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Old 08-08-2011, 08:35 PM   #1
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Do AGM batteries provide more power?

Group 27, flooded cell deep cycle versus AGM

Which provide more usable power from a single charge. Not interested in cost or solar or number of cycles. Which has the most usable power on a cycle from which the battery can reliably recover.
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Old 08-08-2011, 09:12 PM   #2
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According to information I have found the AGM is better because the lead can be more pure. This is because the glass mats in which the electrolyte is absorbed the help support the plates. This eliminates the alloys in the free standing plates of flooded batteries, required to give them the strength to retain their shape.
This allows up to 1.5 times more capacity. Some sources say an AGM battery may be cycled to 80% of its’ rated capacity without damage, compared to 50% discharge for flooded batteries.

In my simple, unscientific use boondocking without solar, the AGMs last notably longer. And, I never have to add water.
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Old 08-08-2011, 10:50 PM   #3
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The answer is here...
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Old 08-09-2011, 12:44 PM   #4
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If you look at the spec sheets ...

The energy density by weight for AGM's is usually just a tad less than it is for wet cell lead acid batteries. That means that, for a given weight, an AGM battery will store a tad less energy than a wet cell. The difference is insignificant in RV terms where a rule of thumb of about 12 usable watt hours per pound covers the ground.

As for power, an AGM can be a benefit because they can usually handle larger discharge and recharge currents. This might help if you run a 2kw microwave with an inverter on a small bank, for instance. In most RV circumstances, I don't see any real benefit here, either. There just isn't sufficient battery to support any power levels for the length of time to make a difference.

For properly used and maintained batteries, the AGM may have a life about 50% longer than a wet cell. They are sealed and rugged batteries but sealed means extra care is needed to not overcharge and other stuff in an RV is going to suffer before a battery does when it comes to ruggedness.

As far as battery capacity in an RV goes, there is no magic bullet. You just can't store much electrical energy in lead acid batteries of any type. A typical group 27 stores a bit over 1 kWh of energy. Best cost effectiveness (see smartgauge.com) is to use only about half of that as a regular cycle depth. Compare that to the typical household energy use of 30 to 60 kWh.
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Old 08-09-2011, 04:05 PM   #5
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Thanks guys. I run a CRAP unit at night. Rated 5 amps, do not know for sure what it draws. Not having any particular problems now. Can go 3 nights maximum with light use for other things now. Was wondering if the AGM would make a big difference. We have a generator and I will just recharge as needed. I have tried to read the spec sheets, but have had a hard time because of the complex relationship between draw rate and maximum draw. And a hard time because many of the manufacturers do not want you to understand it clearly anyway.
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Old 08-09-2011, 06:44 PM   #6
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AGMs usually have very slightly lower capacity for the same size and weight, 5-10%. Their advantages are in other areas.
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Old 08-09-2011, 07:35 PM   #7
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Quote:
Originally Posted by Bill M. View Post
Thanks guys. I run a CRAP unit at night. Rated 5 amps, do not know for sure what it draws. Not having any particular problems now. Can go 3 nights maximum with light use for other things now. Was wondering if the AGM would make a big difference. We have a generator and I will just recharge as needed. I have tried to read the spec sheets, but have had a hard time because of the complex relationship between draw rate and maximum draw. And a hard time because many of the manufacturers do not want you to understand it clearly anyway.
there are a few cpap threads. you can find them here:
Google Custom Search
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Old 08-09-2011, 11:48 PM   #8
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I have 2 group 27 Lifeline agm's in my Airstream and I can go 4-5 days before I charge them. I only use the lights that have led's and the water pump for showers and washing dishes. Best batteries I ever owned but they cost a lot.
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Old 08-10-2011, 12:38 AM   #9
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Bill M,

You can purchase a 12V DC adapter for your CPAP machine.

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Quote:
Originally Posted by Bill M. View Post
Thanks guys. I run a CRAP unit at night. Rated 5 amps, do not know for sure what it draws. Not having any particular problems now. Can go 3 nights maximum with light use for other things now. Was wondering if the AGM would make a big difference. We have a generator and I will just recharge as needed. I have tried to read the spec sheets, but have had a hard time because of the complex relationship between draw rate and maximum draw. And a hard time because many of the manufacturers do not want you to understand it clearly anyway.
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Old 08-10-2011, 09:51 AM   #10
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re: "have had a hard time because of the complex relationship between draw rate and maximum draw" -- this is the Peukert phenomena.

The 20 hour rate usually used to indicate battery energy capacity is about a 60 watt draw for a typical RV battery. For 2 batteries, that'd be 120 watts.

And do use watts for power. The CPAP rating for 5 amps is probably at 120 volts. That means 600 watts. When you use the proper unit for power, watts, there is no ambiguity about voltage.

If your CPAP is indeed 600 watts, then it is running at about 5 times the power level used for the battery 20 hour capacity measure for 2 batteries. That means you will have a lot less capacity available than specified.

Note that the 'reserve minutes' specification for batteries is at a 25 amp draw. For 12v batteries, this is 300 watts (or 600 watts for 2 batteries). That is much closer to your CPAP draw (if I have it right) so should be a much better guide for that need.

You can compare the 20 hour and reserve minutes specifications power levels to get a good idea about what changing power levels can do to battery capacity. I have used them to calculate the Peukert coefficient for battery comparisons.

Also note that you should really only use half the specified available energy for optimum battery cost effectiveness.

You know that you need to charge your batteries when their voltage starts to approach 12.0v after resting with no significant charge or discharge for a half hour or more. 12.2v is usually considered the 50% discharged voltage.

I have encountered several folks needing to use CPAP but none have been very satisfied with the typical 2 kWh trailer battery setup as the overnight power source. One, with an FB, tried using the gensets in his truck bed but that set off the CO alarms.

Seems to me, the RV market might be a prime target for CPAP manufacturers but, as far as I know, there aren't any available that are intended for 12v use and extreme efficiencies.
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Old 08-10-2011, 12:09 PM   #11
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If you have a heater in your CPAC turn it off and go the 12V route suggested by Bill.
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Old 08-10-2011, 12:10 PM   #12
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Thanks for the help and suggestions. And maybe this will help others. My CPAC is a Rem-star and it does run on 12 volts. When it is hooked up to 110 it is through a power supply to get it to 12. The CPAC says 5 amps at 12 volts on it. So 60 watts. Not sure if that is with the heat cycle on or not. The thing is sorta modular but I did not see a amperage listed on the heater portion. I do run it with a 12 volt cord when boondocking. But I generally use the 110 when the trailer is plugged in.
Now that I have a generator I charge after 2 days. Before gen I have run it successfully 3 nights but also had it cut off from low voltage when we used much power for other things.
Have had a couple of entertaining nights. machine quite after I went to bed. The 12 volt socket in the trailer fried a wire and it shorted to ground. But not enough to blow the fuse, just burn and smell. Up for an hour or 2 locating the fuse for the 12 volt outlet.
Later the machine quit again. This time it was the little spring that holds the fuse in the fused cord I was using.
So I go with the 110 when I have it.
From what I have read in the replies I do not think it would be worth it for me to go the the AGM for more power. If I am ever "flush" again (see stock market) and decide to upgrade the power converter I will consider the AGM's. I am getting tired of cutting the converter on and off while the trailer is parked.
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