I'm getting ready for winter in Ukraine, and want to extend the autonomous time of my existing setup consisting of UPS with external 100 Ah lead battery. I need at least 20 hours battery operation for a constant load of about 600 watts (12 kWh).
By connecting under-seat 12 V DC outlets on my 2018 model 3 to the aforementioned battery. I'm going to use 7 meters of 10 mm² copper wire.


simulate this circuit – Schematic created using CircuitLab

Do I need anything in place of [???]? Tesla's DC-DC converter would output 200 Amps, and trip if above.
Do I risk overcharging lead-acid battery with such a setup?
Is it possible that charging current gets over 200 A?

  • \$\begingroup\$ Yes, you need to match the voltages of the batteries you intend to parallel or you will have a big spark/inrush current. \$\endgroup\$
    – winny
    Commented Aug 10, 2023 at 11:05
  • \$\begingroup\$ I'm going to use 7 meters of 10mm² copper wire, at 200Amps it would drop almost 5V. Would it be enough as the current limiter? \$\endgroup\$
    – Gleb
    Commented Aug 10, 2023 at 11:33
  • \$\begingroup\$ hell no, best case the tesla goes in safety mode, worst case you set something on fire. You need a full charge/discharge management to avoid issues, and you need it to be connected to your UPS \$\endgroup\$
    – Sclrx
    Commented Aug 10, 2023 at 11:37
  • \$\begingroup\$ "2018 model 3" of what kind of device? \$\endgroup\$
    – jonathanjo
    Commented Aug 10, 2023 at 11:41
  • \$\begingroup\$ Assuming Tesla (electric automobile) model 3, 2018. \$\endgroup\$
    – rdtsc
    Commented Aug 10, 2023 at 11:45

3 Answers 3


My answer to this question is based on the following assumptions:

  1. You have an existing UPS, with an internal lead-acid battery rated 12V 100AH and you have physical access to the terminals of this battery.
  2. The UPS in charge mode can supply 10A of charging current in total to the battery port.
  3. The UPS in discharge mode draws about 800 Watts of power from the battery port. Assuming 90% efficiency, this means a current draw of about 75A from the battery port (= 800W / 0.9 / 12V).
  4. You have a Tesla Model 3 built in 2018, and the "under-seat 12V DC outlet" you referred to is the cigarette lighter outlet (also called the "Low Voltage Power Socket", refer link 1 below) located in either the boot ("trunk") or in the center console’s rear compartment. This outlet is connected to an internal 12V lead-acid battery, and you only have access to this battery via this outlet - you do not have direct physical access to the terminals of this battery.
  5. You have 7 meters of cable sized 10sqmm (cross-sectional area) available. You plan to directly connect the two batteries in parallel using this cable.

If any of these assumptions are not correct, then my comments below are not valid.

My comments are as follows:-
From assumptions 1 & 3:
If AC power fails and the UPS is in discharge mode, then at 800Watts of load the current the UPS will draw from its battery port will be about 75A Assuming that the Tesla battery and the internal battery share this current perfectly, that means the Tesla battery will be supplying about 37A of current.

From assumption 4:
The power available from the cigarette lighter is limited to just "accessories requiring up to 12A continuous draw (16A peak)." If the load exceeds 12A, then it is not clear what will happen. Best case scenario: the Tesla cigarette outlet has overload protection that will trip and safely disconnect the Tesla internal battery from the load. Worst case scenario: the Tesla cigarette outlet does not have any overload protection, in which case the internal wires between the Telsa 12V battery and the cigarette outlet will overheat, which could cause a fire, or if the insulation melts it may cause a short-circuit which could cause an explosion.

From the above, it would not be wise to do this.

The best-case scenario is this: you go to all this effort to connect the UPS battery port to the Tesla cigarette lighter outlet, and get very little reward, because the Tesla internal protection operates to disconnect the Tesla battery from the UPS the moment the AC power fails, leaving you with just the original battery in the UPS to support your load. You have not extended the UPS autonomous time, and you have wasted your time and your cable. You may have also voided your warranty, and now you may have a cigarette outlet that no longer provides power. To re-set the internal overload protection may require a visit to the Tesla workshop. You can be sure the vehicle has recorded this event - the over-current, and the operation of the protective device, so if this has damaged any internal electronics, then you will not be able to plead ignorance - the car knows that you have overloaded the cigarette lighter outlet, and it will tell Tesla this, so you may have to pay for a very expensive service.

