Heat and Battery Derating

A battery may give less power on a hot day. Makers describe this as the battery protecting itself, and it's related to what's called derating.

What derating means

Derating is running a device below its top rated ability on purpose. Wikipedia defines it as the operation of a device at less than its rated maximum capability to prolong its life.¹

That article is about electronics in general. It says a power part's datasheet can include a derating curve.¹ A battery maker may publish something similar, or may just describe the behaviour in words. Check your own product's manual to see which.

What a battery does when it's hot

A battery has a management system, or BMS, built in. A maker's guide says that if the unit gets too warm, it may cut output or slow charging. It may even shut the unit down.²

The BMS is the electronics inside the case that keeps an eye on the battery.

So on a hot afternoon you may see:

• Lower maximum output than the label says.
• Slower charging than normal.
• A temperature warning on the screen or app.
• A shutdown, if the heat keeps building.

Whether your unit does any of these, and at what temperature, is down to the maker. Your manual should say.

Is it normal?

Makers describe the BMS cutting output, slowing or stopping charging, or shutting the unit down to protect the battery.² If the unit feels unusually hot, shows a temperature warning, or stops charging, stop using it and check the manual before trying again.²

If a hot unit keeps shutting down in a cool room, call the maker's support. Don't open the case.

Why the spot matters

Garages and sheds can get hotter than you'd guess. The guide names a closed garage in July, a sunlit window and a cabinet with poor airflow. Each can keep a unit warmer than you may realize.² For a heat wave it suggests moving the unit to a dry, shaded indoor area with room around the vents.²

Heat also affects charging limits. Battery University notes that many chargers prohibit charging above 50°C (122°F).³

Heat and lifespan

Short hot spells may cost you output. Long ones may cost you capacity. Battery University calls a battery dwelling above 30°C (86°F) one at an elevated temperature, and says elevated temperature hastens permanent capacity loss.⁴ It also says heat is the worst enemy of batteries, including lead acid.³

That's why makers print upper limits for storage and charging as well as for use. If you're unsure how the ranges differ, this explainer sorts them out.

Where to go next

• Find your unit's top operating and charging temperatures in the manual. A guide to finding your own limits shows where.
• Keep the vents clear, as your manual directs.
• For a hard-wired battery, any work on the wiring or the enclosure is for a licensed electrician.