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Old 22-02-2006, 09:33   #31
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Sean, something I don't know, do you have any figures for Battery resistance at flat and fully charged??? It would be interesting to do a calculation.
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Old 22-02-2006, 17:20   #32
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I agree, Wheels. I will see what I can come up with. It's interesting though... Ohm's Law does seem like it should apply... ??

I know when you look at a graphical representation of the bulk phase of charging on the internet, it shows a continuous current going in, and a varying voltage, coming up from whatever the dead voltage is (10.5V?) to the full charge voltage (13.8V).

My thought was that the battery's internal resistance would vary to satisfy Ohm's Law, as it went through bulk charging.

(V=IR)

Then, I was assuming that when the voltage reached 13.8V, smart chargers would then trip over to the absorption phase, where there is then a constant voltage (as opposed to the constant current supplied in the bulk phase). In this case, Ohm's Law would be satisfied, and then a smart charger would provide a voltage, and vary the AMPS as needed to maintain the voltage.

I guess I have to do some more reading, at least to see where it might vary from what I wrote above. Actually, it would seem that you could use Ohm's Law to calculate battery resistance over the entire charge (all phases) by simply plugging in some knowns and leaving R as the unknown.

If (during charging) I=100amps constant current, and V= whatever you measure with no DC load at the battery terminal, then by Ohm's Law, you can arrive at R, the battery's internal resistance during a charge.
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Old 23-02-2006, 02:48   #33
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OK, I think I see the error. An AC transformer is both constant current AND voltage. Simply put, it is a sum of current and voltage. It produces power expressed as watts or more accurately expressed as VA. VA=power available to use or the Apparent power. So the real figure is that a dead short load on a transformer capable of supplying 100A at 50V =5000VA. A transformer that is 100A but has a voltage output of 13.8V = only 1380VA. The impedance of the copper winding is where the loss in voltage/current occurs. The power is turned to heat in the winding and the transformer gets hot. If the winding is heavy enough to maintain the short, it will be able to dissipate the heat build up without burning out,or at least for a while, which is then called the "duty cycle".
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Old 23-02-2006, 04:47   #34
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Battery Ri

R = E / I

Although I don’t know why you would want to, the Internal Resistance of a Battery can be calculated:

First, we need to measure the "open circuit" (E) voltage of the battery. This is simply the voltage at the battery terminals when no current is being drawn. Practically, it's hard to measure voltage without drawing some current, but most quality digital voltmeters have a high enough input resistance (10 megaOhms) that it can be neglected* for mathematica purposes.

Next, we need to load down the battery and measure the voltage at the battery terminals again (V). Let's call the load resistance RL. A word of caution: be careful when using low value resistors with good batteries, as the current draw will be high enough the heat up the resistor (and the battery!). Given all those values, the equation for the internal resistance (Ri) of the battery is:

Ri = (Eo-Vl) / I
or
Ri = (Vs / I) - RL
or
Ri = RL x (Eo / Vl) - 1
derived from
Vl = Eo x RL / (Ri + RL)

where
Ri = Internal Resistance (Battery)
RL = External Load Resistance
Eo = Open Circuit Voltage (Battery not loaded)
Vl = Loaded Circuit Voltage (Battery under load)
I = Current (when Battery loaded)

Expect very small numbers (< 0.01 Ohm) for a new fully-charged battery. Expect a non-linear increase in Ri, as the battery discharges. As noted, you'll want a chart of numerous Ri calculations, at varying state-of-charge.

* Do NOT attempt to directly measure Internal Battery Resistance with an Ohmmeter - very BAD things will happen.

FWIW,
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Old 27-02-2006, 20:16   #35
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Follow up.

After a lot of calculations, we have decided to go with the Itoa charger(s). I'll get two of them and hook them up in parallel, each running off one of my two 30amp power circuits. Possibly two 75's or two 90's, depending on my final battery bank size and some 110VAC power requirements/loads.

Thanks to everyone for helping me through this exercise. In a few years... maybe we can talk solar?

PS: I've been reading that AGM batteries have the fastest charge acceptance of all types of batteries. Could be beneficial in this case since I'll be charging them through bulk phase in 2 hours. Anyone love or hate AGMs? Considering I'll probably fry (sulplhate) them in 2 years, I wonder if it wouldn't be cheaper just to use the standard old lead acid batteries and go through them faster.

Anyway, thanks... this was a good exercise in chargers, both traditional, and outlandish.
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Old 27-02-2006, 20:59   #36
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Only advantage of the AGM i that it is sealed. Flooded will last longer and take slightly more abuse. The charge time is not that much faster. The cost difference is huge.
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Old 28-02-2006, 15:21   #37
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Hi Sean,

There is an AGM thread going on right now on the SSCA board.

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Old 28-02-2006, 17:27   #38
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Hello Sean

Please review the archived submissions in this thread on "How fast you can charge your battery" and "Understanding a model for battery charge acceptance. They may help clarify some of the issues and long-held myths about batteries.

Don't confuse AGM with lead-acid batteries. AGM, Gel, and flooded construction batteries are all sub "species" of lead-acid batteries. There are good and bad versions of AGM and GEL just like there are good and bad flooded-cell batteries.

