Two Definitions of “Discharged”

[Jonah]

Hi all,

I’m reading one of Don Casey’s books on marine electronics. Early in the chapter, he says that 1.75V is ‘dead’ and ‘fully discharged’ for a single cell. He explains how amp-hours and reserve minutes are calculated with 1.75V as the lower limit. This would mean that a 6-cell 12V battery would be fully discharged at 10.5V (6 x 1.75 = 10.5).

Later in the same chapter, he starts referring to 11.8V as ‘empty’ and ‘zero capacity’. This would mean that a single cell voltage of 1.96V is fully discharged.

Which is it? 11.8V? If so, why are amp-hours and reserve minutes based on 1.75V per cell, and why is 1.75V per cell called ‘dead’ or ‘fully discharged’?

 

[Jonah]

Ok, I decided to check in Chapman's Piloting & Seamanship. In the Electronics section, it says that a "fully exhausted" battery will still read 10.5V. But then a few paragraphs later, it recommends never discharging a battery to less than 50% of its capacity, or 12.2V. But if 12.2V is 50%, then 0% must be about 11.8.

So again, I wonder what the difference is between 'exhausted', 'dead', 'discharged', and '0% capacity'. And if I leave my dome light on for a week, should I expect my battery to drain to 11.8V or to 10.5V?

 

[Halfhitch]

...and this matters, why?

 

[Jonah]

Mainly, because I’d like to follow the 50% rule. Or, if troubleshooting a battery, I’d like to know whether 11V is 75% discharged or past empty. Seems like something any of us should know: how low should our batteries go?

 

[seasick]

Battery chemistry is complicated and defining dead vs fully discharged is not so simple. First of all, referring to lead acid batteries, the normal open cell voltage (no load at all, standard temp, new materials and electrolyte) is just about 2.1 volts. So in ideal conditions a 6 cell lead acid battery fully charged at 68 degrees will measure about 12.6 volts. Technically such a battery is fully discharged at 11.8 volts. Remember these voltages are for a standard test cell arrangement and your measurements may be different (although the fully charged cell voltage is determined by chemistry;plate materials, cell impedance and temp. )A fully charged battery during the summer heat would be only 40% charged at 0 degrees.40% charge translates to about 12.1 volts. That means that a fully charged battery in the winter at 0 degrees will read about 12 volts but at 68 degrees it would read about 12.6 volts.

The definition of dead has no specific meaning. The state of charge is what engineers measure. The measured voltage is one parameter used to 'estimate' the state of charge

 

[Jonah]

Seasick, thanks very much. What I'm understanding is that 10.5V (1.75V per cell) is used by engineers as a lower-limit when measuring things like amp-hours in a deep cycle, or reserve minutes in a starter. But 11.8V is what I should use as "fully discharged". So, following the 50% rule means keeping my batteries above about 12.2V.

I hear what you're saying about lot of factors like temperature, but I live where the temperature is almost always above 68°, so I'll use these numbers as an estimate when checking my batteries.

Thanks again.

 

[seasick]

Seasick, thanks very much. What I'm understanding is that 10.5V (1.75V per cell) is used by engineers as a lower-limit when measuring things like amp-hours in a deep cycle, or reserve minutes in a starter. But 11.8V is what I should use as "fully discharged". So, following the 50% rule means keeping my batteries above about 12.2V.

I hear what you're saying about lot of factors like temperature, but I live where the temperature is almost always above 68°, so I'll use these numbers as an estimate when checking my batteries.

Thanks again.

You are confused about two different measurements. Open cell voltage is measured with no load. The amp hour measurement or cold cranking amps are by nature measured with loads. For CCA the load can be quite high. That said, in both load tests the impedance of the cells themselves affect the voltage. The two tests are like apples and oranges as they say If you took a brand new fully charged battery and measured the voltage at the battery terminals you would expect to see about 12.5 volts. Now hit the starter and see what the voltage is. It will be lower but the battery is still fully charged. The internal impedance of the cells affect the voltage. Simple Ohm's law.


I am not sure what you mean by keeping your batteries above 12.2 volts. Is that the point where you would attach a charger and charge the batteries or are you referring to a maintenance voltage as supplies by a battery charger? In the second case, the maintaining voltage should be higher. The key factor in smart chargers and smart trickle chargers is the charging current. Intelligent charges measure the voltage of the battery and adjust the current by varying the output voltage until the desired charging current is obtained. Charge voltages can go as high as about 14.5 volts but it's really the current that does the charging.

 

[Jonah]

You are confused about two different measurements. Open cell voltage is measured with no load. The amp hour measurement or cold cranking amps are by nature measured with loads. For CCA the load can be quite high. That said, in both load tests the impedance of the cells themselves affect the voltage. The two tests are like apples and oranges as they say If you took a brand new fully charged battery and measured the voltage at the battery terminals you would expect to see about 12.5 volts. Now hit the starter and see what the voltage is. It will be lower but the battery is still fully charged. The internal impedance of the cells affect the voltage. Simple Ohm's law.

Nice, thanks. Very helpful. Did not know about how the load could affect voltage without/before draining the battery of its charge.

I am not sure what you mean by keeping your batteries above 12.2 volts. Is that the point where you would attach a charger and charge the batteries or are you referring to a maintenance voltage as supplies by a battery charger? In the second case, the maintaining voltage should be higher. The key factor in smart chargers and smart trickle chargers is the charging current. Intelligent charges measure the voltage of the battery and adjust the current by varying the output voltage until the desired charging current is obtained. Charge voltages can go as high as about 14.5 volts but it's really the current that does the charging.

Thanks again. I meant the first case! I've read, as you say, that the maintaining voltage should be higher, but I trust my onboard charger to get that right.