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Regulated power supply on lead battery bank

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alex0432

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... how about this question.... My electric boat project has 24vdc lead batteries driving dc motors. For extended range I want to use a Honda inverter-type generator (2kw) to drive 24v regulated dc power supplies that would be in parallel with the lead batteries. These are cheaper and lighter than iron core transformers.

How much current would flow from the Reg DC supply ? Batteries have very low impedance, and so being in parallel, at 24.0 full volts batteries would hog the load, right ? But when battery voltage drops just one volt to 23.0 upon partial discharge, would the reg DC supply put full rated current into the batteries while trying to keep up the voltage ?
 

OutToLunch

New Member
your electric boat will have a gas powered generator for extended range? Wouldn't a gas powered motor be more efficient for extended range?

Anyway, why do you need to run the regulated supplies in parallel with the batteries? Or are you expecting the regulated supplies to run the boat and charge the batteries at the same time? I guess it depends on what you want from your system. If you only want the generator supplied regulated supplies to run the boat, then the batteries should be automatically disconnected from the system when the generator is running. If you want to run the boat and charge batteries at the same time, then you need to regulate the charging and take into account the charging current with the load the boat is pulling to size your regulated supplies - and your generator needs.
 

alex0432

New Member
Yes, a gas outboard motor is always the cheapest way to go. However, in this case its an electric drive so as to be a quiet sunset island cruiser. Because the boat is moored off shore there is a generator in a ventilated compartment for battery recharge.

On extended runs, both the batteries and generator are needed for power, neither one is adequate alone. The DC supplies by them selves dont even come close, but are intended to make up maybe 30% running power. My goal for the dc supplies is simply to get more current to the motors while cruising at full power power far away... which I hope is not often.

I already have the generator, and so dont want to add a noisy gasser outboard when ebay power supplies could do the job.

What do you think about how the current would flow ? Would the dc supplies do anything, or provide very little current when connected to 23v system voltage.
 

OutToLunch

New Member
two supplies that are completely independent of each other - one of which is likely to have foldback protection in case of an overcurrent condition while the other will supply tons of current into a fault. They will also have different output voltages and you want to place them in parallel.

Perhaps using the regulated supplies to be inputs to the battery while using the batteries to always run the motor would be a way to go. I suspect, though, that the regulated supplies are voltage controlled with over current protection so that when the battery voltage starts falling, the current from the supplies will increase until OC and then shut down. You'll need some type of current controller in between the supplies and the batteries so the output voltage of the supplies can be maintained while supplying the maximum current they can source to the batteries.

As the voltage differential between the supplies and the batteries increases, the power loss of this current controller is going to increase. The device could be as simple as a bank of power resistors or could be an active transistor bank or...
 

alex0432

New Member
Thanks for your thoughts, OutToLunch, So you do agree that the batteries, when even slightly below the set regulated voltage, will act as a near short circuit to the power supplies? These supplies have pot to control the voltage +/- about 1v. Would adjusting the output voltage to follow the battery voltage be practical, or will it be too sensitive for that ?
 

OutToLunch

New Member
Yeah, The voltage differential between the batteries and the output of the regulated supplies would force the supplies to keep trying to bring the battery voltage up which means more and more current. That's why there should be some type of intermediary between the battery and the supply.

If the batteries and supplies are tied directly, tracking the battery voltage would be an odd type of cyclical control loop. The supplies would be tracking the battery voltage but the battery voltage is the output of the supplies. so, in effect, the supplies are trying to track their own output.
 

tcmtech

Banned
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Realistically you would want to keep the charging system set up to try and maintain the battery voltage at a normal full charge like with a automotive charging system.

Your vehicles alternator output does not follow the battery voltage. It tries to keep it at a constant voltage. It only drops its output voltage when the electrical load is greater than the alternators output can produce. As soon as the load drops back down it starts recharging the battery again. If the batteries are fully charged they draw very little current.
Your actual running voltage on a 24 volt DC battery system is actually going to be around 28.8 volts when the batteries are properly charged.
Same as your cars 12 volt electrical system is actually around 14.4 volts when the engine is running.

I would be inclined to set the charging system up to deliver 28.8 volts and put a thermal limiter on the chargers. If they overheat they just shut off until they cool down and then would automatically start charging again.

The generator and chargers work as the primary power source when they are running. It only when the load exceeds the charging systems output that the batteries get used. And when the load drops below the charging systems limit it starts putting the power back into the batteries until they are at full charge again.
 

crutschow

Well-Known Member
Most Helpful Member
The nominal fully charged, open-circuit voltage of your batteries would be 25.2V. If the voltage drops below 25.2V the batteries will start supplying current. If the voltage is above that, then the batteries will start absorbing current and charging. As noted, the typical maximum equivalent automobile charging voltage is about 28.8V.

