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Charging AGM Battery

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Copper - Posts: 238
Copper spacespace
Joined: June 13, 2008
Location: Florida, United States
Posted: December 04, 2010 at 6:55 AM / IP Logged  
Hey I'm wondering out of all the high and low end Battery chargers, what are some or a good priced chargers I can purchase for my 100ah Shuriken battery? I've been told AGM's need specific chargers and cheap ones from walmart won't be able to do the job as good and other cheap ones my damage it. Its for my car stereo and its starting to sit pretty low in voltage.. also what are the best ways of charging it?
I can't hear you!
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: December 04, 2010 at 8:03 AM / IP Logged  
Never exceed 14.4V @ room temp else whatever the manufacturer recommends. Many cheaper chargers do not have limit the voltage...
Do not exceed its max charge current, though for a 100AH capacity, that is unlikely (I'd expect well over 20A...??).
Some manufacturers seem to suggest a minimum charge current (else battery life is compromised).
Some suggest or require higher than the "normal" commonly accepted "max" charge voltage of 2.4V/cell (14.4V) - the accepted "onset of gassing" voltage for lead-acid batteries.
But again, refer to the manufacturer's recommendations - after all, it is their warranty conditions that you have to meet.
And as with all lead-acid batteries, if charged above their float voltage (2.3V/cell = 13.8V), once they are fully charged, the charge voltage should be reduced to 13.8V (eg, from 14.4V).
But that is from a "max life" POV. Many if not most car alternators stay at 14.2 or 14.4V etc. (But they don't measure battery current nor temperature and hence do not know when the battery is fully charged - unlike the Projecta batter isolator that some how does it remotely though a simple +12V & ground connection! Amazing stuff eh? But I shouldn't highlight their alleged "priority charging"... Charging AGM Battery -- posted image. Charging AGM Battery -- posted image.
Battery chargers however can determine when the battery is (apparently) full, hence their various charge "modes".
"Smart" AGM chargers are the same as wet cell chargers except they lock out boost charging (?? - or whatever... I forget - ie, voltages above 2.4V/cell = 14.4V).
And they may impose a current limit suitable for that AH.
But otherwise they are the same "constant voltage" charger (with different voltages used for each stage; in ye olde days, "constant current" charging was recommended for AGMs).
But the key issues:
- do not overcharge; they do not have fluid to absorb the heat etc. This mainly refers to voltage limits (eg, 14.4V max longterm) but also over-current (heating).
- recharge ASAP after discharge - especially if a full or deep discharge. Unlike wet cells, they cannot be de-sulfated nor given the same rejuvenating resuscitation.
And please do not short circuit their terminals!
Copper - Posts: 238
Copper spacespace
Joined: June 13, 2008
Location: Florida, United States
Posted: December 05, 2010 at 1:45 AM / IP Logged  
Thank you! for the detailed response! would you be able to direct to a good battery charger? On amazon or another online store? I Just need an idea of the specs and things that I'm looking for. The Manufacturer has a charger but its COST more than the battery! and they give no details about it on their website. But I will email them and ask to amke sure like you said.
Also "discharge" means whenever the battery has gone below 10.5 volts right?
because you say "recharge ASAP after discharge"
Thanks again!
I can't hear you!
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: December 05, 2010 at 4:19 AM / IP Logged  
{Updated - corrected Projecta to Pro-User. I must have Projecta on my litigious mind!}
Discharge means any discharge.
But as I said - "especially if a full or deep discharge".
It's really the same with any lead-acid battery - it should be recharged ASAP.
As soon as they discharge, sulphates form on the plates. That impeded efficiency.
The more discharged aka flatter they are, the faster the sulphate forms.
Where is becomes a problem is arbitrary. Technically as soon a you stop charging and the surface charge has disappeared, it starts discharging through its internal resistance. Hence sulfation starts - but at an insignificant rate.
And self-discharging generally isn't a problem for 3 months wet cells or 12 months for AGMs. (I recently witnessed a wet battery that had been untouched for 2 years. It worked fine after (I assume!) a recharge. But that's impossible - it even stretches AGM capabilities....!)
But as per earlier - wet cells can be "reconditioned". AGMs can't - at least not in the same way...
