Mixing Lead and LiFePO4

[KMac55]

Nice.

 

[Sverige]

Good points. In my case I already had the LA I wanted to keep to preserve my investment in it, increase overall capacity and performance by adding lithium, and as mentioned I have an alternator attached (wind turbine) that I want it to stay connected all of the time to a load (like any alternator).
Regarding the self discharge concerns, the difference between the float voltages is not much at night and the actual nightly self-discharge draw of my lead on my lithium seems quite negligible. in any event, I generally keep loads on at night (inverter, refrigerator, computer, mobile router, intermittent well pump, etc.) so the power going primarily from my lithium feeds those draws and keeps my lead acid from dropping much below 25.6v all night, which is one of the benefits of the combined system, frankly.
To be clear, I don't suggest making full-time parallel connections between lead acid and lithium; rather, based on their differences they can be in parallel most, but not all of the time. and I have found the best way is using the BBMS device that combines and disconnects under the right circumstances using a large relay (I use a bi-stable). Clearly, the banks need to disconnect for significant periods to account for the different charging parameters. When the charging source(s) fills the lithium up, for example, they must disconnect if more power is being provided to the system, otherwise the lithium would overcharge. Because after disconnection there is only LA attached to the system, the charging voltage then naturally rises out of the lithium safe range and the lead goes through its normal daily charging cycle (I have not had to change the charging profile on my Morningstar controllers). The banks should only be reconnected when the charging sources have ceased sufficiently for the banks' voltages to align at the same value so as to prevent inrush (and the combiner has to be smart enough to know, based on how full the lithium is, not to recombine and keep the banks connected even if the voltages are briefly aligned mid-day, for example).
The point is that the BBMS is sort of like a battery combiner (linked to above by CB-OTB), but it is a lot smarter than a device that just combines and disconnects on a set voltage. With the BBMS you program with the actual battery capacity, and it measures current in and out of your lithium so it will only charge to a percentage of that capacity (again, whatever you set it at - If I am going to be away a long time I can set it at 50-60%), and then disconnect. Purely voltage based combiners for lead and lithium risk overcharge of lithium - it can overcharge even at a relatively low voltage if kept there too long. So purely voltage based combiners are not sufficient in my view. And as always, precautions need to be taken with these very powerful batteries, including keeping both banks independently and properly fused.
Your mileage may vary.

How is the functioning of this BBMS different to simply having a LFP battery (with conventional FET BMS on the front end of it) connected in parallel with a lead acid battery bank? I mean it seems like the main functioning of the BBMS as you’ve described it is to disconnect the lithium battery from the parallel connection once it is charged to max voltage, then reconnect it at the end of the day when the voltage has dropped.

How is this better than just having a BMS on that lithium battery which stops the charging once the cells are up to 3.6V/cell, then if your charge controllers are set to a voltage level which suits the lead acid bank, charging will continue with the Lithium battery isolated and reconnecting once the lithium BMS sees a lower level on its input?

Is the BBMS solving a problem which does not exist?

 

[wattmatters]

I think the idea is to avoid relying on the BMS for regular switching. BMS should not be used to disconnect too often I’d have thought, more of a “just in case“ scenario. Leave that job to a decently rated robust contactor built for daily switching.

I guess it depends on how long you expect your charger to hold your hybrid system at the absorb voltage for. Which will be a function of how frequently you expect the lead battery to be discharged by a substantial amount.

I don’t think it’ll be much of an issue in my case.

The peak charge voltage of my lead battery is not so high (56.4 V = 3.525 V/cell) and rarely does it sit there for very long. That’s because I don’t cycle the lead much and so it pretty quickly drops back to float voltage (54.0 V = 3.375 V/cell) after charging and I just don’t see that voltage as a problem for the lithium to sit at during periods of excess solar PV.

The only time I expect lead to be discharged more deeply is during a long grid outage.

It would be different when grid power is less of an option such as on a boat and you expect more regular periods of poorer PV output to drain the lead more regularly.

 

[Sverige]

I think the idea is to avoid relying on the BMS for regular switching. BMS should not be used to disconnect too often I’d have thought, more of a “just in case“ scenario. Leave that job to a decently rated robust contactor built for daily switching.

I don’t see why you would not be able to rely on your BMS to switch regularly- if it’s not reliable, how can you go trust it to disconnect when it needs to? It just makes no sense to me to have a device (BMS) which is in the system to perform the vital function of disconnecting the lithium battery when it’s at risk of overcharging, then to say it can’t be relied upon to do this on a regular basis and to expensively install a secondary system which appears to be performing exactly the same function.

