Hybrid Lithium / Lead Acid Battery Bank
This blog is at best quite technical, at worst will be quite dry and boring to a lot of people. Unfortunately there's no way to avoid this. I'll try to explain things in layman's terms, but it's not an exciting subject.
When my boat is away from a dock or a marina pontoon everything on the boat runs off batteries. At it's simplest it is very much like a car. However when you're running the little luxuries we have come to expect in life, as well as several important safety devices, it gets a little more involved.
The basics of a boat electrical system
On Isosceles, like most boats, the batteries are split. There is one battery reserved for starting the engine, that way even if the batteries get drained I can still start the engine and recharge the other batteries. Then there is a bunch of batteries dedicated to everything domestic, that means lights, navigation equipment, TV, radio, fridge and anything else that uses the magic of electricity. The general rule is "if you can't plug it in in a car it won't work on the boat", and yes, that does include the TV!
The batteries
I was bought up with the commonly held belief that all of the batteries joined together in a bank should be identical. You buy multiple of the same battery at the same time, and throw them out together too. Traditionally they were car type lead acid batteries, and variations on these. More recently Lithium has been added to the choice of batteries. Unfortunately using Lithium requires a specific setup in the charging of batteries that is quite different from Lead acid batteries.
Note: While I am using the generic term Lithium for these batteries there are actually six types of lithium battery, in technical terms the ones discussed here are LiFePO4 batteries.
Likewise there are different types of lead acid battery, such as Gel, sealed, AGM and others. I'll use the generalised term Lead Acid to cover all these.
There are several ways of charging the batteries, the two principle ways I use are by solar power, and using the alternator on the boats engine, just as a cars alternator charges a cars battery.
An alternator will overheat and burn out if highly loaded, and lithium batteries will draw a high load from the alternator if the systems are not managed correctly. An alternator will also blow its internal diodes if what it is trying to charge is suddenly disconnected.
Lithium batteries, at least the self contained ones that come ready to attach to the kind of battery system I have, come with a Battery Management System (BMS) built into them. This is what keeps the lithium battery safe and manage the rate of charge being taken from the alternator. These systems are great, but part of their job is to isolate the battery if something happens that is going to damage the battery. The down side of this is that if all of your lithium batteries get shut down for what ever reason then your alternator is going to burn its diodes out and fail because all of a sudden it has all of this power and nothing to do with it.
Note: It is very possible to build your own lithium battery, the cells are readily available, but that is way beyond the scope of this blog. If you do do this it's highly advisable to build in a decent BMS to safely manage the battery.
Lead acid batteries on the other hand are in far more forgiving and tough when it comes to handling the abuse thrown at them while charging, although this will shorten their life it won't kill the battery immediately, unlike abusing lithium.
The differences between Lead Acid and Lithium
There are some fundamental differences between Lead Acid and Lithium batteries
Useable capacity
Lead acid batteries do not like being over drained. To keep a lead acid battery at a reasonably optimal level you shouldn't discharge it below 50%, and it's better if you keep the charge higher than this. It will work below this level, but you will seriously damage the battery and ultimately it will shorten the life of the battery.
Lithium batteries on the other hand can cope with being almost fully discharged, or at least down to 10% or so of their fully charged level.
To put that into perspective a 100ah lead acid battery will safely give between 30 to 50 amps of useable power, a lithium battery of the same size will give 90 to 95 amps of useable power.
Optimum charging
Lead acid batteries like to be fully charged, and will last longest if they are kept in a state of full charge.
Lead acid batteries that are fully charged will settle at 12.8 volts, having some way of float charging them at around 13.2 volts will keep a lead acid battery at its optimal level and if you could keep it at this for ever it would never cycle and would in all probability never wear out.
Most proponents of lithium batteries will say you need to charge lithium to a higher voltage, but this isn't really necessary, in fact lithium batteries do not like to be fully charged all the time, they are happier sitting somewhere in the middle or up to 80 or 90 percent charged.
Note: this is why most phone batteries die. Most people, myself included, will leave them on charge overnight so they are full for the day ahead. this will actually shorten the life span of the battery.
Once a lithium battery is charged, just like lead acid it will settle to a level. In the case of lithium this voltage is around 13.2 volts, and will stay there until most of the power in the battery is used up.
Battery lifespan
Lithium can cope with 1000's of charge / discharge cycles during their lifetime.
Expensive high quality lead acid batteries can cope with 500 charge cycles, possibly a bit higher, but much less than lithium.
The Lithium / Lead Acid hybrid system
Ok, this is where all the old rules are thrown out of the window. A hybrid battery set-up basically takes one or more Lithium batteries (two in my case) and adds one or more Lead Acid battery on the end of the system.
The benefits of this hybrid system
In the example earlier of an alternator burning its diodes out, having a lead acid battery in the system will allow the alternator to send its charge to the lead acid battery if the lithium batteries have shut down temporarily. This has the potential to save the alternator in such circumstances.
The charging parameters of all of your equipment (battery chargers, solar chargers etc) do not have to be changed in the hybrid system, making it much less technical to set up.