Most AGM and gel batteries are killed by low quality charging systems and, therefore, have given them a bad rap in the cruising community.

You may be mis-spending your money in an attempt to parallel two chargers. You will NOT be able to get rated charging current by paralleling charge sources due to the inability of the internal regulators to "track" each other. One regulator will always dominate and the other will cause a decrease or complete shutdown of charge current. This problem exists with all power supplies not designed to operate with "source-sharing" in mind.

The two major aspects in your application is look for a charger with a pfc rating which means that the charger will appear to the shore power as a resistive load and, therefore, you will not have to derate a 30A breaker to a lower value before it opens due to a lagging power factor charger. Ideally you want a SINGLE charge source capable of delivering as much current as the number of Amp-hours "missing" from the battery that you desire to charge in a minimum amount of time. Not many people have the luxury of such a charger as you can imagine, yet that is the limit of what you could use.
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Old 28-02-2006, 17:46   #39
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Rick:
Does ”... a charger with a pfc rating ...” refer to Power Factor Corrected equipment?
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Old 28-02-2006, 19:46   #40
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Re: Hello Sean

Rick... thank you very much for stepping into this thread. I have learned a lot from your other posts. Sorry about using the incorrect terminology regarding AGMs. I knew better.

To expand a little bit on this... my "missing" power is 250ah/day max. My thought was to use a 600-800ah bank to allow me to operate between 50% discharge and 80-90%, using a bulk mode charger to get me between 50% and 90% in 2 hours max.

I had asked you this in a PM, but you can disregard the PM. I should have asked in public where everyone can benefit from the answer:

If connecting two chargers in parallel isn't going to work due to the internal regulators getting out of sync, could I just hook each charger up *directly* to the battery terminals, so each charges its own battery? A previous owner had dome something like this on my boat before. They added an extra charger connected directly to the dedicated DC refrigeration battery, which was also charged by the main 50amp charger.

Instead of the above setup, I thought I might keep the existing 50amp charger turned off during my genset run. Then, I would run a Iota 90amp charger on each battery in my 600-800AH bank. Would this method work, instead of paralleling them?

I'll check out the archived threads now... to refresh. Just when I think I have this DC stuff all figured out, it changes on me.

If there is no "off the shelf" way to take the energy my genset makes and charge my batteries up at a reasonable rate (reasonable for genset running times), I think I'm just going to build something.

I did find some units that would work, but... they were $3000. Ugh.




Quote:
Rick once whispered in the wind:
Please review the archived submissions in this thread on "How fast you can charge your battery" and "Understanding a model for battery charge acceptance. They may help clarify some of the issues and long-held myths about batteries.

Don't confuse AGM with lead-acid batteries. AGM, Gel, and flooded construction batteries are all sub "species" of lead-acid batteries. There are good and bad versions of AGM and GEL just like there are good and bad flooded-cell batteries.

Most AGM and gel batteries are killed by low quality charging systems and, therefore, have given them a bad rap in the cruising community.

You may be mis-spending your money in an attempt to parallel two chargers. You will NOT be able to get rated charging current by paralleling charge sources due to the inability of the internal regulators to "track" each other. One regulator will always dominate and the other will cause a decrease or complete shutdown of charge current. This problem exists with all power supplies not designed to operate with "source-sharing" in mind.

The two major aspects in your application is look for a charger with a pfc rating which means that the charger will appear to the shore power as a resistive load and, therefore, you will not have to derate a 30A breaker to a lower value before it opens due to a lagging power factor charger. Ideally you want a SINGLE charge source capable of delivering as much current as the number of Amp-hours "missing" from the battery that you desire to charge in a minimum amount of time. Not many people have the luxury of such a charger as you can imagine, yet that is the limit of what you could use.
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Old 28-02-2006, 23:47   #41
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Re: Hello Sean

Quote:
You may be mis-spending your money in an attempt to parallel two chargers. You will NOT be able to get rated charging current by paralleling charge sources due to the inability of the internal regulators to "track" each other. One regulator will always dominate and the other will cause a decrease or complete shutdown of charge current. This problem exists with all power supplies not designed to operate with "source-sharing" in mind.
The Iota devices I have on board are designed to work in parallel.

In practice, my two 75 amp units put out the full 150 amps for a while and then the charge current starts to drop off. (This is as you would expect from any charging system.)

Eventually it gets to the point where one unit is working harder than the other, even if the combined output is greater than 75 amps. The only way I know that they don't continue to split the load evenly is that the heat sinks get warmer on one of them.

I have 840 AH of batteries. Based on the observed charge curve, I probably could have good results with a somewhat smaller charger. I'm glad I didn't go with anything bigger. It would have been a lot of trouble and expense for not so much gain.

One nice thing about the arrangment I have is that I can operate just one power supply module at a time. Of course, I only get 75 amps then, but I have fault tolerance and some discretion for my power allocation. For example, if I want to cook something in the microwave oven, I can just turn off AC power to one unit for a few minutes.