But maybe I misunderstood @KMac55 description of how his BBMS (battery bank bms) is functioning, or perhaps it has additional functionality which hasn’t been described. Hopefully he will clarify as I’m interested to learn about this.

 

[wattmatters]

I don’t see why you would not be able to rely on your BMS to switch regularly- if it’s not reliable, how can you go trust it to disconnect when it needs to? It just makes no sense to me to have a device (BMS) which is in the system to perform the vital function of disconnecting the lithium battery when it’s at risk of overcharging

It might be reliable until it isn’t. That’s the point.

Do you know the number of switch cycles the BMS is rated for? I just don’t know if they are designed for thousands of switching cycles.

That’s where having an external contactor comes in. Designed for decades of reliable daily service. Some BMS use an external contactor.

 

[KMac55]

It might be reliable until it isn’t. That’s the point.

Do you know the number of switch cycles the BMS is rated for? I just don’t know if they are designed for thousands of switching cycles.

That’s where having an external contactor comes in. Designed for decades of reliable daily service. Some BMS use an external contactor.

I think of a BMS as a device designed to shut everything down if certain dangerous events happen to a cell or cells. The BBMS shuts off charging too, but not necessarily only on those same dangerous events, and with more control and an eye on optimal charging of the bank. I've read many arguments here that a BMS is not designed to be and should not be relied upon as a daily switching mechanism; rather, it is a last ditch protection device, and I am largely convinced of that. For a time I had things in parallel relying solely on my BMS to do as you described, and I admit it worked okay, but I wasn't particularly comfortable with that setup, nor did I feel it was optimal. The BBMS, by contrast, uses a Hall effect sensor focusing also on charging amp flow, and using math it calculates how full your battery is at any given moment. That way you can cause it to stop charging at say 50 or 90 percent full, and it will do so safely and then reconnect when charging is again needed, not solely based on voltage. It won't, for example, just connect and disconnect off and on repeatedly whenever a cloud or large load happens to drop the voltage to a level where the lead and LFP match. It instead knows how full the battery is, not just voltage and won't reconnect until a lower state. I am convinced it similarly prevents overcharging in a low voltage long duration environment. Mine connects and disconnects reliably with a 300 amp bistable relay, and the BMS is never activated, which I appreciate. Perhaps one might be fine relying solely on a BMS if it has well programmed trigger and reconnect events and robust electronics to do daily switching, but I think this works a ton better and keeps my BMS separate for catastrophe prevention as it was designed to do.

 

[wattmatters]

Just out of interest, I received my LiFePO4 batteries this week and have installed them in parallel with my Sealed Lead Acid bank.
More details in this thread about my system.

I now have a 48 V system with:
200 Ah of LifePo4 (51.2 V nominal)
400 Ah of SLA

When I connected them the SLA bank was fully charged. It has remained that way since.

Been running for a few days now and the behaviour so far is exactly as I expected.

The LiFePO4 discharges and charges preferentially (SLA was full to begin with).

e.g. After overnight running the house from the battery and recharging the bank this morning I noted:

• The LiFePO4 managed all the discharge overnight, while the Lead just sat there happy as Larry like a roadside worker leaning on his shovel. Current flowing from LiFePO4 to Lead was < 0.1 A, so very little leakage, while LiFePO4 supplied all the energy demand.
  
  
• Today when the charge current was ~45 A (0.23C), the LiFePO4 was sucking pretty much all of it while the Lead was sipping less than 0.5 A (0.001C).

 

[Hendo7]

you will be ok with mixing as long as lifepo4 batteries has a BMS

 

[Hendo7]

one has high resistance and the othe has has low,your lithium batteries will charge and discharge over 4 times the lead acid batteries

 

[wattmatters]

you will be ok with mixing as long as lifepo4 batteries has a BMS

They do. My Lead battery banks also each have their own 80A NT00 HRC Fuse in case of an internal cell short or other issue.

 

[Vigo]

one has high resistance and the othe has has low,your lithium batteries will charge and discharge over 4 times the lead acid batteries

Id like to hear the story of your odd mix of 3 types in parallel some time.

