Controlling a battery charger.

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 needed.

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?

Reply to
<RamRod Sword of Baal>
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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?

Bye. Jasen

Reply to
Jasen Betts

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.

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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.

Regards, John Crighton Hornsby

Reply to
John Crighton

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.

Reply to
<RamRod Sword of Baal>

That is exactly what I am trying to achieve

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.

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 find..

I take it you are in Hornsby, Sydney.

Thanks for your help.

Bob

Reply to
<RamRod Sword of Baal>

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 connection.

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 load. 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.

Regards, John Crighton Hornsby

Reply to
John Crighton

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