280Ah prismatic LFP battery winter - Page 1
Forum section: electronics / general

author: Qrdi86
author_rank: Newbie
timestamp: October 06, 2024, 09:59:22 am
content: I would use LFP batteries in an unheated garage, I will make an insulation box for the batteries with a heating element inside. The solar panels would charge the batteries and heat the contents of the box. The charging current would be 0.2C maximum. In winter, there is little sun around here and the temperature can drop to minus 25 degrees. What temperature would be best to keep it in the insulation box? I have heard that at least +10 degrees? And either to keep this temperature there constantly and waste more energy, or when the day comes, the solar panels would first start to heat up the insulation box before charging, and then charging would start when the battery pack is warm enough. Could this constant temperature fluctuation, heating up during the day, cooling down at night, have a bad effect on battery life? What kind of solution to build there?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 06, 2024, 10:31:27 am
content: Really your main enemy is lithium plating, which happens:* Only during charge* More the lower the temperature is* More at higher SoC* More at higher charging currentSome cell datasheets simplify this to a single binary condition: no charging below 0degC, full datasheet charging allowed above 0degC. But you sure can charge even at -30degC if you just do it slowly enough (extremely low current). And if you stop at say 60% SoC then even higher current could be fine.Limiting to max 0.2C charging current already helps, but you might want to think about the case where batteries are nearly full and immediately start charging at 0.2C (at high SoC) at coldest possible temperature. This combination of conditions is your enemy.I would not worry too much about thermal cycling, but uneven temperatures within the box might be a problem, so if you use something like PTC heating cable, prefer a large amount of low-power cable instead of inadvertently creating a hotspot.In the end, getting more insulation and just keep the box at, say, +5degC, could be the easiest way out without having to think about it.And with insulation + the mass of batteries, you won't have much thermal cycling during 24hrs.

author: dietert1
author_rank: Super Contributor
timestamp: October 06, 2024, 12:28:56 pm
content: I guess you want the batteries outside the house for safety reasons. Certainly their thermal insulation box would need several cm of foam, maybe even 8 or 10 cm like we use to insulate outer walls. It might get hot inside that box when loads take more power out of it and when it's warm outside. It may need a high temperature alarm, too.Regards, Dieter

author: rteodor
author_rank: Frequent Contributor
timestamp: October 06, 2024, 02:34:10 pm
content: I can not say how it should be done right but only how I passed over the last 2 winters.It is rather rare here to have lower than -20C but under -15 lows happens in some winters for maybe one week, in others for up to month in total.The packs are in an open attic, almost like outside. The box is 2 cm polystyrene in between 2x1 cm of those woodchips+liant planks (I'm too layzy to search for the exact name). It's the same on all sides.From the bottom going up there is:- 0.3mm Zn plated metal sheet- square pipes of 1 cm (or something about that) from some aluminium. The pipes are spaced about 8cm and the heating wire goes inside the pipes. The wire is cca. 8 or 9 m and consumes about 60W.- over the pipes there is a another aluminium plate 2 or 3 mm thick (can't remember exactly).- another 0.3mm Zn plated metal sheet- polycarbonate sheet 0.5mm- then there were the cellsI said were, because now I added 3 heating film pads and another polycarbonate sheet. The heating wire got me over 2 winters just fine but now I want to get power directly from battery.The BMS is set to reduce charging current to about 19A (1kW) under 7C and stop charging under 2C (this only happened once at low current).To put it this way: you can realize how big/small problem was heating if after 2 winters I still don't have a thermostat in place.More than half of the winter there was no need for heating.When heating was needed (mostly when going negative) I would start heating very early, like 4 or 5 AM. Heating would stop depending on the weather anywhere between 7AM till 11AM. More its no use: self-heating from charging does the rest.The purpose I set was to keep 10C minimum. But its not such a big problem as it seems and many time I would start it manually or have a crontab rule.Sometimes I did forget the heating wire on for the whole day and got like 20...25C.If you have your pack in a garage it might be even easier.TL;DR: What I consider important is to have over 7C, better 10C early in the morning, when charging starts.

author: Qrdi86
author_rank: Newbie
timestamp: October 06, 2024, 03:50:30 pm
content: I have an 80 mm thick foam plastic box there, the battery pack is 12V and for heating I use an aluminum plate that fits under the battery pack. The aluminum plate has one field-effect transistor with a current limiting circuit and a temperature sensor measuring the air temperature both on the aluminum plate and inside the box, the values ​​of both can be changed from the potentiometer. At the moment, I am allowed to heat the aluminum plate up to a maximum of 45 degrees, and it heats the air temperature inside the box up to 10 degrees at the moment and then keeps this temperature there. I can also choose whether it heats the inside of the box all the time from the 12V battery or only when energy comes from the solar panels. So, as I understand it, this 10 degrees would be a suitable temperature to keep, and changing the temperature from day to day would not be a problem? I have only one 450W solar panel in an upright position on the wall, and I don't get a lot of energy from it in winter, there is generally not much electricity consumption there either, but in the meantime I consume up to 0.5C from the battery there even in winter. (I use google translate, so the text may be wrong)

author: Siwastaja
author_rank: Super Contributor
timestamp: October 06, 2024, 06:34:23 pm
content: 10degC is plenty, it's not a problem if it goes a bit below that given that your charge rate is only 0.2C(And probably less than that during winter I would guess?) - I mean, 280Ah*12.8V = 3.5kWh. A single 450W panel during the coldest winter time would probably top out at 300W or so; less than 0.1C charge.To compare, a typical EV with cold battery pack (if heating is not available, or hasn't had time to work yet) would limit charge rate to around 0.1C at -25degC or so; EV manufacturers are fine giving long battery guarantees in such conditions.So I'm 95% certain your LFP pack would be fine even if unheated because charge current is so low. 10degC is more than enough to be on a safe side. Probably thermal insulation alone is enough to even out the coldest hours, especially if there is any battery use causing self-heating. Self-heating (energy output) increases linearly by current so that (dis)charging at 0.5C for 1 hour creates 5 times more thermal energy than (dis)charging at 0.1C for 5 hours, which is why the low-current charge is not going to heat the pack a lot, but your more peaky discharge pattern does.I would concentrate on the insulation, and making sure any heater solution you have is robust against unintended failure. It certainly doesn't need to be powerful. And as dietert1 points out, excessive battery heating might become a problem with good insulation, but maybe you will handle this manually, by opening the box for the summer. Just make sure you do it early enough. If the insulation is too good then the battery might get quite warm early in the spring. And even if it doesn't become too hot to be a safety risk, higher temperature causes calendar life loss of the battery. Even just 30degC is detrimental to lifetime. It would be optimal for high discharge and charge currents, which you don't have, so better to keep the pack a little bit colder to optimize for calendar life.

author: indeterminate
author_rank: Regular Contributor
timestamp: October 17, 2024, 10:44:18 am
content: They do make LFP's that are rated down to -45chttps://en.winston-battery.com/?cnxdc/306.htmlThe set that run my house are now 9 years old and till going strongno heating required

author: Siwastaja
author_rank: Super Contributor
timestamp: October 17, 2024, 01:12:23 pm
content: Quote from: indeterminate on October 17, 2024, 10:44:18 amWinston Chung surely rates his products to any specification customer wants