Since you mention 10.5V, I assume you are talking about a specific discharge rate - ie, 10.5V open circuit means a very damaged battery; and 10.5V at 1A or 10A discharge will soon be as damaged...
I normally talk open-circuit voltages as that is the only universally applicable measurement - hence ~11.3V is fully discharged; 12.7V is fully charge - above that is surface charge else charging voltage; charging above 14.4V is gassing...
And fully discharged is a damaged battery. Crankers should not be discharged more than 30% (~12.3-12.4V), and deep-cycles not less than 50% according to many - even if supposedly 70% and 90% dischargeable. But even 90% discharged is not as low as 11.3V.
To quote eg 10.5V as a level of discharge is applicable only to that discharge rate (for that battery).
My fully charged cranking battery reads 9.5V during cranking yet is is 100% charged.
FYI - I recently saw someone else recommend a favorite of mine from years ago, namely (also aliased as - last updated September 13, 2010. (Hmmm - maybe I should review?)
As to charger recommendations - I have none.
I have a 6V/12V 2A/8A Pro-User BCH8 that I got for $30 (Jaycar).
I use a multimeter to monitor its voltage...
If I need a hard-charge (ie, over 20A to de-sulphate plates or kick AGMs into submission), I connect them to my trusty 1965 ute (with its 1980s Hitachi/Mitsubishi >70A alternator).
However, recently Aldi had a charger that seemed too good to be true for $25 - a switchmode charger with 4 or 5 modes; "microcomputer cointrolled", and from memory, "up to 120AH" - though I think it was about 7A (or maybe 4A?)...
Charging AGM Battery -- posted image.
(the above from
Alas by the time I decided it was something I didn't need but wanted it anyhow (like, I mean, for $25 bucks man!), they were all sold....
As long as they have an AGM/VRLA setting so they do not exceed 14.4V.
And AFAIAConcerned, even 4 Amps is fine EXCEPT if you require faster charging OR if you need to blast off the cobwebs (for which my guru recommends 20A or more for car batteries - ie, too expensive!) OR the manufacturer states otherwise.
There was something about a well known AGM manufacturer claiming a MINIMUM recharge current of significant proportion (like 10A or 56A...?), but I'm not a fan of them... They allegedly suggested giving a free battery for one that had lasted 13 years (that's about 12 years 9 months longer than the ones I have dealt with). LOL! My 11 year old AGM is doing fine despite its unintended car use. (Not that I'm one to admire "the olde" Yuasa batteries.)
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: December 06, 2010 at 1:02 PM / IP Logged  
rfhvhtoo - how do you intend using the AGM battery?
In storage, or a total loss system, or car (cranking) or audio?
Like I said - I use a $30 charger (normal price is ~$60-$70).
My flooded car batteries tend to last a minimum of 6 years despite abuse. (And I only recently upgraded to non-external non-electromechanical voltage regulators.)
I am now using a 10 year old 38AH AGM battery that I obtained when it was 5 years old. I had it in storage until this or last year. (I was reluctant to use it as a cranker, but when my brother said he had been using his for 3.5 years....)
I'd top it up every year or 2 with the 8A charger until it reached ~14.5V (and at times a bit higher... Charging AGM Battery -- posted image. ) and it still seems fine. (An alternator-less night-drive home some time back indicated it still had its original 38AH reserve; I discharged it ~50% over 60 minutes.)
The only battery I had troubles with was a new ~55AH wet cell which was little used for ~24 months. It was topped up every 3-6 months.
Despite seeming fine when charging prior to use as my 2nd battery to run a 12V 5A fridge, the MW728 battery protector (low voltage cutout) was cycling at ~15-30 second intervals.
The battery was dipping to the MW728's 11.2V off-voltage, and then recovering to its 12.5V on-voltage. (A typical problem for voltage-switched isolators...)
But as suspected, it came good after a few minutes with the engine running - I figured it needed the >16A or >20A blast to kick the sulphate off the plates... (I had a 75A alternator rather than the standard 23A generator (LOL!).)
The point is, with some nous, you can do quite well without sophisticated stuff. For one ctek charger I can by 3 cranking batteries. And since I retired my last batteries after 8 years service...
