Testing: Lead and lithium (LFP) battery banks in parallel

Fork lift batteries are also worth considering, and those are capable of sourcing enormous surge current.
A fully rebuilt fork lift battery with five year guarantee has about five times the Kwh storage capacity per dollar here in Oz.

If I was going to do this, I think I would not connect the batteries directly in parallel, too many issues with the fine details.
How about a separate solar controller for each battery (fed off the same solar array) set up properly to suit each different chemistry.
Then couple the two batteries to the inverter through large diodes, so each can source power as required.

Keeping the charge controllers and batteries separate offers some redundancy, and if something does go horribly wrong you only lose one battery, and not everything.

wattmatters said:

Exactly. This is my PV output power (and load) curve for a typical day of my off-grid system. Pool pump mostly (and in this case also a pool cleaning robot for a couple of hours). There's so much unused capacity that efficiency losses in storage are not really a factor. Obviously the size of that Unrealised Capacity area varies with season and weather, even so, it is well above demand on all but the worst days.

The main exception is when the system is occasionally performing its primary task of household backup during grid outage.

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The reason I'm interested in exploring the potential of adding some LFP is I could possibly run our general household circuits from this off-grid system and free up my grid-tied system (a bit) for other loads I plan to move onto it, hot water to begin with and later an EV.

But I do not want to cycle my lead acid storage much, hence having some LFP to look after most of the daily cycling, leaving the lead to be ballast and for grid outage backup.

Most of the household circuit separation is already done by the way I have our transfer switch wired to exclude high power draw circuits which I don't want to operate from backup supply (oven, induction stove, ducted aircon etc). They are only ever going to be grid connected. But I have one or two trickier circuits to solve before I can consider this option.

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Just circling back to this thread for a bit of an update.

Since writing the above in November 2021, I did indeed add some LiFePO₄ to my SLA battery pack. I did that back in August 2022, adding 2 x 5 kW LiFePO₄ server rack batteries to my 20 kWh of sealed lead acid (8 x Enersys Supersafe SBS190F 12 V batteries in two banks of four). I added a extra 5 kW LiFePO₄ server rack unit in May 2023.

The batteries have been in parallel operation ever since. This post shows the system:

Each battery has its own OCPD (fuses for all, and the LiFePO₄ also have breakers). I have a battery voltage equaliser on each of the lead acid banks. The LiFePO₄ batteries use a PACE BMS with an on board balancer (but low current).

The basic operation is the LiFePO₄ does the daily cycling, while the SLA is maintained as reserve capacity for outages. In this way I limit discharge to ~15 kWh from a full charge, unless I need it (e.g. during grid outage). Else the hybrid battery pack voltage rarely falls below 50 V. It was always my expectation that the SLA would give the LiFePO₄ a "soft landing" once the voltages got low (for LiFePO₄). I rarely tap into the capacity of the SLA.

For monitoring I use a Victron Smart Shunt for the whole hybrid battery. In the early days I spent a fair bit of time doing manual monitoring with a DC clamp meter and the LiFePO₄ BMS displays just until I was satisfied the system would operate just fine.

With the Solar Assistant monitoring system I was able to also connect to the BMS of the server rack LiFePO₄ batteries however Solar Assistant has a limitation in that it is not (yet) able to monitor both the Victron Shunt data and the LiFePO₄ BMS data - I had to choose one or the other. While I did track the LiFePO₄ BMS data for a while I ultimately chose to prioritise tracking the Smart Shunt as that was providing the status of the whole hybrid battery and I have home, load and supplemental charging automations which use this data.

I would from time to time manually check the LiFePO₄ battery display panels. After a while I realised it was all working just fine in this configuration.

All the data is integrated with Home Assistant and captured via InfluxDB and can be charted with Grafana. This was nice but I still did not have monitoring to track how the individual chemistries were performing.

But now I do.

I had a spare Raspberry Pi mini computer and so added a second instance of Solar Assistant and have that exclusively monitoring the PACE BMS data from the LiFePO₄ batteries. It's also integrated with Home Assistant / InfluxDB / Grafana.

Usual stuff, combined data plus key stats from each battery. Things like min/max cell voltages and the cell voltage delta (this while I was doing some controlled charging to a higher voltage for recalibrating the BMS SOC values:



But because I am now capturing independent data from the LiFePO₄, I can compare that with the whole hybrid battery data and monitor the difference between them.

