Originally Posted by twistedtree
Good point. That lead in conventional batteries is pretty nasty stuff too.
I think the biggest risk with LiFePO4 is that your charging protocol isn't just right any you trash your $4000 bank of batteries. For all the fussing we do over lead acid battery charging voltages, etc., they actually can take quite the licking and keep on ticking.
That's a VERY good point, and perhaps it warrants some explanation - at the risk of being too technical and writing way too much.
There are several things to consider when maintaining and charging LiPeFO4 batteries. The four most important are:
1 Don't allow any cell to discharge below the minimum voltage
2 Don't allow any cell to be charged at over the maximum voltage
3 Don't allow any cell to be charged at over the maximum current
4 Don't allow the battery to be exposed to excessive heat
Lithium Iron Phosphate batteries actually have a nominal cell voltage of 3.2V. To make a "12V" battery pack for a boat, we put four of these cells in series, which gives us a pack with a nominal voltage of 12.8V.
Then, (in our system) each of these sets of four cells has a capacity of 100Ah, so we have six sets of these in parallel to make a 24-cell pack with 600Ah capacity at 12.8V nominal.
It's important to remember that this is not just a "12.8V battery," though. It really is six sets of four 3.2V batteries/cells in series/parallel. That means we want those sets of 4 cells in series to be kept "balanced". I'll talk more about balancing in a minute.
The normal working voltage of a cell is in the range of 3.0-3.3V. That makes our 12V pack have a normal working voltage of 12.0-13.2V.
With that in mind, let's take the four issues one at a time:
1. "Don't allow any cell to discharge below the minimum voltage."
The minimum discharge (without damage) is about 2.5V per cell, which is 10.0 V for our array.
There are two lines of defense against over-discharge. First, all of the batteries I have seen for marine or RV use have a built-in battery management system (BMS) that automatically cuts the battery pack off if the overall pack voltage drops below some conservative value.
Our BMS cuts off our batteries at a minimum pack voltage of 11.3V (well above the 10.0V where damage would occur). If the overall voltage drops to 11.3, the system cuts the batteries off, preventing further discharge. When the BMS cuts off (and this is true for all the BMS cut-offs I describe below as well) The system lights up a red button installed in our panel. We would then diagnose and fix the problem, then press the button to reset the BMS - re-connecting the batteries. If the problem is not "fixed" pressing the button will not re-connect the batteries. To date, we have never had a BMS cutoff with our system.
Second, we program our Magnum inverter to shut off if the pack voltage drops to 11.4V (to help prevent us from ever hitting the 11.3V BMS cutoff). That way, any AC loads that might be drawing current via the inverter will be stopped before the battery hits the BMS cutoff line.
2. "Don't allow any cell to be charged at over the maximum voltage"
The maximum charge voltage is 3.65V per cell, which would be 14.6V for our array. Damage will not normally begin to occur unless charging at over 4.2V per cell for an extended period, so the "damage" threshold is around 16.8V for our pack.
There are several things we do to prevent over-voltage charging. First, the BMS is set to cut off the batteries any time the pack voltage exceeds 14.4V. So if any combination of chargers, alternators, etc - tries to drive the voltage above 14.4V, the BMS cuts off the batteries (well short of the 16.8 "damage" voltage and comfortably below the 14.6 maximum charge voltage.)
Second, we make sure that none of our charging sources will exceed the maximum voltage, because we don't want the BMS to ever go into over-voltage shut off. We charge via the engine alternator, the Magnum Inverter/Charger, and the solar controller. (The generator and shore power both charge via the Inverter/Charger).
Our engine alternator has a voltage regulator that keeps it below 14.2V. Unless the regulator fails, that should keep us below BMS cutoff voltage.
Our Magnum Inverter/Charger is programmed to charge at maximum current ("bulk" setting) up to a pack voltage of 14.0, then to switch to a constant-voltage charge ("absorb" setting) at 14.0V for a fixed period of time, then shut off ("float" setting) until the pack drops to a voltage of 13.3V. Obviously, this never allows our pack to be charged from Generator or Shore power at a voltage above 14.0.
Finally, our solar charge controller is set to charge at max current (which for solar isn't all that high), up to a pack voltage of 14.2V, then to switch to a constant-voltage 14.2 charge for a fixed period of time, or until the charge current drops below a minimum level. Then, the solar shuts off until the pack voltage drops down to 13.5V.
A couple of notes:
- As you may have noticed, stopping charge at 14.0V means our shore power never brings the pack to a full 100%.
- And, note that the solar has higher "stop" voltages and "start again" voltages than the shore power. This means that shore power will work until we are close to fully charged, then shore power cuts off and lets the "gentler" solar power finish the job. Later, when the voltage drops back after some use, the solar kicks in first - so if solar can handle the job alone, shore/generator power doesn't have to kick back in.
- If your system is set up for an "Equalize" charge (used for AGM and lead-acid batteries periodically to de-sulfate them.) It needs to be disabled. Don't confuse "Equalize" and "Balance". Lithium batteries do not require equlization. You don't want "Equalize" to kick in once per month or something and run the pack voltage up to ~15V-17V (which is the normal voltage range for "equalizing" AGM and lead-acid batteries.)
So, if we never charge the pack above the maximum pack voltage, we're all safe, right?
