[RE-wrenches] Nickel-Iron Batteries

[Exeltech]

Review of Ni-Fe versus lead-acid based on 15 years'
use:


I have a fellow off-grid acquaintance who lives with
a bank of nickel-iron batteries - a bank I gave to him
many years ago, and it's still working.  I too lived
off-grid with an Ni-Fe battery system.  (Free was good
back then - as now.)

What Tom mentioned (below) about the wider-than-lead-acid
voltage swing during charge/discharge is correct.  My
system was configured with 20 cells in series as a 24 volt
battery.  At full charge, the system reached 33 volts.
Almost fully discharged, it was 20 volts - depending on
the load current draw compared to the battery A-H rating.

Yes, this voltage range is a problem for some electronics.


There are some advantages to Ni-Fe batteries depending on
where and how the battery is used, and some disadvantages
as well...



Advantage #1:
The electrolyte doesn't participate in the charge/discharge
process, thus no change in specific gravity.  This makes Ni-Fe
cells almost impervious to extreme cold.  The potassium
hydroxide (KOH) doesn't freeze until the cell temperature is
approximately -55F, at which point it becomes more of a slush.
Even under those conditions, some current is still available
from the battery.


Advantage #2:
Because of advantage #1, there's much less loss of capacity
at very cold temperature compared to lead-acid batteries.
At -40C, the Ni-Fe capacity is on the order of 85-90% of the
rating when warm.  Lead-acid batteries would be reduced to
10-20% - if that.


Advantage #3:
Ni-Fe cells can be discharged almost 100%, then recharged
many times without detrimental effect as would be experienced
by L-A batteries under the same operating conditions.


Advantage #4:
Ni-Fe cells can be left in a fully discharged condition for
extended periods of time, then recharged, and will exhibit
negligible detrimental effects of sitting in a discharged
condition.


Advantage #5:
As mentioned earlier in this thread, Ni-Fe electrolyte can
be removed and replenished, somewhat reviving otherwise "tired"
cells.  Not to full capacity - but nearly so.  (My experience
was 90% or so of the original A-H rating could be recovered IF
the plates weren't exposed to oxygen for more than a short time.)


Advantage #6:
Able to withstand moderate overcharge without harm.  Plates in
Ni-Fe cells are very rigid, and don't warp like their lead-acid
counterparts.  Conversely, long-term overcharge boils away the
KOH electrolyte, exposing the plates to oxygen, which DOES
damage them permanently through oxidation, carbon poisoning,
(as in C02) and perhaps other contaminants.


Advantage #7:
No need for temperature compensation during recharge.



Disadvantages of Ni-Fe compared to lead-acid:

Disadvantage #1:
Poor charge efficiency.  High quality lead-acid batteries
require [nominally] 105%-110% of the energy taken out to
recharge them (new).  Ni-Fe I've used required 110-130%.
In the days of expensive PV, this was a big cost and energy
hit.  Today - less so .. but it's still there.


Disadvantage #2:
Higher self-discharge rate.  The Ni-Fe battery system I used
lost on average 1% per day.  Cold temperatures would slow
this process - hot would accelerate it.  Other Ni-Fe systems
familiar to me exhibited similar traits.


Disadvantage #3:
Much lower energy density (already mentioned in another post).
It takes a physically very large Ni-Fe battery to equal the
energy storage of an equivalent yet smaller lead-acid battery.


Disadvantage #4:
Messy.   Ni-Fe cells out-gas for a long time during the
"absorption" phase of the recharge cycle.  For that reason,
they're also not a sealed battery - ever.  This makes them
a high-maintenance aspect of a system.  Cells often have a
"dispersal" cap intended to allow only gas to escape, but
this doesn't always stop 100% of the vented gas and vapor.
As a consequence, cell tops require periodic cleaning.


Disadvantage #5:
Many connections.  Ni-Fe are typically available as 1.2 volt
cells.  This makes them a hassle to install.  Some Ni-Fe
manufacturers provide custom inter-cell jumpers, which
mitigates some of the installation pain.  I don't know
this to be true of all brands.


Disadvantage #6:
Most charge controllers I'm familiar with today lack the
ability to handle the voltage range of alkaline cells,
especially the higher voltage required for their proper
recharge.  I'm not acquainted with every CC on the market,
so can't say this statement is definitive.

---

In summary, having lived off-grid with both alkaline and
lead-acid technologies, and having worked extensively with
both, I would only recommend the use of Ni-Fe batteries if
specific circumstances warranted it.  Otherwise, lead-acid
are in my opinion generally more user-friendly.


Regards to all,


Dan
Sr. Engineer
Exeltech



--- On Sun, 5/6/12, Tom Duffy <Tom at thesolar.biz> wrote:

> From: Tom Duffy <Tom at thesolar.biz>
> Subject: Re: [RE-wrenches] Nickel-Iron Batteries
> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> Date: Sunday, May 6, 2012, 6:53 PM
> Nickel iron has been around since
> Edison (Thomas) the cells are 1.2 volt and use potassium
> hydroxide as an electrolyte. The problem we had with them 20
> or so years ago was that the voltage range is large, too
> large for electronics. So the useable range (window) is not
> that great. I have actually seen some that the cases were
> nickel... I can't imagine what that would cost today.
> 
> We concluded that while what you read looks amazing, they're
> not all that useful for RE applications using inverters.
> They might be useful if you're doing straight DC circuits.
> But it's a lot of cells.
> 
> Kind Regards
> 
> Tom Duffy
> Senior Solar Design Engineer
>