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Old 18-09-2005, 11:07   #1

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Question Basic Battery Question

Does anyone know how to properly asses the amps a given battery will store?

For instance, I have Interstate Battery (lead-acid) batteries that each say they are SRM-27 (group 27?) and they each have 600 CCA and 750 MCA.

Given that these are deep cycle batteries, and would never be cranking any starters or similar loads, how can I know how many amps this battery actually is? CCA and MCA are ratings that refer to a use this battery will never see.

I am asking because I'm doing all the math now for the electrical system, and am going to try and figure out what the fastest way to (safely) charge the batteries while running a genset is.

Thanks in advance.
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Old 18-09-2005, 12:17   #2
Kai Nui

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Yes there is a formula based on the two figures you gave, but it escapes me right now. Call your local battery retailer, and they should be able to tell you what it is. THe Interstate guys are pretty good on this stuff. If I recall, the group 27s are about 120 amp hour. Charge rate should not excede 10% of te amp hour rating.
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Old 18-09-2005, 16:03   #3
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Sean, if your batteries are showing a CCA rating and no AH rating, my suspicion is that they are not deep cycle types but rather start batteries.

I disagree with Kai Nui; you should be able to charge up to 25% of the capacity of a bank without harming the battery, assuming the charge is managed - the 25% figure is for the bulk charging rate. Those of us who live on the hook routinely carry small frame HiCap alternators that are hot-rated at a max of 108 amps, and these often feed T105 4-battery banks rated at roughly 440 amp/hrs. This is considered a good match and works well.

I'd recommend you use Nigel Calder's reference for these kinds of things, and calling a retailer to check not only your existing batteries capacity but also its type sounds to me like a good idea.

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Old 18-09-2005, 17:10   #4

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Thanks, Guys..

A couple of follow ups, if you don't mind:

Jack: I do show the CCA and MCA ratings (cold cranking and marine cranking amps). However, the batteries also are labeled as "Mega-Tron Marine/RV Deep Cycle Batteries" So.. I assume they are not starting batteries, correct?

(I'm sure hoping they are not! )

Kai Nui: I have been doing a lot of research into charge rates. I think Jack is right regarding a managed charge. 25% of the rating is ok for a managed charge during the bulk charge stage. I would imagine for a non-managed charge, your figure might be accurate.

Basically, I'm trying to figure out how quickly I can charge these batteries up so as to minimize hours on the genset. Thanks.
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Old 18-09-2005, 17:12   #5
Kai Nui

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10% is industry standard. Can you get away with more? Yes, but it will effect the life of the battery. As the charge rate of an alternater varies according to engine speed, and the need of the battery, a bigger alternater is fine. Interstate's Group 27's often do not list the amphr rating. You may or may not have deep cycle, but this is not defined by that fact. Again, check with Interstate. By that, I mean the distributor, not the local auto parts retailer. I have worked with Interstate on all of the off the grid homes I have set-up,(3 of my own, and numerous others professionaly) as well as the battery needs for every boat I have had.
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Old 18-09-2005, 17:23   #6
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Amps and energy

I refer you back to: How "fast" you can reliably charge your battery; and "Understanding a model for battery charge acceptance" both threads appearing under this subject heading (Power Equipment and Electricity). Some of the details are not addressed by Nigel Calder and certainly not generally known in the cruising community in general.

Just to clarify the terminology no battery stores "Amps". In addition, no battery stores "Amp-hours". No battery is rated in "Amps". There is no such thing as "Amps per hour" in this technology. Storage batteries are rated for energy storage period. Just how that energy storage is rated depends upon how the energy is to be absorbed by a load. In the case of lead-acid batteries (flooded, AGM, gel-cell, or whatever) the amount of energy DELIVERED to a specific load will vary according to rate at which the energy is depleated and the temperature of the electrolyte, hence the Peukert effect and formula descriptive of the inability of a battery to deliver an energy rated for a 20 hour discharge rate at a one hour discharge rate.