If the Battery Management Systems on your lithium setup shuts down the batteries then you have a fall back battery that will keep the boats electronic navigation systems going. It is most likely that the lithium batteries will come back on line soon after, but in the case of something catastrophic such as a lightening strike you can manage with just the lead acid battery for a period of time. You may lose some of the nicer luxuries like fridges and TV's (if these haven't already died in such a scenario), but you can get safely to a port and then either reset or replace the BMS in the battery(s).
Faster charging
The BMS on the lithium battery will pull as much charge as it can from the alternator, reducing engine or generator run times, and thus reducing fuel use.
This is also a down side of this system. The lithium batteries will pull the most charge that they can from the alternator for as long as they need charge, this can result in the alternator over heating. Ideally you need to be able to watch the temperature of the alternator, particularly if it is running for some time, for example if your lithium battery bank is particularly discharged.
There are things that can be done to manage this, but in all honesty if you need this kind of advice I would recommend getting in touch with the 12 volt boating group ( https://www.facebook.com/groups/144684286108763 ) on Facebook (as opposed to the 12 Volts Boating Group who seem inclined to refuse to accept that this system actually does work).
Misconceptions
There are a few misconceptions about the idea of a hybrid, or mixed, battery bank. These are usually used to dispell or disprove the idea of a hybrid battery bank.
Lead acid battery draining the lithium batteries down.
Remember the voltage required to float charge the lead acid battery? Its the same as the resting voltage of the lithium batteries. So the lithium batteries will float charge the lead acid battery at 13.2 volts.
The lead acid battery isn't going to drain all of the power from you lithium batteries, it might drain half an amp or so per hour overnight until the solar charge starts to top them all up again. I do want to check this out, so if anyone has one of those clever meters that can tell how much current is going down a cable please get in touch. Lithium batteries have much more power than the lead acid could ever take, unless of course you have one lithium battery and several lead acid batteries being float charged by the lithium. But this system is designed for 2 or 3 lithium batteries and one lead acid as a safety net / backup battery.
Fire risk
Yes, there have indeed been incidents of lithium batteries going into meltdown and even catching fire. Remember when I said there are different types of Lithium battery? Unlike the differences in Lead Acid variants, different forms of Lithium battery are made from very different chemistry's. For example the batteries in your mobile phone are often Lithium Cobalt Oxide or Lithium Ion, which can be thermally unstable. These batteries have caught fire, particularly when they are being over worked by the electronic device or charged by a poorly regulated charger. LiFePo4 batteries are much more stable and have a built in management system to control the charge and discharge of the battery. Unfortunately they are far too bulky for your personal electronic equipment.
It is true that any Lithium mixed with salt water is a volatile combination, but so is Lithium and the contents of a Lead Acid batter. Let's be honest, if there is sea water to the level of my batteries, and it manages to get through the sealed case, and it gets through the sealed case of the battery cells inside, then I think I'll have bigger things to worry about!
All batteries have to be the same size, type and age.
Well I think there are enough people out there doing exactly what I'm talking about to disprove that statement.
To Sum Up ...
For ease of installation and to have some redundancy I would consider a lithium heavy hybrid. If you like gizmos and don’t mind spending more and aren’t bothered about redundancy then stand alone lithium may be the way. The reason I say ‘redundancy’ is that when lithium go flat they just turn off with very little warning, if any depending on settings. With hybrid you still have 50% of the lead to go when the lithium cut off and lead has a very linear discharge curve so you get plenty of warning, the lithium curve at the end of discharge is like falling off a cliff but during 85% of the discharge is almost flat. You can download the power audit in the files section of the group to get a comparison in sizes between the two technologies for your use and also the solar that would be needed throughout the year to service that load.
Disclaimer: This blog shows what I have done to manage the electrical power on my boat, and why I have made the choices I have. If you decide to follow the same or similar ideas then you should do further research and satisfy yourself of the pro's and con's of the system. I take responsibility for my own decisions, I DO NOT take responsibility for the decisions or choices of other people.
References
Here are a few of the references I used to get information. You will no doubt come across a lot of negative comments and thoughts on this subject, as is often the case with new ideas. I completely understand those views as that is where I started out. The fact that we have all made it this far shows that we are at least willing to have a discussion about this.
Adding LEAD to your LITHIUM battery bank by Clark from SV Temptress (Emily & Clark's Adventure) https://www.youtube.com/watch?v=sqT27KbxRec
The 12 volt boating group ( https://www.facebook.com/groups/144684286108763 ) on Facebook
WILDEBUS - Electrics for Campervans and Motorhomes - https://www.wildebus.com/hybrid-battery-theory/
The Will Prouse forum https://diysolarforum.com/ and YouTube channel https://www.youtube.com/c/WillProwse
WILDEBUS - Electrics for Campervans and Motorhomes - https://www.wildebus.com/hybrid-battery-theory/
The Will Prouse forum https://diysolarforum.com/ and YouTube channel https://www.youtube.com/c/WillProwse
Dragonfly Energy - A guide to the 6 main types of Lithium batteries - https://dragonflyenergy.com/types-of-lithium-batteries-guide/
Zwerf Cat - Add lithium power to your ship. https://www.zwerfcat.nl/en/lithium-on-yachts.html A series of articles on lithium batteries, including an article on the hybrid lithium setup.