Worst-case scenario: you burn down your Tesla Model 3, which starts a bigger fire that burns down your house which kills yourself and your family.

I have not even examined the situation when the two batteries are connected together while the UPS is in either "float mode" or "charging mode", which have their own set of problems to deal with.

Even if assumption 5 is not correct (about the parallel connection), and you somehow manage to connect some "ideal device" between the two batteries that ensures the Tesla battery is charged and discharged perfectly, then you are limited to drawing just 12A of current from the Tesla battery. For an 800W load the UPS will draw total battery current of about 75A, of which only 12A is supplied by the Tesla and the rest (63A) is supplied from the normal UPS battery - so the UPS autonomous time will only be extended by a small degree, perhaps only 15 to 20% at most. Is it worth the effort and the risk for such a small gain?

Tesla warns you about abusing the cigarette outlet, here is a screenshot of the website at link 1.

enter image description here

Link 1: Tesla manual regarding "Low Voltage Power Socket":

UPDATE 2023-09-04

The OP (@Gleb) has now disclosed:

  1. The make and model of the UPS.
  2. The fact that the 12V 100Ah battery is external to the UPS.
  3. Voiding Telsa warrantee is not a concern.
  4. The UPS must operate on battery for at least 20 hours continuously, supplying a 600W load, which means 12kW-hour on battery power.

I have also added one more assumption:

Assumption #6. The OP has limited access to materials and components.

Given all of the above considerations, I would like to present for discussion this very simple proposal, refer schematic below, which is your original circuit which I have modified.

enter image description here

The most important point I would make here is that you do not want the Telsa DC-DC power converter, which can deliver 14.7V at up to 200A, to charge the existing 12V 100Ah battery in an uncontrolled way, since 200A can easily destroy the battery and possibly cause a fire/explosion. Yet, we want to use the capacity of the Telsa DC power as much as possible since that is being powered from a much larger battery (the battery in the car). Also, we do not want the battery charger within the UPS to interfere with the Tesla power supply, and vice-versa.

This can be achieved by using two diodes, and one resistor, as shown in the schematic. D1 and D2 must be rated to handle the full load current of the UPS battery port during discharge, but the reverse voltage is quite low (less than 15V max) so I would suggest selecting a low-voltage Schottky diode, such as these:



For both of these diodes the power dissipation during battery operation will be quite high, about 40 to 65 Watts, so be sure to mount them on a good heatsink.

The charging current into the 12V 100Ah battery, regardless is that comes from the UPS or the Tesla, is limited by R1. Select this to be about 10 ohms. In case of a deep discharge of the 12V battery, say, it is discharged down to 8V, then the maximum recharge current will be less than 1A, which will be safe, but will be slow to recharge the 12V battery. Select R1 to have sufficient power rating so that it can handle the worst-case power dissipation: power should be about 5W maximum, but use a resistor rated for 10W or 20W.

You can speed up recharge of the 12V battery by making R1 smaller gradually as the 12V battery re-charges - but you must wait until the charging current has dropped to say, 0.5A, before trying to change R1.

You may be asking: "Why not just connect the Tesla power and leave out the 12V 100 Ah battery?" Well, the answer is that when the AC power fails, the the UPS demands a very rapid input of power from the battery port, and the Tesla 14.7 200A power supply may not respond quickly enough, causing the UPS to shut down and its loads will lose power. This is what the 12V 100Ah battery will do: it will provide very fast power to the UPS and give the Tesla battery-powered DC power supply time to catch up.

The recharge time of the 12V 100Ah battery will be slow - but only if it has discharged. Hopefully, with the Tesla power supply present, the 12V battery will not have discharged too much.

You could design a simple circuit that can manage & automate the re-charge of the 12V battery, but that would just take more time - and I suspect you want a fast and simple solution.

