Login

Lithium implementations A comparison between different implementations

Standard lead-acid installation

Standard lead-acid installation
Standard lead-acid installation

This drawing is provided for comparison. Note that usually charge sources and consumers are added at random somewhere on the single power bus (red wire). There is no separation between charge sources and consumers because there has never been a reason to do so.

Lithium only, no disconnectors, voltage based regulation

Drop-in lithium replacement
Drop-in lithium replacement

This is also called a "drop in replacement" as no modifications are carried out. Note that the lithium battery may have a BMS, but it is unable to offer much protection (if any at all) because it is unable to disconnect the battery in case of overcharging or overdischarging. Don't get lured into the false notion that the battery may have an internal disconnector. It doesn't, and if it had one, your equipment would fry at the moment the BMS activates the disconnector while the alternator is running.

Advantages

  • Single bus: No modifications necessary on existing wiring
  • No modifications necessary on charge sources
  • Drop in replacement. Cheap and easy.

Disadvantages

Lithium only, overdischarge protection

Lithium overdischarge protection
Lithium overdischarge protection

Advantages

  • No modifications necessary on charge sources
  • Protection against overdischarge

Disadvantages

Lithium only, overdischarge proctection, charge termination with interruptor

Lithium overdischarge protect, charge term. by disconnection
Lithium overdischarge protect, charge term. by disconnection

Advantages

Disadvantages

  • Dual bus: modifications necessary in wiring
  • Charge sources need to be able to operate without battery. Modifications/replacement of charge sources likely necessary.
  • When battery is fully charged, equipment immediately starts feeding from the battery.
  • No redundancy. If BMS erroneously activates the overdischarge protection, all equipment goes offline.
  • Lithium battery needs to be large enough to provide all reserve power, will thus remain most of the time in a highly SOC which is bad for its health.
  • No reliable balancing possible

Lithium only, overdischarge proctection, charge termination by communication

Lithium overdischarge protect, charge term. by communication
Lithium overdischarge protect, charge term. by communication

Advantages

Disadvantages

  • Dual bus: modifications necessary in wiring
  • Modifications/replacement of charge sources necessary.
  • No protection against malfunctioning/disobeying charge sources (BMS unable to disconnect the battery)
  • No redundancy. If BMS erroneously activates the overdischarge protection, all equipment goes offline.
  • Lithium battery needs to be large enough to provide all reserve power, will thus remain most of the time in a highly SOC which is bad for its health.

Lead-acid/Lithium hybrid

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

Advantages

  • Single bus: No modifications necessary on existing wiring
  • No modifications necessary on charge sources
  • Properly implemented charge termination
  • Protection against overdischarge
  • Protection possible against low temperature charging
  • Protection against malfunctioning charge sources (BMS just disconnects the battery)
  • Redundancy. If the BMS malfunctions, the system automatically falls back to the standard lead-acid configuration
  • Lithium battery can be smaller, thus gets discharged deeper, which is good for its health
  • When battery is fully charged, equipment will be fed from the charge sources
  • Reliable balancing possible without interrupting normal ship operation

Disadvantages

  • A hybrid installation is heavier than a a pure lithium installation due to the weight of the lead-acid batteries.

Conclusion

A lead-acid/lithium hybrid implementation clearly offers the most advantages.

Despite its clear advantages, the lead-acid/lithium hybrid implementation is relatively unknown. There might be a few reasons for this:

  • Ship owners want to get rid of the pesky lead-acid batteries and only focus on a total replacement by lithium batteries. A hybrid conversion might intuitively feel as a "job half done".
  • Lithium battery vendors want to sell as much capacity as possible.
  • So far there were no Battery Management Systems specifically designed for hybrid installations.

We hope to bring the advantages from lead-acid/lithium hybrid installations more into the well deserved spotlight. Read more about it on our article about lead-acid/lithium hybrid systems.


Comments

Name:
Email:
Characters left:


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?
0
0

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?
0
0

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
0
0

We would be interested in purchasing a Pro Unit.
0
0

Is this Open design available to the public for rebuild?
0
0

Also very interested in the BMS Pro system. Please let me know when it is available. Thank's
0
0

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!
0
0

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.
1
0

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?
0
0

Very interested in the pro system.
0
0

Your BMS sounds very promising and perfect for our situation. Please add us to your list, and thanks
0
0

I'm very interested in your BMS when you start making them, can you notify me with a price when they're ready
0
0

I’d be interested in the pro system. What price roughly? Thank you.
0
0

Frans Veldman
Sorry, I have no insight yet in the cost of a small production run.
1
0

what happens if say the alternators are pumping out 100amps+ I seem to see 50amps as max charge on most .Lithiums?
1
0

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.
0
0

So if I fix cables of a size that would restrict current, say 35qm. Would that keep the BMS happy?
0
0

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?
2
0

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.
0
0

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.
0
0

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!
0
0

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?
0
0

Frans Veldman
I'm not sure why you would be doing that?
1
0

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.
1
0

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!
1
0

contact