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Re: (ET) Fw: [WT] Trickle Chargers
- Subject: Re: (ET) Fw: [WT] Trickle Chargers
- From: "David Roden (Akron OH USA)" <roden ald net>
- Date: Tue, 22 May 2001 02:29:28 -0400
- Sender: owner-elec-trak cosmos phy tufts edu
There are some good ideas here, and some misconceptions and old myths
about batteries too. Although I'm not an engineer or an electrochemist,
I've put a lot of study into batteries. I'd like to add some comments
(though I guess my additions won't be as colorful as the original).
> > As you draw current
> > both plates combine with the sulphuric acid to create Lead Sulfate.
> > The Lead plate also needs some extra O to form the sulfate which it
> > gets by ripping apart the water molecules and scavenging them, this
> > process yields extra Hydrogen left over which is why batteries bubble
> > Hydorgen and you
> > all those "explosion hazard" stickers on batteries.
This is a inaccurate description of the discharge / charge reactions in a
battery. Here's a better one, taken from a paper published by
Negative plate: Pb + SO4 -> PbSO4 + 2e-
Positive plate: PbO2 + 4H + SO4 + 2e- -> PbSO4 + 2H2O
Total: Pb + PbO2 + 2 H2SO4 -> 2 PbSO4 +2 H2O
The e- indicates liberated electrons. That's what makes your tractor go.
The reaction reverses when charging. However, contrary to the paragraph
quoted above, gassing is ~not~ really part of the charging reaction.
When you see bubbles, it means energy is being wasted (inevitable in a
battery). Charging a battery is nearly 100% efficient from about 20% to
80% charged; the major portion of energy lost to heat and gassing occurs
in the last 20%.
Gassing occurs because as the charge increases beyond 80%, the voltage
required to continue to 100% rises above the point at which the water in
the electrolyte begins to disassociate into gaseous hydrogen and oxygen.
> > Anyway, the normal discharge of a battery end up slowly coating all
> > the plates inside with Lead Sulfate.
> > This whole process is very basic to every Lead Acid battery and hasn't
> > changed for a 100+ years ...
> > Since nature isn't perfect, no battery can just sit fully charged up
> > forever. Slowly that rust will form whether you like it or not.
This is because batteries have a small amount of self-discharge --
internal leakage current. Also, when they are charged, gassing causes the
electrolyte to mist out and deposit on the top of the battery. This
provides a path for current across the case -- another gradual discharge.
Because gases are trapped in bubbles on the surface of the plates, this
misting can continue for weeks after a battery has been removed from
> > Having
> > any wires whatsoever hooked up to a battery or even letting one sit on
> > a concrete floor (never do this) only helps accelerate this inevitable
> > process.
Wires that have open switches in them carry no current and have no
effect. However, many newer vehicles draw current from the battery even
when not running -- for the engine control computer, fancy burglar-alarm
stereos, and the like. So in that respect, yes, it can be helpful to
disconnect cables from an idle battery.
I will now probably get myself into trouble by stating that I think the
concrete floor myth is just that -- a myth . But before you fire off a
reply to this, hear me out.
I heard this story for years, and sort of believed it. But recently I
left some of the batteries from my EV on the garage floor for 3 months,
while others remained in the car. At the end of that period, by
hydrometer measure, the average SG of those on the floor was almost
exactly the same as that of the ones in the car. No, it's not a rigorous
test, but I'm about 80% convinced that concrete floors don't affect
batteries any more than wood ones.
Sure, leaving batteries on a garage floor for 6 or 9 months without
charging them or cycling them will cause permanent sulfation. But that
would have happened if they'd been off the floor, too.
> > It actually forms
> > so uniformly that it gets real stubborn to try and remove (when you go
> > to recharge your battery after letting it sit for a year).
More or less correct. Sulfation is pretty much inevitable as a battery
ages, but premature sulfation results from too long a delay before
charging a discharged battery.
> > So what you end up
> > doing when you recharge a battery in this condition is plate Lead/Lead
> > Oxide ON TOP of the tough stubborn layer of Lead Sulphate instead of
> > fully driving it back into solution.
As I understand it, the problem is that the lead sulfate forms large
crystals which flake off the plates and fall to the bottom of the cell.
This is caused by the very slow discharge, and by the time before
Once these crystals have loosened from the plates, there's no way to put
them back, so they represent lost lead and thus lost capacity.
This is also the reason that specific gravity and fully-charged voltage
drop as a battery ages. Look at that equation up at the top. If you
can't recover the lead and sulfate ions, you can't put back the lead and
you can't turn the water back into sulfuric acid.
I should point out here that sulfation (from undercharging) and positive
grid corrosion (from overcharging) are not the only aging mechanisms in
batteries. They also degrade from positive material (lead dioxide)
shedding. This is a natural consequence of charging and discharging, and
like sulfation it represents lost capacity.
