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 Topic: Issue with leisure battery and coolbox
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13/11/2017 at 1:37pm
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Quote: Originally posted by Grampian91 on 11/11/2017
Quote: Originally posted by achrn on 07/11/2017
How are you measuring battery voltage?
If you're doing it with the fridge connected, the voltage reading will be more a function of the instantaneous current draw than of the state of charge of the battery.
That is, while it is (broadly) true that you can gauge the state of charge of a lead-acid cell by measuring voltage, that's only when the battery is open-circuit (ie nothing connected, no current draw). You need to disconnect the fridge, let the cell voltage stabilise, then measure the voltage.
Wrong.
Testing a battery off load can give a totally false figure for its state of charge.
A duff battery can read 12.6v+ and yet when put under any load even 1 or 2 amps the voltage could plummet to below 11 volts.
You're talking about something different.
Specifically, you are apparently discussing a battery with a very high internal resistance (though other faults may give similar effect), which is itself a fault, but one that's not related to state of charge.
If you want to determine state-of-charge of a lead-acid battery, an open-circuit voltage is a reasonable way to do it, and a voltage test of the battery under load is most definitely not the way to do it, because (as I said) that reading will be more a function of teh current than about state-of-charge. However, state-of-charge doesn't tell you that everything is 100% tickety-boo with the battery (and I never claimed it did).
If you have your battery under load and change the load so the current reduces, the voltage will rise. If this voltage actually was proportional to state of charge, it would mean the battery has become more charged without putting any charge in (indeed, while continuing to draw charge out).
In your anecdote, it simply means the battery was near fully charged, but ALSO (in your terminology) 'duff'. I haven't said open-circuit voltage tells you anything about 'duff'iness. It does however tell you state of charge (approximately).
If you want to determine state of charge by means of voltage, the correct way to do it is with no current drawn.
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14/11/2017 at 1:51pm
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Quote: Originally posted by Grampian91 on 13/11/2017
So your off load test shows a large 110 battery at 12.68 volts, would you say it was well charged?
Generally yes.
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Why then will the battery only light a 20w bulb for less than 30 seconds before it dims to a glow worm?
Because it's faulty. It may be fully charged, but faulty (internal corrosion, sulfation, bad connection, etc etc).
Saying that a battery is fully charged is not at all the same as saying that it's in good condition, has low internal resistance, or is capable of providing a particular current for a particular duration.
I haven't claimed that open circuit voltage tells you that the battery is fully operational, 100% fine, good for any and all purposes, or that it has a high capacity. I've said it tells more about state of charge than does measuring the voltage under load.
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