AGM Batteries and the FD
12-06-21, 08:04 PM
#1
AGM Batteries and the FD
In April of 2020, I installed an Odyssey PC952 battery, and I have had an issue recently. I will start by mentioning that I relocated my battery to an LRB Speed tray just behind the passenger seat. Currently, the battery ground is attached to the chassis, just behind the door opening and also to another point along the lower door sill (around where the fuel door/hatch release is but on the passenger side. The positive goes through a 150A breaker, then a battery disconnect switch, then straight to the starter. I have plans to run a ground directly from the block straight to the battery this month. A little over a week ago, I noticed my battery was dead. Onboard voltmeter was showing around 4.5V. I had been using a Battery Tender Jr, which claims to be AGM-compatible, and has worked fine as far as I could tell.
I have always noticed intermittent slow cranking, even though I have the upgraded to the RX-7 nose/RX-8 motor started from Banzai (I think). The behavior is that a cold start cranks nice and fast. If I stop for a short time, a warm or hot start will have the starter spinning much slower than normal. If I immediately key-off and right back on, it will spin fast again. Occasionally it may do this a couple of times before spinning fast again. My grounding plans include grounding the starter, as that has been a documented solution.
Back to the battery topic:
AGM batteries require different charging (in the case of my Odyssey battery) than flooded lead acid batteries, and not all battery chargers are created equal. "What do you mean?" you may be saying. Well, if you were to use a typical "AGM-compatible" trickle charger (think my Battery Tender Jr), know that not all provide the necessay "float" voltage required to keep the battery at 100%, and will never charge the battery to the higher resting voltages that AGM batteries require. This is the exact noted reason the batteries fail prematurely, according to what I have read. Now you're probably thinking, "Well, get your head out of your *** and go buy a proper AGM brand-specific charger!" Of course, you'd be right, and that's what I did. I got a Batteryminder AGM charger/maintainer. Here's some observations and thoughts:
Newer cars that come standard with AGM batteries have "smart alternators" with regulators, and wiring/computers that do not "overcharge" AGM batteries. Those systems also regulate the proper float voltage needed. Apparently, older vehicles (ours) with less sophisticated alternators could possibly overcharge an AGM battery if the car is running too long. "Too long" is not an exact time that I have read about or experienced, but the battery would become hot to the touch, I would imagine.
Anyone running an AGM battery, with stock or stock-based wiring system, would need to buy a proper battery charger/maintainer specific for that brand battery, or risk constantly changing expensive batteries due to constant under-charging. The charger/maintainer would need to be plugged-up whenever (the whole time) that car is not being driven. I would imagine that is nearly all the FDs owned by this forum's members.
I would like to know everyone's experience and habits with AGM batteries - longevity, issues, what charger/maintainer you use, tips, tricks, etc.
I have always noticed intermittent slow cranking, even though I have the upgraded to the RX-7 nose/RX-8 motor started from Banzai (I think). The behavior is that a cold start cranks nice and fast. If I stop for a short time, a warm or hot start will have the starter spinning much slower than normal. If I immediately key-off and right back on, it will spin fast again. Occasionally it may do this a couple of times before spinning fast again. My grounding plans include grounding the starter, as that has been a documented solution.
Back to the battery topic:
AGM batteries require different charging (in the case of my Odyssey battery) than flooded lead acid batteries, and not all battery chargers are created equal. "What do you mean?" you may be saying. Well, if you were to use a typical "AGM-compatible" trickle charger (think my Battery Tender Jr), know that not all provide the necessay "float" voltage required to keep the battery at 100%, and will never charge the battery to the higher resting voltages that AGM batteries require. This is the exact noted reason the batteries fail prematurely, according to what I have read. Now you're probably thinking, "Well, get your head out of your *** and go buy a proper AGM brand-specific charger!" Of course, you'd be right, and that's what I did. I got a Batteryminder AGM charger/maintainer. Here's some observations and thoughts:
Newer cars that come standard with AGM batteries have "smart alternators" with regulators, and wiring/computers that do not "overcharge" AGM batteries. Those systems also regulate the proper float voltage needed. Apparently, older vehicles (ours) with less sophisticated alternators could possibly overcharge an AGM battery if the car is running too long. "Too long" is not an exact time that I have read about or experienced, but the battery would become hot to the touch, I would imagine.
Anyone running an AGM battery, with stock or stock-based wiring system, would need to buy a proper battery charger/maintainer specific for that brand battery, or risk constantly changing expensive batteries due to constant under-charging. The charger/maintainer would need to be plugged-up whenever (the whole time) that car is not being driven. I would imagine that is nearly all the FDs owned by this forum's members.
I would like to know everyone's experience and habits with AGM batteries - longevity, issues, what charger/maintainer you use, tips, tricks, etc.
12-07-21, 10:45 AM
#2
Interesting, I have been running Miata AGM battery since 2013 and I'm currently on my 2nd battery (2015-present). First battery lasted only 1 years, it died after I forgot to put a tender on it in the garage for more than a month during winter.
