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Lithium-Hybrid What is it and why do you want it?

Let's recapitulate what we have seen in the previous articles:

Lead-acid batteriesLithium batteries
  • Hate to cycle
  • Love to remain fully charged
  • Love to cycle
  • Hate to remain fully charged

Lead-acid batteries love to remain fully charged indefinitely but hate to cycle and stay for any time in a discharged state. Lithium batteries however hate to remain charged but love to cycle and to remain forever in a discharged state. It is almost as if they are made to complement each other.

Lithium/Lead-acid hybrid
Lithium/Lead-acid hybrid

A Lithium Hybrid system is a system where you don't replace your lead-acid batteries but just add a BMS-equipped Lithium battery to an existing lead-acid based installation.

How this compares to other lithium implementations you can read in our article lithium implementations.

How can it work?

Discharge curve: Lithium-Ion vs Lead-Acid
Discharge curve: Lithium-Ion vs Lead-Acid

Operating the lithium batteries and lead-acid batteries in parallel is possible because lithium batteries have a much flatter charge and discharge voltage curve.

Discharging

While discharging, the lithium batteries stay above 13.0 Volt until they are almost empty. The lithium voltage is higher than the voltage of a lead-acid batttery under load, so the lead-acid battery will hardly deliver any current, if anything at all.

After the lithium battery has only about 20% of charge left the voltage becomes low enough to allow the lead-acid batteries to gradually start taking over the load. Only when the lithium battery becomes fully discharged and is taken offline by the BMS, the lead-acid batteries will fully take over. With a correctly dimensioned lithium battery, this will only happen in rare situations. Most of the time, the lead-acid batteries will remain fully charged, which is exactly what keeps them healthy.

Charging

While charging, the voltage quickly rises to about 13.4 Volt, a voltage where the lithium batteries absorb all the available current but a too low voltage for lead-acid batteries to meaningfully charge. So the lithium batteries take up all the current until the BMS takes them offline, and only then the voltage rises enough to charge the lead-acid batteries.

When charging the lithium battery, the chargers "see" a voltage which is similar to the voltage of a lead-acid battery which is in its early bulk phase, so the chargers are providing charge current to the lithium battery while "thinking" they are charging a normal lead-acid battery, patiently waiting for the voltage to increase.

When the lithium battery is fully charged it is taken offline by the BMS, the charging continues with the lead-acid batteries only, following a charge trajectory which the chargers fully recognize, allowing them to do their "end-point-voltage-limiting" thing. If the lead-acid battery has not been used, the voltage will rise quickly to the end-voltage and the charge process will be terminated and revert to a "float" voltage.

As long as there is a charge current available the fully charged lithium battery will remain "parked" aside and the on-board equipment will be fed by the charge sources with the lead-acid battery as a buffer. Only when the lead-acid voltage starts dropping below the float voltage, indicating the absence of a charge source, the BMS will put the lithium battery online again.

Note that OpenHybridBMS implements a "lead-acid priority" feature, where after a (partial) discharge of the lead-acid battery, the lithium battery is held offline to allow the lead-acid battery to first complete its bulk charge phase, then to charge the lithium battery, and then to use the remaining available charge current to finish the lead-acid absorption phase.

In practice

So the lead-acid batteries don't disturb the lithium batteries at all. The lead-acid batteries don't have to be "regulated", they just remain connected to the DC-bus all the time. The whole system works naturally, the BMS only has to disconnect the lithium battery to prevent it from overcharging and overdischarging. Practically, the lithium system is just a simple add-on to an existing unmodified lead-acid installation and is the most easy conversion you can imagine.

In a hybrid lithium/lead-acid installation you are going to use the lithium batteries to cover the short time (i.e. overnight) cycles. The lithium cells discharge deep enough to keep them healthy and the lead-acid batteries are just doing nothing at all, just sitting there fully charged the majority of the time, which keeps them healthy as well. Only when there are multiple days without any means of recharging, which usually does not happen very frequently, the lead-acid batteries are used to supply their reserve power after the lithium batteries become fully discharged.

