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I have an off grid solar system, with an 240AH 24v deep cycle battery, and the system (and all of the components in it) is a year and a half old.

i have several questions and i hope that some one could answer, at least some of it:

  1. I had trouble to measure the charge with the hydrometer. is it o.k. to use this table as a reference for any 24v deep cycle battery, or do i have to find my one manufacturer table? is it o.k. to use only the voltage indicator?

  2. In the same article the author is proposing 3 methods for measuring the battery charge: voltage, specific gravity and amp-hour metering. He also says that the amp-hour metering is the best method. is that true? why a Watt-meter isn't just the same? does the amp-hour meter knows when the battery is degrading and therefore has lost amp-hours?

  3. The measurement with the hydrometer is not quite accurate. is there i way of measuring the specific gravity with an electronic hydrometer? if there is, where can i get one of those?

  4. I'm starting to notice that the battery is reaching the 70% red line (Arbitrary limit that i chose to set) far too often. How can i tell what is the condition of the battery?

  5. How can i tell if i new but untested battery is defected?

  6. my solar system produce maximum current of 10 AMPS. someone once told me that a 10 AMPS equalization charge is not enough, and it should be higher for an effective charge. is that true?

  7. an 240AH at 24V have a capacity of 5760 Watt/Hour. is that calculation is right?

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A lot of interesting topics here, but they probably belong as separate questions. Instead of flooding the site with a bunch of questions, I recommend asking 1 or 2 questions at a time, and letting the answers you get help inform the next question. –  Jay Bazuzi Mar 19 '11 at 18:17
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And you might also want to ask on the electronics and robotics site, as that group might have experience with deep cycle batteries, too: electronics.stackexchange.com –  Joe Mar 19 '11 at 19:04
    
thanks. i'm new here :)... i'll keep this in mind. –  Asaf Chertkoff Mar 20 '11 at 4:55

2 Answers 2

up vote 2 down vote accepted

Based on my experience being qualified as a battery charging electrician on submarines and installing/upgrading/maintaining the 12v electrical system in my motorhome.

  1. Battery electrolyte specific gravity is not model or battery specific so your chart is fine.

  2. Amp-hours is a measurement of current consumed (or the sheer number of electrons you've pulled out of the battery) rather than tracking watts which is a measure of power. You can easily track the amount of power used, but it would be impossible to determine the actual state of charge in your battery. It would be like if you tried to gauge how much fuel you had left in your car by reading a "cumulative horsepower meter".

  3. A manual hydrometer is fine - just avoid the super cheap ones available at the auto parts store.

  4. Get a real battery charge monitor that measures amp-hours in/out. I use one made by Xantrex.

  5. You have to test the battery to know its bad.

  6. An equalizing charge involves holding charge current at a low value, which boosts charge voltage - so if your charge controller won't allow you to set EQ current at a value, get a new one. 10 amps is probably close enough to the charge rate appropriate for equalizing - however if your array is only capable of delivering 10 amps with full sun, you might want to reconsider that strategy.

  7. Who cares. Battery capacity is a function of the rate of discharge so that value is effectively worthless to you as a monitoring metric.

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thanks a lot for your answer. as for the specific gravity table, is that correct that for accurate measurement i needed to deep charge my battery at the beginning and the stat the voltage measure as the full battery voltage, and not count on the general specification or on the manufacturer spec. about the battery? cause when my voltage is measured 24.8 the hydrometer is showing a specific gravity of 1.240 give or take... which is giving me the better measurement? –  Asaf Chertkoff Mar 20 '11 at 5:01
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You're on the right track. You want to charge the battery and then check each cell's specific gravities against the specific gravity table. Remember to do your temperature compensation as well. If you have low SGs or divergent readings between cells (greater than .05) that is an indicator of a battery going bad. You may be able to "save" the battery w/ an equalizing charge but many times you have to replace the battery. Here's a link to a good explanation of how to maintain your battery. marine-electronics.net/techarticle/battery_faq/b_faq.htm –  kkeilman Mar 20 '11 at 23:53
    
thanks. i will read it carefully :) –  Asaf Chertkoff Mar 21 '11 at 5:08

I'd definitely go with a amp-hour, or watt-hour meter. I don't live with a solar system, but with an electric bike, and use this http://www.ebikes.ca/drainbrain.shtml for 140$. I use it to count number of cycles, and in each cycle to know how many watt-hours, or amp-hours i have used.

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Hi, Hanan! glad you decided to join! :) interesting device you recommended here, but if i got it correct, there is a lot of features that i won't use, like the speed-o-meter and stuff... do you think that i can find something more specific for my cause at the same price or lower? i got a price offer from interdan for the xantrex amp-hour meter monitor but it is more then twice of the price of this device here, so i'm still looking... thanks. –  Asaf Chertkoff Mar 24 '11 at 14:09

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