# Looking for a device to limit charge current to 125A (at 13-14V) and let any surplus load come out of battery.

Thread Starter

#### LMF5000

Joined Oct 25, 2017
123
In a nutshell, I am using a Nissan Leaf in place of a backup generator to run the house in case of power cuts. The basic circuit is as follows:

Nissan leaf 12V "starter" battery terminals -> current-limiting device -> external 12V buffer battery -> 2500W inverter -> house

The leaf keeps its "starter" battery topped up by means of an onboard DC-DC converter. By tapping into the 12V bus like this, I can extract energy from the car's enormous traction battery without the difficulties of trying to tap into the 400V circuits. I just use an inexpensive off-the-shelf 12V inverter, and as an added plus it will work with normal cars in a pinch as well.

Problem is, I'm looking for a suitable "current limiting device". I want to limit the current drawn from the leaf to 125A since this is the safe limit of the onboard DC-DC converter. For currents under 125A the device should just pass through the current (ideally with no power conversion at all, for maximum efficiency). Once load hits 125A, it should lower the output voltage to keep current at 125A. Any surplus current will then come from the buffer battery.

The intent is to be able to briefly run high-load devices in the house without overloading the leaf. For instance, the microwave draws 170A. I want the leaf to contribute 125A and the buffer battery contributes the remaining 45A. This is workable because the microwave only runs in bursts of 30 seconds or so. The buffer battery then gets recharged as soon as the microwave switches off.

Currently I have a simple 125A circuit breaker there. Obviously it only does half the job, because when the current limit is exceeded it just trips and disconnects the leaf - it doesn't actually let the leaf supply its max safe power by lowering output voltage.

I've looked at DC-DC buck converters with settable constant current output, but it seems very rare to find ones that will do 125A at around 12-14V. Most top out at just 400W. Perhaps I could buy a couple and parallel their inputs and outputs?

I'm now looking at solar panel controllers - I was thinking maybe connecting the leaf to the "solar" input and the buffer battery to the "battery" terminal, it would limit current to its rating (some go up to 100A or more). Not sure though whether it would have suitable behaviour for this application.

I'm curios to hear your thoughts and suggestions. Thanks in advance!

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#### graybeard

Joined Apr 10, 2012
86
No idea on solving your problem, but I think your math is off a bit. I couldn't run a microwave that large with the 100A service that supplies my entire house...

#### Ian0

Joined Aug 7, 2020
8,935
No idea on solving your problem, but I think your math is off a bit. I couldn't run a microwave that large with the 100A service that supplies my entire house...
170A @ 12V = 2kW. Microwaves that big are available.

#### graybeard

Joined Apr 10, 2012
86
170A @ 12V = 2kW. Microwaves that big are available.
I misunderstood. Thought you were inverting it back to 120V.

#### bassbindevil

Joined Jan 23, 2014
793
It might be simpler to just cook using propane. Or consider getting a propane-powered generator.

#### winniest pooh

Joined Jan 17, 2023
7
Hi,

Have a look at this data sheet of a hot swap controller and E-fuse and in PDF page 18 there is a current limiter circuit for 12 V at 100 A, it could work for your application.

#### Ian0

Joined Aug 7, 2020
8,935
Do you know what the maximum current the Leaf will supply to charge the 12V battery?
If your microwave connects to the external battery with short cables, and the external battery connects to the Leaf internal battery with longer cables, and if the external battery is rather larger than the internal battery (hence lower impedance) then the current will mainly be drawn from the external battery.

Thread Starter

#### LMF5000

Joined Oct 25, 2017
123
No idea on solving your problem, but I think your math is off a bit. I couldn't run a microwave that large with the 100A service that supplies my entire house...
Microwave has a 1000W magnetron. It draws 1700W at the plug because of its internal inefficiency. The inverter is itself only 85% efficient, and the voltage at the terminals drops as well due to the large current (despite using 6AWG cable), so the draw would be 1700/12/0.85 = 166A @ 12V approx.

Thread Starter

#### LMF5000

Joined Oct 25, 2017
123
It might be simpler to just cook using propane. Or consider getting a propane-powered generator.
We have a propane-powered stove. And a portable petrol powered generator. But the stove takes 10 minutes to heat what a microwave will do in 30 seconds, and the generator is inconvenient, noisy, and the exhaust stinks up the running area (hence trying to use the leaf in its place).

Thread Starter

#### LMF5000

Joined Oct 25, 2017
123
Hi,

Have a look at this data sheet of a hot swap controller and E-fuse and in PDF page 18 there is a current limiter circuit for 12 V at 100 A, it could work for your application.
Thanks for trying to address my question, much appreciated. Unfortunately I lack the skills to implement that IC. I'm looking more for an off-the-shelf product I could just plug in between the leaf and the battery.