Also, be advised that 10sqmm cable will carry about 50A safely when touching a surface. Be aware that it could get hot, and in that case, the insulation may become weaker and its shape may deform. In a cold temperature environment of less than 15C it should be ok to carry, say, 80A, depending on (a) the type of insulation, (b) proximity to other devices that generate heat, and (c) how the air flows around the cable.

UPDATE 2023-09-05

The OP (@Gleb) has now disclosed (via the comments under this post):

  1. A preference for the Tesla power supply to be connected to the battery for "long periods", where the duration of "long periods" is sufficient to cause accelerated aging of the 12V 100Ah AGM battery if the recharge current & float voltage are not managed.

  2. A preference to use a simple battery charge control with a simple relay contact output, refer link provided by OP:-


My response to these new disclosures is as follows:
The original circuit presented in this answer is presented below with some changes in blue:

enter image description here

The changes are as follows:
A current path has been added across D1 and R1, comprising a relay contact marked K1 connected in series with R2. This relay contact is either part of, or activated by, the proposed simple battery charge control device mentioned in disclosure #2 above.

Selection of R1: This resistor provides a permanent path for battery charging and discharge current from either the Tesla power supply, or the UPS, depending on which of these provides the higher voltage at R1. This current must be limited to being below any current that may cause damage to the battery. I suggest it is sized such that, when Telsa power supply is at maximum Vout, and battery is at minimum float voltage with temperature compensation applied, the current through R1 is limited to below 1mA. Therefore, a typical value for R1 may be in the range of 1k ohm to 10k ohm. Power rating of R1 should be chosen for worst-case current flow in this resistor, which will occur then Telsa Vout is a maximum (or UPS is at maximum, whichever is higher) and battery voltage is at minimum (say, after a deep discharge). If this deep discharge cottage is not known, suggest using a value of 0V for minimum battery voltage (in case of battery disconnected and a short circuit applied across the battery wires).

Selection of R2: This resistor only allows current to flow to or from the battery when relay K1 is closed. Assuming the battery charge controller is working correctly, then it will only permit this current to flow when it is safe for the battery to accept this current. Therefore, R2 can be selected based on a suitable charging current for the battery, which can be from 1A to 10A, and the maximum voltage applied by either UPS or Telsa at the connection to K1, and the minimum voltage of the battery.

A higher current will allow the battery to charge at a faster rate, and have a shorter time to be fully charged.

Assuming the Telsa Vout = 14.7V, and the battery has had a deep discharge event, the voltage applied across R2 may be as high as:
VR2max = 14.7 - Vd2 - 8.0V,
where Vd2 is the forward Vf of diode D2, which will be about 0.3V at small current. This gives:-
VR2max = 6.4V.
To limit the current to 10A, the resistor must be: R2max = 6.4 / 10 = 0.64 ohm.

In reality, the battery voltage will rise very rapidly when charging starts, so the 8.0V may rise to, say, 10V, within a few seconds of 10A charging current being applied. I would suggest starting off with a value of, say 1 ohm, and doing a trial with the battery being fully discharged by the UPS, and observing the recharge current. Then adjust R2 as required.

Of course, R2 must be selected to handle the power rating required, at 10A a 1 ohm resistor will dissipate 100W. Here is a typical part that may be suitable, note that it must be mounted on a heatsink:


NOTE: If the AC power fails while K1 is closed, then some current will flow out of the battery via R2 and K1, in addition to the current in D1. Since the maximum voltage across R2 in this case is determined by D1, this current should be limited to about 0.8A (0.8V / 1 ohm).