Now I'll talk about "balancing" as I mentioned above. Remember that we have four cells at 3.2V each wired in series to make 12.8V? What if ONE of those four cells drops low for some reason - to maybe 2.5V, but the rest are at the normal 3.2V. If we put a "seemingly safe" 14V charge on the whole pack, our poor little 2.5V cell isn't doing its job, so the other cells would be pushed up to over 3.8V each. That means the weak cell would cause the other three to exceed their 3.65 max charging voltage. And, over time, this effect could get worse and worse as the 3 "healthy" cells get more charge and the 1 "weak" cell gets consistently under-charged.
To prevent this, all lithium systems use "cell balancing" circuitry - which does two things: First, it slows down the charging of the highest voltage cells so that all the cells charge up together with equal voltages. Second, it sends an error to the BMS
if a balancing failure is detected (meaning it was unable to balance the cells) which shuts down the whole pack.
In a modular system like ours, if one cell (of the 24 cells we have) fails and causes a balance failure, we could still remove that cell and the 3 "good" ones that are in series with it from the array, and we'd then have a fully working 500Ah battery pack (rather than the original 600Ah) as well as 3 "spare" cells. This makes for a very robust system with almost its own supply of spare parts.
3. "Don't allow any cell to be charged at over the maximum current"
The maximum charge CURRENT is usually stated as "C/1 up to 3.6V" which means that the maximum charge current is the amount that would equal the full capacity of the battery in one hour. ("Capacity divided by one") For our 600Ah pack, that means the maximum charge current would be 600A! (whoa!) Also, that means that the pack could be safely charged from zero-to-full in ONE HOUR!
That's just for "recommended" operation. The actual rating for our cells for "max charge current" is 200A per cell. Since we have 6 stacks of cells in parallel, the real "damage" threshold for our array would be a whopping 1200A!
Let's never do that! (Not even the 600A "recommended", even thought it's within spec and "safe")
We set a MUCH more conservative C/3 maximum charge current (one-third of capacity per hour) - which for our 600Ah pack would be 200A - allowing the pack to charge from zero to 100% in 3 hours.
Our Magnum Inverter/Charger has a maximum charge current of 125A, so no problem there with exceeding our 200A limit from generator or shore power.
Our (theoretical) 520W solar array would never (theoretically) exceed ~45A (520W at ~12V) so that's safe - even at the same time as the maximum 125A Inverter/Charger current. Also, we have never seen more than 30A with our solar system in the real world.
Our 150A engine alternator would not exceed our 200A limit, even with maximum solar at the same time. In practice, we've never seen more than 125A going from our engine alternator to the batteries, because the engine alternator is powering other loads at the same time.
The only combination that could exceed 200A would be if ALL 3 were at maximum at the same time - running the generator AND the engine AND the solar at high-noon in the Sahara... Then we'd be at 150 for the engine plus 125 for the Inverter/Charger plus 30 for the solar - That's just over 300A (which is still below even the 600A "recommended" max. Plus, the charging voltage required to achieve such a high current would automatically cut out both the Inverter/Charger AND the solar (because of the voltage ranges we put above) leaving only the engine alternator charging the batteries.
Whew! OK, I think we're safe.
4. "Don't allow the battery to be exposed to excessive heat"
Don't take this to mean that lithium batteries are particularly heat sensitive. They actually do much better when hot than conventional lead-acid or AGM batteries. But, the life of the batteries is definitely most related to the temperature of the chemistry, so we want to avoid excessive heat.
Our batteries are installed in the engine room, so the ambient temperature is usually warm and fairly constant while underway. While that's not absolutely ideal, none of the temperatures in there are near what would damage the batteries.
The thing that WOULD damage the batteries is causing them to generate too much heat internally. When the batteries are fully charged, and more charge current is applied, the excess energy is dissipated as heat. That means you DO NOT want a "trickle" charge on lithium batteries when they are full. All of the "trickle" energy would act as a heater, constantly keeping your expensive batteries warmer than they should be.
As you may have noticed above, we avoid this by the way we program the various chargers - cutting off before the max "full" voltage of the lithium cells. Yes, that means that the batteries are theoretically never charged to 100% of their "chemistry" capacity. That's OK. Unlike conventional batteries, lithium batteries do not have to be regularly fully-charged. In fact, they are happier long term down in the middle of their voltage range. Most lithium manufacturers recommend 1/2 charge for "long term storage".
And, our "100%" capacity is really well below what the batteries are capable of. Quite a bit more energy could safely be stored there, but it would cause risk of premature failure. In the news recently, it appears that Samsung pushed their phone batteries too hard in their now-infamous "Note 7". We want to play it safe and get a lot of years out of our boat house batteries.
Another note on temperature - you should never charge lithium batteries if the temperature of the chemistry is below freezing. If your boat is stored out of the water, you need to be sure that nothing charges the batteries when the battery temp is likely to be below freezing, or you may want to put something like an aquarium heater pad under the batteries to keep them above freezing.
Below-freezing temperatures do not damage the batteries, but charging them when they are below freezing will cause damage.
Overall, we tried to set up our system so that we were extremely conservative compared with the maximum/damage specs for the batteries, so that we have multiple lines of defense against problems, and so that we take into account likely component failures - without causing damage to the batteries themselves or safety issues.
We also had to be creative (with the help of a great manual from AM Solar) about setting up devices like the Inverter/Charger, the Solar Controller, and the Engine Alternator to charge lithium the right way, even though none of them have a "lithium" setting available. We are using the 3-stage "bulk", "absorb", and "float" settings (intended for AGM batteries) to get the desired behavior for our lithiums.