Float charging a Lead Acid Battery (Leisure)

[andybarrett1]

Hi All

This is a black art to me so here goes :-

I have an application where I need to have a 24v PSU in Parallel with 2x Series 12V leisure batteries.

Long story but ultimately what I need to do is keep the batteries topped up by the PSU (Float if you will)!

Lead acid batteries Gas off at 28.8v (Not good)

But wondering if I set the 24v PSU o/p  (trimmer) at 27v it would act as a float charger for the Lead Acids. ?
Or Maybe 27.5v and put a silicon diode in series with the cells.

I understand this is not an ideal situation but passing it by you for feedback.

BR
Andy


 

[Codemonkey]

I used to work for an emergency lighting manufacturer. We always used to float charge 24V batteries at 27.5V and the batteries lasted years so you shouldn't have any problems there.

You might want to make sure you've got some current limiting in your PSU though and limit the current to 0.1C (so for example, a 20Ah battery would be charged at a maximum of 2A).

[KD0CAC John]

Is you PS built with a battery charging circuit ?
Your title says auto battery , your text says leisure batteries ?
The lack of detail can get a lot of answers that do not help . 

[andybarrett1]

Corrected the Title !!

Is you PS built with a battery charging circuit ?

   No Just a Industrial Grade PSU

BR
Andy

[Ian.M]

You want a diode in series with the PSU output to protect it against reverse current and prevent the battery discharging into it while mains power is off.  Use a heavy duty Schottky diode for minimum voltage drop.    Take the load from the battery side of the diode.

[KD0CAC John]

Corrected the Title !!

Is you PS built with a battery charging circuit ?

   No Just a Industrial Grade PSU

BR
Andy

There you go , you need to add battery charge controller .
The details of the charge controller are dictated by the rest of what you are doing , which the details are incomplete so far .

[andybarrett1]

I know I can use a of the shelf charger... Etc etc

But that is not doable in this application ... Long story .. Only got 600vDC available ... Noise levels (electrical) .... Inverters ... Back feeds .... List goes on..

Attempting to keep thing simple ....

Hence original question :- Can I use what is allready there?

BR
Andy

[KD0CAC John]

You can use what you want .
I think most are trying to keep you from reinventing the wheel , but your welcome to find out for yourself , just make the connections .
I do not know as much as many , not EE , but this seems to be a common thread - ask a question & disregard all advice , till you get some one to say - do it ?

[rr100]

The voltage drop on a diode varies with the current so I would take the measurement/trim the voltage with the batteries fully charged and in the "floating" state. But first try to see what's the current with the power supply unplugged from the "mains" to estimate if you really care about whatever current goes back in the power supply.

27-27.5V final on the batteries or anywhere in between ... take your pick (my UPS "floats" at 27.3).

[andybarrett1]

Thank you ..... This is what I am interested in :-)

[mtdoc]

Not all LA batteries specify the same float voltage. Any decent battery should have available a manufacturer specified float voltage. What you use to provide that voltage should make little difference as long as it can supply the necessary small amount of current.

[tautech]

Not all LA batteries specify the same float voltage. Any decent battery should have available a manufacturer specified float voltage. What you use to provide that voltage should make little difference as long as it can supply the necessary small amount of current.

This ^^^^^^

The old rule of thumb for 12V auto batteries was a charging voltage of 13.8-14.2 but with new LA battery technologies it's wise to consult datasheets for accurate float or cyclic charge voltages.

[rr100]

If you want to make it right also note that the charge voltages are temperature dependent - can be important if the temperature can vary a lot. Also the voltage regulation on your power supply might change with temperature...

It's turtles all the way down!

[nukie]

Just get a BatteryMinder

[rr100]

Do they come in 600VDC versions?

[Ian.M]

Doesn't look like it.  Nor do they have a low voltage DC input model that could be fed by the existing PSU.

[andybarrett1]

Thank you all.

Especially Tautech and Rr100..... This was exactly the info I was looking for 

So next week I will be fitting a Diode and Fuse to my Tram project before upping the wick a little.

Will hope to post progress

BR
Andy