Lithium users

[John61CT]

Again, BMS should be a safety backup, normal charging operations should be handled by the charge sources.

If you are calibrating your SoC meter, that is the only time you need to measure trailing amps (Absorb, dropping to .02C) to measure 100% Full precisely.

And be ready to draw down every time, not good for the bank to sit there Full.

Otherwise just use voltage, close enough and safer.

 

[John61CT]

> Great idea. Is there such a beast?

You supply small cheap sacrificial lead as discussed above.

Only needed for charge sources or sensitive electronics damaged by BMS isolating the LFP bank.

Which is not a normally recommended approach.

 

[John61CT]

> Eventually as lithium becomes more popular there will be a profit to be made in creating charge controllers designed to work with them.

Problem is all hardware manufacturers implement voltages too high, so banks continue to need being replaced faster than vehicles.

My sense is the cost of bare cells won't come down, and getting good longevity without paying for "integrated systems" based on proprietary BMS will continue to be a DIY endeavor.

Open source Arduino or Raspberry Pi plus standard LVD OVD and temp sensors will emerge toward an open hardware BMS / PMS control system, eventually Kickstarter then off the shelf.

Meantime the knowledgeable vendors are cherrypicking the wealthy Yachties and Expedition vehicle market, $5-8K per install no problem.

 

[John61CT]

> As for the small battery....if I just add a second cable connect to the mppt...basically two ring connectors to the same terminal connector on the mppt...that will work?

Just parallel to the LFP bank with an ACR combining while charging only.

Also needed for the Alt, prevents frying its diodes.

Starter batt can do the same thing, no extra gear needed!

 

[tripper]

The high voltage and low voltage cutoffs on the BMS, like a previous poster said, are there as a safety catch, you shouldn't ever hit those. The MPPT solar controllers do fully charge the battery to the voltage you tell them to go to. For example my main solar bank can push 85 amps, typically by the morning my lithium battery will be down a bit (est 60 Ah or so). As the sun comes up the wattage output by the controller may get up to 400 watts or so then will slowly taper down until the solar controller believes it is full and goes to float. For me, it is all working the same as when I had lead acid batteries. My lithium seems to be operating just fine using a conventional MPPT, currently set at 14v bulk/absorb and 13.2 float, but I am going to bump both of those values down to 13.8 and 13.0 and see how that works. I too have multiple charge sources, the auto, the converter and the solar. All I can say is it just works. The voltage is what will cause the BMS to cut the battery out of the system, my auto goes 14.2 max, the converter goes 13.8 and the MPPT goes only to what I set it to. I think my BMS cuts out at 14.6 volts.

 

[VanKitten]

Again... in my system I do not think the mppt has any way to "see" what the charge on the batteries is.

It is only connected to the power busbar, then thru the contactor.
Then in the ground side..again to the common ground busbar, then thru the shunt to the battery monitor

How does a more conventional set up "see" the battery charge?

I would set the mppt to shutdown at a lower point than the battery monitor...just do not understand how it ever has any knowledge on the battery state.

Also, remember there are other chargers in this system too...which the mppt is isolated from as well.

 

[tripper]

I do not know myself, I just know it does, I think it just gets to the point where it senses the battery is no longer accepting amperage and switches to float. My MPPT is just a normal one but it has been working just fine. I believe its the same for lead acid. The 120 converter and the 12v from truck both run concurrently with the MPPT controller. In fact I have 3 MPPT controllers all running at the same time, no problems at all with all of them hooked up at the same time. I do plan on splitting the system soon, I want to run the standard RV 12v stuff off of my 12v Trojan battery and the 320 watts solar - these will also stay connected to the 12v auto and converter system, and run only the 12v compressor fridge and the inverter from the one MPPT controller and 930 watt panels with the lithium. This will protect the lithium from the car alternator going crazy and overvoltage the system or the RV converter doing the same. Also will allow me to use the basic system in extreme cold and not worry about screwing up the lithium in a deep freeze situation as I can turn off the output of the MPPT controller. Plus redundancy in case one system goes down. I would myself be interested in a simple to setup relay that would cut the solar panel input before the BMS shuts down as an extra precaution.

 

[John61CT]

RK. I have no clue why you think it *won't* work?

What is it precisely you think you mean by "battery state"?

Voltage is a reasonable proxy for % State of Charge at the top of the curve to end a charging cycle.

Every charging regulator can see voltage.

When *you* measure battery terminal voltage while charging is at Absorb, how different is it from your reading at the SC output?

 

[VanKitten]

john...I am trying to figure out how the mppt could know the battery State

Since it has no connection direct to either the shunt or the battery, so how could it know the charge being added to the battery by other charging devices?

I am reaching the conclusion I must manually cut off the solar array whenever I have another charge source active.

 

[John61CT]

Again by "battery state" you mean SoC state of charge correct?

Take a battery charger of any type. Connect it with properly crimped wiring to a buss, then to a solenoid (actually relay) and from there to the battery. Large and short enough wire that the voltage drop is minimal.

As long as the relay is closed (connected) while charging, the voltage of the battery is the same as the voltage measured at every point in between and at the charge source. "Direct" has nothing to do with it, the connections are designed to pass the current at that voltage until the circuit is opened (disconnected).

That is how the controller "knows" the battery voltage.

When that voltage reaches 13.8, or 13.9, whatever setpoint you have defined, the battery is close enough to Full for charging to stop, probably 99.8% or so.

Technically to be precise (like when calibrating a shunt-based SoC meter) you would measure tailing amps declining at that Absorb voltage setpoint, and true 100.0% is the point where current falls to .02C, or 2A per 100AH.

But that last bit is irrelevant for day to day use.

Some people say it is a good idea to go to 14.1 or 14.2 once in a while to prevent a memory effect. Doing that is also part of many top-balncing protocols, but these last are even less relevant to the basic topic at hand.

Basic point is your MPPT is perfectly capable of charging any Lithium bank without any help from any BMS.

And having multiple charge sources concurrently active is no problem.

Just make sure they are all set to stop soon after reaching your desired setpoint. If one insists on a minimum Absorb time longer than say 5-10 minutes, then just set the voltage a point or two lower on that one.

 

[VanKitten]

Wouldn't the voltage on the cable from the mppt be lower between the mppt and the power bus bar than the total voltage from the busbar to the battery if there is a second charging source? Is there feed back from the busbar TO the mppt?

 

[John61CT]

It is all (or should be) one circuit, all the same V and A at any point.

Multiple concurrent active sources mean a single "negotiated average" current the combined result of each V/A , plus the battery's state of charge and resistance.

When one source drops V to Float earlier, it in effect may as well not be there.