Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 05-03-2010, 19:23   #1
Registered User
 
Wakadui's Avatar

Join Date: Dec 2008
Location: Portland, Oregon
Boat: Beneteau 49 - WAKADUI
Posts: 36
Simple Voltage Question

I apologize ahead of time for my lack of electrical knowledge, this simple mind was meant to sail and not to tinker!

Have an 800ah 4D (4 batteries) AGM battery bank (Deka batteries) with a Xantrex 40amp battery charger (I know, too small). When 20 amp load placed on battery bank (charger off) voltage will drop to 12.0V. 30 amp load placed and it drops to 11.8V. When all loads off the voltage rises to 12.7 volts suggesting that batteries are full.

Question: Why do AGM's let voltage drop to 12.0-11.8V under a 20-30 amp load? Too rapid a discharge? Faulty batteries?
__________________

__________________
Wherever you go... there you are!
Wakadui is offline   Reply With Quote
Old 05-03-2010, 20:14   #2
Registered User
 
Ziggy's Avatar

Join Date: Apr 2008
Location: U.S., Northeast
Boat: Contessa 32
Posts: 1,421
Images: 2
The voltage drop is due to the internal resistance of the battery (~0.035 Ohms). AGM batteries are supposed to have lower internal resistance than flooded cells, and 0.035 Ohms does seem a bit high. It could indicate end of life condition.
__________________

__________________
Ziggy is offline   Reply With Quote
Old 05-03-2010, 20:44   #3
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
You have to measure the voltage with a good multimeter (like a Fluke) directly on the battery terminals to check if the reading you get is correct. Panel meters often measure at the panel instead of the battery terminals, in which case you measure the loss over the cabling.

cheers,
Nick.
__________________
s/v Jedi is offline   Reply With Quote
Old 07-03-2010, 08:37   #4
Senior Cruiser

Cruisers Forum Supporter

Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 4,038
Images: 4
Yes, by all means measure the voltage with a good multimeter directly at the battery terminals as Jedi recommends.

However, you should know that with an 800AH AGM battery bank a 20A load (representing 2.5% of total capacity) should make virtually no discernable difference in voltage at the batteries.

My bet is that those Dekas are shot. Beg, borrow or steal a good battery analyzer (like one of the Midtronics testers with AGM capability) and see about their condition.

The only other reasonable conclusion -- even if someone wired the meter to the panel instead of the batteries -- is that you have some very bad (i.e., high-resistance) connections in the battery cables, connectors, switch, wiring, etc.

Bill
__________________
btrayfors is offline   Reply With Quote
Old 15-05-2011, 18:29   #5
Registered User
 
sea_goin_dude's Avatar

Join Date: Apr 2011
Location: SOUTHEASTERN USA
Posts: 44
Re: Simple Voltage Question

i know old thread but fact is as batteries get older the internal resistance increases over time indicating that the batteries are getting closer to need of replacement. If you could have a batt with zero internal resistance, you would have a perfect batt and it would last forever.....
This is true with your AA AAA C D 9VOLT batts also. To make them last longer in storage put them in an airtight container, tupperware etc and put them in the fridge. Do not freeze them!!!!just keep them fridged until you need them. They will last much longer in storage and yes they do start to "go away" simply due to age and this is helped by ambient heat. The fridge slows the internal chemical activity thus makes them last longer. I've been storing my spares like this for several years and when i take them out for use they are just like new...(hot and fresh)
__________________
Let me see if I have this right. "the water is below and outside, and the air is above and inside? sounds good to me
sea_goin_dude is offline   Reply With Quote
Old 15-05-2011, 19:52   #6
Senior Cruiser
 
senormechanico's Avatar

Cruisers Forum Supporter

Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
Posts: 5,823
Re: Simple Voltage Question

Todo del previoso es exactamundo correcto.

Sus batterias estan basura, problamente.

El Senormechanico.

I agree, they're probably better left at the recycling center.
Mine are getting there (220 ah/r) but not quite that much drop yet, but they're over ten years old.
Eagle eyed readers may want to argue with my previous posts about battery age, as I've been GIVEN these latest batteries.
They're basically free and work 90% of new. Good enough for my sailing style.

As for yours, assuming they're lead acid, and not gel or AGM,give 'em a good equalizing charge.
Do an internet or CF search for the particulars, I used a 19V. laptop power supply! It worked fine.
Now I have a MPPT solar controller which does it once a month.
Check the water afterwards!





BTW, I'm Norwegian !
__________________
Memento,homo, quia pulvis es, et in pulverem reverteris.
senormechanico is offline   Reply With Quote
Old 15-05-2011, 19:58   #7
Registered User
 
Swede57's Avatar

Join Date: Apr 2011
Posts: 20
Quote:
Originally Posted by senormechanico
BTW, I'm Norwegian !
We won't hold that against you
__________________

__________________
Swede57 is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Simple Raw Water Pump Question Ram Engines and Propulsion Systems 14 29-09-2015 01:35
ask a simple question perchance Construction, Maintenance & Refit 0 02-01-2009 15:19
A simple question, gloss or matt on the ceiling ? ribbony Construction, Maintenance & Refit 6 24-03-2008 02:54
Simple Tranny question By Invitation Propellers & Drive Systems 5 29-06-2007 14:04
A simple, dumb question. Gator81 Multihull Sailboats 2 23-01-2007 13:58



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 16:24.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.