A Word About Lithium Batteries
Thought I would write a few words on Lithium as we are seeing more and more misinformation and huge price variances. This is not about cheap or expensive, this is about giving you some information that hopefully will help you to make informed decisions.
Yes, I am in the trade, and this should be read as general information and is not brand specific.
It is intended as some base information to help you ask the right questions when looking at all the options.
I have split the info into six basic categories.
Motorhome Solar sells Relion, Enerdrive, Invicta and Allion batteries. However this article is not brand specific.
Some are 100% usable and some are not, it’s as simple as that. So it’s a good question to ask! That is not saying that a 40% or 50% max discharge battery is any lesser quality, BUT, you need to know as if you want 100Ah of usable energy you will need either 200Ah of 50% usable or 100Ah of 100% usable.
Of the batteries that are 100% usable, you are NOT damaging the battery by using all its capacity even if you run it until it shuts off, it’s designed to do that.
What many don’t realise is it is technically not going flat, it is being turned off by the BMS, more on that in the next section.
When you buy a 100Ah 100% usable battery, you are actually buying an approx 105Ah battery, as the BMS turns off at 100Ah and the extra capacity is held back to continue running/monitoring the battery. This is the case with every 100% usable battery with built in BMS that I have ever seen.
Some will promote this feature calling it different names and even charge a premium for it, but the reality is, it’s there in virtually every 100% usable battery, so worth knowing to help in your decision process.
You also can’t damage a 100% usable battery by using the full 100%, if you do it a lot, you will shorten its life, but we will talk about that under cycles.
You also don’t need to get the lithium to 100% to maintain health like you do with lead, lithium is happy spending significant periods of time in the mid-range as an example.
Battery Management System (BMS)
There are a few types and they are very different in terms of what they do and or don’t do.
Some batteries come with built in BMS, some have no BMS, so that should be one of your questions!
You can have a battery installed without a BMS, but in most cases if you dig into the battery manufacturers data you will usually find that they do not recommend using the batteries without a BMS.
So now we have a BMS, another question is:
Is the BMS active or passive?
Why is this important?
Safety Safety Safety
A BMS is designed to monitor certain operating parameters, these can include, but is not limited to:
- Discharge capacity
- Internal cell balance
- Charge amps
- Discharge amps
An active BMS has the controls to monitor AND take action when certain parameters are meet, shutting down the battery as an example if you exceed the max discharge rate.
A passive BMS will monitor and report, BUT will not take action, so it is important to know what type you have.
This is an area that is well worth spending a bit of time understanding.
For us in motorhomes we can generally say a cycle is “1 day and night” so a 24hour period.
Cycles are used to state the expected life of a battery at certain DoD (Depth of Discharge).
Some of the most common used with lithium are 100% 80% and 50%, so you could see 1 or more of those on a data sheet and will usually be something like:
- 100% DoD 2000 cycles
- 80% DoD 3000 cycles
- 50% 6000 cycles
- Or similar numbers.
A couple of comments: you can see in the example you get 3 times the life if you only cycle to 50%, so in a real life example, if you know you will use 100Ah a day, you could in theory buy a 100Ah usable battery, but it makes more sense $ wise to buy a 200Ah usable and cycle to 50% and get 3 times the life!!
I sometimes use cars as an analogy to explain cycles.
The faster we rev an engine the harder we are working it. So two engines, one operating at max revs is not going to last as long as the same engine revving a lot lower. The harder they work the shorter they live, with all batteries the lower you take them the harder you are working them, the shorter their life expectancy.
This is where the quality of a lithium can start to show through, as some of the lesser quality batteries have a much lower max charge rate, and can leave you struggling to recharge after use.
This has more impact if you are a medium to high energy user, the majority of lithium batteries have a lower max charge rate than discharge rate, so you need to be sure the max charge rate is enough to meet your consumption requirements.
A common misconception is you don’t need to leave a small load on when not in the rig to keep the battery healthy.
As stated above the max discharge rate is usually higher than the charge rate.
You still need to be aware of the max to be sure it can deliver enough energy to meet your requirements.
And if you run an inverter the maths is easy, take your largest load, say a coffee machine at 1400W, divide by 10, it will draw approximately 140amps, so the max discharge rate of the battery or batteries needs to exceed your max discharge.
A good real life example, there are plenty of 200Ah batteries with a max discharge of 100amps.
There are plenty of 100Ah batteries with the same 100amp discharge, so you would be better spending a little more on 2 x 100Ah batteries to give you a combined 200amp draw and meet your needs, where the single 200Ah battery in this example wouldn’t.
We generally install lithium batteries inside the motorhome, as they don’t like being charged below 0degC.
Between 0deg and minus 15ish the charge in most cases can’t exceed 10% of the capacity of the battery, so if you have 200Ah the charge must remain below 20amps until the temp is back above 0, then your back to the specific battery max charge.
“I have put this together based on my experience, so yes this is one man’s opinions, please do your own research and come to your own conclusions. I hope you find this useful.”
I have kept the examples simple and tried not to be too technical.
The intended audience is people considering buying a lithium battery. Where multiple batteries are used, in these examples they would be in parallel.
It’s also worth noting that many lithium batteries, depending on quality can only be wired in either series or parallel and most have a max number you can have together, two to four is the range we see most often.