Measurement accuracy differences aside, it can at least provide some insight into the role the SLA plays during charge and discharge.

In essence, the Hybrid pack power flow minus the LiFePO₄ pack power flow = the SLA pack power flow. So I have set up some charts to monitor it.

Here is a charting showing this, i.e. the SLA power flow. I have also put the pack voltage (dots) on the right axis:



Can see that during the last two days during discharge and charge the power flow to/from the SLA is not high.

Note the pack voltage range is between ~ 51 V and 57 V. I normally don't charge that high (56.4 V is my normal setting) but I was doing some BMS SOC recalibration yesterday.

The occasional spikes are likely data artefacts (aliasing) as the data sampling rate for each monitoring device is not exactly equal. They could be smoothed out with some filtering but for now I'm just posting the raw data.

Now if I add the LiFePO₄ power flow to the chart we can see just how small that SLA power flow is relative to the LiFePO₄ charge and discharge contribution (note the power axis rescaled from +/- 500W to +/- 5kW):



Zooming in on the period of discharge overnight, this showing from 8PM last night to 7AM this morning:



We can see that initially the SLA is taking a little power from the LiFePO₄. It is in effect a parasitic load. Not a big load (<20W), but it is taking some of the energy. However as voltage drops so does the parasitic load.

At around 3AM, when the pack voltage had dropped to about 51.6 V - 51.7 V the SLA power flow was neutralised and then it gradually started to reverse direction, with the SLA now beginning to contribute to the overall supply of loads. That contribution gradually increases the lower the pack voltage goes. It is a minor but growing contribution, and not long after that the sun came up and charging took over, with voltages rising again.

This is pretty much the same chart, except this one shows the SLA power as a percentage of the total hybrid pack power:



Can see that during my normal daily operation, the SLA is responsible for well under 1% of the total battery energy flow.

On Tuesday this week we have a planned grid outage for some power pole maintenance. Power is expected to be turned off sometime after 9AM and not be back on until early afternoon. So this provides a good opportunity to let the Hybrid pack voltage drop further and allow the SLA contribute much more energy than it normally would.

After the grid returns, it is likely we won't have enough solar PV power to recharge the full pack before the day is done, so I plan to use my electric car's vehicle to load facility to provide a supplemental charge source. It will be able to supply a steady source of charge so I can track how the batteries behave during recharge.

I will report back on how it goes.

Minor correction. I realised the pack voltage I was plotting was using the inverter's voltage reading, not the Smart Shunt reading. I consider the Smart Shunt my source of truth. This is the chart with the smart shunt voltage data:



It is no different, the upshot is the discharge voltage at which the SLA moves from being a parasitic load to contributing to the the supply is around 52.0 V to 52.1 V.

Another minor correction, the proportion chart above was showing fractions, not % as was on the axis. Fixed that and added the LiFePO₄ battery proportion as well:



Can see how this morning the SLA contribution gradually increases as the pack voltage drops. Eventually supplying about 1/3rd of the power at which point the LiFePO₄ was down to 12% SOC.

Then sun came out and it all flips about.

Currently in a grid outage, this is a planned outage for pole/line maintenance. It's cloudy(ish) so batteries are discharging as the small off-grid PV is not quite able to keep up. I may even turn off the PV supply for experimental reasons as I want to see how the batteries perform as voltage drops further.

Lead is dead

Not too many lead acid powered drones flying around LOL.
On the other hand, a second life fork lift battery makes excellent economic sense for home solar on a budget.

sunsurfer said:

Lead is dead

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Hardly.

For backup it's still the chemistry of choice as keeping lithium sitting for months/years on end fully charged ain't a great idea. And if you have existing lead batteries in good condition, then why not extend their useful life?

For the cost of 20 kWh of my backup SLA I would be lucky to get 2.5 kWh of LiFePO₄.

Lead has other applications it works well for, such as ancillary batteries, engine starter batteries etc. Most EVs use a lead acid for their 12 V ancillary battery.

Anyway, I did the discharge test today during the scheduled grid outage. Started the test at ~9:00 AM.

