Tesla 24v batteries not charging with solar after full discharge

Q&A and troubleshooting
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1

Hi,

I currently residing in the PNW where solar is low during the winter months. Last night after running my heater and fridge my 2 tesla battery modules (24v system) were fully discharged (this is the first time they have ever been this low) . This morning I woke up cold with my fridge and heater turned off.

I can see the float LED blinking on my mppt charge controller which should only be blinking at a full charge so i knew something was off from that. Looking at my victron connect app i see it displaying a full voltage of 24.4v but my battery monitor shows the voltage at 19.12v

My victron battery protect also was displaying an error code of e.1 which from what i looked up says there is a short circuit and im not sure if thats true and I think that may be whats causing my mppt to not charge even though there is actually sun today.

I am completely at a lost for what to do. Its cold and the lack of energy is causing a lot of stress.

Things i’ve tried:

  1. ive disconnected the mppt activating the the 60 amp breaker (Amazon.com: RED WOLF 60Amp Circuit Breaker for Boat Trolling Motor Marine ATV Trailer Vehicles Stereo Audio Electronic Battery Solar System Inline Fuse with Manual Reset Switch Waterproof DC 12V-48V : Automotive) and left it off for 15 mins hoping that would be a good reset. That did nothing it just went back to float mode and is not taking the correct reading from the battery.

ive completely removed all the wiring from the battery protect and replugged them in after 15 mins and nothing happened either.

I’ve ordered alliagator clips and am waiting on them to come in tomorrow so i can try to reset the battery protect hoping that will fix the issue but im not sure if that will do it either.

I am so lost.

Please help.

2

This is a 24V pack built from a recycled Tesla battery pack? What sort of BMS was used? Does it show pack/cell status? How does it handle low voltage events? Does it need resetting?

FYI: Charger switching to Float doesn’t guarantee the battery is fully charged, it only means the charger thinks a bulk/absorb cycle has been completed.
Many chargers switch to float as a safety precaution should they encounter a battery condition they don’t know how to handle.

Speaking of safety (and not the subject of your question), Tesla uses several different battery chemistries in their various traction battery packs. Two, Nickel Cobalt Aluminum and Nickel Colbalt Magnesium, are very high energy density, but prone to thermal runaway and fires that cannot be extinguished. Lithium Ferro Phospate is used in some Tesla packs, and almost all consumer batteries due to its temperature stability and inherent safety.
If the packs are NMA or NMC, it would be well to have a thorough understanding of their safe operational procedures, and what the BMS does in terms of thermal management as well as other safety features.

3

yes this is a 24v battery module from a recycled tesla. It is currently connected to a victron bms 712 (not sure the exact model). The victron bms does not seem to show pack/cell status. I’m not sure as this is the first time that the battery has ever gotten this low. If it does need to be reset i’m not sure how to do that either.

yes, i understand that a float does not mean its fully charged, thats why I was suspicious that something was off, how could it be suddenly on float and ready 24v when the battery monitor is reading 19v, theres a disconnect there somehow.
That makes sense that the switch happens for unknown conditions thank you for that info.

I’ve used these batteries for the past 5ish years without any issues until now. And i agree, its a very scary thing that could happen if somehting were to go wrong.

4

I never seen a Tesla battery so I can’t help with the battery part, but this may help troubleshoot. Some bits you already know.

A Victron MPPT will go immediately to float if the battery terminals are not connected to anything that “absorbs” the voltage on the wire.
Usually in a battery with a BMS, the positive of the last cell is connected directly to the positive terminal on the outside of the case, but the negative of the first cell is not - its connected to the negative on the outside of the case via the BMS. I this way, the BMS has the opportunity to break that connection and isolate the battery from the rest of the system.
In a BMS where a panic condition exists (too low or too high cell voltage, temperature extreme, short circuit detected, etc) and the BMS isolates the battery (using a solid state relay or turning off FETs) and sits and watches until it can safely reconnect.
For low cell voltage, or low pack voltage, or ultra high or low temperatures, we usually call this battery protection mode. In this mode, there isn’t really a battery present between the positive and negative terminals, so essentially its an open circuit.

Also, this is where “readings” can get misleading. Your BMS might say 19v, but this is probably the true voltage of the cells. In some batteries, there might be an LCD that shows the voltage, but this will often be the voltage at the terminals (which are disconnected from the cells), so it might be influenced by the rest of the system and not the real battery.
A multimeter on the terminals of the battery might be the same - i.e. show 24.4v, but is this telling you the float voltage of the MPPT? Sometimes turning off or disconnecting all breakers is required to really see where voltages are coming from. Also, some BMS leak a little power, so you might see a small voltage coming from the battery, but this is just what the solid state relay is leaking.

When your Victron MPPT sees some sun, it brings up the voltage on the battery terminals to the bulk charging voltage, and if this power isnt consumed by the battery, the voltage keeps climbing, and the Victron jumps quickly into float. As you have already determined, this is your hint that something is not right.

The common BMSes i work with need to see a “charging voltage” before they will reconnect the battery and come out of protection mode - so that’s probably in excess of 24v.

As @RaySun mentioned, once you have considered all the safety issues, your investigation needs to focus on getting some insight into whats happening in the BMS. There may be an RS232 cable to a laptop and some software that will tell you what alarms are on, or a way to acknowledge those alarms or reset the condition.
If you can get to the power supply to the BMS (usually a red wire with a push-together connector in it) you might be able to power cycle the BMS.

Also, draw us a diagram of the battery, mppt, breakers, and loads - diagrams.net is a good free one.