Flooded NiCads for off-grid solar

Peter_V · February 2011

Hi folks. Even though my house is grid tied, my workshop is off-grid and uses a 24V battery bank.

I need new batteries and since I have a bunch of military surplus NiCad batteries (came out of mothballed jets) I was thinking about using them.

These are BB-600 style NiCads if you are familiar with them.

Anyway, the normal charge routine on these is constant current charging to replace approx 110% of the amp hours removed. Kind of hard to do with solar.
Does anyone know if I can use these with a BZ MPPT-500 charge controller and a 240 watt array?

The MPPT-500 has an adjustable float voltage, anywhere from 25V-32V.
The battery boxes I have contain 19 cells, but I can probably modify them to fit 20 if needed. Each cell is about 35 ah.

Thanks.

PhilS · February 2011

Re: Flooded NiCads for off-grid solar

Unless the units have changed dramatically, IMO you would be wasting solar output by running it through anything made by BZ.

There are other threads, but this one you should read if you haven't already:

http://forum.solar-electric.com/showthread.php?t=1779

There ARE other threads about that company's products but I can't remember Bill's (B.B.) formula for searching here when the search term is small, like "BZ". Something about using Google to search for threads in this forum... I suspect Bill will be by here soon and give directions again.

Phil

BB. · February 2011

Re: Flooded NiCads for off-grid solar

In Google:

BZ site:wind-sun.com

The "site:" tag limits Google searches to the website you are interested in.

-Bill

PhilS · February 2011

Re: Flooded NiCads for off-grid solar

Geez Bill! You didn't get here with a relevent post for 17 minutes!!

I tried your search directions. (every other time, I'd seen it but not tried it)

Had 184 'hits' for comments in this forum. Maybe I'll remember (or maybe you'll be coming along after me to finish up)

Phil

icarus · February 2011

Re: Flooded NiCads for off-grid solar

I wouldn't use a BZ MPPT controller for anything but a paper weight!

Do a search and you will find legions of posts about how poor it really is. The one I tested actually puts out less current than no controller at all through a number of tests!

Tony

PS. Phil,, you want it back?

T

Peter_V · February 2011

Re: Flooded NiCads for off-grid solar

PhilS wrote: »

Unless the units have changed dramatically, IMO you would be wasting solar output by running it through anything made by BZ.

Apparently I got lucky. I've been using mine for 2 years now and it seems to work fine.

Unfortunately my 20 year old Gel Cells are starting to wear out so I need to replace them.

Does anyone have any ideas about using flooded nicads in my system?

Cariboocoot · February 2011

Re: Flooded NiCads for off-grid solar

That particular BZ is rated for a 500 Watt array - and 45 Amps @ 12, 24, or 48 Volts? This doesn't add up, but then ... BZ isn't exactly stellar. 45A on a 48V system would be 2565 watts.

In any case, setting a Float Voltage is not the same as a constant current source. I don't know of any solar charge controllers that deliver a fixed current with variable Voltage; most charging is done to Voltage set points. It might make an interesting experiment to see how the FNiCads work, but I wouldn't want to count on it.

Peter_V · February 2011

Re: Flooded NiCads for off-grid solar

Cariboocoot wrote: »

That particular BZ is rated for a 500 Watt array - and 45 Amps @ 12, 24, or 48 Volts? This doesn't add up, but then ... BZ isn't exactly stellar. 45A on a 48V system would be 2565 watts.

In any case, setting a Float Voltage is not the same as a constant current source. I don't know of any solar charge controllers that deliver a fixed current with variable Voltage; most charging is done to Voltage set points. It might make an interesting experiment to see how the FNiCads work, but I wouldn't want to count on it.

The MPPT-500 does either 40 amps at 12V, 20 amps at 24V, or 10 amps at 48V

I KNOW it doesn't do a float charge. I'm asking if I can charge them with what is effectively a constant voltage charge scheme instead.
It's not ideal, but I've heard of others doing it in the past and I'm trying to figure out how.

I'm pretty sure the Jets that these batteries came out of did NOT use a constant current charger.

niel · February 2011

Re: Flooded NiCads for off-grid solar

i seem to recall a max of about 1.43v per cell, but if fed a very large current the batteries will overheat. some kind of current limiting would be advisable. also note the nominal cell voltages differ from the cells in lead acid type batteries making customization of the voltage mandatory along with something to prevent overcurrent heating. linear regulator ics can be made to be constant current sources.

techntrek · February 2011

Re: Flooded NiCads for off-grid solar

I would NEVER use a lead-acid charge controller with another battery chemistry. For instance, Li-ion are very current-tolerant, many can handle a 100% charge current (unlike a lead-acid's 5%-13%), but you may be calling the fire department if you let a cell get above 4.20 volts. I haven't investigated NiCd's charging profile so I can't comment on its specifics but I'm 100% sure you'll either damage the battery or cause a fire if you use anything designed for lead-acid.

