Author Topic: Battery Chargers  (Read 2146 times)

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Offline Pete

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Battery Chargers
« on: June 17, 2016, 06:09:41 pm »
I have been turning an old Welder transformer into a 24 volt battery charger. There are two issues, the output of the transformer is around 46 volts AC. I connected the windings to put both output coils in parallel and get an AC output of 23 volts.
Now I know that the theoretical DC output from this should only be 20.7 volts but when you add a a capacitor to filter the output the voltage rises to 32.5 volts.
I know that the battery bank will have a fair amount of capacitance also and am wondering if this charger will actually have enough grunt to charge the battery bank.
It is meant to be used only for topping the batteries up when a generator is running.
I could series the output windings again and use a large triac controller on the primary to reduce the output , but want the charger to be fairly bullet proof.
Can anyone please tell me what the optimal output voltage should be for a transformer on a 24 volt charger .
Pete

Offline oztules

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Re: Battery Chargers
« Reply #1 on: June 17, 2016, 08:03:49 pm »
Straight battery chargers  use different styles of transformer than normal, as we want totally different behavior.

Toroids are electrically stiff..., you ask for 24v you get 24v and loading makes little difference... this is useless for battery charging, as it will blow the fuses at lower SOC and wont finish the battery at higher SOC.... so we are looking for the opposite of this behavior.. ie loads of leakage... we want good current at low SOC, and small currents at high SOC.

Dedicated transformers for this regime are ones where the secondary is loosely coupled to the primary. One favorite way was to wind the primary at one end of the former, and the secondary as far away as you could up the other end... so the magnetic coupling was loose.... some even went as far as using shunts between the windings, to short out some of the field, and shield the secondary from some of the flux.

This way when the battery was only 22v, the current would be fairly strong as the voltage differential is great, and when we get up to the 30v mark, it is still charging, but at a lower rate, as it nears the top end that the transformer can do.... but at low SOC, the current was decent without being brutal, because of the soft magnetic circuit.

Your 23v AC will translate to 23x1.414=32v less diode losses.. so yes it has the voltage for charging 24v bank. It will depend on your magnetic coupling as how well this will work out. ( 23vac as measured was rms. with peaks of 32v )

If it is tightly coupled, you will struggle with heat and over power at low SOC.

You can use a ballast  ( inductor) to limit the current on the primary, or a capacitor on the primary will do the same thing but different methodology.

Caps are a cheap way of controlling the power of the charger, and I have used torroids and caps to this effect.

Your welding transformer will be nowhere as tight as a torroid, so may be ok as is. It was designed to be shorted out without taking out the fuse as you tried to strike the arc, so it must have been a bit loose by design.

I expect it has windings separated by a gap, and this will be a good start. If too strong at low SOC you would be able to place steel laminate strips between as a shunt to limit the current if you need to.

Or.. to answer you question directly.... there is no optimum, it depends on how you wish to tackle it, and what style of transformer and winding you use... nothing is simple with batteries is it...

Consider this.. for a 48v system, the transformer needs to operate sensibly from 46v to 60v..... thats a fair dynamic range to deal with, only loose coupling will do it without resorting to electronics.

..............oztules
Flinders Island...... Australia

Offline Pete

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Re: Battery Chargers
« Reply #2 on: June 17, 2016, 09:10:35 pm »
Thanks Oz, yes the welder transformer is pretty loosley coupled. It is a C core type, it did have a moveable core in the centre of the coils to control the current for welding but I have taken that out. The windings are on two formers. Spaced at either end of the C cores.
I have tried it with two small batteries in series and the current is pretty low, will have to get hold of a couple of larger batteries and see if the current is going to be high enough when they are down on charge. So far it looks like I will need to series the secondaries and use the dimmer to control the primary.
At least that way I can use it to charge 12 volt or 24 volt batteries.
Seems like the science is not so rigid as you say for battery chargers.
Hope it is sunny and your panels are powering well.
Pete

Offline oztules

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Re: Battery Chargers
« Reply #3 on: June 18, 2016, 12:43:31 am »
Yes put it on the real battery bank first at lower SOC, and see before you make any severe arrangements. It may be easy to put on 1 or two more turns on the secondary too, as phase chopping on a genny may be a bit rough too.

Remember your only using the peaks anyway for charging most of the time, so your phase chopper/dimmer will be tough on your alternator i would think.

........oztules
Flinders Island...... Australia

Offline Pete

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Re: Battery Chargers
« Reply #4 on: June 18, 2016, 02:27:30 am »
Thanks Oz will try a couple of car batteries. The small Gel batteries I have to test it with don't seem to accept much of a charge. I am guessing that larger batteries would have a lower internal resistance. The theory will be proven when I try larger batteries.
Thanks again
Pete