I noticed that in the night time (dark) the voltage measurement on the battery was lower than in the day time (sun), measured with my voltmeter. I'm new to electricity, but that made me assume it had lost charge.

Voltage in the day in bright sun: 12.56V

Voltage early the next morning: 12.42V

Did I lose charge over night by leaving it clamped?

I'm using a 12v solar trickle maintainer on a 12V sports-bike battery. I am trying to charge it up over several days, and don't want the voltage to drop. The instructions said the battery is done when it reaches 12.7V... I assume it'll take like 5 days.

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    \$\begingroup\$ Most if not all solar panels have a series diode, sometimes a LED, to block discharge of your battery. You can test charge/discharge current flow during light/dark conditions with an amp meter rather than voltage. \$\endgroup\$ – Optionparty Jun 11 '13 at 6:36
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    \$\begingroup\$ I'd have to disagree: Common solar panels do not have a series diode, and probably none of them is using an LED for this purpose since LEDs only allow small forward currents and low reverse voltages. Panels made to be used in a series setup usually have bypass diodes connected in parallel to the solar cells however. \$\endgroup\$ – JimmyB Jun 11 '13 at 11:14
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    \$\begingroup\$ As to the OP: The off-the-shelf (car) battery maintainers (which look like this one) should of course include a diode built-in; otherwise there would be a significant current flowing from the battery through the panel whenever there isn't enough sunlight to keep the panel's voltage above the battery's, depleting the battery and frying the panel. \$\endgroup\$ – JimmyB Jun 11 '13 at 11:30

The voltage on a 12V lead-acid battery will always be higher during the charge state than when it is at rest. So you can expect to see lower voltages 10 - 20 minutes after charging has stopped. This doesn't mean you're loosing power, this is just the natural chemical response of a battery after a charge cycle has stopped. As Optionparty mentioned you have to measure the current to know if charging or discharging is taking place.

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  • \$\begingroup\$ Amazing, thanks so much for you help. So now this makes me wonder. How can I tell when it's fully charged? If the instructions say 12.7V, is that with the charger disconnected? So perhaps 12.8 or 12.9 is ok while the solar panel is connected? \$\endgroup\$ – Jonathan Jun 11 '13 at 19:51
  • \$\begingroup\$ Also, I know I'm not supppppooooooosed to be charging with a maintainer. Am I crazy to do this? :D \$\endgroup\$ – Jonathan Jun 11 '13 at 19:53
  • \$\begingroup\$ I'm afraid to measure amperage with my voltemeter because when I did I saw sparks (connecting the leads to the charging battery/panel clamps) \$\endgroup\$ – Jonathan Jun 11 '13 at 19:56
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    \$\begingroup\$ A battery maintainer can slowly charge a low battery with no problems, as long as you are not draining the battery's power faster than your maintainer can keep up. You have to compare the draining amp-hours vs. charging amp-hours. \$\endgroup\$ – user6972 Jun 11 '13 at 23:16
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    \$\begingroup\$ @JonathanLeaders You likely connected your amp-meter directly to the battery, in parallel. NEVER do this, as it's equivalent to short-circuiting your battery. The sparks happened due to the high current of said short-circuit. Also, small (~5W) maintainers are fine. Your battery can float charge up to about 13.5-13.8Volts. This is roughly 2.25-2.3 volts per cell. If your maintainer is over this voltage, it may damage the battery/cause gassing. Yes, you want 12.6 or 12.7V when disconnected, after the 'float' voltage has come down to the actual charge voltage. \$\endgroup\$ – schizoid04 Jun 24 '17 at 19:44

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