Self build battery - precautions…..

[Marcus]

If i might jump in on this thread (sorry) with another safety question:

Lithium cells generally don't like being charged below 0c - if you have your battery in an unheated location like a garage then the temp could get down to that. Presumably the pylontecs protect themselves but with a diy battery, does the inverter/charger monitor battery temperature? Or do you need a bms with temperature sensing (i was looking at bms's on aliexpress and a lot don't mention temperature protection).

[marshman]

Most decent BMS units will have temperature "protection" as well as individual cell voltage and string current "protection".

Having said that if the battery stack is in an unheated outbuilding / garage then it would be sensible to house them in an insulated cabinet with a thermostatically controlled low wattage heater of some sort, and a fan and vents for the warmer times as well.

[AGT]

An old chest freezer suitably altered to take the weight and any vents would keep them clean and dry..

[Oldgreybeard]

Our batteries and inverter are in an unheated outbuilding, in a narrow passageway to the north side of our house, between the house and a tall wall, so that building never gets any sun and is sheltered from the wind. The building isn't insulated and I was a bit concerned about the batteries getting too cold in winter, so planned to fit one of those oil filled radiators in there, connected to a frost thermostat.

In practice the batteries have never got colder than about 8°C, most of the time they sit at the temperature they are at now, 16°C (just looked and yesterday the peak temperature they reported was 18°C, lowest was 16°C). There's no real indication they heat up when either charging or discharging. For example, they were charging at 3kW for a couple of hours yesterday and the temperature rose by 1°C. This is possibly partly down to us having six US3000Cs, so a 3kW charge is only about 500W per battery.

The main issue I had was with the inverter getting a bit hot when charging. Almost all the heat in that building comes from the inverter, rather than the batteries, and I suspect that the heat from the inverter when charging the batteries overnight in winter is more than enough to keep the outbuilding fairly warm, even with the thermostatically controlled extractor fan I added.

[Marcus]

Good to know, thanks.

I've acquired a few ex- hybrid battery modules with a view to adding to my existing storage capacity (2005 vintage AGM lead acid). But as they're nickel cobalt type i don't really want them in the house, and was considering mounting in a metal box with insulation in an outbuilding - but had a wondering as to how much of an issue low temp might be.

[Oldgreybeard]

Good to know, thanks.

I've acquired a few ex- hybrid battery modules with a view to adding to my existing storage capacity (2005 vintage AGM lead acid). But as they're nickel cobalt type i don't really want them in the house, and was considering mounting in a metal box with insulation in an outbuilding - but had a wondering as to how much of an issue low temp might be.

If your charger or inverter is in the same building I think you'll probably be fine, especially if charging overnight on an off-peak tariff. Our inverter gets a lot hotter when charging than it does when discharging, probably because it only ever runs flat out for a long time when charging, when discharging it rarely runs for long at close to maximum power.

[Oldgreybeard]

Our batteries and inverter are in an unheated outbuilding, in a narrow passageway to the north side of our house, between the house and a tall wall, so that building never gets any sun and is sheltered from the wind. The building isn't insulated and I was a bit concerned about the batteries getting too cold in winter, so planned to fit one of those oil filled radiators in there, connected to a frost thermostat.

In practice the batteries have never got colder than about 8°C, most of the time they sit at the temperature they are at now, 16°C (just looked and yesterday the peak temperature they reported was 18°C, lowest was 16°C). There's no real indication they heat up when either charging or discharging. For example, they were charging at 3kW for a couple of hours yesterday and the temperature rose by 1°C. This is possibly partly down to us having six US3000Cs, so a 3kW charge is only about 500W per battery.

The main issue I had was with the inverter getting a bit hot when charging. Almost all the heat in that building comes from the inverter, rather than the batteries, and I suspect that the heat from the inverter when charging the batteries overnight in winter is more than enough to keep the outbuilding fairly warm, even with the thermostatically controlled extractor fan I added.

I posted this a few days ago but have just been digging through the data from HA and thought this plot of battery temperature over the past week might be useful:

Battery temperature.jpg (37.88 KiB) Viewed 1209 times

As above, the batteries are in an unheated outbuilding. Last night the air temperature just a few feet from that outbuilding dropped to about -2°C, yet the batteries only reported that they got down to about 8°C. This is consistent behaviour, I haven't seen them get below about 8°C since they were installed nearly two years ago. It also looks as if charging and discharging them at around 3kW makes next to sod all difference to the temperature, the variations match the change in outside temperature better than the charge/discharge pattern.

[Caesium]

It also looks as if charging and discharging them at around 3kW makes next to sod all difference to the temperature, the variations match the change in outside temperature better than the charge/discharge pattern.

This tallies with my observations, all the heat in my tech cupboard is from the inverter, the batteries (both DIY and Pylontechs) really don't generate much at all if anything, even when pushing 70A through them.

I guess this means the batteries themselves are pretty efficient and most of the energy you put in must be coming out, all the losses are in the inverter electronics I suppose.

[Oldgreybeard]

This tallies with my observations, all the heat in my tech cupboard is from the inverter, the batteries (both DIY and Pylontechs) really don't generate much at all if anything, even when pushing 70A through them.

I guess this means the batteries themselves are pretty efficient and most of the energy you put in must be coming out, all the losses are in the inverter electronics I suppose.

Seems to be the case. The inverter temperature varies a heck of a lot and can get a bit warm. This is the inverter temperature plot for the same period:

Inverter temperature.jpg (52.91 KiB) Viewed 1196 times