The battery is an electric device that can store energy and provide power. Energy is stored while there is a surplus on the connected electric grid; the charge capacity is 600 Wd (watt days). Each battery provides an unlimited amount of power if there is a deficit, until discharged. Charging power is unlimited as well, but happens at only 50% efficiency, ie. half the energy provided is lost. Batteries self-discharge at a rate of 5 W, even while disconnected. Batteries need to be operated under a roof, or they will frequently short circuit in rain or snowfall, causing an explosion.
Batteries are built, moved and stored like other kinds of movable furniture. They will keep their charge while stored ("minified") or disconnected, only discharging at the normal rate of 5 W (self-discharge).
With regards to power production, they behave like the other electric buildings in the game. All power consumers can connect to batteries in the same way as to other power sources: within a 5-tile radius, a device can directly connect to a battery (or array of batteries), without power conduits in between.
In order to charge a battery, it has to be directly connected to a power grid with excess power. It is not possible to connect to batteries "through" power consumers (this is because consumers can not be connected to more than one power grid at the same time).
Just like the other power producing buildings, a battery acts as a power conduit on the 2 tiles it covers.
Installed batteries can not be switched off manually. As long as at least one consumer is connected, the battery will provide power during a deficit, and also be vulnerable to the short-circuit event.
Installed batteries need to be kept dry. If outdoors, a roof has to be built over the tiles occupied by the battery. If a battery becomes wet (either due to rain or snowfall), there is a very high chance of a short circuit, causing an explosion and fire (probably spreading to adjacent batteries and connected devices). At least the rain will usually extinguish the fire...
Damage to a battery does not change its other properties and does not cause discharge (eg. a 1 hitpoint battery has the same capacity and wattage as a 100 hitpoint battery).
Batteries as power supplies
As a power supply, the battery behaves just like any other power source in the game (eg. generators or solar panels), but each battery will deliver an unlimited amount of power. This means that any wattage can be delivered until the battery is completely discharged. It makes the battery not only convenient as buffers for wind turbines and solar panels, but also to cover arbitrary spikes in power usage, usually caused by turret arrays and other heavy power consumers that are only brought on-line sporadically.
The power output of a battery is theoretically unlimited. Even a huge colony that demands, say, 30,000 W of power can be powered by a single battery (but only for about 1 in-game hour if the battery is fully charged). Adding more batteries is only necessary to lengthen the time span.
If more than one battery is on the same power grid, all batteries with remaining charge will share the power load evenly.
The maximum amount of energy provided per battery is 600 Wd (watt-days) for a fully charged battery. Eg. if exactly 595 W were drawn, a single battery would last exactly 1 in-game day (taking into account the 5 W of self-discharge). If only 295 W are drawn, it lasts for 2 days, etc. Since maximum power draw is unlimited it is – theoretically – possible to empty a full battery in 1 tick of game time (instantly).
A disconnected battery will discharge at a rate of 5 W. This means that a fully charged battery can be stored for 120 days (1 in-game year) until depleted.
A battery will automatically keep charging while it is connected to a power grid with excess power available. Selecting the battery will show the current charge status in the information window.
The power used for charging is unlimited; charging batteries will always draw all available power, but never cause a power deficiency on the grid.
All batteries in a power grid will evenly share the excess power for charging, but can only use half of it (the charge efficiency is 50%). In other words, to fully charge an empty battery, 1200 Wd of energy have to be provided, as well as 5 W extra to cover the self-discharge.
As an example, connecting an empty battery to a power grid with exactly 1205 W of available power will charge it to full in exactly 1 in-game day. If more power was available, charging would complete more quickly. 5 W of charging power would keep the battery at exactly the present charge level, merely nullifying the self-discharge.
In practice, the self-discharge can mostly be disregarded.
Constructing, moving and storing
Batteries behave exactly like other furniture items in the game: they have to be constructed on solid terrain, using the construction skill. It takes 14 work units to build, and can be done even at construction level 0.
After construction, they can be uninstalled like furniture, and then moved to storage, taken to a character's inventory or on a caravan. Uninstalling and reinstalling does not cause additional charge loss, beyond the 5 W of self-discharge that always applies.
It is possible to take charged batteries on caravans as a power source for camps and newly founded colonies.
Batteries that are not installed can be stored in the open, also in rain or snowfall, as they can not short circuit. Like other furniture, they are not subject to decay.
Batteries are very likely to explode in rain and snowfall. This can be completely avoided by keeping installed batteries under a roof; keeping them indoors is not necessary. The resulting explosion will not cause any discharge, but it will heavily damage the battery, and possibly surrounding structures as well, also causing a fire.