On the other side of your system, your inverter's cut-off voltage will be wrong so you'll need a new circuit to cut the battery off at the right time. Again, I only know Li-ion, and one of those cells must be cut off at 3 volts. Much below that the internal protection circuit can blow a fuse and your cell is dead for good.

So lets say you combine 3 Li-ion cells, that gets you 12.6 volts at full charge and 9 volts when empty. Your lead-acid controller will try to charge them well above that - and again the internal protection circuit will either blow or they'll catch fire. Not to mention that Li-ion lasts longer if you don't fully charge them (unlike lead-acid). Your inverter will cut-off at 10.5 volts, which is good for long life but it leaves a lot of unused capacity, too.

So to get around the 10.5 volt problem you combine 4 Li-ion cells. That gets you 16.8 volts fully charged, a bit high for some inverters. Easily fixed by lowering the max charge voltage - a lead-acid charger pushing 13.5 -14.5 volts is ok here. That extends the life of the Li-ion batteries too. But now your low voltage cutoff should be 12 volts, and your inverter will try to go to 10.5 before cutting off - it is designed with lead-acid batteries in mind. At that point your Li-ion cells are dead for good.

Then there is the difference between the charge stages of a lead-acid battery vs. the charge stages of a NiCd (or Li-ion). Looks like NiCd has 4 stages and is constant-current. Li-ion has 3 stages and MUST not be trickle-charged unlike a lead-acid battery. Finally temperature compensation will be different, too for every chemistry.

I think using alternative chemistries for PV is really cool. If you can afford the price difference - in your case "free" is great. I would love to build a Li-ion based system. For starters Li-ion has an efficiency in the 97-99% range (lead-acid is maybe 75%) and doesn't have memory issues. But if you build a system like that you must be very careful to use a charger designed for that type of battery and somehow implement a different cut-off voltage.

westbranch · February 2011

Re: Flooded NiCads for off-grid solar

this link is for SAFT NiCd batteries, this may provide some insight

http://www.evdl.org/docs/Stm5_chg.pdf

and this

http://www.battcon.com/PapersFinal2001/LansburgPaper2001.pdf

and this

http://www.saftbatteries.com/doc/Documents/stationary/Cube780/tm_bb_en_0407.6f92351c-93f1-4404-b97d-bc4669140ace.pdf

the rep I spoke to also told me to split a 12 v into 2 x 6v parallel banks to get a constant current charge from a constant V charger, it works , took 30 hours to recondition them...

did it outside in the cool spring weather due to the heat generated.
cheers,
e

Peter_V · February 2011

Re: Flooded NiCads for off-grid solar

techntrek wrote: »

On the other side of your system, your inverter's cut-off voltage will be wrong so you'll need a new circuit to cut the battery off at the right time. Again, I only know Li-ion, and one of those cells must be cut off at 3 volts. Much below that the internal protection circuit can blow a fuse and your cell is dead for good.

FWIW the nominal voltage of Flooded Nicads is very compatible with Lead-Acid. 1.2V per cell nominal, so a 24V NiCad battery has a similar end of discharge voltage to a 24V PbA battery. My inverter cuts out at 21V, technically the NiCads are dead at 20V but there is not a lot of energy left in that last volt so I don't mind not getting a full discharge.

The only question I have is about using a set float voltage instead of a set amount of returned current, which is what the NiCads normally want.

Peter_V · February 2011

Re: Flooded NiCads for off-grid solar

westbranch wrote: »

this link is for SAFT NiCd batteries, this may provide some insight

http://www.evdl.org/docs/Stm5_chg.pdf

and this

http://www.battcon.com/PapersFinal2001/LansburgPaper2001.pdf

and this

http://www.saftbatteries.com/doc/Documents/stationary/Cube780/tm_bb_en_0407.6f92351c-93f1-4404-b97d-bc4669140ace.pdf

the rep I spoke to also told me to split a 12 v into 2 x 6v parallel banks to get a constant current charge from a constant V charger, it works , took 30 hours to recondition them...

did it outside in the cool spring weather due to the heat generated.
cheers,
e

I think that is the info I was looking for.

