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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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Hi, I'm interested in your BMS! I’d be very interested in a 12v version. Please add me to your list. Dank u wel.
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I would like to build this project myself, any chance of sharing the software and schematics as suggested on Github?
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Hello Frans, I just want to suggest also to give a look at the BOS LE 200 manual that makes what you already designed with your BMS. https://www.manual.bos-ag.com/le300/ One interesting feature of BOS battery is the sleep mode to avoid deep discharge of the Lithium battery. I don't know if you have already thought to this with "Extensive voltage checking on cell level" or not, anyway I took the occasion to highlight the manual to you! Thanks again Daniele
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Hello Frans! I'm very interested in purchasing your BMS for my boat (if you are going to sell it, of course). I have also some contacts to distribute it in Italy, if you want :) Please let me know if you could be interested. May you have fair winds and following seas! Daniele
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Beste Frans, jouw aanpak lijkt precies datgene wat ik zoek: een LifePo4-aanvulling op mijn loodaccu's (360 Ah) die zonder heel veel gedoe kan worden ingepast in het (12V-)systeem. Lever je kant en klaar (met/zonder LifePo4-accu), en wat zou dit dan moeten kosten? Ik verneem het graag om zo de afweging te kunnen maken.
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In respect to Lithium-hybrid, I see two problems that difficult his implementation: *You need special alternators that can work with Lihium banks (with temperature sensors and external regulated). *Due to his different internal resistance, until the Lithium don't reach his maximum charge the lead bank will be unable to be charged.... that can result in a bad SOC of the lead bank during too much time.
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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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Hi Neil, I'm looking at fitting the hybrid system and wondered if you managed to get it up and running,and how it's doing?
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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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