Charging a Battery

[precisiontek]

Here goes my silly questions of the day..

Lets say I have 8 24V Commercial batteries hooked together in a series...192 VDC output..(I think that can be done)..If I had them under constant load of 14amps what amount of power would it take to keep them fully charged?

When rectifing AC power to DC is normal to see a drop in amperage and an increase in voltage?

[richhagen]

Use this information at your own risk, basically:

In - Out = Accumulation

If you have a 192V bank discharging 14 amps, you have 2688 watts of power being constantly drained out of your system.  If you had a power source of this size constantly supplying your batteries, and no internal losses to your battery system, then you would be at steady state.  In the real world, your batteries would self discharge at a rate that is a fuction of the type of battery, its size, the temperature, and other factors.  One amp per hundred amp hours or so ought to more than cover that for flooded batteries, so if your 192V bank was a hundred amp hours in capacity of flooded batteries, then another 200 watts or so ought to do.  If you have sealed batteries, then you would have to be especially careful about not overcharging them, and regardless of the type, I would use some type of charge regulation on a system of this voltage and likely size.  For an intermittently available charging source, your sizing of the bank, and the sources gets a little more complex as you don't want to discharge your batteries to deeply in order to not shorten their life expectancy too drastically.  I would need more specifics on your system in order to venture a guess of a recommendation.  Rich Hagen

[precisiontek]

Thanks Rich

I am a machinist.I don't have a clue as to what you mean when you say "One amp per hundred amp hours"  Please forgive my ignorance.

Are you saying that If I am drawing 2688 watts of power that I would nee roughly 2888 watts being continously fed back into them?

[pyrocasto]

precisiontek, batteries are rated in Amp Hours and voltage. He's saying batteries drain at a rate of 1% per day. You would have to know what the Amp Hour rating on the batteries were for sure, but you could guestimate how much extra power to have(overguestimate for saftey). I would say, 16 amps or so charge, if you're draining at a rate of 14.

[chux0r]

I'm sorta surprised no one else has recommended to be real careful with 192VDC.    In my paranoia, I'd put a fuse every couple of batteries or so, just in case I cause something to arc.  A 120VAC arc is not so bad, since it crosses zero potential 100 or 120 times per second or so, but a 192VDC arc would be very hot, and possibly tricky to extinguish while you're panicking your arse off.  (At least that's what I'd be doing.)

However, it does bring to mind a question I have had in the back of my mind for a while.  If you put 10 12V batts in series, you'd have 120VDC.  For 10-20 bucks worth of parts (timer, some power transistors) you could make a cheapie squarewave inverter and power certain items.  I'm thinking CFL's and stuff with transformers like computers and stuff.  (The transformers themselves would probably smooth out your wave enough to work properly.)  Actually, if it's a squarewave you'd probably have to diddle with the duty cycle and/or input voltage to get the correct RMS, but you probably get the idea.

Or if you have a fast circuit, you may even be able to use some PWM to simulate a MSW.  (Yeah, I just used those two TLA's together!)  Say, if you could switch on/off with accuracy down to 1ms or so, then you could use a 1ms on, 9ms off duty cycle when the wave is supposed to be at 12V, 1:2ms cycle at 40V, 1:1ms at 60V, 2:1ms at 80V and so on.  You could get pretty close to a sine wave that way, and it would be quite efficient.

Hmm... have I gone nuts?  Or is that how MSW inverters work, basically?    But my point is, it could be way more efficient with 120VDC coming in instead of 12V or 48V, right?  Or does it even matter?

If it's less electronics, and less current to use 120VDC instead of 48VDC, it's quite tempting.

[Jerry]

Hi chux0r

I have a 120 volt DC re system at my store. We use the 120 volts DC directly for power tools with high rpm brush type motors and resistive heaters.

Its great we love it. Oh incandecint bulbs also but very raerly.

                    JK TAS Jerry

[pyrocasto]

Also, CFLs will work on 120DC. Anywhere from 90volts to 1something.

I would like to go higher voltage too, but I have a 12 volt inverter so I would have to get a transformer. Though, if I go 12 volts, I would probably get a transformer anyway to use higher voltage on the genny...

[stevesteve]

High voltage DC can be nasty if you short it with youself!

AC has the advantage that it tends to make muscles vibrate as the polarity alternates (I have 'buzzed' myself once or twice). I have heard of cases where someone held a high voltage DC source... and the muscles controlling the fingers cramped closed making it impossible to let go of the live component.

I don't know where the cut-off is between DC that will not drive a current through a human body (eg. 12V, 24V) and a voltage that can fry you... take care with your system!

 

[BeenzMeenzWind]

I was doing a power supply swap on a large mainframe computer at an insurance company about 10 years ago. Think it was a Stratos, or something similar. We had to read a long warning notice before we were allowed to touch it.

The computer tech told us this was because someone had been killed by a 5V supply in a similar machine a few years before. Apparently they use some kind of 'current pump' generator thingy. (No, I don't know what that is, either?) I do know the 5V output was rated at 2600 amps and had capacitors the size of 5 litre paint cans!

If this hearsay report is true, it suggests that ANY DC voltage can be dangerous under the right, or wrong, circumstances.

I've certainly had 'wake-you-up' type shocks off 50V DC lines.

