battery capacity question

[steff bugielski]

What type of device measures the capacity, or state of charge of a battery. And just how does it do it.
We have a smart charger and it can be set to small, medium or large battery size. It then gives a read out of say 65% how does it know? What is the method it uses?

[Windy in Kansas]

It depends on the kind of battery you are asking about. Rechargeable dry cells such as flashlight use or wet cell batteries such as autos use?

I'm assuming you are speaking of dry cell rechargeable batteries. In that case you would check battery voltage to get a vague idea of the state of charge. For capacity the only proper way is to use a more sophisticated charger which goes through charging and discharging cycles to fully determine the storage capacity of the battery.

This article is enlightening: http://www.naturalnews.com/021847.html

With wet cell batteries you would measure the specific gravity of each cell with a hydrometer.

[wy_white_wolf]

Originally Posted by steff bugielski

....We have a smart charger and it can be set to small, medium or large battery size....

make and model number might help us answer your question.

[Wheeloptions]

You can determine the remaining capacity of a battery yourself. Assuming you were testing a nominally 12 volt battery, say perhaps a 72 amp hour size (group 24): Obtain a 12 watt, 12 volt light bulb (1 ampere), wire it up to the positive and negative posts of your fully charged (13.2 volt) battery. Mark the time and let it run down to empty (10.5 volts). If it runs 72 hours then your battery has 100% capacity. If it only runs 36 hours, then your battery has 50% capacity. Or any other percentage of 72 hours running the bulb will give you your answer. You could cycle the battery more quickly with multiple bulbs and adjust the math, however at some point the current draw would be more than reasonable to work with or get good numbers with.

[steff bugielski]

Well that helps but not really what I am looking for.
I am first off talking about flooded lead acid batteries.
I really want to know HOW does the charger know the capacity. I assume there is a instrument to do that. If so what is the instrument called.

I do not know the make or model # of the charger but I would consider a number of different batteries in the medium size range so what exactly is it measuring ?

[Windy in Kansas]

You could determine the state of charge with a hydrometer.

A smart charger would perform a cycled charge, discharge, charge cycle and determine from that how much capacity the battery has.

[farmerj]

you would either check the specific gravity with a hydrometer (already mentioned) or else a battery load tester.


Hydrometer would give you a state of charge, load tester the ability of the battery to handle a heavy load.

[ace admirer]

the smart chargers are measuring the rise in voltage/time of the batt, the small med large switch is adjusting the "taper" or the charge curve to prevent damaging currents to smaller batts,,,,,but allowing shorter charge time (more aggressive charge curves ) of the larger batteries.