Several things I have learned over the years that help keep my second battery healthy, these probably apply to all batteries and not just AGM.
Be sure to get minimum of 13.5V at idle and above 14V during cruise, that ensure the battery is being charge when the car is running. Run proper sized pulley and get a good alternator, I'm running IRP 140A. I used to get 12.5V-13V at idle with OEM alternator and underdrive pulley, I notice slow crank between drive unless I put it on a tender even in the summer time. Now I rarely tender the battery in the summer but it is always disconnected at the breaker when parked, and if I don't drive it for 2 months I'll trickle charge it before taking it out. During the winter months I remove the battery and store it inside, also trickle charge it once a month, with it being inside I can go as long as 2 months without putting a tender on it. Chargers are nothing special, an old Diehard charger that support AGM battery I use in the garage, and a Deltran battery tender when I have the battery inside during winter.
Several things I have learned over the years that help keep my second battery healthy, these probably apply to all batteries and not just AGM.
Be sure to get minimum of 13.5V at idle and above 14V during cruise, that ensure the battery is being charge when the car is running. Run proper sized pulley and get a good alternator, I'm running IRP 140A. I used to get 12.5V-13V at idle with OEM alternator and underdrive pulley, I notice slow crank between drive unless I put it on a tender even in the summer time. Now I rarely tender the battery in the summer but it is always disconnected at the breaker when parked, and if I don't drive it for 2 months I'll trickle charge it before taking it out. During the winter months I remove the battery and store it inside, also trickle charge it once a month, with it being inside I can go as long as 2 months without putting a tender on it. Chargers are nothing special, an old Diehard charger that support AGM battery I use in the garage, and a Deltran battery tender when I have the battery inside during winter.
12-07-21, 07:14 PM
#4
I have not tested the idle voltage at the posts, but the onboard (I have a PLX AFR gauge that show voltage) shows somewhere around 13-ish volts at idle, sometimes 14V.
The thing that keeps coming up whenever I read about AGM battery charging is that they are sensitive to over and under charging. I do know one thing, though - it's that Odyssey calls for "Charger voltage at 68°F (20°C): 14.4V to 14.8V" in their literature. Float charge/maintain charge is: "The trickle charge voltage measured at the battery terminals must be between 13.5V and 13.8V" Also, Odyssey specifies maximum voltage that I believe any FD alternator is not capable of: "It is imperative not to exceed 15.0V as this will cause the pressure valves to open and out-gas hydrogen, oxygen and water from inside the battery. This will shorten the life of the battery and cause premature failure."
All this to say that I have not logged alternator output, but I suspect that it likely comes up a little shorter than 14.4V and probably never gets to 15V. I would also venture a guess that, on a fully-charged battery, my FD alternator is not "floating" between 13.5-13.8V.
My conclusion is that the only time my Odyssey battery is being "properly" charged is when it is on the charger/maintainer. Of course, the real world is never nominal conditions, so the real goal here is to take any AGM battery requirements you have, and try to stick as close to them as possible, I guess,
The thing that keeps coming up whenever I read about AGM battery charging is that they are sensitive to over and under charging. I do know one thing, though - it's that Odyssey calls for "Charger voltage at 68°F (20°C): 14.4V to 14.8V" in their literature. Float charge/maintain charge is: "The trickle charge voltage measured at the battery terminals must be between 13.5V and 13.8V" Also, Odyssey specifies maximum voltage that I believe any FD alternator is not capable of: "It is imperative not to exceed 15.0V as this will cause the pressure valves to open and out-gas hydrogen, oxygen and water from inside the battery. This will shorten the life of the battery and cause premature failure."
All this to say that I have not logged alternator output, but I suspect that it likely comes up a little shorter than 14.4V and probably never gets to 15V. I would also venture a guess that, on a fully-charged battery, my FD alternator is not "floating" between 13.5-13.8V.
My conclusion is that the only time my Odyssey battery is being "properly" charged is when it is on the charger/maintainer. Of course, the real world is never nominal conditions, so the real goal here is to take any AGM battery requirements you have, and try to stick as close to them as possible, I guess,
12-07-21, 08:08 PM
#5
Full Member
I've went through 3 batteries in 20ish years so approximately one battery every 7 years. I've used a 925 and two 525s. Battery is behind passenger seat connected with 0 gauge cables. Ground is short and to the chassis near the base of the battery. Positive cable long goes to the connector on the engine bay fuse/relay box, i.e. same place the stock battery cable was connected. The bigger battery cranked the engine quickly and consistently. The smaller batteries worked well, cranking just a little slower but as long a no drain while engine was off they would get the job done. Car is garage kept but has been driven in during freezing but dry weather and would start fine after sitting 4 or 5 hours in the cold. A Battery-Tender trickle charger has served me well for at least 15 of these years. Also still using stock original alternator with a pulley kit.
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