More batteries

I got a whole boat load (well, trunk load really) of batteries from a friend to test. He inherited them from someone who moved out of state and didn't want to take the whole load with him. There are some that are unused (still in their original boxes). I calculated that on the way home, I had 800 Amp Hours of 12 v power in the vehicle. That would have been at full rated capacity of course. These are all AGM batteries (like this one) that were either used or destined for use in wheelchairs.

Anyway, the first couple I tested were not so good; they are rather old and had been in use for quite some time (one was dated 1998). These 70 AH rated batteries topped out at 30 and 50 Ah after three cycles of charge/discharge.

Next, I tested (well, I'm still working on them) two of the "unused" batteries. They look fairly new, but I can tell that they've been hooked up to something before - even if only for a short time - because of the marks on the terminals. These two are doing much better! I charged one of them up partially (using my new Schumacher WM-2500A charger). The first time around it tested at about 50 AH. On the next cycle after charging it fully, it was up to 67 AH under a 0.1C load (this is actually its rated capacity - 75 AH * 0.9 = 67.5). On these big batteries, 0.1C is about the highest load I can use on my tester.

The second "unused" battery is undergoing its second test. I charged this one up completely for its first test and it measured 63 AH. The trace on the current test is running somewhat higher than the first one so far, but I'm only about 10 AH into it.

I'll probably cycle these two batteries one more time after this to see if they continue to improve significantly.

Interesting notes:

1. I learned from the spec sheet I listed above that they recommend a maximum 15 AH charge rate. I had read elsewhere that you can use a rate up to 40% of the rated capacity of the battery. This was why I selected the 25 AH charger to begin with for these 70-80 AH batteries. It's not that big of a deal to use the 12 AH rate; it still takes overnight anyhow. I like having the capability of getting a battery (or multiple batteries) charged up quickly if need be...
   
2. Even though a charger is rated at 25 Amps, you can't charge a dead 75 AH battery in 3 hours. It takes overnight or so (each battery is a little different) for it to absorb that last 10% of energy before it's "fully charged."
   
3. My fancy new charger does a few cool things to keep the charge rate up. Initially, it hits the battery with as much current as it can while keeping the voltage under a certain level (it seems to use about 15.5 volts or so). This seems high, but it only does it for a short time to gauge the battery. Then, it just dumps as much charge into the battery as it can (it peaked at 30 Amps according to my Whatt Meter in the 25 Amp mode) until the voltage nears the maximum for whatever battery type it's charging. It tapers off until the voltage tops out at whatever its cut-off voltage is and it goes into "float mode" (where it seems to hold at about 13.8 volts - which is perfect for AGM batteries). This taper period is why the charge takes longer than just Capacity / Charge Rate. On batteries that are badly sulfated (at least I think that's why it was doing it in one of these batteries), the voltage quickly goes high and the charger went into float mode. It was putting 2.5 Amps constant into the battery for several hours before it tapered again and then started cycling up and down to a maximum of several hundred milliamps while holding at 13.8 volts.
4. The "float mode" of this charger is really a cycle where it raises the voltage to 13.8 volts and then shuts down until the voltage drops a few tenths of a volts and it ramps back up again. It does it quickly so it's hard to get a good reading on my digital meter.
5. Because of this pseudo float mode, getting the battery completely and fully charged would take quite a while on this charger. I would say that charging it up most of the way with the fast charger and then putting it on a power supply (like a UPS or a Ham Radio supply) that holds a constant voltage is a faster way to get a full charge.