(I'm not like other Aussies that buy $300-$500 dc-dc converters to charge their auxiliary batteries. I prefer the big-3 to the big-300 or $500! Besides, I'd rather boost alternator output and then chop it down if required (instead of chopping "up" with a dc-dc converter).
I happened to see the CTek charger (US7002) with max 7A charge rate as recommended for up to ~150AH, whilst the US3300 (maybe 3.3A max?) is for up to 75AH or 90AH. So maybe a 4A charger is enough....
I got some 4A chargers for $5 each at a sale. I merely replaced the blade fuse with a circuit breaker as they don't like real flatteries (my Pro-User handles them fine - presumably it has current limiting).
But bigger is good if not too expensive. (Battery chargers have become another vvankathon with their 2200, 2500, 2700, 3300, 4200mA etc. Unbelievable!)
Not that I could find the specs on the Shuriken - I'd refer to the equivalent Kinetik else the Battery-FAQ link....
Beware though - I was amazed at the misinformation - like one place saying the Shuriken SK-BT100 is a Gel-Cel; or the above ctek 3300 for "up to 75AH" or "up to 90AH"...
I also saw another site's Dual Battery Wiring sticky/tutorial.... IMO it matched the quality of their discussion generally (eg, batteries are "poor" because they perform worse in dB comps. That means they have a lower internal resistance and are better batteries! But yes, that means lower audio peak-dB).
But it used the old matched batteries.... (except for location and non-diagonal wiring).
At least they agreed battery life is compromised by paralleling... yay!
Sorry - I said I'd go to bed an hour ago. The sun is just rising and I'm still warm from yesterday.
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: December 08, 2010 at 12:06 AM / IP Logged  
Update -
Reading as updated 7Dec2010, Darden recommends linking to as the primary source.
I also recommend downloading the file (as linked in the above).
Just unzip it to a directory of your choice and, once started from any of its hatemail pages (.htm), it should page fwd & back and link correctly.
[ EG - Under my D:\Technical\Batteries I created a "BatteryFAQ.20101207" directory and moved to it.
Extracting (from) should then place files under a \battery sub-directory.
Create a link/shortcut to "index.htm" and place it wherever you want. (In windoze, I usually right-click & Send to Desktop to create the shortcut and then move to wherever... ]
PS - I'm glad I left my review of BatteryFAQ till today! (Let's see where we disagree this time... LOL!)
PPS (after initial posting) - Reading " 1. WHAT IS THE BOTTOM LINE AND TIPS?" (carfaq1) slowly and thoroughly should be MANDATORY.
Copper - Posts: 238
Copper spacespace
Joined: June 13, 2008
Location: Florida, United States
Posted: December 08, 2010 at 9:33 PM / IP Logged  
Sorry it took a while to reply my computer went down for a couple of days. But I dont have too much to disagree with you on this one lol You KNOW batteries. I just didnt realize how much maintenance you have to do on maintenance free batteries! Compared to what you're saying I've been neglecting my battery horribly for about a year now. I need to get it charging ASAP But its my Main battery so I can't leave it charging for days. And the charger you recommended is similar to the one that SHuriken actually has on their website
but i found this one as well
how does it look?
I can't hear you!
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: December 09, 2010 at 1:54 AM / IP Logged  
Not that I waste time researching...
I was gonna say that cteks are expensive, but ~$110 is cheaper than I thought. I have only read good things about them. (I guess some Swedes are good at design & build. Charging AGM Battery -- posted image. Go Grippen, but I still want a Viggen!)
And having looked into this $30 Schumacher SC-600A "high frequency" bullsh...   I reckon go for it.
Its owner's manual ( here) clearly states AGM compatibility. Even a desulfating mode. I like it! Charging AGM Battery -- posted image.
I thought I did well with my $30 8A "linear" (heavy transformer) non-auto charger. But a $30 uProcessor-controlled charger with "high frequency" either referring to its SMPS supply (at under 2.8lbs - it must be) or its desulfation mode. (Add-on kits for that feature alone sell for $50 to $100 here!)
The SC-600 seems good. For $30 vs $110, or even $65 vs $110, I'd want to know what justifies the >$doubling for a ctek (based on the published blurb).
