Controlling a battery charger.

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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 output 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 around 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?

Re: Controlling a battery charger.

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not as such. but you cant use cheap dimmer or speed control circuits to
regulate typical transformers on their imput side.

This means you need to consider controling it on the output side,
where voltages are lower can currents are higher.

Assuming your large charger consists of a big transformer with it's primary
directly connected to the mains you seem to be looking for a way to regulate
its output.

what other parts are there in your large charger?


Re: Controlling a battery charger.

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There is only the 240 volt main transformer with various taps on the
primary, and the diodes with their heat sinks. It has circuit breaker
protection on the primary limiting the current draw. That is about all.

Re: Controlling a battery charger.

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Lets say you had access to a large bench power supply capable
of supplying  30 amps at 27 Volts.  
A power supply that is voltage regulated but adjustable and
with a current limit mode that can be set for 30 amps max.

You could connect your battery bank in parallel with
this power supply. Your battery bank would be float
charged. A small trickle charging current will flow into
your battery bank when no external load is connected
to this combination.

Now lets say you connect your inverter to the power
supply/battery bank.
Lets say the inverter is to power a small to medium load.
All the input current to the inverter will come from the
regulated power supply. The battery would be loafing,
doing nothing, not supplying any current to the inverter.

Now lets say you connected  a few more 240V appliances
to the output of the inverter so that the input current
to the inverter was more that 30 amps, say 40Amps.  
The regulated 27 Volt power supply would go into
current limit mode at 30 amps and the power supply's
output voltage would droop slightly.
The battery bank would then be supplying current to
 the inverter also, around 10 amps.

When the extra 240 V appliances are switched off
the battery float charges back to 27 Volts and the
regulated power supply will provide all the current
to the inverter.

When the mains fails. The battery will supply all
the current to the inverter.

When the mains is restored. The regulated power
supply will supply current to the inverter and also
supply a charging current for  the battery bank.
The inverter current and charging current to the
battery can only be 30 amps max.  When the
battery voltage reaches 27V all the current from
the power supply  will go to the inverter.

Is that method automatic enough?

What I am leading up to is maybe you could use
your battery charger as part of a 27 Volt regulated
power supply with a current limit facility.
All you need to add is a bank of electrolytic
capacitors a regulator chip such as a 723 and
several power transistors on a large heatsink
plus other nick nacks, shunts, meters, box.

Here is an article on 13.8 V power supply
construction for you to get some ideas.
Google for more power supply schematics.

Just looking at that power supply construction
article. You could put two high current 13.5V
supplies in series to give you 27 volts.

Ten to twenty dollars will be enough to pick up a
dud repairable 13.8V  transceiver power supply at
a ham radio trash and treasure meeting.  
You'll  need two that are similar current wise.

Just a suggestion.

John Crighton

Re: Controlling a battery charger.

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That is exactly what I am trying to achieve

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This sounds what I need, and I will look into it. I think that the highest
voltage I can get out of the battery charger is around 30 volts DC and hope
that will be enough to achieve the 27 volts DC after it has passed through
the electronics you suggest.

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Not sure if we would have ham trash meetings here, I am in Cairns
(Australia) with a population of around 130,000,  I will see what I can

I take it you are in Hornsby, Sydney.

Thanks for your help.


Re: Controlling a battery charger.

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Hello Bob,
yes I'm in Sydney.  Since Ham radio junk, such
as power supplies are a bit thin on the ground
up your way, how about experimenting with
a couple of alternators from old abandoned cars
just for fun. Don't spend any money, find them
for free if possible or next to free.  I am thinking
of the type of alternator with its own in-built
voltage regulator.

Visualise a heavy piece of timber such as a piece
of a builders/painters plank with two car alternators
bolted on to it and a 240V electric motor driving
both alternators via belts and pulleys.

I am suggesting using a lump of wood for a base
plate as it is an insulator and we don't want the
bodies of the alternators to touch.
Get each car alternator and car battery to work
separately to your satisfaction. Use car headlamps
as load resistors for testing.

When you are happy with each alternator/battery
setup, connect both batteries in series to give
you a 24 Volt supply. Well close to 27V with
the 240V motor spinning the two alternators.

Down here in Sydney we have periodic council
clean ups where householders can place unwanted
large household appliances and other unwanted
goods as well as plain junk on the footpath to be
collected by the council.  Sometimes people will
put serviceable items on the footpath outside
collection times because they would like someone
else to take it and get some use out of it. Wardrobes,
chest of drawers, clothes dryers, microwave ovens
that sort of thing.  Is that sort of recycling thing
done up in Cairns?  I am all for it.  My scanner
and printer were found on the footpath.

Computer junk is plentiful on the footpath down
here. I hesitate suggesting to modify a computer
power supply but some people have successfully
isolated the common connection. Nah! We wont
go there! Have a go at the car alternators while
looking for a cheap regulated 24Volt supply
or two 12 Volt regulated supplies for series

Getting back to the car alternator idea.
You might have to use one electric motor per
car alternator, depends on what junque you
can find.  Washing machine motors etc.
Nice one horse power motors are a bit thin
on the ground even down here.       :-)

Getting back to your battery charger as the
basis for a 24 Volt regulated power supply.
Find some large electrolytic capacitors and
hook them up to the output of your battery
charger and check what the DC Voltage is.
Find a couple of car headlamps. For example
old sealed beam types with the high beam intact
and the low beam blown, a helpfull wrecker
will let you have them for free. Take a bucket
Connect up your two series connected head
lamps and check the DC output voltage of the
charger using the electrolytic capacitors on the
output. Connect up a second pair of head lamps
and a third pair if necessary. Checking the DC
output votage.   Let us know what you find.

John Crighton

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