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Control of battery charger

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Posted by <RamRod Sword of Baal> on December 10, 2005, 6:26 pm

I have a basic 24 volt 40 amp battery charger. It has the basic transformer
and rectifier in it. The input voltage is 240-250 volts 50 cycles AC.

I am trying to control it to charge a set of batteries (4 x 6 Volt T 105)
that in turn run an inverter (24 volt to 240 volts AC; 2.5 Kw) that in turn
runs 3 computers, 3 printers and the phone system. Eventually this system
might be set up as an emergency power supply to run the fridge if we have a
power failure, with a manual change over switch for the fridge.

There is a manual change over switch for the computers etc so they can run
either from the mains or the inverter, and they are currently running from
the mains until I overcome this overcharging problem.

The 40 amp charger's transformer has some tappings, but tends to overcharge
the batteries. The input voltage tends to vary somewhat from the incoming
mains (236-250 or so)

Changing the tappings manually does not work, as when the load or input
voltage change the amount going into the batteries can change a lot.
(Excluding what goes into the inverter).

I have a second battery charger (automatic) that seems to do a fine job, and
it holds the batteries at around 27.2 volts with a trickle charge of point 7
amps. This charger cannot supply enough power to the batteries when the
inverter is coupled up hence the need for the above listed 40 amp charger.

The problem is that the 40 amp charger seems to vary its voltage and
charging rate quite a bit, causing the batteries to use quite a bit of
water, or at lower tappings undercharge the batteries.

I am looking for some way of controlling the input power so as to hold the
output voltage stable.

I am looking at something like an electronic motor controller or dimmer that
can hold the output voltage around 27 volts DC (with a pot so I can trim the
voltage if ever required).

It needs to be fully automatic, so it needs to sense the battery voltage,
and trim in input power accordingly.

I would think something that can handle around 6 amps @ 250 volts would be

BTW running both chargers together does not work, already tried that.

So far the 40 amp charger has not had to put out any more than 30 amps when
supplying the inverter, but if the batteries get a bit low, that could
increase to the full 40 amps

The power going to the inverter varies with the load, and so this controller
needs to be able to sense the DC voltage and boost or reduce the output of
the transformer accordingly to maintain a constant DC voltage.

The second choice is something that will switch between the tappings to
control the 40 amp battery charger transformer, but this could lead to
hunting, so an electronic controller such as a motor controller seems to be
a better choice.

So I am looking for something that will sense the battery voltage of around
27 volts DC and will then automatically control a motor controller or dimmer
circuit that will vary the power going into the 240 volt transformer and
hold the DC voltage constant.

Anyone know of any circuits that will control this battery charger for me?

Posted by Robert Morein on December 10, 2005, 8:12 pm

Go to Lowes and pick up a high frequency car charger for about $0. Throw
what you have in the dumpster.

The damage done to batteries by unsophisticated chargers such as the one you
describe are enormous. It really isn't possible to fix, because your charger
is missing 90% of the circuitry in a modern charger.

Posted by SQLit on December 10, 2005, 8:51 pm



Kinda doubt your going to find a 50 CYCLE anything at Lowes

Posted by Robert Morein on December 11, 2005, 12:37 am

High frequency chargers don't care.

Posted by nospam.clare.nce on December 11, 2005, 5:45 am

On Sat, 10 Dec 2005 19:37:08 -0500, "Robert Morein"

How about that 240 volt primary???

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