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Lithium advantages The advantages of Lithium batteries

In this article we will discuss "Lithium batteries". In fact there is a whole family of "Lithium batteries", all with a specific chemistry tailored for a specific use, optimised for weight, size, voltage and/or temperature range, etc. For use on sailing boats, only LiFePO4 (also sometimes referred to as LFP) is a sensible choice. In this article read "LiFePO4" wherever we write "Lithium". The statements made might not apply to other types of Lithium batteries.

Comparison

If you own a sail boat which spend most of its time off grid, you are probably very aware of the problems associated with lead-acid batteries. Let's compare them with Lithium batteries, side by side:

Durability

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Lead-acid batteries
Lead-acid batteries on sail boats typically don't last very long. Lead-acid batteries perform very well to generate short term heavy current, like for starting an engine. They also last very long when they just need to provide backup power very occasionably. In both situations the lead-acid batteries spend most of their life fully charged. But lead-acid batteries hate to be discharged, and they hate to have to cycle (from charge to discharge and back again). Most manufacturers specify somewhere about 500 cycles, which is, with a daily cylce, a lifetime of less than 2 years! There is not much that can be done about it, it is just their chemistry which make them suffer from each single charge-discharge cycle and spending any time in a discharged state.
Lithium batteries
Lithium batteries love to cycle. They can perform a virtually infinite amount of cycles. The specifications often claim 5000 cycles and with very good care for the battery, it can be even more. But Lithium batteries hate to spend much time in a fully charged state. You might have bad experiences with Lithium batteries in consumer electronics, the typical "hardly ever used" batteries. This is usually due to the common mistake to charge these batteries after each use and let them spend most of their time in a fully charged state. On a sail boat, where the batteries are being discharged every night, the Lithium batteries will not suffer from spending too much time in a fully charged state, especially if the capacity is choosed wisely (not much more capacity than actually needed) and a good Battery Management System is being used.

Economy

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Lead-acid batteries
Because lead-acid batteries usually don't last very long on an off grid sailboat, they are not an economic choice, despite their lower price. The lifetime is about 500-1000 cycles, provided you don't discharge them too deeply. The deeper the discharge, the shorter their lifetime. To get any meaningfull life out of lead-acid batteries, you can only use 30% or even less of the rated Amp-hours capacity.
Lithium batteries
Lithium batteries might be more expensive, until you realise that you can safely use 80% of their rated Amp-hours capacity. In practice, this means that to replace a 600 Ah lead-acid battery (which you could only discharge for 30%, which is about 200Ah), a 250 Ah Lithium battery (which can be discharged for 80%, which is 200Ah) will give you exactly the same usable capacity. Also, it lasts much longer, at least 3 to 5 times. If you don't buy the Lithium batteries from a marine shop but directly from the importer, and make the very conservative assumption that the battery will last 10 years, the lithium battery is by far the most economic choice.

Weight

Lead-acid batteries
The typical specific energy is 33-42 Wh/Kg. With about 30% usable capacity, this would equate to 10-13 Wh/Kg.
Lithium batteries
The typical specific energy is 100 Wh/Kg. With about 80% usable capacity, this would equate to 80 Wh/Kg.

This means that the lead-acid batteries are 6 to 8 times heavier per Wh storage than their lithium cousins. That might be important, especially on fast boats. After all, for every kilo of "boat", you will have to displace 1 kilo of water to move forward, which translates into drag...

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".

Charging

Lead-acid batteries
Lead-acid batteries suffer from a very frustrating charge curve: From around 80% of charge upwards, they don't accept the full available charge current anymore. In technical terms they transit from "bulk charge" to "absorption fase" (which we used to call "spoon feeding" on our sailboats). It is frustrating to have a very powerfull alternator or a high capacity solar array and not seeing the full charge rate going into the batteries. The battery voltage is limited to around 14.4 Volts (depending on configuration) and the current has to taper of to prevent exceeding that voltage. At the end of the charge, hardly any current is going into the batteries anymore.
Lithium batteries
Lithium batteries accept the full available charge curve up to the point where they are 100% charged. You don't have to run your engine/generator so long anymore. In fact, you have to be careful not to burn out your alternator, because the Lithium batteries absorp much more power than you have ever seen flowing into your lead-acid batteries.

Discharging

Discharge curve: Lithium-Ion vs Lead-Acid
Discharge curve: Lithium-Ion vs Lead-Acid
Lead-acid batteries
The output voltage of lead-acid batteries is depending on load and state of charge, and in general the output voltage during discharge is much lower than the voltage during charging. Also, the output voltage sags substantially when you switch on a heavy load and you will see the lights flicker.
Lithium batteries
The voltage curve of Lithium batteries is nearly flat over most of its capacity. From about 20% State Of Charge up to 90% SOC the voltage will stay around 13 Volts. It is higher than the discharge voltage of lead-acid batteries and also more stable. At the same time, Lithium batteries have a lower internal resistance so the voltage doesn't sag very much when you switch on a heavy load.

Safety

You might have heard about the safety hazards of "Lithium batteries". Most problems however are associated with other Lithium batteries chemistries than LiFePO4, and usually some type of mismanagement is involved. In reality, LiFePO4's are the most safe Lithium batteries known. Read more about this in our article Lithium-butwhatabout.


Comments

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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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