NiCad Batteries [RE-wrenches]

[Robert Warren]

John,
 I have had a fair amount of experience with Ni-cads. I love them 
because they won't freeze, they can survive years without watering and 
even a lot of abuse in terms of overcharging and overloading. On the 
other hand, they are expensive, so it seems that people in the off-grid 
market, unless they have oodles to spend, only get ahold of used 
Ni-Cads, with no indication of how old they are. I have personnally seen 
Ni-cads which were 30 years old, and given them a fresh charge of new 
electrolyte (KOH) and gotten 3 more years out of them. 
 But they do take a bit of care and feeding, so to speak. I don't like 
using them on a 12 volt system, because it is too hard to match the 
charge/discharge curves which inverters are used to because of the 
global lead/acid oriented battery industry. No, you don't want to drop 
to 9 batteries on a 12 volt system: stay with 10.
 If you are only getting 8 amps from 200 watts of PV, it may be because 
of the charge controller limiting the voltage, as it is also used to a 
typical lead/acid curve. The Ni-cads have a very flat curve: they hold 
their charge for quite a long time when discharging without dropping 
voltage, and then the voltage drops through the floor at the end of its 
discharge cycle. When charging, they also have a fairly high voltage 
compared to Lead Acids, as they may not accept the charging at the lower 
voltage a charge controller is trying to put out. They thrive on a high 
voltage input.
 To get them fully charged, basically you want to remove your charge 
controller from the system.  But pay attention keep all your fusing in 
place, as you never want to jeopardize a system by removing safety 
devices. Now, the problem you may encounter with this strategy is that 
the battery voltage will get higher than the high battery cutoff of the 
inverter. I lived with a system like this for awhile, with a 12 volt 
Powerstar inverter. When I came home at night, the batteries were fully 
charged because of the PV, but the inverter had gone into safty shutdown 
mode (doesn't hurt a thing). I had a couple of 12 volt lights installed 
in my kitchen, so we always had light even if the inverter failed. By 
switching on a couple of these 12V lights, after a minute or two the 
voltage would drop one volt or slightly more, and then the inverter 
could start by itself.  I wouldn't want one of my customers to live with 
this type of system, though.
 The situation you describe with it behaving as though the batteries are 
momentarily disconnected when the inverter switches on, is another 
undesirable quirk: they are slower to respond to quick changes in 
discharge. Even on larger systems with Ni-cads, I would have problems 
with starting a well pump, for example, and that took an extra tweeek to 
fix that problem. 
 One other possibility is that the electrolye needs to be replaced, if 
the batteries are more than 7 or 8 years old. What happens with the KOH 
in alkaline batteries is that it gets contaminated over the years. You 
only want to use distilled water, like yoo would with any battery. But 
rather than mess with mixing chemicals for this customer, which can be 
dangerous and tricky, just sell her some new batteries, if you haven't 
already!
Robert Warren
www.solarcentury.co.uk  



Brad Bassett wrote:
> John,
> 
> My understanding of NiCd's is that they are not affected by staying less 
> 
> than fully charged like Pb acid batteries. Also, that they will charge 
> up to about 90% of full at regular PV system voltage ie: 15 volts or 1.5 
> 
> v/cell with 10 cells. So my assumption is, without direct experience, 
> that you just don't use the top 10% of the battery's capacity. I have 
> also read that if held at a lower voltage, 1.5v/cell, given enough time 
> it will eventually fully charge the battery. I've specified a few NiCd 
> battery banks to northerly climes and have not had any negative feedback 
> 
> with this method, for what it's worth. It's been a number of years since 
> 
> these batteries went in and there was no way to equalize them either. 
> Actually they were large enough that after 5 years they might be just 
> about discharged for the first time. The system had about 3 years 
> battery autonomy, they are being used for seasonal storage (smalll 
> load). I wish I could do that! 
> 
> The problem with going to 9 cells in a 12 volt system is that the 
> voltage can drop too low for the loads, 1.2 x 9 = 10.8v running voltage.
> 
> Brad
> 
> 
> John Blittersdorf, Cent. VT Solar & Wind wrote:
> > Jay,
> >     I just learned about using 19 instead of 20 but the DR manual says 19 is 
> >     
> >     
> > OK  but this is a 12 V system with two strings of 10 and they say not to 
> > 
> > go 
> > to 9.  Anyone had luck sawing these batteries apart?  Looks like nicads 
> > and 
> > 12V systems are not compatible unless someone else has some ideas.  This 
> >  system was set up with golf cart batteries originally and the owner 
> > took it upon herself to buy some used Nicads to get longer life.  Maybe 
> > I should  trade with her my 8 Trojan L16's for her 20 nicads and keep 
> > the 20th one  for a doorstop. 
 > Jeff W., this may be one of your customers as she lives only a few 
miles from  you.  I got involved as she is a dear friend of one of my 
new employees  and I  had a repair to do nearby (another friend).  
> > 
> > John
> > CVSolar
> 
> Brad Bassett
> Schott Applied Power 
> Tumwater, WA office
> bsbassett at earthlink.net


>I'm looking for the Nicad experts. I have a client with 20 nicad 
>batteries 
and a DR1524. We can't seem to get a charge rate above 8 to 12 amps. A 
neighbor with the same batteries and an SW2512 unhooks one battery from 
each 
string to get the charging voltage down to a level the inverter can deal 

with. Has anyone come up with a less labor intensive way to get a good 
charge into NiCads. Also we are having a strange (I haven't documented 
it 
myself) situation where the battery voltage will climb rapidly up to 
inverter 
HVD when power is coming in from 200 watts of PV. When the inverter is 
off, 
the battery voltage drops back down. The system does not have an amp 
meter 
and the day we checked it out, there was no sun so we haven't done a 
good 
test yet. It almost acts like the batteries are being disconnected when 
the 
inverter comes on....??? We checked all the battery cables and 
connections 
throughout... no obvious points of concern. Any input on nicads will be 
helpful. BTW the generator is a 6000 watt propane Honda. 

John Blittersdorf
Central Vermont Solar and Wind 
 


What will we do if there is another fuel shortage? 
Did you know that there is a pollution free, completely
renewable fuel which you can use right now in your own car? 
Don't replace the engine, replace the fuel.
Check out my website on making your own fuel: 
http://running_on_alcohol.tripod.com 
robertwarren at mail.com

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