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CaptainK · 18-02-2006, 11:56

Thanks Kai.

Where on the internet would you find this downloaded program at?

And if I have to search for this myself. What listing is it generally under?

Thanks for that last post. I made a copy for a future referance starting point!!

18-02-2006, 12:00

Just Google off the grid system design worksheet. Many of the alternative energy catalogs have a worksheet in them as well.

CaptainK · 18-02-2006, 12:17

Alrighty Kai.

I'll give it a shot!!

Thanks!!

**GordMay** · 19-02-2006, 06:10

What Size Battery Charger?
Deciding on the required charger size is a series of compromises. Higher charging rates reduce both re-charge cycle time & battery life.
For more information, Google: "Peukert's Equation" (Law)

10%
Users with a charger of 10% or less of the battery capacity (A/Hrs) generally complain of excessively long recharge times. Their batteries do however last a long time.

20 %
Users with chargers of around 20% to 30% of the battery capacity tend to seem reasonably happy with the compromise they have of battery life versus recharge time.

40%
Other users (myself included) tend to go slightly higher than this and generally run a charger between 35% to 50% of the battery capacity, enjoy the slightly shorter recharge times, but accept the (again) slightly shorter battery life.

ssullivan · 19-02-2006, 09:29

Very true, Gord. I'm leaning toward the same end of the spectrum you are in. Did you find a charger that is simply a constant-current DC source? I can't find one anywhere in the amp rating I need.

I can't see paying thousands of dollars for a 3-stage or smart charger when all I want is to turn a switch on and blast my batteries with juice while the genset runs.

I basically need a car battery charger, but huge.

ssullivan · 19-02-2006, 10:10

Stupid question:

Assuming that during the bulk phase of charging, you are supplying a constant current - what voltage should the 100 amps be pumped in at?

I know the "no-load" battery terminal voltage goes through an increase during this phase of charging, but what voltage should the DC source be outputting in order to correctly charge the battery bank?

**Alan Wheeler** · 19-02-2006, 10:51

Mate, this has to be the fastest running thread in history. I yawned once and then spent the day trying to catch up.

Answer to the last question Sean, =14.8V
Any "ugly" car charger will do the trick for you. Just find one of sufficient current capacity for your needs. You could build your own, but I have always found it is cheaper to buy one than build one. But if you build, you don't need anything special. Just a very high current capacity Trany, a good high current rectifier if you want it nasty, but you will find it will place a lo0t of "noise" on the DC system, so a good capacitor as a filter system will take care of that. If you want some regulation, that can be added as well.
The main difference between trany and switch mode is, switchmode is light weight, and less bulk. They also use the sales gimick of AC/DC isolation, but a transformer can also do the same thing. It is jut a case of your earthing setup.
You can place transformers in parrellel to gain more current. You can NOT place smart multi stage chargers in Parrellel to gain more current. It mucks up their sensing and they won't charge correctly.

19-02-2006, 11:25

FWIW, when we needed a charger for our fishing boat with a 24 volt system, we purchased it from the MAC Tool truck that came to our work. We got a good price, and the unit was very heavy duty.

coot · 19-02-2006, 18:03

Regarding charge rate: It depends on the battery. I chose AGM batteries partly because they can accept more charge current than similar sized flooded batteries. Different manufacturers specify different maximum charge currents. I would always believe the manufacturer's specification for a specific battery over general statement I read on a web forum.

Using multi-stage chargers while on shore power: I've never heard of a problem with this before. If anybody has more information on the failure mode, I'd like to hear about it. (I can't think of any reason it should be a problem with a well designed unit.)

Iota chargers: The IOTA chargers that I have are really just robust switching power supplies.

To make it into a three-stage charger, you use an external controller that tells the power supply what output voltage to use. That controller cost me something like $20 or $30 dollars in addition to the two power supply units. If you don't want the multi-stage charging, you can leave this part out. The power supply voltage can be switched manually.

The expensive part of this device is the 900 watts of regulated power, not the multi-stage control chip.

The thing that makes a battery charger "multi-stage" is that it looks at either the battery voltage or the charge current to make an estimate of the state of charge of the battery, then adjusts the output voltage accordingly. The idea is to put more voltage (and therefore more current) into the battery when it can take it, but limit the voltage (and therefore the current) when the battery CAN'T take it.

About constant current: The whole point of a constant current source is that the voltage changes, going as high as necessary to keep the current constant. If you want 100 amps going in to your battery, the charger has to turn up the voltage exactly high enough to cause 100 amps to flow.

As the battery starts to fill up, the current flow will drop a little bit and the charger has to turn up the voltage a little. This continues as the battery charges, with the charger output voltage slowly creeping up.

At some point, it will take more than 13.8 volts (or whatever) to keep 100 amps flowing. At that point, your charger should stop turning the voltage up even though the current flow will drop. i.e. It is no longer a constant current source -- it is a constant voltage source.

You can choose that cutoff voltage from your battery spec sheet. For Lifeline AGM batteries, the bulk charge is 14.2 to 14.4 volts; float is 13.2 to 13.3. Other battery manufacturers specify different values.

