Two battery banks of different ages - How to use one bank as a back up for the other.

[cortijo]

I used a 24v - 390ah lead acid battery bank for 10 years. It started to get a bit weak so I bought some new batteries.
The new batteries are 24v lead acid 750ah.

The old batteries are not too bad and could power my house for a day.
There are currently sitting next to the new batteries fully charged, they have a charge controller and a solar panel but they are not doing anything.

I would like to use the old batteries as a backup so that when the new batteries get down to below say 24v the old batteries will take over or help.

So what is the best solution to use the old batteries as a back up or support for the new batteries?

First I have looked at automatic switches and relays which would change the connection from the inverter from one set of batteries to the other when at a certain voltage.

Then I have looked at a make of inverter called MUST. It has a battery charger which accepts 240v and would start charging at any chosen voltage.
In that case I would need 2 inverters. One to feed 240 to the other one when needed.
Is that a good solution?

In this forum I just read about connecting 2 battery banks in parallel.
What would happen if I connect the old battery bank with 24.8 volts to the old one with 23.9 volts?
Wouldn't that be dangerous?
Would the old batteries charge up the old ones just by being connected in parallel ??

It would be great to know if anyone knows a good solution.

[cortijo]

Nobody answered my question so I worked it out myself.
Forget dealing with relays with 24 volts etc. Install 2 fully functioning systems .

The main battery system works as usual.
The secondary system has a couple of solar panels and a cheap charge controller just to keep them at 100% ready for action in the event of the first one going below 24v

Then fit a simple emergency automatic transfer switch connected to the inverters of both systems which switches over to the secondary power system when the main one fails.

I have set my main inverter to switch off below 23.9
That makes the other one which is always kept at 100% to save the day.
At 100% charge it will probably keep going for 24 hours but I doubt that will ever happen.
Because the first one will recuperate the next day with a bit of sun.

You can buy cheapo "dual power automatic transfer switch " on internet but I went to an electrical supply shop and the assistant sold me two high quality automated switches and a wiring diagram to serve as an automatic transfer switch.

With that level of DOD (depth of discharge) I expect my batteries to last 15 years.


So now you know what to do when you buy some new batteries before the old ones are shot.

[nwdiver]

I recently added a bank of L16s to an existing 2 year old bank of GC2s and a 5 year old bank of GC2s. I don't think parallel banks of different ages has an adverse effect on performance. Also due to Peukerts law you're probably better off running them in parallel instead of one bank at a time. Reducing the current draw will increase the effective Ah of the total bank.

[robeyw]

Generally the older batteries will prefer a lower float voltage - that is their current after charging at the same voltage will be higher so it is possible that the new cells will slowly charge then old ones when there is no PV input. I think that is the first thing to check. Batteries of different manufacturers and ages float differently so there is no definite rule.

[nwdiver]

Generally the older batteries will prefer a lower float voltage - that is their current after charging at the same voltage will be higher so it is possible that the new cells will slowly charge then old ones when there is no PV input. I think that is the first thing to check. Batteries of different manufacturers and ages float differently so there is no definite rule.

I agree there would likely be some minor efficiency loss due to paralleling banks of different ages but this would be more than made up for by decreasing the load of the bank by running them in parallel plus the extended life of the bank due to shallower cycles.

[Mike90250]

Once you deviate from lot matched batteries, you are unable to make any valid comparisons. You can record factual data, but with mismatched batteries, it always becomes a race to the bottom as the best cells assume most of the load, until they are as bad as the worst. Toss some different chemistries, sizes and acid mix variations, and you have a lot hopes and dreams.

[nwdiver]

Once you deviate from lot matched batteries, you are unable to make any valid comparisons. You can record factual data, but with mismatched batteries, it always becomes a race to the bottom as the best cells assume most of the load, until they are as bad as the worst. Toss some different chemistries, sizes and acid mix variations, and you have a lot hopes and dreams.

A bank of new PbSO4 batteries will last longer paralleled to a bank of old batteries than if they were on their own as long as you're still producing enough daily to fully charge.

When I upgraded my 16 GC2s with 8 L16s what was my best option? IMO it was to just add the L16s in parallel. The GC2s still help reduce the load on the L16s so the cycles are shallower... As long as the additional charging requirements of all batteries don't prevent a good charge during the day how could keeping the GC2s be anything but beneficial?

[PNjunction]

It seems so at first. But like trying to save a man drowning who can't swim very well, the bad/old battery wants to take the good one down with it.

It's one of those things that seems to play out well on paper, but in the real world, it is taxing the good one prematurely. It takes time, but it happens.

4-wheelers who run dual-battery setups, find this out the hard way if they are not using an isolation switch, when they replace just one of the batts with new in parallel with the dying one. Due to their smaller size than house-banks, they find out much sooner than we do.

Personally, what I would do (and have done on a much smaller scale), is keep the old bank separate, but able to be a dc-charger source for the new bank to help keep it in good condition with additional charging if needed when the sun goes down on the new bank.

