Piggy backing 24V nickel iron alkaline bank on 12V lead acid bank

[offgrid1]

Not in the same room of course for the electrolytes will contaminate one another. Maybe you started out with a 12V lead acid system and could never keep it in that top 10 to 20% of discharge? If you're like me you realized 24 V was possible and more efficient at higher levels of homestead consumption by adding 2 24v 3500 watt inverters to the original 12V 2500 watt inverter as more solar cells, wind generators, batteries, etc were installed.  I tap off the NiFe bank for about 15V max voltage at high noon to make sure the 12V PbSo4 bank gets fully charged. Because the NiFe bank has a wider voltage spread between full and empty the charge to the PbSO4's is 'regulated' to a degree. One may still need diversion loads but with enough mass of batteries a system can lumber along without a controller. The NiFe's discharge before the Pbso4's, and the Pbso4's charge to max before the NiFe's. As the NiFe's are less sensitive to over and under charge the PbSO4's are held higher in their range for longer life. One can also alter the balance between the battery banks, maybe totally draining the NiFe bank on a cloudy day before the PbSO4's are strained. The 24V bank of solar cells can be added to the 12v batteries if necessary using just the lower power inverter. Don't forget that tapping off the cells of a battery bank improperly can cause an arc which can ignite pockets of hydrogen splattering hydroxide or acid impressively :-)

[Mary B]

Tapping a battery bank at midpoint for any amount of current draw is a quick trip to dead batteries.

[Flux]

For lead acid I totally agree, I think he is relying on the claimed indestructability of NiFe cells to get away with this.

I suspect the NiFe will survive under these conditions but they are not very energy efficient anyway and using them like this won't help any with one half having to be boiled to keep the other half charged. Not what I would choose to do but if he has plenty of input power then it might work.

Flux

[Bruce S]

Most of want you say is "close" to being usable, and others like Flux say it is possible.
However, the wider voltage range is only for charging, and should not be considered as part of the usable voltage.
They are rated by any reputable resellers at 1.2Vdc/cell no-load no-charging. Therefore they should be rated as such when a novice person is making a purchase decision.
You certainly can tap the mid-way point for 15Vdc IF the incoming charge has them at this point, but that is not a given.
AND at max-charge even Edison said no more than 1.7Vdc/cell; so I'm suspect of the no charge controller idea for novices as well.
Do you currently have this as a setup? would you be inclined to post a pic of the setup? Those of us who have longed to be able to afford these; would enjoy seeing a full NiFe setup working and in full use!
7Kw of 24V inverter would be interesting as well.
     

[offgrid1]

Yes this is an operating system. 40 TN-500's from Changhong (2 banks in parallel) for the alkalines and 20 golf cart batteries for the lead acids. Interface 12v 2.5 kw and a pair of Outback 24v 3.5kw inverters. The outbacks have no problem handling the alkaline voltages. The distilled water levels must be watched but after awhile is routine. I prefer diversion loads so the system has never had a charge controller. Have to clean things up a bit for pictures :-)   

[kitestrings]

Tapping a battery bank at midpoint for any amount of current draw is a quick trip to dead batteries.

You can 'tap' and use an equalizer/balancer.  We've done this for years.  It is probably not ideal, but can make a quick trip considerably longer than without it.

~ks

[offgrid1]

images

[offgrid1]

solar panel banks to support images of NiFe PbSO4 piggybacking: 1.5 KW 12v, 2.4 KW 24v

[offgrid1]

Also 2 house circuits - one for each 3524 inverter.

[BillBlake]

Not in the same room of course for the electrolytes will contaminate one another. Maybe you started out with a 12V lead acid system and could never keep it in that top 10 to 20% of discharge? If you're like me you realized 24 V was possible and more efficient at higher levels of homestead consumption by adding 2 24v 3500 watt inverters to the original 12V 2500 watt inverter as more solar cells, wind generators, batteries, etc were installed.  I tap off the NiFe bank for about 15V max voltage at high noon to make sure the 12V PbSo4 bank gets fully charged. Because the NiFe bank has a wider voltage spread between full and empty the charge to the PbSO4's is 'regulated' to a degree. One may still need diversion loads but with enough mass of batteries a system can lumber along without a controller. The NiFe's discharge before the Pbso4's, and the Pbso4's charge to max before the NiFe's. As the NiFe's are less sensitive to over and under charge the PbSO4's are held higher in their range for longer life. One can also alter the balance between the battery banks, maybe totally draining the NiFe bank on a cloudy day before the PbSO4's are strained. The 24V bank of solar cells can be added to the 12v batteries if necessary using just the lower power inverter. Don't forget that tapping off the cells of a battery bank improperly can cause an arc which can ignite pockets of hydrogen splattering hydroxide or acid impressively :-)

Offgrid1,

Quite the unusual set-up.
Facinating !
How close do you think (at new specifications) in kWh the
20 (3 Cell) Lead Acid batteries
would be with
all 40 (1 Cell) Ni-Fe batteries.

(The picture looks like more than 20 LA batteries.)

How long have you had the Nickel Iron Batteries?

How's the Electrolyte holding up?

Thanks,

Bill Blake

[kitestrings]

OG1,

Be careful with the vice-grip terminal connections.  Even if temporary (which I assume they are), it's risky.  An untimely drop of an uninsulated tool can lead to some pretty intense arching, heat and worse.

You may also want to consider venting the battery space separate form the inverter and switch-gear.  I'm not sure which is which in the photos (half are lead acid?).  Even a plastic drop/separation would help reduce corrosion.  And, the connects you can see and service aren't necessarily the most damaging.

We drilled the vent caps and used (maple) sugaring tubing to collect and vent, via pvc pipe, to the outside.  No fans; no controls; dirt cheap.  It significantly reduces the maintenance needed on terminal connections over time.

Best, ~ks

[offgrid1]

Back to piggy backing NiFe's on PbSO4's do to overcast from winter storms here in California. with a new addition! Piggy backing Ni Hydride bank on the NiFe on the Lead acids :-)

PS. still using visegrips and 99 cent Harbor Freight squeeze clamps.

WARNING: do not attach wires, clamps, etc when there is a possibility of exploding hydrogen with an arc in the vicinity of batteries.