Maximum discharge rates of AGMs

11 Posts
Oct 7, 2008 06:39 pm
Maximum discharge rates of AGMs

Hi everyone,

This is my first post here.  I just picked up 24 5W solar modules (.35A x 15V) at about $3.33/watt. I'm looking to put together a simple off-grid system that will usually be called upon to run a floor lamp (45 watts) or to charge a laptop (90 watts).  Occasionally, however it would be nice to charge some power tool battery packs with it at about 350 watts (3A @ 120V for 1 hour).  I wouldn't try drawing over 400 watts for any length of time. 

I need help choosing an AGM battery that can support these intermittent loads.  Do I need a 100AH battery to support the 350WH load, or do I need something approaching 200AH?

Additionally, I'm looking at a charge controller (Solar Boost 2000E 25A, 12V) and a pure sine inverter (Samlex 600W, 12V).  Are these overkill for my project? I'm probably not going to add more panels in the near future (Not until I invest in a full 3-5KW grid-tied system).

 
27 Posts
Oct 8, 2008 10:59 am
Re: Maximum discharge rates of AGMs

Regarding the batteries - I think you need to get the largest AH available to you.  I have (2) 276ah wet cell 12v batteries that I have never seen drop below 12.4 volts.  I run music, lights and have charged multiple Dewalt 18v battery packs on these days.  If you go bigger from the start the piece of mind about not needing to worry "will my battery last?" is wonderful.  Based on a very basic calculation of 350watt drain with 80ah available(80%drain on 100ah agm battery) = 1.14 hours of run time - not enough in my opinion.

I am a fan of my Xantrex C40 charge controller with a CM faceplate as it is PWM, shows battery data as well as panel input data(all at once, with backlit display at push of a button), with a battery temp sensor, for about the same price as that Solar Boost unit

I have the SK(light blue anodized chassis) series Samlex/Cotek 1000watt pure sine inverter(need pure sine for charging those battery packs!!)and it has performed flawlessly so far(2 months).  I purchased that from donrowe.com.  Check out my gallery for some photos.
 
11 Posts
Oct 8, 2008 11:51 am
Re: Maximum discharge rates of AGMs

Thanks for the information Michael.

Based on a very basic calculation of 350watt drain with 80ah available(80%drain on 100ah agm battery) = 1.14 hours of run time - not enough in my opinion.

I'm curious how you came up with that number.  Did you plug in numbers for a 6 or 12 volt battery?

350 watts / 12 volts = 29 amps
80 amp hours / 29 amps = 2.75 hours

From my math, if I ran 350 watt-hours from a 12V 100AH battery I would have approximately 71AH left which is well within the capacity limits of the battery.

However, my real question is if a 12V 100AH AGM battery could sustain a 29 amp discharge rate for any length of time, or is it limited by the plate surface, battery chemistry, heat, etc?  I don't want to ruin a battery by discharging it too quickly.  Obviously you can't discharge a battery in 1 minute or it's likely to overheat and die. I just haven't found any literature that lays out discharging guidelines.
 
27 Posts
Oct 8, 2008 12:05 pm
Re: Maximum discharge rates of AGMs

I used:
http://www.donrowe.com/inverters/inverter_faq.html (click on "how long can i run my inverter" calculator)

I screwed up and put in 12volt batteries, it is 2.28hrs with 80ah at 350watts with a 24volt system per this online calculator, which may or may not be entirely correct...
 
11 Posts
Oct 8, 2008 12:20 pm
Re: Maximum discharge rates of AGMs

I believe I found an answer from the "How to size a battery bank" page at the university library link.

Daily use in watt-hours: 500
Depth of discharge: 50%
Ambient temperature: 60 degrees (1.11 modifier)
Battery voltage: 12

By those calculations I'd need at least a 92.5 Amp-Hour battery.  But like you said, Michael, by getting a bigger battery I could lessen the DoD and prolong the battery life.  With a 105AH battery like the MK-8A31 I'd get a 44% DoD which might last 6-7 years.
 
27 Posts
Oct 8, 2008 12:47 pm
Re: Maximum discharge rates of AGMs

You can never have enough capacity, I have 552ah @ 12v and want more! Smiley  Once you get a taste for that offgrid power it just snowballs.  I don't know your setup, but for me it is such a cool feeling to be running lights, music, charging battery packs, and working on my cabin; with the convienances of 120vac power, while just using the power of the sun.
 

Disclaimer and Disclosure

The Alternative Energy Store, Inc reserves the right, within its sole discretion, to refuse or delete any posting or portion thereof, or terminate or block the access to this forum.

The opinions and statements posted on this forum are the opinions and statements of the person posting same, and do not constitute the opinion or act of the Alternative Energy Store, Inc (AltE). The Alternative Energy Store, Inc does not endorse or subscribe to any particular posting. No posting shall be construed as the act or opinion of the Alternative Energy Store, Inc.

Click here for BBB Business Review

McAfee SECURE sites help keep you safe from identity theft, credit card fraud, spyware, spam, viruses and online scams
Desktop Website | Mobile Website

Share

Click on an icon to share! If you don't see the method you want, hover over the orange "+".

Feedback

What can we do to help you?

Please enter a summary
Sorry, the copyright must be in the template.
Please notify this forum's administrator that this site is using an ILLEGAL copy of SMF!
Copyright removed!!