  • \$\begingroup\$ Assumption 4 is incorrect. Tesla's dc-dc converter is good for 200A, see marc.merlins.org/perso/cars/… \$\endgroup\$
    – Gleb
    Commented Sep 3, 2023 at 9:59
  • 1
    \$\begingroup\$ If you are referring to the DC-DC converter that takes the 400VDC from the main battery, and converts it to 12V (or 14V?) with a capacity of 200A, then the comment I make is this: why go to all that trouble and mess with your car with all the risk that entails, when you could simply get a couple of GLA batteries from a car yard and double or triple your autonomous time permanently with much less risk and certainly much less effort. The risk-to-reward ratio for your proposal is not good. Cheers. \$\endgroup\$ Commented Sep 3, 2023 at 10:23
  • \$\begingroup\$ I have to get ready to 20-hours blackout, based on Russian missiles experience of last winter. 12kWh battery pack would cost way beyond any contraption I need to get this amount of energy from the car I already have. Tesla does not operate in Ukraine, so warranty is not an issue \$\endgroup\$
    – Gleb
    Commented Sep 3, 2023 at 21:46
  • 1
    \$\begingroup\$ I now understand your situation a little better. Perhaps you should include this important information in your post: you need 20 hours battery operation for a constant load of about 600 watts (12kWh / 20 hours = 600 watts). The following data woudl also help: The make and model of the UPS, & the make and model of the battery in the UPS. Measurements of (a) UPS battery voltage during float (not discharging) and during discharge into your load. (b) Tesla DC-DC output voltage at the points where you intend to connect your cables with 3 loads: no load, 5A, and 75A. Best of luck to you! \$\endgroup\$ Commented Sep 3, 2023 at 22:00
  • \$\begingroup\$ PS: I can help with this, I have over 40 years of experience in electrical & power electronics, with 15 of those years directly related to batteries & chargers. The datasheets and product manuals may say one thing and give good guidance, but measurements are where the real wisdom is to be found. However, most of my experience has been in high-temperature environments (Australia), rather than the cold of your Eastern European winters. I wish you luck and strength. Cheers. \$\endgroup\$ Commented Sep 3, 2023 at 22:06

14.7 V "at room temperature" wouldn't help the stationary battery's life expectancy.
I'd be tempted to add a diode (for simplicity) from Tesla to stationary. For 800 VA inverter output (640 W@PF 0.8 to 800 W), a current of about 80 A should be enough, for that current it shouldn't cost an arm and a leg.

If you can't guarantee a voltage difference low enough (have to deal with a deeply discharged lead-acid battery), some form of protection from overcurrent seems advisable so neither diode nor battery take damage.

Conventional "full cycle" charging of a 100 Ah lead-acid "deep cycle"/"solar" accumulator would be about 25 h@5 A to 13 h@10 A (as shown on the left); much more current would be hard on the accumulator - and probably impossible to reach at 14.7 V even at 30° C.

A 600 W load (exceeding max constant power for the UPS hyperlinked in the question: 500 W according to manual) at η .9 would mean 667 W inverter input, about 55 A@12 V. (I get a voltage drop of about .67 V for 7 m copper 10 mm² - double for 2×7 m, add contact resistance/drop: at 55 A, there seems to be no need for additional protection of the stationary battery from getting overcharged, leaving overcurrent, especially when connected in a deeply discharged state.) Most of this current/power/energy would have to come from the Tesla's.
For varying loads "up to 800 W", most of the time not exceeding 99 W (10 h for a deep cycle of a 100 Ah accumulator) and "the Tesla connection" mainly for cycle depth reduction, I suggested just plugging a diode.
With about 90% coming from the Tesla, Edin Fifić's suggestion to feed the inverter directly from the Tesla low voltage looks perfect if the inverter is specified for the voltage (14 V, see above).

  • \$\begingroup\$ What's low for voltage difference? I can't find it in Inverter's specs, but can't imagine its cut-off battery voltage too low. \$\endgroup\$
    – Gleb
    Commented Aug 30, 2023 at 15:34
  • \$\begingroup\$ seems you missed to include the picture "shown on the left" \$\endgroup\$
    – Gleb
    Commented Aug 31, 2023 at 10:36
  • \$\begingroup\$ I'm referring to the "UPS" to the left of the diagram in your question. \$\endgroup\$
    – greybeard
    Commented Aug 31, 2023 at 16:53
  • \$\begingroup\$ @Gleb A better question is: what is the maximum voltage your UPS can take from the battery? If I'm correct, the Tesla battery output goes up to 16.6V or is it regulated to 14.7V? \$\endgroup\$ Commented Sep 1, 2023 at 9:15
  • \$\begingroup\$ @EdinFifić, 16 is on newer models, my particular make of 2018 operates on 14.7 \$\endgroup\$
    – Gleb
    Commented Sep 2, 2023 at 10:02

Your problems stem from these facts:

  1. The UPS needs about 80A at full power, which shortens lead-acid battery's life. Typically no more than 10-20% of a battery's Ah rating is recommended for a load, in this case 10-20A at the most for extended load times.

