How to use Astron VS-50M as a Battery Charger

[John_AZ]

I am looking for some advice and best practices on how to us an Astron VS-50M as a battery charger to EQ and Float charge my 12V 130A-hr FLA batteries.

I just purchased a used one off of eBay and hooked it up to a battery with a SG of 1.175. I'm using 6ft of 4AWG cables.

I set the Voltage dial all the way up and I measured 14.95V. I adjusted the Current dial, but the meter won't go any higher than 12A, no matter how high I turn the dial. I was looking for 16.25A for C/8. I have to put the dial at 1 o'clock (~60%) for it to get to 12A. It will go below 12A, if I turn the dial down.

Am I doing something wrong?
Is there something wrong with the Astron I got?

Why only 12A no matter how high I turn the dial?
Also, as time passed, the Current dropped to 8A.
I thought this unit could be used for constant current charging? Am I misunderstanding something?

I did read here about turning it on before hooking up the battery. Where should the dials be when attaching the batter?

Any other advice or mods needed to use this safely?

Battery specs say EQ at 15.5V. Is there a way to modify it to go up to 16V?

I'm a ChemE not an EE, so I know simple circuit design, Ohm's law and how to use a multi-meter, but that's about it, so best treat me like a noob.

Thanks for all the wonderful advice you guys give.

[Sunking]

John I can help you out, and tell you what is going on. Not only am I an EE, I am also a Ham Radio Operator and have a couple of Astron Power Supplies. Since you have the basic fundamental math and engineering skills I can explain what is going on. Lets start with the Current Limiter. First nothing is wrong, it is working like it should.

So here is what is happening, pure simple physics and Ohm's Law. Batteries are self Current Limiting as a result of Ohm's Law. All batteries have Internal Resistance (Ri) and this limits the current they will accept. So lets do the Math.

Charge Current = [Charge Voltage - Battery Open Circuit Voltage] / Ri

The Current Limit Adjust just limits how much current the Power Supply can deliver so it does not destroy itself. How much curren any circuit will take is a function Voltage and Resistance.

Current = Volts / Resistance.

What you are witnessing is Ohm's Law in action. So lets run a couple of models. Assume we have a 12 volt battery, 50 amps charger, and the battery Ri = .010 Ohms, and Battery OCV = 12 volts. We set the Charger Voltage to say 14.4 volts. Now we connect it and see what happens.

Initially from the above equation we have:

14.4 volts - 12 volts / .01 = 240 amps.

But that is not what happens because the 50 Amp Current limit in the charger limits the current to 50 amps. So the Current Limiter Folds Back, or lowers the supply voltage. Again using Ohm's Law Voltage = Current x Resistance. So now the charger lowers the voltage to 50 amps x .01 Ohms + 12 volts = 12.5 volts. Now the math works. 50 amps = 12.5 volts - 12 Volts / .01 Ohms.

OK as the battery charges up the battery voltage rises. The charger keeps pumping 50 amps until the battery voltage rises to 13.9 volts. The moment the battery voltage goes above 13.9 volts, the current starts to taper off toward 0 Amps. When the battery OCV = 14.32 volts, charge current = 8 Amps. When Battery OCV = Charger Voltage, Current = 0 amps. This is called Saturated Battery.

This is what you are seeing, and it means your battery is fully charged and just about fully Saturated. To fully charge a 12 volt Pb battery, you set the Charge Voltage to 14.4 volts, and when charge current tapers to 3% oc C where C = the battery AH capacity. Example for a 100 AH battery you terminate the charge when current drops to 3 amps.

If you want to see 50 amps, discharge the battery. Then charge the battery and set the Current Limit knob to 16 Amps or whatever you want. To EQ the battery, set the Astron PS voltage as high as it will go, and charge until the Specific Gravity in each cell are roughly equal. Forget about the voltage and current. Well not completely, because you do want to Current Limit to be whatever you want, but just remember once the battery is charged, you are not going to see full amps. Think of it like a Speed Limit. Just because the limit is 50 mph does not mean you have to go 50 mph, just no faster.