Thread Starter

#### LMF5000

Joined Oct 25, 2017
123
Do you know what the maximum current the Leaf will supply to charge the 12V battery?
If your microwave connects to the external battery with short cables, and the external battery connects to the Leaf internal battery with longer cables, and if the external battery is rather larger than the internal battery (hence lower impedance) then the current will mainly be drawn from the external battery.
It's rumoured to be able to supply 130A (so around 1500W). I have implemented exactly what you mentioned - I ran 1.5 meters of 6AWG cable from the leaf to the external battery, then connected the external battery to the inverter with 50cm of 4AWG cable. The idea was to get the voltage to drop enough over the long run that at high loads it would be coming from the battery.

I tested the setup with a DC clamp-meter with a 1000W heat gun as external load, and I measured just over 100A in all the cabling - both the one coming from the leaf, and the one going to the inverter. So the current wasn't mainly drawn from the external battery as we suspected. I think the reason is that the leaf's output voltage is 13-14V (since it has to charge its internal battery). The external battery, when fully charged, only supplies a terminal voltage of 12.6V - so the current will always be primarily drawn from the leaf due to its higher voltage. I don't think it droops if overloaded, I think it just shuts down or burns out - and I don't want to test it and find out !

#### Janis59

Joined Aug 21, 2017
1,773
In general case the power loss is small only at digital stabilizers (like SMPS), but at the concrete ocassion I see the on-state will be extremely short for any starter, thus the power loss may be any. Then simplicity and trustability is the first hand choices and then I cant name any more appropriate circuit topology as following: input goes through n-ch mosfet Drain with an appropriate current qualification. There are plethore of mosfets having 125 and more Amperes. The output is the same mosfer Source via the very small but powerful resistor. Probably 125A*0.01 Ohm=1.25 V may be appropriate. From that mosfer Gate is coming out the reciproke Zener TL431 cathode, but anode to GND. The 431 Gate is connected to mosfet Source. The mosfet will need the cooler for 125 A*(0.1...0.5)Ohms=13-60 W. If natural air flow then each Watt demands the 26...40 cm2 area of both side taken together cm2. Reference: http://1.bp.blogspot.com/-G3KRUK7ZC...RcguM0/s1600/TL431_MOSFET_PS_skeleton_sch.png
Reference using the bipolar transistor(but then 125 A looks biggish) https://www.diyaudio.com/community/attachments/tl431-jpg.405657/

#### Janis59

Joined Aug 21, 2017
1,773
RE: Ian0 <<I have implemented exactly what you mentioned - I ran 1.5 meters of 6AWG cable from the leaf to the external battery, then connected the external battery to the inverter with 50cm of 4AWG cable. The idea was to get the voltage to drop enough over the long run that at high loads it would be coming from the battery.>>
Every lovely agricultural seazon the sunday-peasants like me tries to enstart their century old tractor with ten years as dead battery with just cable from light vehicle. Yes, You are right that with standard 6 awg cabling bought at shop one may throw it in bushes or create the rope for pant drying. For electricity this wire have no any application, sorry to tell the bad news. So, then I made a 3 meter long starting cable from 8 mm thing copper wire. The result was much better but still, hmmm, insufficient. Then I made a cable from 12 mm thick welding wire and it serves just brilliant. Original accu is 240 Ah, my car have 95 Ah, and it enstart tractor with the first turn, while with shop-bought I even not hear any difference is it connected to my car or is not.

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#### AnalogKid

Joined Aug 1, 2013
10,774
As I understand it, the Leaf battery is charged up from your mains service during normal times, and used to run the home during a blackout. If so, note that a fully-charged 30 kWh battery will deliver only about 20 kWh after some aging and all of the conversion inefficiencies. That's less than 1 day's worth of refrigeration plus other stuff, at which point the car is dead, and not driveable until many hours after power is restored.

I understand what you are trying to do and why, but if the car is your only mode of transportation, I think it needs to be preserved for that. A 2 kW generator from Harbor Freight is under 50 pounds, eliminates the need for the battery, quiet, efficient (variable speed), \$619, and cannot damage your car.

https://www.harborfreight.com/gener...enerator-with-co-secure-technology-59135.html

Perspective: I have 100% of the parts and skills needed to design and build from scratch what you want to do. If I had your problem, I'd buy the generator. Nothing is more important than mobility, especially during a blackout.

ak

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#### bassbindevil

Joined Jan 23, 2014
793
If your electricity supply is that unreliable, maybe it's better to invest in a bank of LiFePO4 batteries and a few kW of solar panels. Weather conditions aren't likely to get any less extreme in the future.

#### strantor

Joined Oct 3, 2010
6,743
only about 20 kWh after some aging and all of the conversion inefficiencies. That's less than 1 day's worth of refrigeration plus other stuff, at which point the car is dead
I agree with your sentiment about having reliable transportation but 20kWh can go a lot further than that.

https://www.eia.gov/tools/faqs/faq.php?id=97&t=3
In 2021, the average annual electricity consumption for a U.S. residential utility customer was 10,632 kilowatthours (kWh)
10632 / 365 = 29.13kWH per day.
That's with HVAC turned on to whatever level the average person uses.