I think for the most part the only reason you need any hardware between lithium and lead is to protect each side from a fault on the other side. I have lifepo4 and FLA in parallel and that factor has been the reason for all my experimentation. If i didn't care about that it would have been hook up and go. Especially if you don't feel that you need to charge your lithium all the way up to the disconnect point! If you simply stop your lithium charging at a voltage that is safer for the lead you will still have 90% of your lithium capacity (maybe more?) and vastly reduce the concerns/complexities of mixing the two. As far as i can tell, anyway.

 

[Texas-Mark]

To add a bit to the discussion, I also just added a 100ah Lifepo4 to my existing 800Ah FLA bank. So far I am seeing the same results as already posted. During initial discharge, the Lifepo4 supplied nearly all of the current. As the bank voltage dropped, current flow started to even out between the two. When my Lifepo4 got down to 5% remaining and the bank is sitting at 12.4V, there is practically no flow coming from the Lifepo4. and the FLAs are providing the bulk of the current. This was all exactly as I expected.

I think some people overthink the mixing of the two. If one keeps the charging withing the FLA voltage range, I really don't see a problem. And I don't see the BMS ever tripping any more or less than would normally happen.

 

[conra]

Also did the same on my system, I have an isolated solar system, had two sets of 110AH flooded batteries (4S, 2P) at 48V, for a total of 48V nominal at 220AH. this was not enough to maintain my loads over night, I bougth 4 LIPOFE4 12V batteries 100AH, each has its own BMS and can be set up to 4S and 4P configurations. I connected this in parallel to my older bank (lead) and since then I have enough charge and no issues.

 

[CB-OTB]

is there a reason why the lead acid banks are generally higher capacity than the lifepo4 banks? Could you safely do 600Ah of lifepo4 with 200Ah of lead acid?

 

[conra]

is there a reason why the lead acid banks are generally higher capacity than the lifepo4 banks? Could you safely do 600Ah of lifepo4 with 200Ah of lead acid?

In my case, already had lead. So lipofe4 are an incremental to my previous setup. I'm my case lipofe4 are more expensive. I bought a 48v 200ah battery pack in Alibaba but takes over 2 months to get here. So these ones I added I got on Amazon meanwhile I get the bigger one. My idea was to only use lipofe4 once the new ones arrive but so far I like how is working today and may just leave it

 

[Substrate]

I think some people overthink the mixing of the two. If one keeps the charging withing the FLA voltage range, I really don't see a problem. And I don't see the BMS ever tripping any more or less than would normally happen.

Wait - what about the fact that lead-acid has a Peukert effect, whereas with LFP, there is practically none?

I know, active balancers! These may seem like fun-hacks, but not something to take seriously.

If I saw something like this on a boat, I'd jump overboard.

 

[KMac55]

Wait - what about the fact that lead-acid has a Peukert effect, whereas with LFP, there is practically none?

I know, active balancers! These may seem like fun-hacks, but not something to take seriously.

If I saw something like this on a boat, I'd jump overboard.

What about the Peukert effect and what about active balancers? Not understanding what you mean with those references.

 

[Substrate]

What about the Peukert effect and what about active balancers? Not understanding what you mean with those references.

It's for this very reason I would be jumping overboard.

Puekert - with lead acid, the larger the current you pull from it, the faster the voltage drops. But your paralleled LFP doesn't. Instant imbalance. And the active balancer to compensate for this mixed-chemistry phenomenon was meant as a joke.

Look up Peukert. Some battery manufacturer's even provide a chart.

 

[KMac55]

It's for this very reason I would be jumping overboard.

Puekert - with lead acid, the larger the current you pull from it, the faster the voltage drops. But your paralleled LFP doesn't. Instant imbalance. And the active balancer to compensate for this mixed-chemistry phenomenon was meant as a joke.

Look up Peukert. Some battery manufacturer's even provide a chart.

So, I think we all know what the Puekert effect is, and I'm going to assume you meant that in a joking manner. But your statement simply begs the question. I'm waiting for your explanation as to how this manifests as problematic in a mixed system. Can you please enlighten us?

 

[Substrate]

So, I think we all know what the Puekert effect is, and I'm going to assume you meant that in a joking manner.

No, I'm serious. This is the reason why NO reputable manufacturer will ever put their blessing on doing a mixed-chemistry setup. Aside from other reasons.

But go ahead and have some backyard fun. As long as you are safe.

Instead of asking me, how about asking those who do know more about batteries than you or I do, like a manufacturer. Of either chemistry.

Otherwise we're just playing paper-engineer bench-racing charades and having a good time goof.