So you would adjust the power supply voltage as desired to either supply external current and let the batteries supply some current (less than 25.2V), just supply external current (25.2V), or both charging and supplying external current (between 25.2V and 28.8V).

In operation you may want to but an ammeter in series with power supply and adjust to voltage to get the desired current for your particular operating conditions. If the power supply can operate safely as a current limited supply then you could just increase the voltage until you reach the current limit and let it run at that point.
 

MikeMl

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Most Helpful Member
...
Your vehicles alternator output does not follow the battery voltage. It tries to keep it at a constant voltage...
WRONG!!! The vehicle system voltage is determined by the battery's State of Charge. If the battery is not fully charged, the Alternator behaves like constant-current power supply with a current-limit of about 50A. A discharged battery will absorb all the current that the alternator can produce and then some.

This causes the initial bus voltage to be determined solely by the battery terminal voltage. With a mostly discharged battery, the initial system voltage could be as low as 12.2V; the battery voltage will gradually (linearly) increase as it accumulates charge. When the battery voltage approaches the set-point of the Voltage Regulator, the VR will reduce the output CURRENT of the alternator by reducing the alternator's Field current. The VR modulates the Field current of the alternator. The alternator acts as a current-controlled current source. Iout=k*Ifield, where K ≈ 35 for most alternators.

Back to the original poster's question, a sure way to blow up a non current-limited power supply is to connect it to mostly discharged battery. You MUST provide current-limiting, or the power supply must have been designed for battery charging. In the case of an automotive alternator, the current-limiting is build-in via magnetic flux saturation in the stator. In the case of battery-chargers, the current-limiting is implemented various ways, but it is ALWAYS there.
 
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MikeMl

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... So you do agree that the batteries, when even slightly below the set regulated voltage, will act as a near short circuit to the power supplies?
The only thing that will limit the current if the power supply is set higher than the battery voltage is the resistance of the wires, and the internal battery resistance (≈ 5-15mΩ) for large batteries. ANY practical battery charger has current-limiting as well as voltage regulation. See my other post to this thread.
 

tcmtech

Banned
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WRONG!!! The vehicle system voltage is determined by the battery's State of Charge. If the battery is not fully charged, the Alternator behaves like constant-current power supply with a current-limit of about 50A. A discharged battery will absorb all the current that the alternator can produce and then some.
Attention deficiet strikes again! :(

Aparantly you did not get to the next sentence in that pargraph so here it is again! :D:)

It only drops its output voltage when the electrical load is greater than the alternators output can produce.
Man! the OCD detail nazies are out in force today! Too bad they stopped taking there ADD meds!:D ..l.. :rolleyes: :p
 

alex0432

New Member
MikeM, you hit on the key aspect of the question here.... that is, do the regulated power supplies have built-in current limiting so that when driving into that 23v low-impedence battery, does it see a short circuit and shut down, or does it go to full current, full power and just stay that way.

The reason I say 23v, is that this auxilliary power source would only be used in conditions when the battery are getting run down and are below the 2.0v/cell.

... back to magnetic saturation current limiting in transformer-type chargers. What would a nominal 24vac RMS transformer with bridge rectifier do here ? Would the voltage peaks above 24v feed current into the batteries, then zero current for the entire rest of the wave cycle, making a very low average current ? And, if so does that mean if I use the transformer and diode method for current boost, that I should modify the transformer winding to make say, nominal 28vac RMS ? This, I would think would use more of the wave cycle, but saturate and flatten out on the peaks leading to increased heating ???

Any better ideas on getting 24vdc current boost from that little generator ?
 

crutschow

Well-Known Member
Most Helpful Member
The efficient way to use more of AC waveform and smooth the peak currents when generating DC is to use a large inductive choke at the output of the rectifier. But for the currents you need here, that would be a very large choke for the 120HZ rectified waveform. But if you want maximum efficiency that would be the way to go.
 

alex0432

New Member
Yes, the weight is an issue here and that is why I want to avoid iron core transfomers... it would be about 100 lbs of them !

I spoke with a power supply engineer and he says the voltage regulated supplies are generally not current regulated and would overload into that 23v battery bank. He challenged me to dream up some kind of feedback circuit for current limiting.... yeah, sure... in my spare time..

Maybe Ill just take the kids sitting the dock fishing rather than electric boating : (
 

tcmtech

Banned
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Actually an iron core transformer in the 3 KVA capacity is not all that heavy.
I am fixing a 24 volt 120 amp golf cart battery charger supply right now.

The guy just dropped it off about an hour ago!

The transformer is around 35 - 40 pounds and the whole charger could be reduced to a 1 foot cube and weigh under 50 pounds if really need be.

The old transformer growls horribly and he wants me to change it out. The one it uses is a common 120/240 input to a 48 volt ct type. and its not a ferro resonant current limiting type either.
The actual open circuit DC is around 34 volts. It's rated to hold 28 volts at 120 amps.