Storage battery energy is given in terms of Amps multiplied by time multiplied by voltage. An Amp-hour rating implies current in Amperes multiplied by the time in hours and leaves out the variable voltage, which means that an Amp-hour rating is NOT an energy rating and is bandied about as though it is. That is why an AGM battery having a normal standing voltage of 12.9V and a 100 Amp-hour rating has MORE stored energy than a flooded-cell battery having a standing voltage of 12.6Volts and also rated at 100 Amp-hours. Comparing the two is like comparing apples and orages without the voltage versus time Amp-hour discharge curve known. I have made many such measurements.

Some people say "Amps per hour" when they really should be saying, "Amps for the duration of one hour (or fill in the number of hours). Mathmatically the word "per" means divided by and, therefore, "Amps per hour" would be current divided by time which is Coulombs per second per second which is the acceleration of electrons as they leave the source, not a meaningful concept in battery terminology.

Although popular usage bastardises original usage it might be interesting to know that Amps is formally capitalized as is Volts because they are names of Ampere and Volta, the people for whom the terms honor. Naturally the adjective and adverb forms are not capatilized like voltage and amperage.

Storage lead-acid batteries designed as deep-discharge batteries are rated in terms of Amp-hours for a specified time. In the case of 12V nominal batteries the end of the discharge time is noted when the battery reaches 10.5 Volts at the rated load. For example, a 100 Amp-hour battery rated at a 20 hour rate (those that we are interested in because if they are rated at a 1 or 8 hour discharge rate we cannot translate that energy meaningfully into a once per day charge/discharg cycle like many of us do living on the hook) is rated at a 100 divided by 20 equals 5 Amperes load. At the end of 20 hours with a 5 Amp load expect the voltage to be 10.5 Volts regardless of the starting voltage.

Like Euro Cruiser wrote, if the battery is specified by cranking current you can assume that it is NOT a true deep discharge battery. It may be a compromise (translation: shitty at one thing and shitty at the other yet it will do both) design.

If you do not know assume that lead-acid batteries can deliver 1 to 1.5 Amp-hours per pound of weight. In my experience most Amp-hour ratings are optomistic yet in one case I actually measured an AGM 4-D case size battery at over 200 Amp-hours. In another case I measured a flooded battery in an 8-d case at 110 Amp-hours. You just don't know until actually making the measurement.
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Old 18-09-2005, 18:23   #7

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Thumbs up Wow...

Extremely useful responses.... thank you so much to everyone who already responded. I am learning a lot (and remembering a lot of old knowledge) reading these posts.

Rick, I have to re-read yours a couple times to really get it 100%. I also found one of your posts in another thread that was extremely helpful.

Thanks to Kai Nui and EuroCruiser (Jack) too. Much appreciated. I'm off to an OK start of analyzing this electrical system now that I am getting a handle on battery capacity.

Thanks again.
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Old 18-09-2005, 18:34   #8
Kai Nui

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As always, Dr Rick you are correct. Amps are a measure of the flow of electricity, not the stored amount. Voltage is a measurement of stored electricity. The simple fact is that most of us size our storage battery systems based on watt/hr or amp hr usage, so such a rating is critical to determining the size of the bank, and the size and type of charging system.
As for the batteries not being good at either, well, sort of true, but consider the level of battery we are talking about. Group 27 are essentialy automotive type batteries. (Even though thay are noted as marine). Any group 27 or 31 is going to give about the same performance. (slight variance by brand). If you want a battery that truly performs as a "deep cycle battery", you need to get into another class of battery (e.g. Trojan L-16's)
I do agree on your thoughts about AGM's. THese are a great improvement over old technology.
Anyway, Sean. I hope some of this has been helpful. If you are still shopping for batteries consider the AGM's. If you are looking at just the charging system, there is a lot more involved than just a good smart charger, as it sounds like you know. The 25% figure will probably not give you a noticeable reduction in the life of the batteries if you equalize them, and make a point of not letting them drop below 60%. 80% is better.
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