  2. The lead-acid battery needs a slower charge (about 10% of its Ah rating, or about 10A at most for at least 10 hours if completely empty - which is also not recommended), and this slow charge would be impossible with 200A from Tesla if they were simply placed in parallel without any regulation.

  3. The lower voltage lead-acid battery stands in between its charger/UPS and the higher voltage Tesla battery, while the more powerful Tesla battery should be in the middle because it is a path of higher voltage AND current, as well as capacity, so the lower current from the lead-acid wouldn't damage it.

First, to answer your questions:

  1. Yes, you DO need at least one additional thing in place.
  2. Yes, you DO run a risk of overcharging the lead-acid battery, and significantly shortening its life.
  3. Yes, it is possible for current from Tesla's battery to go over 200A.

Here is how I would do it:

  1. Disconnect the lead-acid battery from its load (input to UPS inverter), but leave it connected to the UPS charger.
  2. Connect the Tesla battery directly to the UPS inverter, but place an 80-100A (at the most) fuse if there isn't one in place already.
  3. Use a step-up or boost converter to raise the voltage from the lead-acid battery's 12-12.8V to the Tesla battery's ~14.7V:
    a) The boost converter will naturally block the current from Tesla to lead-acid, and only allow the flow in the opposite direction when it is operational. Even so, you should place a 10-15A fuse between them.
    b) You should use a boost converter with a synchronized rectifier to increase efficiency and it should have both a CV (constant voltage) and CC (constant current) feature, allowing the setting of both.
    c) You should set the CV to slightly below 14.7V, like maybe 14.5 or 14.6V, so that it doesn't run and waste lead-acid battery unless Tesla battery is loaded and its voltage going down.
    d) You should set the CC to no more than 9A so that the load on lead-acid is no more than 10A. The amount of time you want to run the UPS during power outage should determine the amount of current: if you need it for a short time (up to 10 hours), you could set the CC to about 8-9A. If you need it for a longer time, you set CC to a lower value, like half the value for twice the run time, etc. This would keep your lead-acid battery lasting longer and prevent its early depletion.

Here is a rough diagram below:


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ @greybeard I'm sorry, my sentence is a bit confusing. I meant the Tesla's battery would overwhelm the lead-acid and charge it too quickly (if current limit or fuse doesn't kick in) if they are simply placed in parallel. I have edited and clarified it, let me know if it's good now. \$\endgroup\$ Commented Sep 1, 2023 at 15:52
  • \$\begingroup\$ 3. seems to assume the Tesla is "always on" when the inverter is needed. \$\endgroup\$
    – greybeard
    Commented Sep 1, 2023 at 16:09
  • \$\begingroup\$ @greybeard Thank you, I have corrected the error. According to the o.p. and a few discussions on Tesla forums, there is a low voltage DC output of around 12-14.7V providing up to 193 or 200A. \$\endgroup\$ Commented Sep 1, 2023 at 19:25
  • \$\begingroup\$ Point 3a) doesn't make sense, unless you're meaning something unconventional by 'current flow' \$\endgroup\$
    – Gleb
    Commented Sep 2, 2023 at 10:13
  • \$\begingroup\$ @Gleb Since the maximum voltage for lead-acid is 14.4V and you have 14.7V on Tesla, the current would flow from Tesla to battery and it would damage it due to high voltage. A boost converter has a diode which is turned from the lower to the higher voltage, blocking the current from the higher voltage towards lower voltage. I don't know if Tesla's 14.7V is stable, regulated voltage, or if it actually goes lower. Connecting batteries of different chemistries together is a tricky business where you have to pay attention to all the details. I will draw a diagram in my answer. \$\endgroup\$ Commented Sep 3, 2023 at 7:45

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