Small 12 V battery is flat

Some electric motorcycles have a small 12 V auxiliary battery besides the big traction battery. It powers the dashboard, lights, horn, alarm and the computers that switch on the main battery. When the bike is on, a DC-DC converter recharges it from the traction battery. If the 12 V battery goes flat, the bike can stay dead or refuse to go to ready, even with a full traction battery (SolarQuotes). Many small commuter bikes have no 12 V battery at all; check your manual first.

Is it safe to ride?

The bike usually cannot be ridden. If it does wake but the 12 V battery is low, lights, horn and indicators may fail, which is unsafe in traffic.

Quick checks first

• Does your bike have a 12 V battery? (Manual.)
• Has the bike stood for weeks? SolarQuotes describes a 12 V battery going flat after about two weeks parked.
• Was anything left on: lights, a USB charger, an alarm, a GPS or fleet tracker?
• Are the 12 V terminals loose or corroded?

Diagnose it step by step

1Does your model have a separate 12 V battery?

• No: the low-voltage supply comes from the DC-DC converter. See Dashboard is dark and the bike seems dead.
• Yes: go to step 2.

2. Are the terminals clean and tight? Clean and tighten battery terminals.

• No: clean, tighten, retry.
• Yes: go to step 3.

3. Measure and charge the 12 V battery: Check the 12 V auxiliary system. Use a charger that matches its type (lead-acid/AGM or lithium). For lead-acid, see Test and charge a 12 V battery. Does the bike wake after charging?

• No: check fuses and the main harness, then the dealer.
• Yes: go to step 4.

4With the bike on and in ready, does the 12 V reading rise (where the manual says it should)?

• No: the DC-DC converter or its fuse is not charging the battery. Workshop.
• Yes: go to step 5.

5Does it go flat again after parking for a few days?

• Yes: something drains it with the bike off. Find what is draining the battery. Trackers, alarms and added accessories are common causes. If no drain is found, the battery itself may be worn: replace it.
• No: it went flat from standing. Before long parking, follow Store an electric motorcycle; Zero, for example, advises a trickle charger on the 12 V battery for most of its models.

What happens if you jump-start from a car or another bike: you may damage the bike's electronics. Only do it if your manual describes how.

Causes, most likely first