[ausev] New EV Use

[Chris Robison]

Very good advice here. I'll add some comments (and a question):


Sarah & Erik wrote:
> As for not killing your batteries - they're going to die sometime,
> your goal is not to speed that process up more than you need to.
> Things that damage batteries include:
> chronic undercharging - flooded batteries must be equalized every so often

Sufficient regular charging (avoiding chronic undercharge) and 
equalization are actually two separate but similar issues.

Chronic undercharging results in deeper daily discharges, since your 
pack never gets entirely full each day. Deeper discharges age a battery 
faster than they would if they weren't discharged as deeply.

Equalization solves a different and equally serious problem. Within a 
series string of cells, it is guaranteed that some cells will have a 
slightly higher/lower capacity, higher/lower internal resistance, etc. 
When cycled (discharged and charged), it is therefore given that some 
will discharge slightly more or less than their neighbors relative to 
their own total capacity. Continued cycling causes the "imbalance" to 
worsen. The "low" batteries get discharged more deeply than the others, 
until eventually you discharge them beyond the critical threshold 
(1.75Vpc), even though the pack doesn't appear to be discharged. In some 
cases you may even discharge some cells below zero volts, which is 
called "cell reversal". These conditions cause immediate damage to the 
battery -- the former shortens its life substantially; the latter makes 
the battery pretty much worthless.


> chronic overcharging - there's a point where more hard charging doesn't help

This is one I'm not completely clear on; maybe someone can help out. 
It's been my impression that over-charging flooded batteries effectively 
just wastes energy and water and makes an unnecessary mess, and if the 
level is low it risks exposing the plates. I'm not sure if there's any 
other problem associated with keeping the batteries on charge too long.

> leaving them sit quite discharged for a while - basically, charge as
> soon as you can. I don't believe this is quite as scary as most make
> it out to be

Here's the situation as I understand it. Lead acid batteries like to be 
full or as close to it as possible. Aging of a lead-acid battery is an 
unstoppable process, but the more full the battery is at any given 
moment, the slower it is aging.

If you leave a battery (of any chemistry) to sit, it will 
"self-discharge" at a rate that is particular to its chemistry, design 
and construction. Lead acid batteries generally have a low 
self-discharge rate. I've seen it recommended that you top them off 
every 6 months for a good lifespan, and this doesn't seem unreasonable.

Leaving them to sit in a discharged state means to leave them sitting in 
  a state at which their aging is accelerated. The primary process 
taking place is "sulphation". When discharging, sulfur moves out of the 
electrolyte as sulphate ions and bonds to the lead in the plates. It 
bonds in an amorphous form, from which it's easily released back into 
the electrolyte when charging. After remaining on the plates for a long 
time, this lead sulphate compound has the opportunity to self-organize 
into a more crystalline form. The sulphate crystals grow and become more 
difficult to release back into the electrolyte -- the crystals become 
"stuck" to the plates. Every bit of plate surface that's covered with 
these crystals is surface area that's unavailable for participating in 
the battery's chemical process. Internal resistance rises, capacity 
decreases.

When a lead-acid battery is more fully discharged, there's a lot of lead 
sulphate on the plates, and the sulphation process occurs more quickly.

> abusive discharges - Lee Hart on the EVDL had a guideline for
> discharging the batteries nicely. I believe it was try to keep your
> continuous discharge amps lower than your AH rating on your battery.
> So a 220 AH battery shouldn't hold more than a 220 amp discharge. Half
> again that much is fine for short accelerations.

There's a jargon term you might sometimes see for this. The term "C" can 
be thought of as a value of current in amps equal to the capacity in 
amp-hours. So, discharging a 220Ah battery at 220A is called a discharge 
of 1C. Discharging at 440A would be called 2C. 110A = C/2, etc. So Erik 
is talking about a continuous discharge no higher than 1C (regardless of 
battery capacity) and 1.5C may be a good upper limit on usual 
low-duration discharges for getting up to speed.

> It isn't necessary that new EV owners kill batteries, but they can be
> unfortunately easy to ignore. I had let my batteries run a little low
> on water, and the tops of some were peeking out of the water. I found
> it hard to add a consistent amount, so I added water until I could see
> it was a little below the bottom of the battery fill neck. I started
> and used about a gallon on the first 4 batteries. After a trip to the
> grocery store for more water, I ended up using just shy of 5 gallons.
> I was proud that I'd added water and taken care of this chore for a
> good long while. After a few charges the acid had mixed in with all
....
> good intentions. I thought I was doing things right, and then found I
> didn't know how much water to add. I added a bunch, and now I'm

Watering flooded lead acid batteries is a significant chore. There is a 
way to eliminate the hassle almost completely, although it's a very high 
one-time expense. There are "watering caps" that automatically fill the 
batteries to a specific level, and they also function to limit the 
amount of acid that escapes during charging. They are installed and 
connected with tubing in a daisy-chain fashion, to create a single 
connection for watering. You plug in, the water flows, they individually 
shut off as each cell gets full, and in seconds it finishes and you 
unplug the hose. We'll be offering this kind of system for approximately 
$16-17 per cap (one per cell). At that price it's a lot cheaper to do it 
yourself though, if you have the time. It may be a good choice for part 
of your pack if it's especially difficult to access for watering.

I think an important point regarding all of this is that flooded lead 
acid batteries are the cheap way to go, but there is a price in terms of 
labor. More advanced batteries require an electronic battery management 
system (BMS). With flooded batteries, *you* are the BMS. You perform the 
function by doing occasional equalization charges, and replacing water.

   --chris