AGM Battery Technology Primer
16 June 2008
AGM (Absorption or Absorbed Glass Mat) battery technology was developed in the 1980's for military aircraft.
In AGM batteries, the acid is absorbed between the "plates" and immobilized by a very fine fiberglass mat.
The "plates" in an AGM battery may be flat like wet cell lead-acid battery, or they may be wound in a tight spiral. Their unique construction (as they are supported in large part by the mat) also allows for the lead in their plates to be purer as they no longer need to support their own weight as in traditional cells.
Some of the liquid material will escape during charging thus decreasing the overall capacity of the battery. The lids (covers) allow safe dispersal of any excess hydrogen that may be formed during overcharge. They are not permanently sealed, but are maintenance free; and they can be oriented in any manner, unlike normal lead-acid batteries which must be kept upright to avoid acid spills and to keep the plates' orientation vertical.
Many modern motorcycles on the market utilize AGM batteries for the combined benefits of reduced likelihood of acid-spilling during accidents, and for packaging reasons (lighter, smaller battery to do the same job, battery can be installed at odd angles if needed for the design of the motorcycle).
Specific things AGM's do not like (i.e. impair their working correctly):
• Parasitic loads - Any small continues load that is on 24x7 such as theft alarms.
• Short rides - Because the battery is difficult to recharge completely, 35 mile (50 kilometers) rides are a minimum. Potentially a once a month ride of 200 miles (300 kilometers) may give enough of a topping off to the charge to keep the battery happy.
• Charging voltages which exceed 15 VDC. Most chargers are not so tightly controlled for over voltage protection.
Things I have found out:
• The cover can be removed.
• Using a syringe distilled water can be added (do not use scented water made for steam irons).
• The water is absorbed slowly so be patient (20 minutes is not abnormal for a couple of ml to be absorbed).
• While charging a discharged battery the battery can handle a high current (20 A for 10 minutes, 10 A for 20 minutes, etc.) and in fact a high current is useful to charge the deep recesses of the battery.
Some people have had very good success augmenting their battery charges with various AGM specific battery tenders that seem to take care of the problems of parasitic loads and short rides.
If you do not have the facility to deliver power to a garage area as is needed with a battery tender you could do what I do!
I have had my battery pronounced dead and used up on several occasions! On each of those occasions I have done the following procedure.
Equipment and supplies needed:
• A wrench to remove the battery from the bike.
• Distilled water (the cheap good stuff, no scents for steam irons, etc.).
• An old hypodermic needle (look around most play ground areas or contact a junkie).
• A flashlight.
• A plastic bag to protect your table surface from acid.
• A battery charger with a voltage protection, or limiter that kicks in at 15 VDC. Maximum current can go to 20 Amps, but minimum should be at most 2 Amps.
• A Volt meter to check resting voltage.
• A screw driver to carefully pop the cover off.
• Removed the battery from the bike.
• Carefully open the cover of the battery. If you break off a few plastic studs (as I have done) you can use some tape when it comes time to put the cover back on, but be sure to leave some breathing room around the cover (i.e. don’t tape it completely sealed).
• Pop the lids off the cells.
• With a flash light look into the cells.
Any cell that appears to be dry will need some distilled water, I use a hypodermic with the metal needle removed, and I add 4 ml at a time to each cell that appears dry.
1. Add water.
2. Wait 20 minutes.
3. Recheck cells condition.
4. Add water as needed and repeat steps 2 thru 4 until the surface of the cell seems to keep a moist appearance.
5. When the cells stop absorbing distilled water put the charger on at maximum current.
Note: It is possible that the battery will not charge at maximum, do not worry, it may need some TLC. I have dropped the charging rate down to low (trickle, or about 2 A) and monitored the water adding it as needed to maintain the moist appearance of the cells. If you over fill your cells, when the battery is put on high charge, later in the cycle, the excess fluids will bubble out the cell access points (be careful with the bubble over because it is acid).
• If the battery absorbs maximum current (depending on what that is) stop the maximum current at an appropriate time and continue with a trickle charge over night.
I have found that checking the battery every 2 hours while on trickle is sufficient to keep it from going dry, if you apply 4 ml above the barely moist cell condition.
• Until the battery will maintain a 12.x VDC (plus or minus) with the charger leads disconnected over night and has been charged for a period of time between 24 to 48 hours with a trickle and a short duration high current charge it is not up to snuff.
• Let the battery finish off by trickle charging it until you observe the moist (but not wet) cell condition on all cells evenly.
You may need to let the battery charge longer to remove excess water, or add water to some cells as the fluid goes down to get them all looking similar.
Alternative water reduction techniques for the impatient:
• Excessive water that remains on the cells after your battery is fully charged can be removed by putting the metal needle tip back on the hypodermic and sucking the excess water out of the cells.
• Tip the battery upside down in the sink and see what comes out.
BECAREFUL with the fluids that the battery yields via any mechanism, they should be diluted with water and flushed down the pipes because they are acid. It will put a hole in clothes and cause an open cut to burn (leaving a scar).
Most important is not to exceed 15 VDC while charging.
At 160 euro a pop, I have saved myself 480 euros so far with this technique.
Ken "motts" Applegate, Paris (France not Texas)
AMA, MGCB, MGWC, MGCGB, FEMA Friend
California EV, V10 Centauro GT
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