Also, these little lead dioxide particles detach from the positive plates
and float around in the electrolyte. During charge, they plate onto the
negative plates and cause dendrites. This is called mossing (and that's
what it looks like inside the battery -- moss growing on the plates).
This phenomenon is irreversible. If the dendrites grow too close to the
positive plates, you have a shorted cell. (Thanks to Nawaz Quereshi, an
electrochemist at US Battery, for explaining this.)
> > Deep
> > cycles, then full recharge repeated over and over can often work their
> > way through that layer, but not always.
Sulfate that has been shed from the plates is lost forever. But long,
slow equalization will do the best possible job of turning the lead
sulfate still on the plates back into lead and acid.
In addition, repeated cycling of a battery always builds capacity,
regardless of its condition. Why, I don't know. It may be that it's
related to more complete conversion of lead sulfate back into lead and
> > Trickle chargers help to prevent this by keeping a tiny charge current
> > flowing in the battery to counteract it's own natural tendency to self
> > discharge ...
Careful! Most trickle chargers are low-rate constant current chargers.
They do more harm than good, causing overcharging and positive grid
corrosion. This is more destructive to the battery than sulfation, and
will ruin it faster.
A ~float~ charger is different. It is a constant voltage charger
calibrated to the float voltage of the battery -- the voltage at which
current will naturally stop flowing when the battery is 100% charged. A
properly regulated float charger can be left connected indefinitely.
> > The newer pulse chargers
> > seem to work better because they let the battery rest between pulses,
> > so it kind of
> > through a more natural cycle of charge-discharge-charge-discharge but
> > on a smaller scale than starting an engine and recharge on an
> > alternator.
The value of pulse charging is still being debated. I've read reports of
spectacular success, and reports that found no benefit at all. Something
else appears to be operating here.
I will say that most of the pulse charging schemes I've seen are less
energy efficient than regular charging, because they deliberately carry
out short partial discharges, effectively wasting energy, during the
BTW, pulse charging is not the same as the $100 pulse "desulfators" sold
through solar power dealers. I think these are at best overpriced, and
the same thing could be accomplished with careful equalization. However,
there are some who think they work.
I do know of one case in which these gadgets ~seemed~ to have an effect.
However, within a month after they were disconnected, the batteries were
worse than before.
> > Also crystals always seem to
> > form around contaminates. You always put nothing but spotless
> > distilled water in a battery, right?
I don't know if this is the failure mechanism, but minerals in a battery
are definitely bad news. Use distilled water or water from a reverse
Warning: some supermarked bottled distilled water is not very clean! Boil
some down before you assume that it's what it claims to be!
Don't cheap out and use water from your air conditioner or dehumidifier.
It will contain dirt and mold spores -- nasty stuff.
> > The best thing for a battery is to use it
Absolutely! Batteries are like athletes. If they're allowed to get lazy,
they lose capacity.
> > That extra few
> > seconds of running all your lights/headlights on battery instead of
> > the alternator will leave your battery spending the evening in a state
> > of partial discharge and will cost you at least a couple years of
> > battery
> > in the long run.
I don't agree. The charging circuit in most cars is somewhat crude, and
always either over- or under-charges. That will make more difference in
the battery's state of charge than burning the headlights for a few
And besides, it isn't worth giving up domestic tranquility to fight with
your mate about little things like this. <g>
> > Keep your battery top clean (just wipe it off, pouring baking soda
> > over it can actually end up hurting you as it inevitably works it's
> > way inside and neutralizes the acid inside)
True. There is some danger of soda solution entering the cells if you
have poorly fitted cell caps, and I've seen lots of them.
The best thing I've found for cleaning battery tops is a solution of
dishwashing liquid (Joy, Palmolive or whatever) and a sponge. Rinse with
clear water or sponge clean and dry. I think drying the tops off instead
of letting them air dry is better.
> > If you must
> > store it, store it 100% charged, completely disconnect it, remove it
> > (steel battery trays conduct electricity even better than concrete)
> > and put in on a block of wood.
Keeping the top clean is all that's really necessary. That eliminates
leakage paths across the top. If you let the battery get gunky, it won't
matter whether it's in a steel frame or not. And concrete doesn't conduct
electricity any better than wood. They are both insulators.
However, if your vehicle or battery is stored in a hot area, it's a good
idea to move the battery in a cool basement to slow down self-discharge.
But watch out for moisture, as this can also accelerate self-discharge.
If you don't have a float charger, charge with a good quality automatic
shutoff charger every month or two while the battery is in storage.
Note: if yours is a valve regulated (sealed AGM or gel battery), self
discharge will be appreciably lower. There's really no need for float
charging these batteries, and you need a maintenance charge only about
every 4 to 6 months.
David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1979 General Engines ElectroPed 24vdc
1974 Honda Civic EV 96vdc
1970 GE Elec-trak E15 36vdc
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