[nwdiver]

It seems so at first. But like trying to save a man drowning who can't swim very well, the bad/old battery wants to take the good one down with it.

It's one of those things that seems to play out well on paper, but in the real world, it is taxing the good one prematurely. It takes time, but it happens.

4-wheelers who run dual-battery setups, find this out the hard way if they are not using an isolation switch, when they replace just one of the batts with new in parallel with the dying one. Due to their smaller size than house-banks, they find out much sooner than we do.

Personally, what I would do (and have done on a much smaller scale), is keep the old bank separate, but able to be a dc-charger source for the new bank to help keep it in good condition with additional charging if needed when the sun goes down on the new bank.

I added a new GC2 bank to an old GC2 bank several years ago with no ill effects. Last week I added 8 L16s to the group. Everyone still seems happy. Last night I discharged the combined bank down ~30%. This would have been a ~50% DOD cycle with the L16s alone resulting in FAR more wear on the new bank. Having them in parallel with old batteries will greatly extend their life.

I agree that adding new cells in SERIES would likely have a negative effect but not in PARALLEL. In a 24v setup if you have an old 12v that is full and sitting at ~14.4v but it's new friend still needs juice and is at 13.5v you're gonna end up with an overcharged old battery and an undercharged new one. But in PARALLEL this won't happen.

How exactly would an old battery in parallel 'tax' a new battery? Will it cause more load during discharge? No... it will result in slightly less load. Will it cause the new battery to not charge at the right voltage? As long as the common terminal can still reach absorb voltage also no. As long as there are no cells completely dead an old battery in parallel can only help prolong the life of a new bank. If you essentially have a 12v in parallel with a 10v... yeah... that would be bad.

[Mike90250]

.......
How exactly would an old battery in parallel 'tax' a new battery? Will it cause more load during discharge? No... it will result in slightly less load. Will it cause the new battery to not charge at the right voltage? As long as the common terminal can still reach absorb voltage also no. As long as there are no cells completely dead an old battery in parallel can only help prolong the life of a new bank. If you essentially have a 12v in parallel with a 10v... yeah... that would be bad.

How exactly, is the new cell in parallel, nearly always has a lower internal resistance. The better a battery, the lower the internal resistance.
Being lower in internal resistance, the new cell supplies 80% of the load, which cycles it faster till it's only as good as the older cells, then they start racing to the bottom, with the best cell remaining, taking the load, till it becomes 2nd best. Lather, rinse, repeat.....

[nwdiver]

How exactly, is the new cell in parallel, nearly always has a lower internal resistance. The better a battery, the lower the internal resistance.
Being lower in internal resistance, the new cell supplies 80% of the load, which cycles it faster till it's only as good as the older cells, then they start racing to the bottom, with the best cell remaining, taking the load, till it becomes 2nd best. Lather, rinse, repeat.....

Ok... lets say it takes 80% of the load. Am I still not better off WITH the old bank attached and the new bank taking 80% instead of 100% of the load? How is this a 'race to the bottom'? The new bank is still going to do better with the old bank helping a little than if the old bank was removed.

It's been ~3 years now since I added 8 new GC2s to 8 old GC2s. There has been no 'race to the bottom'.

[Mike90250]

Well if you have ended up with 2 banks, and you are happy with the results, I congratulate you with beating the odds. I guess if each bank is large enough on it's own, they can share better then when a bank is undersized.

[nwdiver]

Well if you have ended up with 2 banks, and you are happy with the results, I congratulate you with beating the odds. I guess if each bank is large enough on it's own, they can share better then when a bank is undersized.

I didn't beat any 'odds'... it's physics; A new 200Ah Bank is always going to last longer and 'perform' better if it has an older bank parallel to it (so long as all cells are functional).

200Ah battery + a 10yo 200Ah battery > 200Ah battery alone

The overall round trip efficiency might be a little lower but you'll have more storage and the new bank will last longer... again (so long as all cells in the old bank are functional). Paralleling a 46v battery with a 48v battery probably won't work very well.

[robeyw]

I think there are two things to check: As you approach full charge based on voltage and current, are the old batteries taking power that would be better used to charge the new ones? (Old cells tend to have a higher current at the same voltage at full charge) and after charging and no load, do the old cells continue to charge from the new ones? The answers will be yes but is it enough to be important?

[nwdiver]

I think there are two things to check: As you approach full charge based on voltage and current, are the old batteries taking power that would be better used to charge the new ones? (Old cells tend to have a higher current at the same voltage at full charge) and after charging and no load, do the old cells continue to charge from the new ones? The answers will be yes but is it enough to be important?

It's unlikely to matter since a well designed off-grid system should over produce PV more often than not. Also... there's puekerts law. For Example: A 400Ah bank uses 16% less energy than a 200Ah bank at a 30A draw so that would help to offset any decrease in round-trip efficiency of the older pack.