If you choose the float voltage, it will take longer to charge your batteries from the generator. If you choose the bulk voltage, you need to manually turn of the charger before the batteries are full, to avoid overcharging them.

ssullivan · 21-02-2006, 06:10

Quote:

Alan Wheeler once whispered in the wind:
Mate, this has to be the fastest running thread in history. I yawned once and then spent the day trying to catch up.
Answer to the last question Sean, =14.8V
Any "ugly" car charger will do the trick for you. Just find one of sufficient current capacity for your needs. You could build your own, but I have always found it is cheaper to buy one than build one. But if you build, you don't need anything special. Just a very high current capacity Trany, a good high current rectifier if you want it nasty, but you will find it will place a lo0t of "noise" on the DC system, so a good capacitor as a filter system will take care of that. If you want some regulation, that can be added as well.
The main difference between trany and switch mode is, switchmode is light weight, and less bulk. They also use the sales gimick of AC/DC isolation, but a transformer can also do the same thing. It is jut a case of your earthing setup.
You can place transformers in parrellel to gain more current. You can NOT place smart multi stage chargers in Parrellel to gain more current. It mucks up their sensing and they won't charge correctly.

Wheels (or anyone who can answer this one):

Regarding the 14.8V charging voltage... I just recalled the old V = I*R equation from basic Physics.

Where I = 100 amps, and R = the changing resistance, as determined by the battery.

Following this formula, wouldn't V be a variable that is in fact controlled by the battery's internal resistance? (Assuming a constant current)

I'm pretty sure this is true. Comments?

So given the above, I would have to carefully measure a temperature-compensated voltage on the battery terminals in order to determine when bulk charging is done, right?

Also, I think I found a decent constant-current source for about half what a battery charger would cost. A DC welder. Just have to find the appropriate size and I should be good to go.

ssullivan · 21-02-2006, 06:16

Mark,

Just read yours after posting my reply above. Makes sense to me, and agrees with V=IR.

Thanks!

I'll look into the IOTAs again in comparison to cheap car chargers and DC welders.

**Alan Wheeler** · 21-02-2006, 10:55

Sean, most welders will have an OC voltage of 55V upto about 70V being the norm. Way to high for battery charging.

The Ressitance and temp and blah blah, is why the real real real good multistage chargers do infact have temperature sensing. Temperature sensing is the way to charge correctly. Unfortunately, I have seen a cheap one yet. The more "everyday" type multistagers conduct the excersise in a slightly different way. They are purely sensing voltage. When it reaches a set level, it goes into it's next stage. When it reaches the next level, it goes to it's third stage. It is these units you have to be careful about loads on the batteries at the same time. If the load is such that the charger can not get a voltage up high enough to go to the next stage, it will remain locked in a high current charge stage and cause the batteries to boil.

Sean, I made a mistake. 14.8V should read 13.8V . 14.4V is the max and I don't recommend it unless you have a higly regulated and temperature compensated unit OR some form of smart charger that has a "float" stage set at 14.4V. 14.4V should only be a float stage, not a charge stage.

ssullivan · 21-02-2006, 17:56

Quote:

Alan Wheeler once whispered in the wind:
Sean, most welders will have an OC voltage of 55V upto about 70V being the norm. Way to high for battery charging.

Sean, I made a mistake. 14.8V should read 13.8V . 14.4V is the max and I don't recommend it unless you have a higly regulated and temperature compensated unit OR some form of smart charger that has a "float" stage set at 14.4V. 14.4V should only be a float stage, not a charge stage.

Wheels, gotta trouble you again. Welders are very new to me - I've never used one at all, so this is a bit theoretical. My understanding is that a welder is a "constant current source." It does in deed have an Open Circuit voltage of 55V to 70V. No question about that.

But... when you touch the leads together, doesn't the current go to zero?

Since it's a constant current source, as R->0, V will also approach zero. V=IR

Doesn't that mean it will self-adjust to whatever voltage the battery requires in order to maintain the constant level of amps?

**Alan Wheeler** · 21-02-2006, 20:47

No, umm well yes and no. You are kinda on the right kinda track, but the track is still the wrong one.
Basically the welder is just a big mother AC transformer. Nothing flashy about them. Only the way the current is controlled. Either they are solid state controllers or manually via a large piece of core material called the "choke" that is slid in and out. The OC(open circuit)voltage is just the voltage across the outputs when no load is applied. So you have no regulation. Without regulation, there is nothing stopping the battery from being over charged and over votlage supplied. A 12V battery can be taken toa very high voltage by the way. It will destroy it eventually, but the voltage can climb significantly.
Next factor, you will need to rectify the voltage from AC to DC. that is a mother of a rectifier. Turning AC to DC means the voltage will now be even higher.
Next point, for the Ohms law of V=IxR to work, you assuming R of the battery is going to be the correct value and remain correct through out the charge range. Unfortunately it doesn't work that way. Even if you were lucky enough to have the correct battery resistance to get a 13.8V supply to start, the resistance will not track accurately enough to rely on it to regulate the charge voltage through out the process.
I can get even more complicated if you want me to, just ask. But the end result is, you need a regulator to keep a lid on things. It is easier to regulate switching power supplies, thus one of the many reasons why we see SMPS in use now.