Merging a Xantrex SW2024 to a Tesla Lithium battery

[howardwheeler]

I have a Xantrex SW2024 that states in its manual that under a custom battery setting that the bulk charge can be set anywhere from 3v to 32v. It also states that when the voltage rises to the absorption voltage that it will switch to absorption from bulk. Well, unless we are really missing something, it appears neither is the case. Multiple calls to Xantrex have been fruitless. Problem? I have installed a 24 v Tesla in the place of four size 31 FLA batteries. Everything is working great on my solar charging system, and we were able to program it perfectly to the Tesla parameters. Not so the Xantrex in spite of its clear statements in its manuals. We cannot get it to be set below a bulk rate of 27.2 v which will fry the Tesla. We caught it before catastrophe. But I need to occasionally be able to charge from shore power via generator or plug in. But I'd have to sit there watching the charger to make sure the voltage does not rise above 25.2 We have an automatic shutoff solenoid attached to our BMS in case we overlook it sometime, but that shuts everything off. So two questions: Does anyone know how to set the bulk limit to 25.2, or second, does anyone know why it does not shift to absorption when the absorption voltage threshold is reached, which would effectively limit bulk voltage to safe levels even if we couldn't actually set the upper limit to bulk voltage? Does anyone understand Xantrex inverter chargers well enough to give us some clue as to why you cannot harness the bulk voltage to a 25.2 volt limit? The battery works great by the way, and I've shaved 479 lbs of my pin weight by removal of the FLA batteries and generator. I'm happy with the battery but perplexed by Xantrex.

[howardwheeler]

We have apparently made a mistake in trusting the specs Xantrex publishes for their 24 volt units. If we could get it to do what it is, in fact, capable of (you can indeed set proper voltage parameters for absorption so it will work) then we would have no problem. I sure hope we can make this thing work because selling it will bring a big hit. But it has to work right. It has all the hardware capability we need. It's just the way it's programmed. I wish someone knew how to hack into it. We are in any event going to install a cut off contactor on the AC coming into the Xantrex as a safety. I believe MrWizaed is right about splitting the units into two. But I'm financially married to this Xantrex until I get grounds for abandonment.

[MrWizard]

No !
Not parallel
It's inline and has to be switched out of circuit
Completely removed
Remember what happens to most charge controllers if there is applied input voltage but no battery load

Neither device buck or solar controller can have a reverse current flow
Or
A reverse polarity on it's inputs

A buck converter might work inline with a power supply instead of charger/converter
Charger converters are smart and do voltage sensing, which they cannot get with a Bucker inline

[BFL13]

Since the charger is only for when the solar can't do it, the buck converter type controller attached to the charger can be attached in parallel with the inverter/charger with a switch so it only needs to be one way?

[MrWizard]

Imo
Charge controllers and Buck devices are not going to work, they only allow current flow One Way

The $&_:; Xantrex is a combo unit inverter/charger
And must have current flow into the inverter when not charging

If theses were discrete separate devices, a charge controller would be a good choice for regulating converter voltage

A resistor is not going to work either
Because it will be inline the current path when using inverter power, thereby reducing the input voltage for inverter use

The basis of these problems is the Tesla battery is Not A 24v battery, it's a 21v battery that charges with 25v,
Most DIY RV installs are using Victron Multiplus inverter/chargers

Depending on monies available, I would dump the Xantrex and go with separate charger and inverter
And install an mppt controller between charger and Tesla battery, also add the contactor on the charger input 110v , just in case the charge controller failed
Separate units while taking up more space, are usually less expensive per pair than the combo units
And if one fails are cheaper to replace or repair and also less inconvenient

[pianotuna]

A buck type device might work. The problem is at large amperages such a device may be pricey.

[BFL13]

I think an actual resistor is not the correct answer what with V=RI, but there is a buck converter way to do it. You might even be able to use an MPPT controller--those things have a buck converter in them.

You said your own solar works with the Tesla, so can you make your Xantrex the input to that controller?

It might be an issue that the input voltage is rising instead of steady? Or maybe it would be steady at something above 25 in that set-up. Input has to be about 2v higher than the output ISTR.

Somebody here should be able to say if that would work.....

[howardwheeler]

Anyone have ideas of how a resistance load could be put between the charger and the battery to insure it never exceeds 25.2 volts even while the charger is outputting 27.9 volts? Or any other solution. This Tesla battery charges fast! Very nice. Went from 6% to 90% in less than an hour and a half as the Xantrex dumped 50 amps at 24 volts into it.

[BFL13]

MEXICOWANDERER wrote:
Mex uses 12 or 24 volt lamps to limit current not volts.

At least I got it right that they are lamps! 🙂 Sorry, Mex.

[MEXICOWANDERER]

Mex uses 12 or 24 volt lamps to limit current not volts.

[BFL13]

The Victron monitor some Li batt guys have, has a voltage limiter built-in of some sort, not sure of details. It came up in those Li threads a few months ago. Maybe it has the right numbers for the Tesla when in 24v mode.

[howardwheeler]

This is a wonderful example of thinking controlled by the “box.” Until Mr. Wizard threw out the idea of shutting off the AC going into the Xantrex, I was completely focused on breaking the link between the charger and the battery as my “safety valve.” This solves a major problem. By the way, no the BMS is separate from the Tesla battery. The Tesla BMS isn’t suitable for our application. Thank you for a wonderful suggestion. I don’t think I ever would have thought of such a simple solution.

[howardwheeler]

That is a great idea! Instead of cutting the charging circuit, cut off the AC voltage into the Xantrex. The way the Xantrex is set up, the contactor inside the Xantrex is operated by the 24 v system, so right now when overvoltage is sensed it shuts off the charge circuit, which on a Xantrex is also the main cable from the battery back to the inverter. So the inverter shuts down. A pain but a better alternative to the battery frying. I’ve never thought of using a contactor to break the AC into the Xantrex! Excellent. Now if we can just get the Xantrex to work according to spec, we will have a working system with built in guards against catastrophic failure.

[MrWizard]

Tesla cars are Not hybrid
No gas engine no ALTERNATOR
A tesla powerwall would have a built in charger
I've read articles and watched rv YouTube videos of people using re-purposed tesla and Chevy Volt batteries, I don't recall anything about 25.2v charging for a 24v setup

Is the bms over voltage disconnect internal to the battery ?
Why can't that be used to control and external relay between the converter and battery no need to disconnect the whole system
Or
I would use it to control the shore power going to the XAntrex converter via an additional transfer switch, if voltage is too high disconnect the 110vac going to the charger/converter
Leave the DC circuit wiring connected

[BFL13]

Mex uses 120v light bulbs to limit voltage while holding amps IIRC. Too complicated for me.

That "24v" battery that can be recharged at 25.2v is just weird, so normal chargers will have a hard time. What charger is recommended for that battery in any blurb on it? Do you have to steal an alternator from a
Tesla car?

[howardwheeler]

We have considered some sort of resistor setup to keep the voltage down but aren’t real clear on how to do it. You can set the voltage for absorption, but the bulk voltage does not stop at absorption level and switch to absorption. It keeps right on going up. We don’t know how far it will go because we don’t want to fry our battery. But our understanding is that you should limit the charging voltage of the Tesla battery at 25.2.