Thanks!

techntrek · February 2011

Re: Flooded NiCads for off-grid solar

Good to hear that electrically they are similar, I'll have to read up on that stuff tonight when I'm on my sofa. I would still worry about the charging characteristics and stages - voltage vs. current and trickle vs. no trickle.

Don't NiCds have memory issues? Maybe I'm thinking of the older technology - I remember having cell phones and cordless phones in the 90's that seemed to need new batteries every year (although I learned that a quick jolt with a much higher voltage would clear up most of this problem, but most people didn't know that then).

westbranch · February 2011

Re: Flooded NiCads for off-grid solar

as the batteries I got had an unknown history for age, charge, float, etc, the recommendation was to apply a 20% load and deplete to an avg of .75V per cell (8 v for a 12 v bank) then charge, apparently this total discharge gets rid of the 'memory' but is not recommended for sealed batteries... it also identifies any bad/suspect cells that may need the electrolyte replaced. (or chucked)

e

PS the load is applied for 5 hours with hourly monitoring, of each cells voltage, for first 4 hours then 2 readings at 1/2 hour intervals, if achievable.
The last hour is where weak cells show up, if in poor condition you will see it before that. A spread sheet is useful.
Depletion cycle/recharge can be applied up to 5 times.

Peter_V · February 2011

Re: Flooded NiCads for off-grid solar

techntrek wrote: »

Good to hear that electrically they are similar, I'll have to read up on that stuff tonight when I'm on my sofa. I would still worry about the charging characteristics and stages - voltage vs. current and trickle vs. no trickle.

Don't NiCds have memory issues? Maybe I'm thinking of the older technology - I remember having cell phones and cordless phones in the 90's that seemed to need new batteries every year (although I learned that a quick jolt with a much higher voltage would clear up most of this problem, but most people didn't know that then).

The so called "Memory Effect" is misapplied.

The true memory effect only really effected things like satellites that were precisely discharged to exactly the same point, and then recharged. Eventually they learned this level and could not be discharged any further.

What people normally call the memory effect on NiCads is actually a condition that results in a depressed voltage, this is normally caused by float charging.

The battery still has it's full capacity, however it can only deliver it at a lower voltage. So their end of discharge voltage drops from 1.0V per cell to something like 0.9V.
The problem is, most electronic devices are setup to call the battery "dead" when it get's to 1V and the device shuts down. So now you only get 1/2 the capacity out of the cell.

Eventually charging the batteries like I'm planning will result in this depressed voltage. However, it's pretty easy to fix. All you have to do is periodically drain the batteries to zero volts, and recharge.

techntrek · February 2011

Re: Flooded NiCads for off-grid solar

Finally got the chance to read through those links, the last one was the most useful. Sounds like they are so similar to lead-acid that you could get away with using a lead-acid charger. Since you can take them down to near-zero DoD I'm still not sure if you can rely on the voltage cut-off level - I didn't see any voltage-to-DoD charts to compare with lead-acid. Interesting lifetime lengths and cycle stats, though.

dwh · February 2011

Re: Flooded NiCads for off-grid solar

techntrek wrote: »

For instance, Li-ion are very current-tolerant, many can handle a 100% charge current (unlike a lead-acid's 5%-13%)

Unlike *some* lead-acid's 5%-13%. Quite a few AGMs are perfectly happy with a few hundred percent charge current.

techntrek · March 2011

Re: Flooded NiCads for off-grid solar

Any update on your use of those NiCads? Curious how they're working out.

This thread really got me on a flooded NiCad kick, I've done lots of reading on them in the last few weeks. When its time to replace my current battery bank I need to include a quote or two from flooded NiCad suppliers and not just lead-acid suppliers. Between their cycle life, being fairly indestructable and their tolerance for cold I'm sold. Its just their price that I'm worried about.

Peter_V · March 2011

Re: Flooded NiCads for off-grid solar

techntrek wrote: »

Any update on your use of those NiCads? Curious how they're working out.

This thread really got me on a flooded NiCad kick, I've done lots of reading on them in the last few weeks. When its time to replace my current battery bank I need to include a quote or two from flooded NiCad suppliers and not just lead-acid suppliers. Between their cycle life, being fairly indestructable and their tolerance for cold I'm sold. Its just their price that I'm worried about.

To many projects and not enough time.

I might have the opportunity to purchase a full pallet (24 ea) of golf cart batteries soon. Since my electric truck only uses 20 batteries I'll have 4 spares which I'll probably use for the workshop and leave the nicads for another project where I can use the correct charger on them.

Flooded NiCads for off-grid solar

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