One thing you can try to get a little more voltage out is open up the Power Supply and find Trim Pot VR and turn it up all the way. Be careful because there are 3 Trim pots inside. One for Current Meter Adj, Voltage Meter Adj, and the one you are looking for Voltage Regulator Adjust. Here is the schematic to your PS. Find the POT in the schematic connected to pin 5 of LM723.It is called 2.5 V ADJ. Hopefully that will get you to 16 volts for EQ Charge. Do not confuse that with R5 which is the Pot on the Front Panel to adjust voltage on the knob.
EDIT WARNING.

It is perfectly fine to use the Astron as a battery charger. Thousands of Ham Radio Operators do it including myself. But there is a monster waiting and hiding inside to bite you if you loose AC Power, and when AC power comes back on. The power supplies are designed as a Power Supply and not a battery charger. It is not a flaw, you just need to modify the circuit. Click Here to see mod.

Last note. Make damn sure you use a fuse between the Power Supply and Battery. Connect it to PS Positive Term Post, and the output of the fuse to the battery positive term post. If not, and if the Crowbar Circuit ever operates will put a dead short on the battery. That would fry your power supply and set the battery wires on fire.

[John_AZ]

Thank you so much for sharing your knowledge.

I now understand what I am seeing with the meters on the Astron. I think engineers become engineers because we want to understand 'why' not just 'what' so we can come up with a better 'how'.

I've ordered the parts to make the recommended mod to the PS.

Thanks again for all the time and info you put into your replies around here.

[John_AZ]

But there is a monster waiting and hiding inside to bite you if you loose AC Power, and when AC power comes back on. The power supplies are designed as a Power Supply and not a battery charger. It is not a flaw, you just need to modify the circuit. Click Here to see mod.

Last note. Make damn sure you use a fuse between the Power Supply and Battery. Connect it to PS Positive Term Post, and the output of the fuse to the battery positive term post. If not, and if the Crowbar Circuit ever operates will put a dead short on the battery. That would fry your power supply and set the battery wires on fire.

I made the mod and I have a 50A fuse on the positive terminal like you suggested.

Now, I am wondering about removing the Crowbar Circuit? This is the thyristor SRC S0565J, right?

What would be the pro's and con's of removing the Crowbar Circuit?

Thanks in advance for your thoughts.

[Sunking]

I made the mod and I have a 50A fuse on the positive terminal like you suggested.

Now, I am wondering about removing the Crowbar Circuit? This is the thyristor SRC S0565J, right?

What would be the pro's and con's of removing the Crowbar Circuit?

Thanks in advance for your thoughts.

Hey John sorry for the delay, just back from South Carolina playing golf and flying. Correct, the SCR is the Crowbar Circuit.

The Crowbar Circuit is an Over Voltage protection for the output transistors. Keep in mind this Power Supply is designed for ham radios connected to an antenna, and thus susceptible to static and lightning discharges which can raise the voltage seen at the output and damage the output transistors. So Astron put the circuit in as protection aka TVSS.

The Monster I spoke of using it as a battery charger is if you loose AC power or turn the Power Supply off. If and when it happens, the SCR circuit sees the battery as an Over Voltage when you loose power, and when it turns back on, BANG! The SCR fires and puts a dead fault across the output. If you have a battery connected, it will burn up the SCR, traces on the circuit board, and possible battery wires. The SCR is the Sacrificial Lamb. Cheap and easy to replace. It takes a lot queer electrons to blow it.

No big deal if you know about it and how to work around it. I use a Diode and fuses. One down side to using a Diode, you loose 0.6 volts. Again no big problem if you crank up the 2.5 volt reference, but most likely you will not get more than 15 volt output. For radio operators that is no problem because we Float our batteries at 13.6 to 13.8 volts. In other words you loose 16 volt EQ ability. No bid deal, it just takes longer to do an EQ charge.

To disable the Crowbar Circuit, just remove the SCR. If it were me, I would use a Diode and fuses. Lots of info on the web on Astron, all from ham radio operators. Kust Google and use the search term ? "Astron Power Supply Mod" Lots of stuff on Repeater Builder and QRZ.