The big advantages of a Hybrid installation

Simplicity
You don’t have to change anything on the ship to convert to a hybrid installation. You can continue to use the existing alternators, wind generators, solar controllers, etc. You don't have to split the DC-bus into a dual-bus with a "chargers" and "dischargers" branch. The lead-acid batteries remain connected to the power bus all the time, just as before. The lithium batteries are controlled by their BMS which can take them offline whenever it likes to, because the alternators then still "see" the lead-acid batteries which they were designed for. This is the only way to correctly use Lithium power on a ship without redesigning the entire existing installation.
Redundancy
You have a plan B. If something fails with the lithium battery or its BMS, one has still the unmodified lead-acid system to automatically fall back to. Sure, lithium batteries are very reliable if treated well, but they can not work without a BMS, which comprises of complex electronics and is sensitive to salt water, lightning discharges, connectors, sensors, corrosion, etc. If things go wrong, nothing beats the simplicity of a lead-acid battery.
Lead-acid charge curve
Lead-acid charge curve
From around 80% SOC upwards, the charge voltage reaches a ceiling and consequentially the charge current drops of sharply. It takes a long time to increase the SOC from 80% to 100% because the battery doesn't accept the full available charge current anymore.

The trajectory under A is called "bulk phase", B is called "absorption phase".

Optimisation of charge time
The problem with lead-acid battteries is that they rarely accept the full available charge current after they are charged above 80%. The current tapers off and to get them close to 100% charged means hours of charging, despite the high output alternator you might have installed. But with a hybrid system you usually only used power from the lithium battery so only the lithium battery needs to be recharged, and they accept the full available charge current right up to the moment they are fully charged. This means that the charge time remains as short as possible. Finally you see the full rated output of your alternator flowing into the battery, right up to the end.
Longevity
Both the Lithium batteries and lead-acid batteries are enjoying a maximum life time because they are operating in a way that suits them best: The lithium batteries are cycled daily and don’t have to spend much time in a highly charged state, the lead-acid batteries are rarely cycled and spend most of their time being fully charged.
Economical
Because of the stretched longevity of both batteries in a hybrid installation, it is the most economic configuration possible. Also, installation wise, it is the most economic as no major modifications of the wiring need to take place and no charge sources have to be replaced or modified.
Maintainability
The ship will continue to use standard alternators and other chargers, with standard replacement parts commonly available, with no ties to a specific brand. No dual bus topology is used, a topology which would likely be confusing to engineers unknown with the lithium technology. This system is understood by engineers all over the world and can be serviced just like a regular system.

