I've recently acquired a Northstar group 27 battery(930 CCA, 89.9 amp hours at the 20 hour rate). It was the older model that had a blue top where as the new ones are gray.
They dropped 30$ off it for being the older one, and its resting voltage before installation was 12.36v, which I was not really happy about.
Northstar says a fully charged healthy battery is 13.0V
Anyway, I drove the 27 miles home from the battery shop, and it was taking 11 amps initially, not sure what it was on engine shut off, but it read 12.7 an hour later.
Which I was not happy about.
I tried to charge it with a Schumacher sc2500a on the 12 amp AGM setting. This charger is on Odyssey's approved list of chargers, but only approved for a 75 amp hour capacity battery ( or less) and only when set on the 25 amp setting.
The voltage climbed to above 15 within seconds and the charger quit, went into fault mode. I've seen this happen often when applied to a fully charged battery, or a sulfated one.
I then ran the headlamps for a bit to drop the voltage below 12.8 so the charger would at least attempt to charge it again, instead of seeing 12.8v+, and going instantly into float mode.
This time it took 12 amps( I have a DC clamp on Ammeter), rose up to 14.6v and then the amps started to taper. Within 10 minutes it was back in float mode, 13.6 needing under one amp to hold 13.6.
I was still not convinced the battery was fully charged and was regretting buying the older blue top as the resting voltage dropped to 12.7 again.
When researching batteries for my application, I had talked with an Odyssey tech person about their batteries requirement that it be initially fed a 0.4C rate( 40 amps for a hundred amp hour battery) until the voltage reached 14.7, then held at for 14.7 for 4 hours(as the amps taper), before dropping to a 13.6v float voltage, at 77f. This requirement was for a deep cycled battery. When used as a starting battery and not cycled deeply their battery did not require this rather extreme charging regimen to return to full energy density.
He said a Deep cycled Odyssey battery not fed this charging regimen would not be fully charged, never get fully charged, or perform as well or last as long as one that was. It would likely last as long as a well treated flooded battery, but at 3x the price. He said a significant discharge was required then the correct High current recharge applied to reach ~ 90% energy density. That another 50% discharge was required and another correct 0.4C rate for 4 hours before the battery could be considered at full energy density.
I figured since Northstar is very similar to Odyssey, I'd try this treatment, but I needed to discharge it first.
I ran 255 watts of resistance heaters through an inverter until the voltage dropped to 11.6, the voltage rebounded to about 12v fairly soon after the loads were removed. Around 45% charged, perhaps less.
I put the Schumacher charger on it, 25 amp AGM setting and let it go for about 5 hours. The amps had started tapering when the battery was about 14.4 about 90 minutes in. The Voltage rose up to 14.65 about 2 hours into the charge and stayed there. The amps continued to taper and another 3 hours they tapered to 1.65. I then removed the charger, there were no loads on the battery but my engine computer. 7 hours later the voltage read 12.99.
The engine cranked faster than I ever heard it crank before.
I was impressed.
I was happy.
Still, Today I went back and paid the extra for the newer gray top battery and swapped them out.
This newer graytop Northstar was reading 12.89v on the shelf in the store, and did not crank the engine with the same impressive authority as the previous blue top I had subjected to the above charging regimen, and the engine was already warm, not overnight cold.
It also is behaving the same way when I attached the Schumacher charger to it after a 27 mile drive home at highway speeds, but with somewhat higher resting voltages than the bluetop had.
So tonight I am going to bring it to ~50% state of charge and feed it the 25 charging amps again to wake it up. That 930 CCA of the fully charged bluetop was fairly incredible to witness.
The Low and slow trickle charge mentality will not work for an AGM Discharged below 90%. Perhaps some other AGM's are more tolerant of a lesser initial charging current, but not those AGM's with these very high CCA numbers when compared to a flooded battery of the same size.
My opinion is does not work good for flooded batteries either. An old wives tale.
Low and slow for perhaps a battery in the 95% range, not to recover one discharged deeply. When discharged deeply, they are very thirsty. Let em drink!! Save the trickle charger for occasional topping up, or for the smaller batteries.
