AGM Battery Question

H

HarlenW

Member
Member
Joined
Mar 30, 2007
RO Number
25699
Messages
473
I have four 8D AGM batteries as my house bank and the rated capacity at 20A/h is 840 amps (4 x 210). Does this mean that I can use the full 840 amps at a rate of 20 amps per hour before I need to recharge?

Thanks.
 
Discharging a battery to below 50% will shorten its life.
 
No, but the manufacturer probably publishes recommendations about how low you can draw those before recharging. Some say recharge when they reach 12.0v (50%), some say 11.7v, and so forth. Many will also give the number of cycles you can get, given depletion to -- say -- 50%, to give you an idea of expected life.

-Chris
 
No, 50% is really as low as you want to get... ideally, 60% will make your batteries last longer
 
More for completeness sake than anything, you should generally consider the capacity to be available between about 95% charged and 45% charged. ( as stated above, about half the "official" capacity )

Reasons:

You will often not start with a truly fully charged battery so use 95%.

While some battery mfgrs allow very deep discharge, you will be well-served by keeping the battery more or less above the 50% charge state.

Don't forget that several things affect that stated capacity. Two important ones are age ( "how many charge cycles old" ) temp. De-rate capacity as the batteries get older and de-rate if the battery temp is at "mid-summer afternoon high".

These variables ( and several others, such as discharge rate ) actually determine just exactly what the total capacity is, and overall makes it somewhat difficult to calculate with real accuracy. Good rule of thumb is to assume that only about 50% available.
 
I have heard that 50% floor is for flooded batteries, not AGM. I believe Bill (BillyIII) has posted here that good AGM batteries can withstand deep discharges with little if any loss in performance or life. Although as expensive as those 8D batteries are, I think I'd stick to the 50% floor.
 
"good AGM batteries"

note the "good" part :)

It is always a good idea to browse on the battery mfgr website and take a look at the tech data. Each mfgr has a different notion of what the min charge "should be", and their charts and graphs will give you a good idea of actual performance.
 
I have seen same recommendation in terms of do not go below 50% for both flooded and AGM. AGM is really same technology as flooded
 
Good to me means Lifeline for AGM's. When used as suggested as stated above 50% of the last charge cycle put into the battery bank (fully charged). I have seen good AGM's completely discharged that were 3-4 years old and come back to life with no measurable loss in capacity. With that being said also look at the ratings mine are 255AH's per 8D. As a general rule if you only discharge to 50% they might outlast your motors. However more importent then the discharge point is proper charging and this applies to all batteiries. Follow the manufactures instructions and also set up your Perkoff's equation/ exponent on your charger if you can program it. This makes sure the battery system (house bank) gets the proper charge to fully recharge itself. I personally think if your going to spend money on a quality AGM battery bank make sure your charging system is modern and has active temperature compensation and don't skimp on the DC wiring type or size. Wires that are warm to the touch = losses why waste your amp hours heating your wires? Also, absolutely no Fero-resonent chargers on appliance timers when using AGM batteries..
Harlan you can probably have a true capacity based on your numbers of 420 amp hours and remain trouble free. Unless you get the one in a million AGM that melts down something I have never seen.
Bill
 
Thanks, everyone, I believe I understand it now. A stat on the manufacturer's website was misleading.
 
According to the Lifeline website "For maximum battery life in cycling applications, do not discharge the battery bank below 50%. Discharging the bank repeatedly to 100% will shorten the battery life." The literature they provide, that I could find, on discharging and charging was basically nothing. I did not see: how many cycles, at what temperature, at what rate of discharge, at what rate of recharge, what the optimal voltage/current is, etc... They should know that and publish it. As a general rule the faster you dischage a battery, including AGM, and the deeper you discharge them the shorter the life. Life of the AGM could be less than 200 days to over 10 years.
 
Harlen,

Considering the boat you have and the batteries you have, is it safe to assume you have a battery monitor? I don't know how people can manage without one. But then again, many of the boaters I talk to don't even know what size or type batteries they have!
 
quote:
Originally posted by Pa Mikee

According to the Lifeline website "For maximum battery life in cycling applications, do not discharge the battery bank below 50%. Discharging the bank repeatedly to 100% will shorten the battery life." The literature they provide, that I could find, on discharging and charging was basically nothing. I did not see: how many cycles, at what temperature, at what rate of discharge, at what rate of recharge, what the optimal voltage/current is, etc... They should know that and publish it. As a general rule the faster you dischage a battery, including AGM, and the deeper you discharge them the shorter the life. Life of the AGM could be less than 200 days to over 10 years.

Click to expand...

I'm on my 6th season and I had one bank fully discharge last winter. Brought the group of 3-31 series back up one at a time and the discharge rate is the same as it was the day they were installed.
Considering they had a 5 year pro rated warranty and a 2 year no questions warranty no asked when I purchased mine 200 days seems a bit skewd Mike.
Bill
 
BillyIII, thanks for providing links but it only adds to what I stated. The only provide minimal information. Where are the white papers? The links are esentially generic info or provide capacity info for their battery.
Here is an example of what they should be releasing considering, by you, they are the gold standard: <http://www.enersysreservepower.com/documents/US-GPL-AM-003_0906.pdf >

Edit: to make link workable.
 
I did some further research. For flooded, AGM, or Gel batteries, discharge past 50% is not good. Recharge to less than 100% is not good either. However, it's rather time consuming to recharge batteries via generator to 100% since the last 10-15% takes so long to accomplish. Thus, what I read as recommended is to avoid discharge greater than 50% and recharge to 100% as often as possible.

Bill, as usual, is right about the type of charger and temperature sensors playing an important roll as well. Today's chargers should be set to the correct battery type (each battery type has it's own specific charging needs) and temp sensors should be included or purchased as accessories as the temp of the battery changes how a charge should be applied.
 
Kurt I think the biggest problem many boaters face is generating enough DC amperage to charge AGM batteries to full capacity. Perkoff's is basically how much additiuonal charging is needed to reach a full charge. I have 130 amps of controlled DC charging and can set my Perkoff's percentage on the charge rate.
The last 5% to 10% of the charging cycle is the hardest to get into the battery.
Bill
 
quote:
Originally posted by mixman

Harlen,

Considering the boat you have and the batteries you have, is it safe to assume you have a battery monitor? I don't know how people can manage without one. But then again, many of the boaters I talk to don't even know what size or type batteries they have!

Click to expand...
Yes, I do have a battery monitor and it is very useful.
 
quote:
Originally posted by mixman

Harlen,

Considering the boat you have and the batteries you have, is it safe to assume you have a battery monitor? I don't know how people can manage without one. But then again, many of the boaters I talk to don't even know what size or type batteries they have!

Click to expand...

+1
 
You must log in or register to reply here.
Back
Top