Comments

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Hallo Frans,ik ben zeer geïnteresseerd in de door jouw ontworpen BMS en dan wel de Pro versie. Ik heb momenteel 420 amp loodaccu's en heb 8 cellen Eve 304 besteld. Verder heb ik 1600 watt solar panels.
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Any possibility of a 48v version to manage a bank for powering a propulsion motor (as opposed to just a "house bank")?
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https://www.zwerfcat.nl/en/bms-news.html Project was revived April 2022, according to that blog.
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I’m also very interested in this project but also cautious because of the lack of updates. Maar hé, wel een geweldig project!
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Hello I.m interested on the assemble version of the OHybridBMS. The project you made is great, thanks for the work, really usable
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Hi Frans, did you build this OpenHybridBMS? I want to buy it!
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Hi, I expressed interest in your BMS more than a year ago - maybe 2! I’d be very interested in a 12v version. Please add me to your list. Dank u wel.
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Hi...really useful, and a set up I hope to build into a new UK canal boat soon. However can anyone advise as to how we then monitor the combined battery, ie which do I watch, and how to know when charged (or needs charging). Expecting to use Victron BMV 702 energy display Thanks
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Is there any news with this BMS project? I'm very interested in either buying or building one.
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Goedendag, We zijn erg geïnteresseerd  in de pro-versie van de BMS voor 24 volt. Kun je een indicatie geven wanneer deze leverbaar is en wat deze gaat kosten?
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Is de pro-versie van de BMS beschikbaar? En werkt de hybrid Lithium aanpak zoals verwacht? Ziet er zeer doordacht uit en zou graag zelf willen ervaren.
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Its very iteresting concept. However in my opinion the voltage difference between lead-acid and lithium batteries will cause the lithium battery continiusly charging lead-acid battery. To stop that from happening you need a diode to be placed at the positive terminal of the lead-acid battery.
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No, the voltage of a fully charged lithium battery is less than the voltage needed to charge a lead acid battery. The lead acid battery will very quickly settle on the "float voltage" presented by the lithium battery, and current will stop flowing.
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Please note the depth of discharge graph in the top of the article (red line) shows discharge curve for lifepo4 (13.7V when fully charged) chemistry not traditional Li-Ion (12.6V when fully charged). Li-Ion chemistry has more linear dicharge curve that looks closer to Lead-acid. Therefore the Li-Ion therm in this artical is sort of mental shortcut more then actual chemisrty name.
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When talking about lead acid, does this include Gel and AGM types, they have slightly different charge profiles to typical Lead Acid chemistries, but will they still work in this situation for a hybrid setup?
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Is this a dead project? There's been no update on Github for 2 years and Frans hasn't commented for 6 months.
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Is de pro-versie van de BMS beschikbaar? En werkt de hybrid Lithium aanpak zoals verwacht? Ziet er zeer doordacht uit en zou graag zelf willen ervaren.
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Any chance of you publishing what you have at the moment on GITHUB? This would allow others to peer review, build, and suggest modifications.
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I am very interested in the BMS Pro system. Is this still an active project? The GitHub repository shows no activity for a while.
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There are various FB groups dedicated to electrics on boats/motorhomes and LFP battery installations. Much is said about having to control or replace the alternator as it risks burning out with LFP, even in a hybrid system. I have installed LFP and kept the system as a hybrid by keeping the old LA batteries. You say that the system needs no modification. Can you reassure me! Thank you. Great articles, by the way.
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Very interested in this. When are you going to publish on Github? and any idea of when the Pro will be available and cost yet?
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1 more comment: my LA and Li battery banks each have their own charging source (LA - solar, LI - wind). The plan is to connect both the LA +/- outputs to the single +/- inputs of a substantial DC to AC inverter. Any comments or thoughts?
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I am going to build this. Parallel connection from each battery type to a single inverter. Inverter has + / - terminals and the Lithium + along with the L Acid + to the inverter +, and same with the negatives. This should work as the author describes, I believe. Any comments?
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I'd love to have a go at building your BMS, when you're happy with it would you mind posting the details of how you built it so others can copy it
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We would be interested in purchasing a Pro Unit.
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Is this Open design available to the public for rebuild?
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Also very interested in the BMS Pro system. Please let me know when it is available. Thank's
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Frans, what you say doesn't fit publishedcapacity/voltage curves of LFP batteries. Check this: https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/ You see in the 2. plot the 12V version with 13V at 40% LFP capacity. If you discharge from, say, 90% to 40% that leaves you with only 50% of the possible 80% LFP capacity. So loweing the discharge cutoff to 12.6V would still leave the LA batt. rather full and deplete LFP to 15%. Much better!
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Hi Frans, thanks for suggesting this simple solution! Why would you want to develop your own BMS? Can't you simply use one that is available like Electrodacus? https://www.youtube.com/watch?v=TrTu9uehOFg you can set the cut-in low voltage separate from the low cut-off voltage, you can set the high cut-off voltage and many more. It also has a battery overtemp protection aswell as an batt undertemp protection (LFP batt should be chared above 5°C only). The starter batt can then float when LFP is full.