The bad SC-600 reviews seem like warranty issues, or people not being as expert as you on batteries (ie, after charging, a battery was 13V, der... surface charge... d'oh. The O-manual mentions surface charge for the test mode in the other models.)
But please confirm with your own search & destroy.... (mine was quick - not that I spend,, waste time researching....)   
Remember that I'm happy with mine... I set it for 8A and then reduce to 2A when the voltage hits ~14.4V, or when I see it at 15.x Volts hours later...
Or I leave it on 2A for a reasonable safe limit - though it will still exceed 14.5V eventually. (I could use a pluggable lamp timer if it mattered...)
But I do have a DMM. Several actually - & I'm thinking of fitting an ~$8 eBay blue backlit LCD voltmeter permanently. Read: Tape to the handle or charger case and run its 2 wires to the alligator/battery clamps etc.
A voltmeter is essential, but every car person should have a DMM - often $10-$20 is enough & rarely more than ~$25 for most needs.     
Having now answered your question with no care and no responsibility, I can relax.
And maybe now ramble a bit....
(I'll leave Dual battery setups till later in case you have seen my "Ultimate Intelligence" Battery Isolator (Patent Pending - in about 20 years... geez I'm so impatient!).)
& BTW - I thought 8A was too little for a 100AH battery, but it is enough. It may even be "too much" (Darden recommends against Fast Charging - but that's a lifetime issue). But a car battery might take all the left over alternator output (eg, a 120A alternator that CAN put out (say) 90A @ 3kRPM and the rest of the vehicle is only using 60A - that's 30 for the battery) even though it might only take 2A if fully charged, else soon drop below 10A if not...
Sorry - I delayed my unintentional ramble....
rfhvhtoo wrote:
I just didnt realize how much maintenance you have to do on maintenance free batteries!
I was going to say "but it's AGM!"
But you are correct... Not that it necessarily needs maintenance, but as BetteryFAQ says, ye oldendays setting of 13.6V to 13.8V for cars is too low.
And if sitting or not running (recharged) for long enough...
As to whether to increase or decrease for AGM, well...
You went on to say... "Compared to what you're saying I've been neglecting my battery horribly for about a year now. I need to get it charging ASAP But its my Main battery so I can't leave it charging for days....".
Me saying? Don't you mean BatteryFAQ? Or did really I cover that...? (LOL!)
But it shouldn't need days... overnight should be fine.
Yes, the lesser the discharge, the less frequent the recharge (cycles) etc etc, the longer than battery lasts.
But as per bFAQ (batteryFAQ), crankers are designed for 4 starts per day.
But AGMs aren't "crankers" are they? (bFAQ says what I have said - AGMs do not state CCA or CA. Maybe that's because their internal resistance makes that pointless. But maybe ...)
But you are using for audio. Does that mean you have a dual battery setup, or just one (and in the boot/trunk)?
You should be able to set your alternator voltage to suit the AGM if it is charging at too low a voltage (unless it is a D+ alternator - ie, only the D+ charge-light output, aka single-wire; we ignore the heavy B or B+ & ground).
But it's in non-colocated dual/multi-battery setups that charging voltage can be an issue. The paralleling of wet cell & AGM is a lesser issue. (I always assume parallel batteries are NOT connected when not charging, and when not being used in combination for a heavy load. But there are those that don't care about longetivity and /else dispute or don't understand the other issues bFAQ & I point out - eg, JM & OJ.)
Copper - Posts: 238
Copper spacespace
Joined: June 13, 2008
Location: Florida, United States
Posted: December 09, 2010 at 5:33 PM / IP Logged  
Yeah its for my car audio as well as the rest of my electronics. the single large battery setup could handle it (back then I only had 1000watts). Now that I have made upgrades in amps and subwoofers over the past yearim near 4000 watts rms. I then upgraded my alternator to 220 amps which ofcourse helped tremendously but can't power my system alone. I've got a stinger voltmeter about 6 months ago inside of my truck actually and is how I'm starting to notice the voltage and trying to learn more about it as I see the performance trickle down. The stinger is always -.2v less than what my DMM reads. I'm starting ask a lot of question now though because When I start my truck it'll drop to about 10.6v and it used to only go down to 11.4v range, and I live in florida so its usually the same 75-85 degree weather isn't that big of a factor I don't believe in my situation. Oh man and I've gotta show you Amazon also has a schumacher sc-1200 with voltage and amp displays for only 6$ more, so many options! But with only 2/8/12 current ratings. But if 8 may be too much ill just stick to the sc-600. But take a look at it anyway before I finalize my relationship with the 600 lol
Thanks for sticking with me through this! Lol
I can't hear you!
