a question which i would like answered, i have a 12 volt battery charger its old and dosnt regulate it produces 0 to 16 amps depending what buttons you press, the question is how do i know what amps to pump into a flat battery? batterys obviously have A/HRs stated on the side but do i need to work it out with a formula of some kind?
battery question
- Thread starter markymark2406
- Start date
You should charge a battery with a slightly higher voltage than the output of the battery, so for a 12v battery 13v would give you a trickle charge & 15v would charge it quicker. Most chargers I have seen have a variable voltage not current, output.
Although marked as a 12v charger the output will be higher ie 13.8v
the current limiter switches are there to limit the current that a flat battery would demand and if left to charge at a high current would cause overheating of the plates and possible boiling off of the battery fluid(ionised water).
the current limiter switches are there to limit the current that a flat battery would demand and if left to charge at a high current would cause overheating of the plates and possible boiling off of the battery fluid(ionised water).
Just as a rule of thumb, to avoid excessive gassing etc. aim for a charge rate that would charge the battery in about 12 hours.
So for a 48 Amp Hour battery, about 4 Amps would be appropriate, or 6 Amps for a 72 AH battery etc. etc.
The higher the charging current, the more gassing occurs, and the greater the chances of loss of electrolyte etc.
Higher charging currents may cause a dangerous pressure build up from the gases produced in 'sealed' batteries, hence the need to remove filler plugs etc. when charging.
Edit:- The internal resistance of a lead-acid battery is not much more than about three tenths of nothing, so the charger output voltage doesn't need to be much above the battery voltage to produce quite a high charging current.
So for a 48 Amp Hour battery, about 4 Amps would be appropriate, or 6 Amps for a 72 AH battery etc. etc.
The higher the charging current, the more gassing occurs, and the greater the chances of loss of electrolyte etc.
Higher charging currents may cause a dangerous pressure build up from the gases produced in 'sealed' batteries, hence the need to remove filler plugs etc. when charging.
Edit:- The internal resistance of a lead-acid battery is not much more than about three tenths of nothing, so the charger output voltage doesn't need to be much above the battery voltage to produce quite a high charging current.
Speaking from personal experience. The gassing from an over-charging battery is oxygen and hydrogen in the exact mixture for explosive combustion. Do NOT remove the croc clips with the charger switched on.
I did......the spark ignited the gas mixture, two cells lost their tops, a window in the shed was cracked ( flying plastic ) and I was deaf in one ear for several hours.
My excuse is that I had not checked to charger was switched off, it was a three way switch 12v 0ff 24v and I had flicked to off but it had gone on to 24v and I hadn't noticed.
Most manufacturers seem to quite C/10 charge rates, i.e. charge at 1/10th of the capacity of the battery in Ah. The switches on your charger will likely control the maximum charge current to the battery. As the voltage across the cells rises this current will naturally decrease.
The charger could be constant current, but it doesn't seem likely if it's old - it would also need to have some kind of multi-stage system to terminate the charge when complete.
The charger could be constant current, but it doesn't seem likely if it's old - it would also need to have some kind of multi-stage system to terminate the charge when complete.
A 12V charger usually varies between 13.8 (trickle charge) and 14.4 (charging) some have a boost of about 16V for a proper dead one.
The ampage only affects what rate the battery charges at (simplified view a 50Ahr battery charged at 10A will be full in 5Hrs, at 1A, 50Hrs.)
The ampage only affects what rate the battery charges at (simplified view a 50Ahr battery charged at 10A will be full in 5Hrs, at 1A, 50Hrs.)
We are all assuming that your battery is lead-acid. Is it lead-acid and, if so, is it sealed or open? An open battery has plugs you can remove to check the water level and top up if necessary. Sealed batteries - which always have a pressure release vent - may be full of liquid or have gelled electrolyte. The gelled ones go by such names as dry-fit and can be used in any position.
A 12V lead acid battery will start gassing off when the charge voltage exceeds 13.8V. That's 2.3V per cell. This is something you must avoid with a dry-fit or you will ruin it. The charging circuits in cars may go up to 14.4V but car batteries are always wet.
It is quite likely that your old charger doesn't regulate anything at all and those buttons are marked with expected current delivered into a 'typical' battery. As electronicsuk says, a charge current of one tenth of the battery's capacity is reasonable, that's 5A for a 50 A.hr battery. This is commonly known as the C/10 rate. Some would argue for C/20.
With an open battery you can hear the gassing and you might notice that some cells gas before others. This is not unusual in old batteries. When they're all gassing it's full.
To find out what's really going on in a sealed battery you need to check the terminal voltage as it charges. At 12V it is not full. At 13.2V it is. The next 0.6V is not real charge but a surface charge on the plates prior to gassing. It will disappear very quickly when you start drawing current.
Dry-fit batteries require chargers with current regulation and voltage limit. Don't bring this thing anywhere near one until you know what it's doing!
A 12V lead acid battery will start gassing off when the charge voltage exceeds 13.8V. That's 2.3V per cell. This is something you must avoid with a dry-fit or you will ruin it. The charging circuits in cars may go up to 14.4V but car batteries are always wet.
It is quite likely that your old charger doesn't regulate anything at all and those buttons are marked with expected current delivered into a 'typical' battery. As electronicsuk says, a charge current of one tenth of the battery's capacity is reasonable, that's 5A for a 50 A.hr battery. This is commonly known as the C/10 rate. Some would argue for C/20.
With an open battery you can hear the gassing and you might notice that some cells gas before others. This is not unusual in old batteries. When they're all gassing it's full.
To find out what's really going on in a sealed battery you need to check the terminal voltage as it charges. At 12V it is not full. At 13.2V it is. The next 0.6V is not real charge but a surface charge on the plates prior to gassing. It will disappear very quickly when you start drawing current.
Dry-fit batteries require chargers with current regulation and voltage limit. Don't bring this thing anywhere near one until you know what it's doing!
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