battery box vent

ws9876

I built a battery box in a pit I have in the unheated shop. The pit was a 1000 ga water tank that was removed.I cleaned up the pit and built walls and store bigger items
that I dont use much.Good use of space. Anyway I dug down another 22 inches on one side of the pit and built a battery box and dropped it in there. So the bottom of the battery box is about 4.5 ft below the dirt level in the shop. And the shop is 1 ft below outside ground level.Dirt floor. Uninsulated shop.
I did this because I dont want to have any chance of freezing expensive batteries and its cooler in the summer. We get hot days in summer and can have 40 below for a day or so in winter but rare.Mostly around 0 farenheit. So I drilled a couple of vent holes in the box lid to pipe in air to the bottom of the box and the other one will take air from the top of the
box to vent. Maybe un needed but I have the time.Some people say that the intake vent will just bring cold air into the box where it will reduce battery efficiency.
Do you think that is true??The box will have AGM batteries.

mike95490

Charged flooded batteries will not freeze, but any battery will have lower power at cold temps. At 0°F that could be just 50% capacity.
Discharged batteries are subject to freezing, as sulfur drops out of the electrolyte and it approaches plain water.

Mountain Don

IMO if there is an intake that extends to the bottom and if there is an exhaust higher up, then cold air should infiltrate. Theoretically AGM do not require venting so you might want to do w/o the vents in the winter, under normal operation. If an AGM vents it is not good for it.

We have a pit for storing items we don't want freezing when we leave our cabin for weeks+ throughout the winter. It has a solid insulated lid. The pit is nearly 7 feet deep. Temperature in there stays at about 50 degrees.

Mike is correct. A fully charged lead acid battery will not freeze until it drops to something like -90 F. Of course at that temperature the capacity will be hugely decreased.

Johann

mike95490 wrote: »

Charged flooded batteries will not freeze, but any battery will have lower power at cold temps. At 0°F that could be just 50% capacity.
Discharged batteries are subject to freezing, as sulfur drops out of the electrolyte and it approaches plain water.

If I remember right,
A fully charged battery will freeze at minus -92 F, but a 50% charged/discharged battery will freeze at minus -16F