm for the temps. Have two computations, one for the target battery temp (desired PWM), and another for the max heater plate temp (PWM limiting). Compare the two, and use whichever is lowest to set the PWM output. This allows for rapid warm up (up to 100% duty cycle), without risk of overheating. Pushing the plate up to 120F is totally fine, and won't harm the case or battery.

 

[john61ct]

I prefer simple robust-reliability.

But whatever you do, realize cell-internal temp is all that counts but is not available

I imagine measuring cell resistance might be a good proxy, build up a correspondence table over time.

Meantime, be very conservative wrt when you actually start charging and at what rate.

For me with a bank worth thousands, if coming up from below zero conditions, 24hours of the box ambient sitting at over 25°C. Maybe more, for cells over 100Ah.

 

[hour]

An update, FWIW. I was previously running the floor of my battery at a constant temperature (tests at 70*, or 80*, 50*, whatever) and it wasn't really translating to temperatures measured at the BMS or the BMS' probe. This was done via PID control so the floor plate silicone heater was always run at the perfect voltage to maintain the perfect temperature, just not as measured where it counts.

I abandoned PID control as sweet (really just fascinating) as it was for the uh... bang-bang approach of heating (relay on/off as needed). Something better done with a solid state relay to handle a ton of on/offs, I would guess.

I didn't swap my MOSFET setup out to a solid state relay, but simply told it to write a PWM value of 60 (~30 watts) or 0 (off) as needed. So unlike a SSR, I can actually limit the wattage in this approach.. and I believe it can handle a bajillion cycles like an SSR - without the chance of getting stuck ON (I hope)

If the average of BMS' onboard temperature and it's remote probe is < 50*F, I heat the plate at 30 watts until the average returns to 50*F OR the bottom plate hits 105*F. It'll maintain 105*F until 50*F+ is achieved.

What I've found in this approach is that the average of the probe + bms doesn't drop to ~50*F until like 3am on a 30*F night. The ammo can enclosure heats up a little during the day as the truck's camper shell warms up, and as the BMS warms up a little taking in charge. The heater has been running once or twice between 3am - 8am, for no more than about 5 minutes. This is costing me practically nothing in capacity, a lot less than maintaining the bottom plate at a constant temperature. And it's effective - I haven't dipped below 49*F in a week and a half with this current implementation.

I'll follow up during hell week, usually some time in December or early Jan where we don't get above freezing even during the day. Build your heated system around insulation and heating technique if you're gonna do it. But if you're like me and had limited options for insulation (because of enclosure dimensions, and things already being assembled) just do some data logging.