OT question about float charging AGMs

[Peter VanDerWal]

I realize this is OT, but it relates to batteries so I figure this is the
best place to get an answer.

I'm building a DC UPS using a 12V ~20ah AGM. Probably one made by MK
Batteries, unless someone can suggest something better.

Since it's for standby use, not frequent deep cycles, I'm wondering what
is the best way to float charge AGMs?
Do I need a three stage charger or can I just use a constant voltage power
supply with current limit?  If so, what's the best voltage for long life,
13.56V? or do AGMs need a higher float voltage?


FWIW:
 In order to improve reliability and reduce energy requirements, I'm
replacing my web severs with small, low power computers (Nagasaki
MS-1200s).
I expect them to use approx 10 watts each.  These have a 5vdc input, so
I'll need a 12V-5V DC/DC (~30watt).  Any suggestions?
I'm also building a router/firewall using an ALIX2 board, this has a 7-18V
DC input, so I can run it directly off the battery/charger.

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[Roger Stockton]

Peter VanDerWal wrote:

> Since it's for standby use, not frequent deep cycles, I'm
> wondering what is the best way to float charge AGMs?
> Do I need a three stage charger or can I just use a constant
> voltage power supply with current limit?  If so, what's the
> best voltage for long life, 13.56V? or do AGMs need a higher
> float voltage?

Best bet is to check the datasheet/manufacturer's recommendations for your specific battery.  The standard approach is to use a plain constant voltage charger, ideally with a temperature compensated voltage.  The current limit is generally not critical; for many gels and AGMs a maximum charge current of 0.3-0.4 of the C/20 rating is recommended.  The specific float voltage recommendation will vary from one manufacturer to another.

Longest life will probably result from not leaving the battery on float continuously, but rather charging it fully and then having the charger turn off and restart to top the battery up at regular intervals or after a discharge.  Some automatic (not necessarily 3-stage) chargers will do this for you, or you can add this capability to a simple constant voltage charger (e.g. use a timer to limit its runtime to 24-48hrs at a time, and another to restart it once a month or automatically after any outage).

Cheers,

Roger.


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[David Roden-3]

Please reply to Peter directly, rather than to the list.  Thanks.

David Roden - Akron, Ohio, USA
EVDL Administrator

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[Lee Hart]

Peter VanDerWal wrote:
> I'm building a DC UPS using a 12V ~20ah AGM. Probably one made by MK
> Batteries, unless someone can suggest something better.
>
> Since it's for standby use, not frequent deep cycles, I'm wondering what is the best way to float charge AGMs?
>  
The best way to float charge AGMs is "don't". Don't float charge them!
Charge them up with a good charging algorithm that takes them up to
14.7-15v per 12v battery, then turn the charger completely off. Let it
sit until it gets used by the UPS, or for up to a few months of self
discharge. Then fully charge it again, and repeat.

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[Ryan Bohm]

Lee, all,


The best way to float charge AGMs is "don't". Don't float charge them!


I'm going to play the 3-year old here ...

Why not?

-Ryan
--
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E-mail: mailto:ryan@...
Toll-free: 1-877-215-6781
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[AMPhibian]

EVDL Administrator wrote:

Please reply to Peter directly, rather than to the list.  Thanks.

David Roden - Akron, Ohio, USA
EVDL Administrator

 Really?  Proper charging of AGM's seems like valid information for the list no matter what they are used in.  I'm interested in all opinions on them.

John