Batteries make the short circuit event ("Zztt...") more dangerous: all affected batteries will instantly discharge, causing an explosion in addition to the fire that is normally caused by the event. The more batteries are connected, the larger the explosion. Note that the event is unrelated to batteries – it can still happen if no batteries are connected, but will then only cause a small fire and no explosion. Empty batteries will not affect the event.
Note that it is not the batteries that explode, but the area around the power-conduit that is affected. This could be right next to a battery, but it is never the battery itself that causes the explosion.
While it is possible to lay the power conduits only under fireproof walls, this will not avoid the explosion damage, and any fires that might start as a result.
To mitigate the hazard from the short circuit event, batteries can be intermittently disconnected from the grid, and only brought online on demand. Note that for this measure to be effective, not a single power conduit must be on the disconnected power grid. The short circuit event is related to the power conduits – if no powered conduits are on a network, the event can not occur on this network. So in order to effectively protect the batteries, a power switch must be placed directly adjacent to the battery array, with no other power conduits directly connecting to the batteries. It is also possible to place the batteries adjacent to a power generator, and then decouple the generator with a switch – again, no power conduits can be behind the switch.
It is also possible to completely avoid building any power conduits at all. This is usually only practical for smaller, self-contained grids. The batteries themselves, as well as other power generating buildings, have built-in power conduits that are not susceptible to short circuits. If you can bridge any distances by these means alone, you can create a power grid that is completely safe.
If the above measures are impractical, firefoam poppers should be installed where an explosion would cause a lot of damage, such as a hydroponics facility or a storage area.
Battery performance is not affected by temperature, but like many other structures they are highly flammable and may catch fire if the surrounding temperature is very high. Somewhat contrary to the in-game description, the battery is not more susceptible to heat than other, comparable flammable structures. Even the most extreme heat waves will not make a battery explode or catch fire. It is therefore not necessary to keep batteries climatized.
If only renewable energy is used, at least one battery is mandatory, because sufficient power supply can never be guaranteed. Solar panels produce no power during the night and during an eclipse, and wind turbines are completely unreliable. If the colony has no other backup power source, it is highly recommended to keep some additional batteries behind a power switch on a separate network (see Hazards above), so they can be brought online in the case of a short circuit event.
Alternatively, some batteries can also be stored in a warehouse, and then placed down anywhere on the power grid on demand. This makes switches unnecessary.
Batteries can also be used to make fueled generators more economical. These generators provide constant power, but they also burn their fuel at a constant rate – no matter how much power is actually used. If you connect some batteries, you can at least catch half of the otherwise wasted energy, and then disable the generator until the batteries run dry. This will save 100% of the fuel for the time the generator is offline. This is especially helpful in the early game, where you might not even have enough power draw to fully load a single generator, and fuel will be usually scarce as well.
The energy stored by even a single battery is very high. 600 Wd are enough to power, for example, three coolers at full power draw for an entire day, with no other source of power available. Therefore, building a lot of batteries ("just to be safe") is not necessary, or even harmful. Unless you take complex and expensive measures to avoid the short circuit event, having too many batteries online is a liability. Examine closely how much energy storage you need, and do not build more than that.
When you calculate the battery capacity that is necessary when using renewable energy, keep in mind that charge efficiency is only 50%. That is, only half of any excess energy will be effectively usable. This is particularly relevant when planning a greenhouse setup, of hydroponics basins and a sun lamp. The sun lamp will draw 2900 W during the day, and 0 W during the night, for an average demand of 1450 W. However, it is not sufficient to generate only 1450 W at all times, hoping that a 3-battery array will even out the difference in power draw. This is because during the night, only half of the extra 1450 W will effectively charge the batteries, storing 8,700 Wh of energy (~360 Wd). This will only be enough to power the sun-lamp for 6 hours during the day. For the remaining 6 hours, the missing 1450 W will have to be provided by other means (usually a solar panel). Simply adding more batteries will not change this. Two batteries will provide enough buffering in this scenario.
The example setup, including 24 hydroponics basins, can be fully powered by one geothermal generator and a solar panel, connected to a single buffer battery.
Even in colonies that do not use wind or solar energy, batteries are useful to cover spikes in power usage, usually caused by turret arrays and devices like crematoriums, smelters and mineral scanners that are only brought online intermittently. A battery can cover arbitrarily high power needs, the size of the array only changes for how long that is possible.
As an example, an array of 20 mini-turrets and 4 autocannon turrets draws 3200 W of power. Instead of building another geothermal generator that would cover this, even a single charged battery can power this entire setup for more than 4 in-game hours. An array of 5 batteries can power the defenses for an entire in-game day. The battery array should be isolated with two power switches from the security grid and main power grid. This makes it then also very easy to power the entire setup with a single switch, as well as keep the batteries safe from the short circuit event.