I am switching it out with a 24 VAC 3 KVA buck/boost transformer. I just need to switch its DC side from a duel half wave bridge to a full wave bridge.

The control circuit is a simple voltage monitor. When the voltage gets over 28 -29 volts it disconnects an SCR that bypasses a big 3 ohm resistor. The resistor gives it a low current trickle or float charge.

It's old but well built, and the guy says it has never had any problems charging golf carts back to back all day! He picked it up from some golf course that he frequents that was clearing out old stuff.
They told him it worked very well for many years but it just started growling so bad it made people worry that something was wrong with it!
He had it in his garage mounted to a wall for a few days and he said he could here it hum through the whole house! The wife said fix it or trash it!
 

alex0432

New Member
120 amps in 40lbs thats some serious amps/pound ? I can live with that.

I thought 120-amp chargers were for fork lift banks ! Did you mean 12 amps ?

Maybe Ill just go with a couple beefed up chargers. They already have the built in smart circuits.
 

tcmtech

Banned
Most Helpful Member
Most commercial high use battery charging systems I have seen often run continuous duty rate chargers that can supply current at around 1/2 of the batteries amp hour rate. For a golf course I can see where having a unit that could charge a dead cart with 250 Ah batteries in under 3 hours would be practical.

It may not obey the "super ultra multistage micro possessor battery charger" rules, But so what if the batteries fry in a year or two. What do you think those outrageous golf corse fees go to? :confused: ;)

Here is a picture of the actual charger system all taken apart. I used a pop can as a size reference.
3 KVA is physically not all that big. As transformer core dimensions go up the VA per inch of cross section goes up too!
A 10 VA from a wall power pack may only have a 40 - 50 VA per square inch capacity. But large ones like these can easily equate to a 150 VA per square inch of core area.

The old transformer is on the left. It still works good but it does have a horrible growl. I may try re varnishing the windings and core to quiet it down some. If not I just got a good 10 pounds of copper! :p

My old bathroom scale says the old transformer weighs 43 pounds. The new one is slightly smaller but still rated the same KVA wise.

The output leads on the new transformer are doubled up 6 Gage. And the bridge rectifier I am using is a standard international rectifier 150 amp full bridge. So yea I am pretty sure its not a 12 amp! :p
 

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alex0432

New Member
Thats interesting tcmtech, so that would be 24/48 rms vac transformer, with the 34vdc being the peak of the rectified wave.... So maybe standard transformers and diodes would be an o.k. way to go.
 

tcmtech

Banned
Most Helpful Member
I will be the first to admit it's low tech but it is a very rugged and reliable system. As long as it has a voltage monitoring circuit to turn it off or limit its charging current at some preset voltage level they work very well!

I know there are thousands of charging systems like this used every day!
I dont see why a basic modern battery charge control IC could not be implemented into it for further control too if needed!

And I cant see why it would have any problems running off of one of those inverter generators.
 
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alex0432

New Member
O.K. hot news, what do you think of this ? 24v bank of lead batteries = 12 cells, fully charged = 2.12v/cell = 25.4 volts. LEAD ACID BATTERY CHARGING INFORMATION, BATTERY TYPES AND OPERATION - SOLAR NAVIGATOR WORLD ELECTRIC NAVIGATION CHALLENGE, NELSON KRUSCHANDL, BLUEBIRD ELECTRIC LAND SPEED RECORD CARS

At 70% depth of discharge, cell volts = 2.05 = 24.6v this is where I need the supplemental power supply needs to kick in. Going below this voltage level shortens battery life.

So the regulated dc voltage supplies, rated 20amps each @ 24v, are adjusted up to 25v. Now, taking the battery bank as a zero-ohm impedence, 24.6vdc constant voltage source.. at this discharge time.. the current flow from the power supply with a .05 ohm series resistor would be (delta V)/R = (25-24.6)/.05 = 8 amps.

The power supply(s) can handle this, but the total draw down by the motors which is much 3x more than the electronic power supplies can provide, will eventually bring the battery bank down to 50% depth of discharge = 2.01 v/cell = 24.1v for the bank.

In this reduced battery condition, the regulated 24vdc power supplies, with .05ohm resistors in series will see (delta V)/R = (25-24.1)/.05 = 21 amps

So, it looks like with a .05ohm resistor in series with the 25v set-point power supplies, they will be working withing their standard ratings.

Feed back, please !


The max resistive power loss would be IxIxR = 400x.05 = 20 watts ??? I can live with that !

That resistor could be #14 coiled copper wire ( @ 2.58 ohms/1000ft) = 20ft wire = .0515 ohms

With extended runs that draw batteries below this voltage, the electronic power supplies would have to be disconnected. Thats no biggie.

So, I guess Im answering my own posting. If you have observations on this , they are greatly appreciated. My goals for using these switch-mode power supplies is to save cost and weight on the boat.
 
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