1. The battery
What kind?If you measure your whole bank (several batteries wired together), pick the bank voltage.
Its total capacityIt's on the label. Two 100 Ah batteries wired side by side (in parallel) make 200 Ah. The label only shows “reserve” minutes? Convert it to Ah.
2. The multimeter reading
12.20 V
Slide to get close, then fine-tune to the hundredth with − and +.
How did you measure it?3. The battery is at about
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How long to charge it? →
Battery voltages
At rest, around 20 °C. The numbers vary a little between brands: if the manufacturer gives its own table, it's the one that's right.
Why not go below 50%?
- A lead-acid, AGM or gel battery wears a little with each discharge. The deeper you drain it, the more it wears: drained to 50% each time, it lasts about twice as many cycles as drained to 80%.
- Left low, it sulfates: crystals form on the plates and it loses capacity for good. Charge it fully as soon as possible.
- Below 20% is a deep discharge. A few of them are enough to damage a battery that isn't built for it.
- In winter, an empty battery can freeze and crack. A full one won't.
Measuring right
- Set your multimeter to DC “V” (the V with a straight line), red on +, black on −.
- Right after charging, the voltage stays high for a few hours (“surface charge”). Wait, or turn on a light for 2-3 minutes before measuring.
- In deep cold, the voltage drops a little: the battery is a bit fuller than shown.
- For a lithium battery, the voltage barely moves between 20% and 90%: trust the battery's monitor.