When you say "refrigeration" do you refer to air conditioning or the freezer keeping meats from spoiling? I assume the latter since most folks with generators don't even run their HVAC system in a blackout.

https://blog.sense.com/how-much-energy-does-your-refrigerator-really-use/

440kWH/yr = 1.2kWH/day

If you turn your AC off and anything you're not using, 20kWH can go a long way; several days I would expect, maybe a week if you're frugal.

But even if you just want to run a window unit to keep one room cool in Texas July, That's going to take about 20kWH all by itself.

#### strantor

Joined Oct 3, 2010
6,743
@LMF5000 I do not know of anything off-the-shelf that does just what you want. Regarding the already proposed (and already tested) idea of limiting current from the leaf using insufficient wiring from leaf to battery, you can dig deeper. I think you are correct in assuming that the intermediate battery won't start contributing until voltage coming from the leaf drops to 12.5V volts or so. But we can make it drop, across that wiring. Let's do some math, but using 100A instead of 125A, just to make sure we don't damage your car:

14V - 12.5V = 1.5V
1.5V / 100A = 0.015Ω

so a 15mΩ resistor between leaf and intermediate battery would cause the voltage supplied by the leaf to drop from 14V to 12.5V by the time it gets to the intermediate battery, if 100A flows.

Instead of a resistor we can use the wire itself. Here is a chart of wire resistance per 1,000ft.
You're using 6AWG which is 0.3951Ω per 1,000ft, or 0.3951mΩ per foot. In order to make 15mΩ we would need:
15/0.3951 = 38ft (11.5m)
You're only using 1.5m. If you used about 8X more wire, you would get the desired effect.

Note that while this strategy is simple, it is not very efficient.
1.5V will be dropped across that wire, with 100A flowing through it. That's
1.5 * 100A = 150W
Put your hand on a 150W heat lamp and that will give you an idea how much heat will need to be dissipated by the wire. Granted 38ft of wire has a lot more surface area across which to dissipate the heat, but still, it's something to consider. Lay that wire out on the ground, snaked, not coiled up. Coiled up and generating that much heat is bad news.

All that is assuming your intermediate battery maintains at least 12.5V. If the load exceeds 100A for longer than it takes to nuke a hot pocket, that intermediate battery will start to drop off in charge, and then more than 100A will flow in the wire and more than 1.5V will be dropped across it, so exponentially more watts of power will be dissipated in the wire. As the wire heats up its resistance will increase so there should be some theoretical "balancing point" but don't bet your car or your house on that.

NOTE: Do not get the bright idea to use smaller wire so that you need less of it. 6AWG is good for 100A and reasonably "safe" to attempt this. Anything smaller is not safe.

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#### prepka

Joined Oct 5, 2020
29
I think what you want to do may be problematic. Microwaves draw full power continuously unless you program 50% power and then it will cycle on at full power and off for half the time. II have never seen a power factor corrected microwave and they typically have a 0.6PF. The peak AC current requirement is much higher than you think Even if the electrical system in the car is rated at 125A, it is probably a peak value and it probably is not rated at that level continuously and/or when the vehicle is stationary. There also may need to be some minimum amount of cooling necessary. The converter has some efficiency losses in the power conversion from the main battery. You may need to leave the car turned on to run the converter further depleting the main battery. Those little buck regulators will drop some voltage over their output FETS which will be full on until current limit then will drop their output voltage which your DC/AC converter won't like. A whole lot of engineers at Nissan spent a lot of time making the power systems in your car meet the needs of the car and nothing else. You seriously risk damaging your car severely and hurting yourself and/or someone else in the process I think what you are trying to do may be quite clever but is really impractical. I would recommend getting yourself a nice portable generator instead to power your microwave.

#### AnalogKid

Joined Aug 1, 2013
10,774
Microwaves draw full power continuously unless you program 50% power and then it will cycle on at full power and off for half the time.
Old ones, yes. Newer, better ones, no. From Wikipedia:

An inverter type, however, can sustain lower temperatures for a lengthy duration without having to switch itself off and on repeatedly. Apart from offering superior cooking ability, these microwaves are generally more energy-efficient.

ak

Thread Starter

#### LMF5000

Joined Oct 25, 2017
123
I don't intend to rely on reducing the power of the microwave. Mine is a non-inverter microwave so I need to be able to supply it its full power, which is 1700W AC.

So, to reiterate, I'm looking for a way to supply 170A at 12V DC where 100 to 120A come from the leaf's starter battery, and the remainder come from my buffer battery.

I have the inverter connected to the buffer battery. I want a device to place between the leaf and the buffer battery that will conduct up to 100A, and then start dropping voltage past that. Pretty much a constant-current DC supply.