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Serious question, please bear with me.
It seems that some boaters are supplementing tired AGM batteries with extra LiFePO4 batteries in parallel. My understanding is that this shouldn't be done. But I hear that many are pleased with the results.
So
Are there any gains?
Are there drawbacks?
Are they fooling themselves?
Are there any dangers involved?
The only issue is that as the lifepo4 has a higher sitting voltage is that it will slowly discharge into the LA. Basically charging the LA all the time. Also until the system voltage drops below 12.7 nothing power wise will be supplied by the LA. So the LA will basically do nothing and the lithium will do all the work.
Now if it's a starting battery then the above will occur but the LA would supply amps during starting or if the lifepo4 bms turned off
If you do it correctly, then you can have Lead Acid and Lithium in Parallel. In fact, as a fortunate happenstance, the chemistries complement each other very nicely.
As far as the Lead being constantly charged by the Lithium, that doesn't really happen even though logic might suggest it does. (I know this as I have independant dedicated Victron BMVs on the Lithium and the Lead, so I can see all currents between the batteries as well as the overall system via a overall system BMV).
I've been running a dual chemistry Hybrid setup for quite a while now and it performs exactly as the maths suggest it should do, and exactly how you would want it to, e.g. The Lithium gets used primarily until the SOC goes down to a point where you would want to stop using Lithium and the Lead takes over. And on recharge, the Lithium takes most of the charge thus maximising the charge efficiency of the system.
Here is an example of this setup in action from last month, currently with 300Ah of Lead Carbon and 100Ah of Lithium ....
You see the Lithium (top) doing all the work and then when it goes to around 15% SOC, the Lead starts doing the work. Then at the end, the Charger is plugged in and the Lithium recharges nice and quick.
In [discharge] use, this behaviour is automatic; When recharging, you need however to take into account the different charge characteristics, and the voltage especially, of the different chemistries. I have a special micro-programmable relay (of my own design configuration) that comes into play when charging to connect/disconnect the Lithium into the Hybrid Bank to allow full charge of both Chemistries with the single charger.
If you want to read more, I have a full writeup and explanation here - https://www.wildebus.com/hybrid-battery-bank-introduction/ .
It would be interesting to try out this setup with some Victron Smart Lithium Batteries, but I have this setup for my own use so cannot afford to just buy those out of curiosity.
No, not at all.
If you look at the maths, Lithium is ~13.3V and Lead is ~12.7V (full battery levels), so IF there were any constant current flow, it would be FROM Lithium and TO Lead, so if anything, it would be a state of permanent discharge from the Lithium.
Now a Lead Battery does not need a constant current to stay charged unless it is faulty, so there is not a flow of current anyway (My BMVs allow me to monitor this).
As per the write up (hopefully you read it to understand further?), the relay is in place to manage the CHARGE aspect. When not charging, the relay just stays on and plays no part other than allowing the two types of battery to be connected together.
Lead generally needs ~14.4V over an extended period to charge (the exact voltage depends on the battery, but 14.4V is a good ballpark number to use to discuss). And in cold weather with temperature compensation that charge voltage can go upto 15V.
Lithium generally never wants to see over 14.4V (and many Lithium Batteries BMSes will disconnect over 14.4V with an over-voltage error) so there is an inherent incompatibility there if the charging defaults are used.
Lithium also does not need a float - You charge .. It takes it ... When full, it wants the charger off. If you monitor the charge characteristics of a Lithium Battery in practical terms, you see they are fully charged at around 14V, so what I can do is have a charger setup for Lead, charge my Lead and Lithium in parallel and disconnect the Lithium (automatically via the programmable relay) from the charger when it is full, at which point you see the Lead continue to charge. The Screenshot shows this charging pattern
As with any non-standard setup, you should understand the characteristics of the products and how they interact within the system. I have the extra monitoring on my system not because it is required but so I can check how everything is working and if it corresponds to the theory and mathematics (which it does). The Relay operates independently of any monitoring or Victron kit . I can however configure and adapt the operational parameters (voltage for connect and disconnect, disconnect and reconnect delays, overrides, etc) via a App over Bluetooth.
With the new Custom Widget in VRM allowing you to combine different devices in the one graph, I thought I would show the two BMVs (one on Lead, one on Lithium) together and how the Lithium disconnects and reconnects from the DC system.
The Green is the Lithium, the Blue is the Lead....
Very interesting! I came here looking to possibly combine a 6kW 48V LiFePo CANBus battery with 4x 12V AGM batteries in series. I don't need more capacity, just better peak current tolerance as the BMS as you mentioned can be "sensitive".
I read the conclusions from the provided link at wildebus.com and it is great but has a slightly different priority from what I need with just peak currents
The system works very well, apart from rare shutdowns by the battery with ugly ripple and low battery warnings. That is what I want to address.
My tests indicate this happens in two scenarios:
1. when attempting to start up the MultiPlus II 48 5000 when there is no PV at all.
2. when you get "unlucky" with several peak currents happening at the same time.
Freezer, waterpump, somebody using a handtool etc.. Discipline by users will also help ;)
The main battery is as mentioned a 48V 6kW CANbus battery that is handled by DVCC from a Cerbo GX. I assume the DVCC part may create some problems.
The battery developer says it is fine to charge the battery with standard LA/AGM algorithms.
Grateful for any comments! Many thanks!
I have a server rack Lifepo4 battery with a senstitive bms that trips wiht fast big current like a compressor or well pump kicking on. Keeping the FLA in parallel as described prevents that, improving the functioning of the Lifepo4. Also, I see no significant discharge into the lead at night, but what little does keeps them both at 25.6. Also, the lead in the system keeps my wind turbine (think alternator) safe in case the bms on the Lifepo4 were to kick off. The lower resistance of the lifepo4 prevents the turbine from shutting down too because it just accepts the current without voltage spikes that occur with FLA from large gusts. Lots of benefits for my system.
I have seen where this works well as stated. https://www.youtube.com/watch?v=tAuPfgZgXec Basically in a nutshell is a cost savings for not having a full Lithium house bank. How I understand it one should size there banks so that Lithium will be the work horse and the AGM/ Lead acid will be the backup or reserve power if needed. Lead acid rarely gets drawn down and charging is accelerated do to the Lithium. Neat idea! And if I am think correctly when charging from an alternator one would be fine if BMS shuts down, voltage spike would be absorbed by Lead or AGM. I think I would try it first on the RV then if all worked well then the SV. I have talked to a large manufacturer of LiFePO4 batteries and they didn't have much to say as they are interested in selling their product.
Interesting avenue by Kevgermany and thanks Wildebus for the graphics. What is interesting is that the Victron Smart BMS 12 200 does exactly that by having the AGM start battery combined (through the BMS) with the Lithium battery for charge - not for discharge. So it led to the question, how to configure the alternator regulator so as to provide valuable energy to the Lithium while not overcharging the AGM since they are on the same wire.
From my understanding of the charge profile of the Victron AGM and of the Victron Lithium, having a short bulk around 14.5 would be an happy moment for the AGM and rapidly go into absoption at 14.2 for the Lithium which will draw all the amperes. If the charge with alternator is less than 2 hours then chance of gassing the AGM are limited i guess. Your thoughts ?