I also kept the original 40 amp charger for use when I am at the dock for a time. So I have 3 charge sources on board. The big AC charger (made of two 75 amp iota units), the small AC charger (original 40 amp unit), and main engine alternator (80 amp internally regulated).

I never use more than one at once, precisely because they are incapable of cooperating for the reasons you describe.

Ideally, I would also like someday to add enough solar panels to top off the batteries from the "mostly charged" state that the generator yields to a complete 100% charge. Charging the batteries to completely full is better for them.


One note about iota chargers: I see that jackrabbitmarine.com now lists "Iota battery chargers" and "Iota power supplies". I have the latter, with the external charge controller. It isn't clear to me whether the device labeled "battery charger" can be parallelized.

I also see that both types of device are now explicitly not recommended for AGM or Gel batteries because they are not temperature compensated. This wasn't mentioned when I was buying my system a couple years ago.
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Old 01-03-2006, 00:13   #42
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Re: Re: Hello Sean

Quote:
If connecting two chargers in parallel isn't going to work due to the internal regulators getting out of sync, could I just hook each charger up *directly* to the battery terminals, so each charges its own battery?
Of course, this will work, but you will need to disconnect the batteries from each other. If you use a switch, you'll have to remember to take one of the batteries off the bus before you charge it and put it back on when you are done.

It can screw up an amp-hour meter, though. I really like my AH meter. It is a Cruzpro VAH-35. Basically, it measures DC bus voltage and battery current, and integrates over time to compute the state of charge of the batteries. It is only an approximation, but I find it convenient.

Depending how far you go down this path, you may find yourself with two battery banks instead of one. i.e. two batteries, two chargers, two ammeters -- maybe you just get a 1-2-all battery switch too. I generally don't like multiple house banks (you've probably seen a list of the problems before), but engineering rarely has a ONE TRUE ANSWER for all cases.


About the 90 amp Iota charger: Look at how you are going to wire the AC side of that before you buy. I had to upgrade AC wiring to provide a 20 A circuit for the 75 amp charger. You might find the AC circuit for the 90 A charger impractical.
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Old 01-03-2006, 09:28   #43
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Yes, separating the batteries to receive their own respective chargers is a fine way to get the job done. Naturally it would be "nice" to be able to do all of this automatically without having to throw switches and think about it.

I can't overstress that utilizing a pfc charger (essentialy they are all a class of "switchers" which are high frequency switch-mode supplies, or hybird topologies) not only enhances the maximum possible output of a genset, they help improve the small power genset waveform. For example, a run-of-the-mill inverter/charger or triac controlled charger rated to deliver 130A of "12V" power will require 20 to 25A of ac input current leaving little left for other house use, like a heater, microwave or vacuum cleaner without blowning a 30A breaker (if that is what is available). A 130A pfc charger will only require about 14A-15A of ac input current.

Your solution to buy two chargers is probably the most economic solution due to the dearth of high current units capable of multi-step charging. What you really want to look for is a unit which will allow you to set the acceptance voltage as high as 14.7 to even 15V and then you will be able to more closely follow Amp-hour law charging to minimize charge time safely.

Should you opt to charge the batteries separately do not discharge them separately. Discharging separately more than doubles the loss from the internal resistance (assuming equally sized batteries) and associated parasitic losses. Keep in mind thast the internal resistance increases non-linearly with depth of discharge which makes an even more compelling reason to not separate the batteries during discharge.

You can still find E-meters for less than $100 on-line at a few places which will let you keep track of separate charging and discharging. You will also be able to note that the batteries track each other in terms of percent source current contribution related to any size differences between the batteries, should you have different sizes. If one day you notice a lack of tracking you can deduce which one is degraded.
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Old 01-03-2006, 19:21   #44
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Hi Rick,

I hate to pin this on you, but can you suggest any brands?

I understand what you're saying about the genset's waveform, and how the PFC unit would make more sense in this situation. I hadn't even considered this. Your advice will save me $$... where do I send the check?

Are there any chargers in the price range of the IOTA 90 that are of the PFC type?

I've spent some time trolling around online and came up with chargers in the $1500 a piece range.

For clarity, I have a 5KW genset running through twin 30 amp circuits. One circuit is primarily for air conditioning and had an old, broken battery charger on it. This circuit was added after the fact.

The other 30 amp circuit has all of my "house" loads, such as hot water, outlets, 50 amp charger, etc...

I plan to definitely run the battery bank as one whole unit on discharge. I understand about the depth of discharge affecting life and cycles, as well as getting me into a situation of diminishing returns as the discharge rate increases. I'll discharge as a single 600-800 AH bank, and charge as individual 200-300AH batteries. Haven't picked the exact configuration due to the charger question.

So, ideally, I'll purchase 2 (or three) monster batteries and a PFC charger for each. (genset waveform is the key) Any ideas about manufacturers?

Thanks again...
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Old 02-03-2006, 10:30   #45
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Following up here, everyone appears to be correct. The Iotas at JackRabbit can be combined using an external regulator. This allows them to run without "stepping on each other's toes."

I'm currently working on getting information regarding the PFC factor with JackRabbit. I'll post the results when they come in.
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