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Old 07-13-11, 04:23 PM   #31
MetroMPG
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Batteries! (Used batteries.)




The 2 on the left (12 volt, 12 Amp hour) read 13.1v each. That's a good sign.

The 3 on the right (12v, 7.2 Ah) are a dog's breakfast: one dead @ 0.0 v, one at 13.1 and the other at 12.6.

Let the capacity testing begin!

If the pair of 12 Ah ones are close in capacity, I think that's what I'm going to start using in the mower. They'd make 24 Ah total (rated) capacity vs. 28 Ah from the OEM battery.

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Old 07-13-11, 08:37 PM   #32
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Capacity testing, part 1...

So far it looks like all the smaller 7.2 Ah batteries are probably duds.

I tested the "best" one first (the one with the starting/resting voltage of 13.1), using a 40 watt, 120 volt incandescant light on an inverter. It pulls 3.7A from the battery at the start (when batt voltage is high), but it goes as high as 4.15A as voltage falls close to 11.0. The inverter shuts off automatically when the battery gets below 11.0v. So let's call it a 4 amp load.

Note that different inverters vary on their shut off points. And some capacity tests go as low as 10.5 volts to evaluate a 12v battery - eg. How to Measure Capacity – Battery University

The "best" 7.2 Ah batt could only deliver 4 amps for 7 minutes before the inverter automatically shut off when the battery slid down to 11.0 volts.

---

The 12 Amp hour batteries look more promising. I've been pulling 4 amps from one of the pair for 45 minutes now, and voltage is at 12.06 and falling reasonably slowly.

EDIT: pulled 4 amps for 53 minutes before the inverter shut things down @ 11.0v. Obviously voltage falls much faster at the end of the discharge - but you probably already knew that!

---

It'll be interesting to see how the mower treats the batteries: what amperage the motor pulls under normal conditions, and at what voltage it shuts things down to protect the batteries (assuming it has an automatic shut off).
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Old 07-13-11, 09:47 PM   #33
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Capacity testing, part 2...

Results of the other 12 Ah rated battery:

- pulled 4 amps for 57 minutes before the inverter shut things down @ 11.0v.

Since the first 12 Ah battery went for 53 minutes, that means I've got a reasonably well-matched pair and can use them together in parallel without one running out of juice significantly faster than the other.

I'm not sure what the mower motor pulls, but I have a feeling these two are going to be able to do the job.

Next step: recharge & install the batts in the mower, then do a test-mowing!

(That'll be next week, because I mowed with the 12A plug-in beast today. And the grass isn't growing terribly fast these days.)
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Old 07-13-11, 10:46 PM   #34
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Nice work! Did you top off all the batteries before discharging them?
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Old 07-14-11, 07:12 AM   #35
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Nope! The three batteries I tried had resting voltages of 13.1 when I got them, so I assumed they were fully charged and just stuck them on the inverter.

(They came out of a system this week that was keeping them charged up.)

I realize a better test would be to partially discharge, then fully charge the batteries and let them rest for a day before doing any testing.

I was impatient. :P
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Old 07-14-11, 01:57 PM   #36
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Capacity testing, part 3...

Did another one of the 7.2 Ah rated batteries, and this one's good!

- pulled 4 amps for 45 minutes (!) before the inverter shut things down @ 10.9 v (why not a 11.0v cut-off like the others, I'm not sure - maybe a function of how "stiff" the battery was - voltage dropped much more slowly & steadily than any of the other 3).

Compare to 4 amps for 7 minutes, which was what I got from the other 7.2 Ah batt. (The one I thought was probably best, based on its higher 13.1 resting voltage when I got them.)

So, as a proportion of rated capacity, this one's the healthiest of the four batteries tested so far.

Unfortunately, one good 7.2 Ah battery does me no good. Need 3 or 4 of these to put in the mower. (Or 3 to put in the electric car... or...)
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Old 07-14-11, 02:24 PM   #37
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Quote:
Originally Posted by MetroMPG View Post
Unfortunately, one good 7.2 Ah battery does me no good. Need 3 or 4 of these to put in the mower. (Or 3 to put in the electric car... or...)

If all of these batteries are 12V then why not add this one the two 12Ah batts in parallel? Then you'd have 12+12+7.2=31.2Ah, which is more than the OEM 28Ah.

And the discharge times aren't that different. How long would you be mowing with this mower, anyway? 30-40 minutes?
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Old 07-15-11, 05:14 PM   #38
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Not exactly sure what my mowing time would be (minus dancing with the extension cord, that is) - but definitely no more than 30 min.

"the discharge times aren't that different."

True, but we also don't yet know the current draw of the mower's motor. The capacity test was with a 4A draw. That's a big wild card.

Also, if we do exceed the run time of the battery with the lower capacity, the mower will probably murder it before it does an auto shut-off.

I've murdered a few batteries in the forkenswift this way. The healthy batteries will continue to work after the weak one has given up the ghost, so it masks the damage you're doing -- unless you actively monitor the voltage of the weakest batt(s).

Anyway, we'll have our answer next week!
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Old 07-19-11, 01:30 PM   #39
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Battery murderer!

So, a few mysteries solved:

  • First, the motor pulls about 9-10 amps once up to speed - that's just spinning the blade (not cutting grass).
  • Second, and most suprising, the mower doesn't seem to have a low voltage cut-off!

I hooked up one of the 12 volt, 7.2 Ah batteries, and when I started the mower, it initially dragged it down to 4.8 volts, pulling ~20 amps!

Of course, as the blade speeds up over the course of 2-3 seconds and RPM stabilizes, current drops, and battery voltage rises. But the voltage only recovered to 9.5 volts @ ~10 amps, which is way too low to safely use a small lead acid battery (risk of cell reversal = battery murder).

Of course, with a larger pack powering the motor, voltage sag will be much less.

I'm trying to track down an owners manual. Maybe it'll shed some light on the subject of how owners were supposed to know when to shut it down for the health of the battery!
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Old 07-19-11, 01:33 PM   #40
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Oh, and some good news: the two 12 Ah batteries will fit under the hood of the mower with some slight modifications (need to trim back some plastic bits in the housing).

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