[stevesteve]

I think the risk with that type of supply probably doesn't come from direct skin contact. The high resistance of skin should mean that only a small current would flow.

I believe that one of the bigger risks from high current capable supplies is when people wear jewellery. If this makes a short it can carry whatever current the supply can manage. In the case of your mainframe supply that could be a wrist watch carrying 5V at 2600 amps. That would be a 13kW heater on your wrist... not nice.

[BeenzMeenzWind]

Yeah, makes sense! That would be REALLY nasty, wouldn't it? Jeez! We were all told at the beginning of our technical training not to wear jewellry. I suppose that's why.

Must admit, it worked! I don't even wear a watch.

[pyrocasto]

I was told in high school about a guy working with high current and wearing a ring. The ring made a short and they said it cut the guy's finger off.

Dont know if it's true, but you would thing it could heat up enough, fast enough, to do it.

[richhagen]

When I went through some first aid training, we went over a section on 'de-gloving' injuries, usually involving jewelry that became entangled with machinery of some type, Think of your flesh as the glove and the bones in your finger as your hand.  Nasty wounds resulting in amputation.  I don't wear jewelry near any type of equipment.  Rich Hagen

[BrianK]

I have some 8 volt train batteries that i use and they are to be charged with about 22 amps. That is if I read the info correctly.

  .

[nanotech]

While in the navy working on the SeaSparrow missile system, there was a cabinet called the SDP (Signal Data Processor) that turned the analog radar return signals from the receiver into digital information the computer could work with.

It had (I think, it's been 10 years) 376 seperate circuit boards all tied into a single 5V buss bar.

There were all sorts of safety measures we had to go through before doing the current limit test on each of the 375A 5V power supplies for this buss bar.  MANY more safety measures than for working on the 10KV, 10GHz Klystron (of which I'd LOVE to get a hold of 4 of.  They had REALLY strong magnets in them!! )

The safety procedures for the SDP had been developed after 3 servicemen (in seperate incidents) were "plasma'd" when they grounded out the power supply through themselves.  One of them literally had his arm vaporised to the elbow, of which it was totally cauterized instantly!!!  :O

[precisiontek]

Safety is the foremost thing on my mind when dealing with this type of power...It does get a little "boogery" working around this stuff...Whats really scary is the DC output I am dealing with..600+ Volts...

Thanks for info all..

Gary

[stevesteve]

Ouch... that's got to smart somewhat!

[BeenzMeenzWind]

I wasn't going to mention this, as it's an alternative energy board not a description of military technology but...

I also heard a story about someone that walked in front of an over the horizon missile detection radar while it was running. The frequency for these wierd black-magic things is fairly low for a radar, between 900 & 1900 Mhz, typically. I honestly don't remember the type, but I've worked on NATO Type 93 and you ain't allowed to go within 100 metres of that thing. Big phased array panel fed by 6 HUGE generators. Apparently this OTH job was LOTS bigger and more powerful.

The story goes that our man casually strolled across the front of the planar array while it was operating at full uni-directional output, presumably during a test run (they set fire to nearby trees if they don't sweep, usually). The individual pulses for Type 93 are in the high Megawatt range at that kinda distance, so what was this baby chucking out?

Anyway, the story goes that our hapless friend actually made it across the width of the radar's gaping maw before dropping to the ground and dieing three days later in hospital.

WE occasionally get news reports of dead tramps (hobos) here, who've decided to sleep on the platforms of microwave telecomms repeaters (usually a few hundred watts or so). Apparently it's warm there. I bet it is.

[nanotech]

Yup.  The Mk48E surface and air search radar that so many US Navy ships use is 250MW of microwave radiation.  

They cannot run it at full power within 50 miles of land, as it will cause severe interference with ALL types of electronics!!

Within 25 miles of any airport, it will effectively "jam" the airport's air traffic control radar.

Sorry for the OT distraction.  Even though I've been out for over 10 years, the stuff we used to play with (yes, I was young and stupid.  I tracked a seagull with a fire control radar so as to cook the bird in flight.  Ew.) still fascinates me......

[nothing to lose]

"I tracked a seagull with a fire control radar so as to cook the bird in flight."

Ohh, yummy, and they say the military does not eat well. Was that duck, or wild turkey?? I know, pheasant under radar

[Tyler883]

W.r.t. Direct current....ohm's law is always present.....

IMO, a super capacitor at 5Volts is very harmless...there it noway that you can find a perfect enough short - be it from a ring, or your skin - to have a low enough resistance to correlate with the kind of current you will need from a 5V supply for it to be dangerous.

I beleive that there is truth to your story, but I just think that there is more to it.

[RC in FL]

You want to have a charger that voltage regulates at 212.8 vdc.

If you truely have a constant 14 amp load the charger to have the capability to put out 212.8 vdc @ 14 A + trickle charge current.  Trickle charge current depends on how big the batteries are.  

For rough guess, lets say 212.8 vdc @ 16 A charger.  That's 3400 watts.

I doubt you really mean 14 amp drawn 24/7.  If so, the charge current should not exceed 20% to 30% of A-H rating as charge amperage (20A for 100 A-H battery)

[RC in FL]

PS, this is potentially lethal voltage stackup, so be careful.