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Nice article. 1 remark though about charging. You can FloatCharge CCCV lithium @ < bms-cut-off-voltage till it's saturated at the voltage setpoint. Advantage hereof is that the battery bms never disconnect the (solar)charger. Under floatcharge I mean just charging with one voltage set-point, e.g @ 14.0v for a 12v battery. Good idea or not?
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Very interested in the pro system.
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Your BMS sounds very promising and perfect for our situation. Please add us to your list, and thanks
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I'm very interested in your BMS when you start making them, can you notify me with a price when they're ready
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I’d be interested in the pro system. What price roughly? Thank you.
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Frans Veldman
Sorry, I have no insight yet in the cost of a small production run.
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what happens if say the alternators are pumping out 100amps+ I seem to see 50amps as max charge on most .Lithiums?
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Frans Veldman
My BMS will disconnect the lithium battery if the charge current is too high. Ofcourse it is better to dimension the system in such a way that the charge current is compatible with the lithium batteries.
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So if I fix cables of a size that would restrict current, say 35qm. Would that keep the BMS happy?
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Hi there, enjoying your articles, thanks. Does the alternator voltage need to be adjusted in order to run a hybrid system? I have one boat which has an alternator which outputs 14.4v, and one which outputs 14.8v via an external regulator. Would these be compatible with a hybrid system?
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Frans Veldman
As long as the lithium battery is charging, the voltage will not be able to go up because the lithiums are absorbing all available current. The BMS will disconnect the lithium battery as soon as it is fully charged. After that, the alternators etc can do just their own thing, increasing the voltage to whatever value they want. So the programmed voltage doesn't matter for the lithium battery, as long as it is higher than the voltage of the lithium charge voltage.
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I am very interested in your BMS. You have a very well thought out system but I noticed on GitHub there are no recent postings. Where do you stand on development? Thank you.
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Frans Veldman
I have been unavailable for a while, but I will soon continue the project. I'm in the process of developing the PCB's. The prototype is working fine on our ship!
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Hi Frans - I am very interested in this because I would like to add a LiFePO on my canal boat. It has twin alternators, a 35A for the starter battery (110Ah SLA) and a 70A for "house" batteries (345Ah SLA). My idea is to connect a LiFepO (120Ah) in parallel with the starter battery for charging via a VSR (ie "split-charging") only when the engine is running, via an ignition controlled changeover relay, which then connects the LiFePo in parallel to the house bank when the engine stops. Any comments?
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Frans Veldman
I'm not sure why you would be doing that?
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Nice work, i too have built a Battery Monitoring system based on a Particle Photon and the ADS1115 with voltage dividers, amazed to discover such a similar approach. I push the data out to Thingspeak with a web hook.
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I have been looking for information about using LFP and LA in parallel and happened upon your comment in the DIY Mobile Solar forum. I have just an off-grid cabin, and this is exactly what I have been dreaming about - a hybrid solution for multiple charging sources at 24v, robust and temperature variable (central plains of the USA here). This solves practically all of my issues. Although I am not much of a help with coding I will follow along eagerly. Thanks so much!
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So, when the lithium pack switches off because of low voltage, how does it recognize that it is ok to switch on again? Manually? If you have two circuits (charge and discharge) like usually with LF packs, you will need to redesign a lot.
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This is a good question. I assume that when the charging source raises the LA voltage to an acceptable level for the Lithium BMS to "reopen" for business. Correct?
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Frans Veldman
If the lithium is disconnected, the BMS will continue to monitor the bus voltage. As soon as it rises above the lithium battery voltage, it reconnects the lithium battery to the bus.
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We both seem to have adopted exactly the same approach. I too use esp8266 and wireless connection for everything including remote displays, web interface, remote monitoring, etc etc.i have the hybrid BMS as a stand alone unit that can perform all required protection and control of the battery environment and that communicates via MQTT with a Pi and direct to other esp control units including proportional dump load control and various other power control and optimising systems. I think hybrid rocks!
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Dank je wel voor een zeer interessant artikel!
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Frans, finally someone who sees a hybrid system with the same view! I have been struggling with some of the same questions, even the same thoughts about hardware for a BMS. I would enjoy hearing more as you move forward, and could possibly work on the Pro a bit. I am not an EDA expert, but I have worked with it a bit.
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Frans we zijn zelf bezig met het voorbereiden van een li ion verhaal. Wat ons nog niet duidelijk is, is hoe je omgaat met je alternator. De meeste alternatoren branden door als ze li ion laden zonder dat er een regeling is in functie van de temperatuur ten gevolge van de hoge laadstroom? Hoe vangt jouw systeem dat op? Voor ons is juni 2021 of 2022 de vertrekdatum en hopen we jullie ergens in de pacific nog te ontmoeten. Bert (schip: Evarne)
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Well, for what it's worth: there is not much you can do about the heat dissipation in a 12V alternator. You can switch to a 24V system (half the amperage and heat at the same wattage), get a large frame alt, get an oil cooled alt, get a double fanned alt, lay a vent hose to the back of the alt fan-ventilated. Limit the amperage by an external regulator like Balmart.
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Lädt man die Lions über einen BtoB Ladebooster z.Bsp.BB1260 Sterling brennt die Lima nicht durch da ja nicht mehr als 60A geliefert werden.
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