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post #1 of 13 (permalink) Old 06-09-2013, 10:56 AM Thread Starter
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Hi guys any one know about building battery chargers. transformer 220 to 12 volts 120va with rectifier connected I am getting 11.3 volts I then put a capacitor 10,000 uf 75 volts my reading is now 17.25volts what do I have to do to get down to about 13.4volts please help one dumb fool .regards carl

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post #2 of 13 (permalink) Old 06-09-2013, 09:57 PM
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Hi guys any one know about building battery chargers. transformer 220 to 12 volts 120va with rectifier connected I am getting 11.3 volts I then put a capacitor 10,000 uf 75 volts my reading is now 17.25volts what do I have to do to get down to about 13.4volts please help one dumb fool .regards carl
What type battery are you trying to charge? requirements vary some with battery type(vary a lot with lithium-ion!). The filter cap makes the circuit peak reading. The voltage will drop quite a bit when actually hooked up to a load. Lead-acid batteries are rarely charged with a well regulated charger.

So, the short answer is it depends on what you are trying to charge!

I have found that hand tools are the best choice when I want to make mistakes at a slower rate of speed.

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post #3 of 13 (permalink) Old 06-10-2013, 12:39 AM Thread Starter
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Default battery charger

Hi Duane trying to charge 12volt car battery used for back up for my lap top. I run a 1kw inverter off the battery when mains power is down. regards carl.

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post #4 of 13 (permalink) Old 06-10-2013, 03:11 AM
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Hi Carl, it sounds like you are up and at it again! I hope you are not trying to lift that battery. The battery itself will act as a capacitor. My suggestion would be to hook it up and monitor the charge voltage and current (or watch for bubbles in the battery) to the battery. 17.25 volts is not to much for the open circuit voltage (no load) once you apply a load the charge voltage will drop. Watch the charge and see where it stabilizes. On a lead acid battery it should be no higher than 14.5v, with the charger connected. Once the charger is disconnected the battery voltage should be 13+v. I hope this helps. Are you using an old backup unit as a charger?

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post #5 of 13 (permalink) Old 06-10-2013, 04:49 AM Thread Starter
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Default battery charger

Hi Dick I am using two deep cycle batteries for 12 volt in through an inverter to give me 220 out for when mains power is out. will hook up the system and give it a try out this afternoon. have to finnish order for 20 frames. regards carl

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post #6 of 13 (permalink) Old 06-10-2013, 06:58 AM
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Hey there Carl, with lead acid batteries it is not so critical, as the other guys said. Even car charging circuits usually deliver a volt or two above 12 volts. Pretty much the same for Ni-cad power tools - check your nominally 12-volt wall plug-in chargers - you will be surprised.
But if it is an issue, you can get solid-state charging regulators locally, they automatically reduce charge current to a trickle when the battery voltage reaches a predetermined level. The better nicad chargers do the same, and i think all Li-ion ones do.
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post #7 of 13 (permalink) Old 06-10-2013, 07:38 AM
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Carl, alternator/ regulators in auto charge at 14.5- 15+ volts. Just hook up your charger as it is and you should be fine. At 120 VA on the transformer, and 2 batteries the charge rate should be fine.(max of 5A per battery, and that won't be reached all the time! Watch or listen for bubbles as mentioned, but the charge rate will drop anyway as the batteries charge.

I have found that hand tools are the best choice when I want to make mistakes at a slower rate of speed.

I don't suffer from insanity. I enjoy every minute of it.

Last edited by Dmeadows; 06-10-2013 at 07:40 AM.
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post #8 of 13 (permalink) Old 06-10-2013, 07:53 AM
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as the fumes from the bubbles can blow up

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post #9 of 13 (permalink) Old 06-10-2013, 09:18 AM
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Hi Carl - it's usually not a good idea to charge 2 batteries in parallel. Their internal resistances will never be exactly the same, so one will tend to get all the charge at the expense of the other, resulting in over-charging. Should be no problem discharging in parallel, but for charging do them one at a time. My charger delivers about 18V on open circuit.
Welcome back, by the way - Rob
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post #10 of 13 (permalink) Old 06-10-2013, 09:41 AM
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Hi Carl
The transformer should have an amperage rating. Put an amp meter in the circuit and measure the output. As long as you don't exceed the rated amperage rating you should be ok. If the transformer gets too hot you are probably exceeding it rating. It only charges the battery during the part of the cycle when the transformer voltage exceeds the battery voltage. Battery voltage rises as the battery charges which is why car alternators charge at 14+ volts. Simple battery chargers consist of a transformer and rectifier. The amperage tapers off as the battery charges because the battery voltage increases so it is charging less of the time.
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