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: December 09, 2010 at 7:03 PM / IP Logged  
Finally - WHY and HOW you want it.... You are making up for too small an alternator or too large a load.
So hence an external charger to the top up required.
Though dual batteries are usually used in your situation (cranking independence and audio performance), that doesn't change the undersized alternator. (You may replace more discharged power, but they still won't be fully recharged.)
In that situation, many people find they replace batteries often - especially those that have 2 or more batteries permanently connected in parallel (battery sales love 'em!). [Hence isolators are well worth it - even if the voltage sensing types (aka smart isolators LOL).]
An any situation, bigger charger capacity is nice IN CASE you ever need a quick charge; emergencies like wedding days etc.
In this situation - go bigger for its voltmeter etc; it is still good bang for bucks - a few more dollars for a larger proportion of capacity increase.
BUT - don't charge at 12A if (say) 7A is the limit for your battery. (Though what if say 8A isn't enough to recharge?)
The recommendation is that you set to the least charge current for the time you have.
IE - if set to 2A and it's charged in the morning, fine. Else 4A etc.
Remember, the lesser the charge rate, the better for the battery. (Reconditioning and maintenance excepted.)   
As to extra price merely for a voltmeter? YES!
IMO a voltmeter is essential - in a vehicle it tells you all you need to know. Except actual capacity - but an ammeter won't do that anyhow.
For a charger, the voltmeter is piece of mind etc. It tells you that it is NOT exceeding specs - eg, 14.4V or 14.5V whatever, or maybe 16.2V for reconditioning (wet cells only).
And upon initial battery connection, its voltage may indicate SOC (state of charge) - but not if or once it has been charged (ie - surface charge).
BTW - SOC is theoretically linear between about 11.3V & 12.7V. (Theoretical? In practice, the battery might hit some level and then just collapse. Modern constant power loads don't help either.)
But for SOC I assume 0.1V = 10% DOWN from 12.7V or 12.6V because that's easy to figure out - hence 11.6 to 12.6V etc. For limiting depth of discharge, that's nicely conservative.
BTW - Darden quoted "about" 2.11V per cell fully charged, hence 6x2.11 = 12.66V for a 12V battery. Hence displayed as 12.6V or 12.7V, but the real voltage can vary through construction differences,temperature, etc.
And for car voltmeters - only ever 3 digits (eg, 12.7V).
4 or more digits for "battery condition" applications. Darden reckons >0.5% voltmeter accuracy. I reckon 8-bits is insufficient (1/256 = 0.4%) but 10 bits is - ie, 1/1024 = 0.1% resolution for digital readers.   
FYI - bFAQ reckons no more than 20% discharge for crankers. I've been saying 30%. But we seem to agree on 50% for deep cycle.
Ammeters are not essential per se (and pretty much useless in a vehicle for any general use!), but IMO chargers should have some current indication.
EG - my 2A/8A charger has 4 LEDs. I see on 8A if all are fully lit and then gradually dim & drop. (At 2A I might switch to 2A to lessen the charge voltage.)
If I know it was flat and it doesn't take 6 or 8A, then the battery is weak... sulfation perhaps.
On the 2A range, only the 2A LED lights (though the 4A lamp may be dim for a while), but that's ok for typical car batteries.   (For small batteries like alarms and motorbikes, even 1A might damage them!)
I hope that helps. Some is POV etc, but hopefully you'll find big agreement between me and the Dardens and other battery "experts" in this world.
For those that think otherwise - it is often a misunderstanding of a basic concept (like only ever paralleling matched batteries, or 12.4V means 52AH remaining), or that battery life or reliability is not an issue for them (eg, permanent paralleling of ANY batteries), or that they really don't know.
To me it seems simple, but I had the luxury of being taught by some some great experienced experts.
As one said, batteries are living and breathing... they are more of an art than a science...
But to quote me, they are still built on a science.
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