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Old 16-04-2009, 13:36   #1
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Charging Capacity & Time to Recharge

Here's the situation:

House Bank of 675amps made up of six 6volt 275amp AGM-Gels (Toyama Hybrids)

Know max charge rate is 25% for wet cells, up to 33% AGMs (Calder)

Options: Install one 100 amp charger or a pair of new chargers (either a 100amp and 70amp, or two 100amp chargers).

If I will be drawing down to 50% 2x a day, with recharging in the morning and in the evening), which above three option will bring me up to 80%:
· the fastest, i.e. the least amount of time? (BTW, I think I know the answer to this one, though I could very well be wrong)
· how much time will it take for each option to going from 50% to 80%? (i.e. the 100amp charger, combo of 70amp and 100amp chargers, two 100amp chargers)
· which if any option(s) would also enable me to go beyond “bulk charging” and do some “absorption charging” as well within a 1 ˝ to 2 hr time period?
Again, keep in mind I want to limit recharging time to no more than 1 1/2 to 2 hours 2x day, while also at the same time cool my fridge 2x a day, and also make water (Alternating Current driven system) while recharging in the evening.

I look forward to the electronics experts on this forum to provide a mathematically based definitive answer.

Thanks in advance.

William
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Old 16-04-2009, 14:00   #2
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time-t0-charge, etc.

Please review the threads on AGM batteries and charging batteries. As good as Nigel Calder is he is wrong (as are most others as well) about how fast (how much current as well) any lead-acid battery can safely charge accept.

Review the Amp-hour law in the other threads for the answer.

A flooded-cell battery can be charged from zero to full in just under 4 hours. An AGM battery can be charged from zero to full in just under 3.5 hours safely.
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Old 16-04-2009, 14:49   #3
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With due respect Rick, and as previously stated, I value your postings and have learned a lot from them.

However, this is one time when I'm just intersted in "knowing the time", not "how the watch is made".

A would apprciate the answers to the questions asked.

Respectfully

William
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Old 16-04-2009, 15:55   #4
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Fastest time

Close to "ideal" would be a 300A charger yet, obviously, two 100A ones will do well if they can be set up for an acceptance voltage of 14.6 V or more. This will yield the fastest bulk charge time. The lower the acceptance setting the longer the bulk time will take.

Because no one has chargers that source current share equally, the first 100A charger to "see" its acceptance setting will back of charge current ant the other charger will keep going at full current until it "sees" its own acceptance point. From there on out only one charger will drive the battery bank, the one with the highest regulation point.

You should be able to get 200 Amp-hours in just over an hour, about 75 min. That will put you very close to the 80% state-of-capacity point. You will then be able to close to another 70 Amp-hours for the next 45 min, but not quite.

The math is that one solves the equation Ic=Im(e*-t) for time where Ic is the charge current available and Im is the number of Amp-hours "missing" from the bank as time goes on. One integrates Ic versus time to get the total accumulation of Amp-hours for a time period. One quick way to visualize this is that one can get about 63% of Im in the first hour and one can get 63% of the value of the remaining value of Im (at the end of the first calculation) in the next hour. This will give you a very close calculation assuming that the battery is charge accepting and knowing when the charger becomes voltage limited (thereby not being able to put out rated current).

It becomes obvious that one needs to know just what acceptance voltage values are available from what charger in order to make the calculations.

Sorry that I didn't difinitively answer the first time.
Regards,
Rick
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Old 16-04-2009, 16:07   #5
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Firstly Rick, you have absolutely no need to apologise for anything mate! You are an encyclopedia of knowledge when it come to this subject. Thanks so much for a very understandable answer.

While I'm on a roll , I know one should NOT combine batteries of different types and ages in any bank. That said, what if I were to decide I needed to expand my House bank another 275Ah (i.e. two more of the same batteries) say 1 or 2 yrs later.

What would the down side, and/or consequences of adding a pair of the EXACT SAME type of batteries to the House Bank?
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Old 16-04-2009, 17:22   #6
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another myth

It is a myth that one cannot mix different age batteries in one bank. The one caveat is that the batteries must have the same designed specific gravity. One can, in fact, mix gel-cel and AGM batteries in one bank. It is easy to discover the specific gravity of those by measuring "full", standing (no surface charge) voltages of each to make sure that they are the same.

voltage = #cells(S.G. = 0.85) Volts
Solving the equation for specific gravity, S.G. one can determing a sealed batterie's S.G.

What I have discovered over the years testing various batteries, all fully insturmented, is that although old batteries suffer time capacity degradation they will track younger batteries in terms of charge acceptance and current contribution. Such tracking will suffer if one does not achieve at least 14.4V acceptance @ 20 deg C for a sufficiently long time when recharging. Keep in mind that more batteries are murdered by undercharging than overcharging, in general.

I have no qualms in recommending that you can add batteries later as long as you have a good charging system. If not you will probably be replacing the original ones anyway.
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Old 16-04-2009, 18:39   #7
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Quote:
Originally Posted by Rick View Post

It becomes obvious that one needs to know just what acceptance voltage values are available from what charger in order to make the calculations.
Regards,
Rick

To answer your above query, I will be installing a Mastervolt 12/2000/100 Combi (Charger/Inverter combination) and with it either a Mastervolt Chargemaster 12/100-3 or Mastervolt Chargemaster 12/70-3

Does that help you to offer even a more definitive answer?
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Old 16-04-2009, 19:09   #8
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My experience shows that the maximum rate of charge a battery will accept in amps is approximately equal to the number of amp-hours a battery is discharged.

In your case a 670 amp-hour battery is being charged from 50% to 80%. Or being charged from down 335 amp-hours to down 135 amp-hours. The total amp-hours to be replaced is 200 amp-hours.

A 200 amp charger would charge at 200 amps for 40 minutes at which point the battery would be down 200 amp-hours. The acceptance rate would then reduce the charging rate from 200 amps to 135 amps over the next 25 minutes at which point the battery would be down 135 amp-hours or 80% charged. Total time = 65 minutes.

A 100 amp charger would charge at 100 amps without encountering the battery acceptance limit. The 200 amp-hours would be replaced in 2 hours.

Regards
Chuck
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Old 23-04-2009, 19:19   #9
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I have written a small spreadsheet that calculates charge time based on the Amp-Hour Law maximum rate of charge. I would be happy to send it to anyone who requests it.

Regards
Chuck
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Old 23-04-2009, 20:41   #10
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Thumbs up

I would really appreciate having a copy of that spreadsheet Chuck.

Thanks in advance.

William
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Old 04-05-2009, 17:53   #11
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I have 6-6V Tojan 125s as my house bank. I have a 3.5kW genset which likes loads around 2.5kW or lower.

Question for Rick, what is the best chargering approach to run off the genset to achieve fastest charging results? I am presently looking at an 80amp Charles Charger.
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Old 05-05-2009, 12:16   #12
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Switch-mode chargers

Trim50; Small gensets can be loaded better with switch-mode chargers that are designed with harmonic correction. Most switch-mode chargers are. In addition they are lightweight. They usually cost more.

I believe that the Charles chargers are triac-controlled 60 Hz chargers which will badly distort the already poor waveform of a 3.5kW genset making it overheat at close to 50% of the genset output power rating.

Harmonically corrected chargers extract their energy from the entire conduction angle of the genset waveform. Triac controlled chargers extract energy only at the peaks of the waveform therefy "flat-topping" the output. No availabe triac controlled charger can pass EU specs for harmonic content or EMI or RFI.
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Old 05-05-2009, 20:06   #13
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What is the best switch-mode charger on the market?
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