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Old 24th June 2005, 09:38 PM   #1
taloyd is offline taloyd  United States
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Default battery charger = difficult


I'm working on a battery powered Hypex UCD400. So I have 4 sealed lead acid 12V batteries in series for each + and - voltage rail for each stereo amplifier channel (=16 batteries total).

Now, the easiest way I can think of charging the 8 batteries, is to run 96 volts at low current across the two sets of four batteries, as the polarity will be - on one side of the 4, and + on the OTHER side of the OTHER four batteries.

negative -- l|l|-l|l|-l|l|-l|l|--*--l|l|-l|l|-l|l|-l|l|-- positive

where the * is the ground voltage between the voltage rails.

So, after listening for a while to music, they'll probably be around 13-13.5 volts each. Which means right around 105 volts across. Which is only slightly less than the 110 volts coming out of the wall here in the US. So, if I were to full-wave rectify the AC, add some simple filter caps, and maybe throw a few LEDs there to indicate that it's charging, I could be around the right voltage.

So the next issue, is how would I limit CURRENT to the batteries? Obviously, connecting 8 sealed lead acids, directly to rectified AC is a very bad idea, as the internal resistance (or lack thereof) of the batteries would soak up enough current to immediately blow them up.

So what's the easiest way to limit current? A current-divider seems like it would work, but I suspect the internal resistance of the batteries is going to fluctuate to the point where it would be difficult to construct a consistent current-divider to achieve what I want, which is to trickle-charge the batteries when not listening to the amplifier. The batteries would be hooked up through a DPDT switch to either the amplifier or to the trickle charger.

All advice is appreciated...


-tal allweil
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Old 24th June 2005, 09:48 PM   #2
Mark Kravchenko
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Default Things to do and things not to do

Normal caharging voltage is 14.5 volts. That is per battery. If you set them up in series yes you can charge them at a higher voltage.

But the idea of rectifying the mains is not very smart. After rectification your voltage is 1.14 times the original. So 117 x 1.14 = 138 volts. You can't do this off the wall without having an isolation transformer.

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Old 24th June 2005, 11:00 PM   #3
Stocker is offline Stocker  United States
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IIRC lead-acid batteries current-limit by their nature during charging.
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Old 25th June 2005, 10:01 AM   #4
AndrewT is offline AndrewT  Scotland
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DO NOT rectify straight off the mains !!!!!!!!
Extreme DANGER !!!!!!!!
You must use a transformer to generate the voltage you need.

The series charging will overcharge the more efficient batteries and produce excess gassing and temperature leading to low electrolyte levels and excess maintenance.

I would recommend a relay system to connect the batteries in series for your PSU duty and in parallel or separate for the recharge period.
Separate current, voltage and temperature monitoring for each battery during charge will maximise the life of your batteries. With the cost of semiconductor circuits Vs battery cost this initial capital outlay may be worthwhile.

Modern batteries do not like deep discharge cycling unless specifically designed for this duty. Otherwise they end up with reduced life and accelerated replacement cycles.
regards Andrew T.
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Old 25th June 2005, 12:39 PM   #5
paulb is offline paulb  Canada
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Lead-acid batteries can be charged with a constant voltage, and floated indefinitely at a particular voltage, but constant current is not recommended (at least not for extended periods). Read up on the UC3906 (datasheet, app note) at for info.
Charging in parallel, however, may lead to problems unless the batteries are all kept at the same charge condition. A more-charged battery will drive current into a lesser-charged one.
One factor is, how long is acceptable to charge the batteries for? If you're using them for short periods and can accept long charging times, it would simplify things.
A big old DC-DC voltage converter, running from smaller number of larger batteries, may be a better solution.

p.s. I like your Goering quote.
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Old 25th June 2005, 02:15 PM   #6
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Default Re: Things to do and things not to do

Originally posted by mwmkravchenko
After rectification your voltage is 1.14 times the original. So 117 x 1.14 = 138 volts.....

Mark [/B]
it's 117 x 1.414 = 165.5v!!!

(At least i think "I think" im correct? sombody cross check me on this!)

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Old 25th June 2005, 03:14 PM   #7
imix500 is offline imix500  United States
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I'm with you there on the math Zero.
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Old 25th June 2005, 04:53 PM   #8
Mark Kravchenko
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Default You stand in good company

Yep I screwed up the number. Sorry

1.414 is correct

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Old 25th June 2005, 05:44 PM   #9
cowanrg is offline cowanrg  United States
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i do have 4 isolation xformers for sale in the trading post...
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Old 1st July 2005, 06:11 PM   #10
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Don't lead acid batteries need a constant current source? I remember seeing a website where this guy rectified AC mains through a diode and fed the DC in series through a 100-W light-bulb directly to a 12-Volt car battery.

"$1.00 trickle charger" or something like that....

As someone else said that is obviously a very dangerous idea and I *definately* not do it but it might give you an idea

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