low battery warning - Page 2 - Electronic Circuits Projects Diagrams Free

**Nigel Goodwin** · 1st August 2006, 07:36 PM

Quote:

Originally Posted by Hero999

A 12V battery is normally 13.8V when fully charged, which gives 2.3V per cell so seven cells would give 16V which isn't that far off 15V. I agree though it's highly unlikely, I've never seen a 15V gell cell before but I wouldn't completely rule it out.

I would!, they are multiples of 2V, so can't be 15V - admittedly under certain conditions of charge it would certainly be possible to read 15V (for a short time), but that doesn't mean it's a 15V battery.

zevon8 · 2nd August 2006, 12:50 AM

Actually, a fully charged flooded cell battery, like a car battery, will read 12.65 Volts. Each cell will produce between 2 and 2.1 Volts, depending on age, plate wear sediment and electrolyte balance. A "hot" battery, one that has just come off charging will sit at very close to the charge voltage applied, say 13.8Volts in a car. 13.8 Volts is halfway between minimum charging voltage 13.2 and 14.4, where gassing will occur, and you boil the battery. This is why 13.8 was chosen for cars. Once the battery has settled, and cooled down, it will read 12.65 again. Cool down and normalization depends on the mass of the battery and its ability to cool.

Gel batteries tend to run a little higher, but not much. They should be charged differently also, constant current to get them up to nominal voltage, then constant voltage float for maintaining them. Mostly in alarm sytems a fixed voltage regulator keeps them alive, since tehy go in new, and rarely get flattened.

oldtimer · 2nd August 2006, 03:00 AM

A lead acid cell fully charged will have an open cicuit voltage of 2.2 Volts. In a 12Volt battery there are 6 cells in series giving 13.2 Volts. Do not allow more than 14.1 volts across it or you will puncture holes in the plates the debris will then fall to the bottom causing shorts. Check the elecrolyte with a hydrometer the reading depends on temprature but should be about 1280.

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2nd August 2006, 12:50 AM	(permalink)
zevon8	Actually, a fully charged flooded cell battery, like a car battery, will read 12.65 Volts. Each cell will produce between 2 and 2.1 Volts, depending on age, plate wear sediment and electrolyte balance. A "hot" battery, one that has just come off charging will sit at very close to the charge voltage applied, say 13.8Volts in a car. 13.8 Volts is halfway between minimum charging voltage 13.2 and 14.4, where gassing will occur, and you boil the battery. This is why 13.8 was chosen for cars. Once the battery has settled, and cooled down, it will read 12.65 again. Cool down and normalization depends on the mass of the battery and its ability to cool. Gel batteries tend to run a little higher, but not much. They should be charged differently also, constant current to get them up to nominal voltage, then constant voltage float for maintaining them. Mostly in alarm sytems a fixed voltage regulator keeps them alive, since tehy go in new, and rarely get flattened.

2nd August 2006, 03:00 AM	(permalink)
oldtimer	A lead acid cell fully charged will have an open cicuit voltage of 2.2 Volts. In a 12Volt battery there are 6 cells in series giving 13.2 Volts. Do not allow more than 14.1 volts across it or you will puncture holes in the plates the debris will then fall to the bottom causing shorts. Check the elecrolyte with a hydrometer the reading depends on temprature but should be about 1280.