It was as expected with the LiFePO₄ doing most of the work until pack voltage dropped enough at which point the SLA started to contribute. The neutral point with low loads is ~52.0 V.

It was going very slowly at first, so after a couple of hours in order to hasten the discharge I disconnected the off-grid PV just after 11:30 AM then ultimately ramped up the household loads.

Eventually the SLA was carry the full load and even recharging the LiFePO₄ at times. The LiFePO₄ state of charge hit a floor of around 8% SOC. The SLA had fully taken over, giving the LiFePO₄ a nice soft landing.

Charts below show the SOC (reported by the BMS) of the individual LiFePO₄ batteries, while the second chart shows the AC output / loads. Can see how in the latter part of the test I ramped up the loads somewhat. Indeed it turned out we had an overload as folks came home and randomly started doing stuff adding to my already deliberate extra loads. Oops. No matter, I was going to end that part of the experiment after about another 15 minutes anyway and I had the data I wanted. If I were alone I'd have taken it much further to find the limits but alas there are occupants of two homes to consider.



The following shows the proportion of load provided by each battery, the SLA in orange and LiFePO₄ in grey:



It's interesting to see the nature of the transition from LiFePO₄ to SLA. With the heavier loads added in the final hour it was faster than it would normally be in an outage scenario (I wouldn't be deliberately running such loads during an outage).

Under heavy fluctuating loads, both batteries would chip in during periods of peak power draw, then when loads backed off the SLA would cover it all as well as provide some charge back into the LiFePO₄. Hence seeing > 100% for the SLA at times and negative numbers for the LiFePO₄.

Experiment ended with the accidental overload a bit after 12:50PM. For the sake of everyone else I switched back to normal operation.

Part II of the experiment, the recharge, started soon after using the car's V2L supply to provide a steady charge to the home battery. More on that later. I decided to interrupt the recharge after about 5 kWh had been transferred as we had a nasty storm hit and I decided to put the car back under cover in case of hail. I'll bring it back out once it's cleared. Probably leave it to run overnight with an automation to stop once the home battery charge status reaches a yet to be determined threshold.

I dumped our LA engine starter batteries 10 years ago replacing them with a 4 cell 300Ah LiFePO4 battery that has started the Canter 3.9l turbo diesel engine perhaps a few thousand times with ease. It also powers the motorhome attached.
Lead will never enter our equation again.

I have no doubt that lead will still prove useful to a few others for a while yet though.

This is not a thread to discuss the relative merits of lead acid versus other chemistries. So before it gets further derailed I'll point out there are plenty of other threads on that topic, e.g.:

This thread is about testing lead acid and LiFePO₄ batteries in parallel. Please stay on topic.

Just quantifying the SLA discharge energy during this period:

In 5-min net intervals:



Pretty much nothing from 9:00 to 11:45, a net charge of 87 Wh into the SLA. Thereafter it discharged a net 1.413 kWh up to 12:55 (test ended).

wattmatters said:

Hardly.

For backup it's still the chemistry of choice as keeping lithium sitting for months/years on end fully charged ain't a great idea. And if you have existing lead batteries in good condition, then why not extend their useful life?

For the cost of 20 kWh of my backup SLA I would be lucky to get 2.5 kWh of LiFePO₄.

Lead has other applications it works well for, such as ancillary batteries, engine starter batteries etc. Most EVs use a lead acid for their 12 V ancillary battery.

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Of course, I agree. I was referring to solar cycling. One of my systems still has lead but its on its way out soon. 1/3 of the batts have already been removed due to shorted cells.

If I had to build another standby backup bank, the Enersys EC-M cells are really nice for long term standby. They have a 20 years standby life. Construction is top notch. O-ringed terminals. Charge/float volts are unusual but that is a well made cell. Only flaw is the fill caps hinges like to break, otherwise an amazing battery. Hard sell to management due to large upfront cost but long term a great option.

Running Lead and LFP banks in parallel is kinda like turning your underwear inside out to extend time between wash cycles. Its possible, but do you really want to try?

Never throw anything useful away.
If you have it, use it.

And the other solution is simple.
Go commando.

timselectric said:

Never throw anything useful away.
If you have it, use it.

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This is the reason I can't get in my garage!

sunsurfer said:

Running Lead and LFP banks in parallel is kinda like turning your underwear inside out to extend time between wash cycles. Its possible, but do you really want to try?

Click to expand...

I think the difference in my case is I had the lead acid already and it's one job was outage backup, not regular cycling.

They were cheap, and the data centres here seem to swap out their Enersys units on a fixed schedule. I load tested them, they were in great condition. They were perfect for the job. It was just continuing what they were designed and originally used for.

Since then I've added LiFePO₄ to do the new and extra job of daily solar cycling for the home.

Meanwhile the SLA is still there, ready to provide outage backup. It does bugger all most of the time. I am not cycling the SLA, except for the top few percent when the LiFePO₄ gets low sometimes.

So while I have 35 kWh of total hybrid bank capacity (15 kWh of LiFePO₄, 20 kWh of SLA), I only cycle the top 15 kWh of it. Below that and my system switches to grid supply.

But I always wondered about how they played together and since I added some extra monitoring/tracking capacity, I ran the experiments.

The nice thing is the SLA gives the LiFePO₄ a soft landing without even trying very hard.

And if I do need the reserve capacity, it's there ready to go.

timselectric said:

Never throw anything useful away.

Click to expand...

This is why I keep reminding my wife how useful I am.

wattmatters said:

I think the difference in my case is I had the lead acid already and it's one job was outage backup, not regular cycling.

They were cheap, and the data centres here seem to swap out their Enersys units on a fixed schedule. I load tested them, they were in great condition. They were perfect for the job. It was just continuing what they were designed and originally used for.

Since then I've added LiFePO₄ to do the new and extra job of daily solar cycling for the home.

Meanwhile the SLA is still there, ready to provide outage backup. It does bugger all most of the time. I am not cycling the SLA, except for the top few percent when the LiFePO₄ gets low sometimes.

So while I have 35 kWh of total hybrid bank capacity (15 kWh of LiFePO₄, 20 kWh of SLA), I only cycle the top 15 kWh of it. Below that and my system switches to grid supply.

But I always wondered about how they played together and since I added some extra monitoring/tracking capacity, I ran the experiments.

The nice thing is the SLA gives the LiFePO₄ a soft landing without even trying very hard.

And if I do need the reserve capacity, it's there ready to go.

Click to expand...

You know, this test is something most of us have thought about doing at one point. Most on here would not admit it but they have thougtht about it.

I found 2 dozen 31 series truck starter batteries on the battery(Interstate) truck once as cores, they where pulled from a fleet and only 1 year old. All good. I was able to swap them for dead cores. I"m now down to about 10 batteries left, the have been a good buffer bank for when clouds roll by but I never cycled them over night. All free.

Another good use of old lead when they get really tired is just a low draw LED outdoor night light. As long as there aren't any shorted cells, they still can run a couple of LEDs

The recharge part of the experiment didn't quite go as planned. A nasty storm arrived in the afternoon so I put the car away and stopped the recharge from V2L test after about 2.5 hours.

This was the result of the sharing of charge energy during that time:



Initially the SLA was discharging into the LiFePO₄, then once voltage had risen to ~ 50.5 V the SLA stopped discharging and began taking some of the charge. Loads were being serviced by the grid during this period with an exception for 20 minutes between ~14:45 and 15:05. My system decided to swap back to battery (which under normal circumstances it should do), so I disabled those automations and took manual control.

As the charge session progressed the SLA accept a little more of the total charge on offer, up to ~ 20%. The LiFePO₄ was taking on the bulk of the charge the whole time, which makes sense as it was mostly depleted.



Later in the evening (~18:00) I ran a cord from the garage to the charger and let it run overnight, drawing a steady 1.1 kW for 11.5 hours. that was enough to cover household loads plus some charge back into the battery.

Today is sunny, life back to normal.

I would not do this with flooded LA. The voltages they need are too high, but with these Enersys SLA unit, it seems to work well.

The SLA are specified to be charged at 2.35-2.40 V/cell, which in 48V form is 56.4 V - 57.6 V. As our temps are generally pretty warm I normally use the lower end of 56.4 V, which suits the LiFePO₄ perfectly (average of 3.525 V/cell).

Float is specified at 2.27-2.29 V/cell which is 54.5 - 55.0 V, again that's equivalent to 3.40 - 3.44 V/cell for LiFePO₄.