How to charge a 4 cell module out of the car

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Well-known member
Dec 18, 2015
Saga continues. One of the 4 packs in my drive battery is a laggard according to Hobdrive. Cell 41 is at 3.79V when all the others are at 4.1V so it triggers the turtle for the whole pack before one gets very far. Bought a good 4 cell pack and plan to switch out. I read on this forum that one must charge the replacement pack before putting it in, otherwise it will take a bunch of cycles to get this replacement pack up to speed. I get that. But what kind of charger does one use? I have a Noco Genius that is supposed to be capable of charging a "12V" li ion battery. This is a 16V pack. I also have an RC type charger specifically made to charge lipos that has a balancer but this is going to take forever as it's of limited current capacity. I really would like to cycle this replacement pack to check that it's good before I go to all the trouble of putting it in the car.

Any suggestions?
i like to use an adjustable power supply such as might be used on an electronics work bench or in a laboratory. They usually have current and voltage limits that can be dialed in, and use it as a charger.

Above all the one thing you need is a really good quality volt meter. You want to match the new 4-pack somewhat close to the average of all the cells in the pack.

Some laptops use 18 to 20 V power supplies with 3 to 5 Amp capability. One of these could be used as long as you were to keep a close watch with the voltmeter.

Your 12V charger could be used to charge all 4 cells up to 3.6V/cell, assuming it would shut off at 14.4V. It might go higher just have to measure it under load to see.

If you needed higher than 3.6, then use it to top up 3 of the 4 cells, and use the RC charger to top up the 4th.

Good luck let us know how it goes,
I see kiev beat me to it, more elegantly, as below is what I wrote, FWIW...

Piece of cake as, unlike LiFePO4, our batteries don't sag when the voltage source is removed.

I would also say that it's not necessary to fully charge everything before extracting or reinstalling. You need only to measure the voltage of the other modules, exactly, and then charge your new module to match that voltage, exactly.

Any number of ways you can charge either your new module or its individual cells, the crudest being a 12v battery charger feeding a 12v battery and that feeding your module with a small resistor in series. I would use the 12v battery with the charger as the battery will attenuate any ripple from the 12v charger.

A regulated power supply is a more-elegant solution. It's a good idea to feed the output of a power supply through a power diode just in case the power supply does not have built-in reverse voltage protection, and be sure to measure the pack or cell voltage after the diode.

I would divide the desired module voltage by four, exactly, as that is what your target per cell voltage is. Then charge up this module you bought while monitoring both the total voltage and each cell's voltage to ensure they are all identical. If not, you can manually bring up the individual cells later.

The four-cell module is only 45Ah, so charging time is little different than charging a conventional automotive 12v battery if you have a higher power 12v charger.

A typical 12v automotive charger will bring the voltage up to around 14.4vdc, which is only 3.6v per cell, so after it's done that, you need to continue manually charging either one, two in series, or three in series of the individual cells using this crude method (dangerous, and you must stay with it to ensure you don't overcharge a cell), or else using your RC charger individually, to bring up the final voltage of each cell, exactly.

Looking at the Noco Genius10 datasheet they don't provide voltage specs for their various 12v battery settings. I suspect that when they say "Lithium Ion" they mean LiFePO4 (3.65vpc max) and not 4.1vpc.

I'll repeat: if you are charging less than four cells in series from a 12v battery&charger (with resistor in series) you MUST monitor the cells all the time to ensure you do not overcharge any individual cell, unless you want to go to the trouble of rigging up an overvoltage shutoff circuit.

Does you RC charger have a Lithium Ion setting and not just LiPo and is its voltage adjustable? If it's adjustable then you can safely finish charge each of your four cells, either individually or as a pack with balancing.

I've had terrific success using a couple of PowerLab8 RC chargers for very many years for charging both my LiFePO4 as well as Lithium(NMC) packs and cells.

Once again, your goal is to have the module voltage match that of all the other modules in the pack before you install your new module. After installation into the car, the car will take care of any final balancing.
My RC charger is a "Triton" and it says it charges both Li Ion and Li Po batteries, up to 4 cells (which is nominal 14.8V) 2.5A max charge current. The manual says when set to Li Ion mode, it uses the "constant current/constant voltage" method, ie you set the current and it supplies that up to 4v per cell, then it switches to constant voltage. When the current falls to 100mA, the charger assumes the battery is charged and it shuts down.

I also have a separate Equinox "balancer" that hooks up to the charger in series with the battery. Or this can be used to balance the 4 cells with the charger off. But to balance 4 cells, there are a bunch of wires that need to be connected to the battery in some prescribed manner (RC batteries are prewired to a plug that can only be hooked up one way). I am not sure what the prescribed manner is for the Mitsu battery. If someone can give me a diagram for that, it would be helpful.

I have a thermal detection probe that can be hooked up as well.
The replacement 4 pack showed up today. Each of the 4 cells measures 3.56V. So not dead and no bad cell. I plan to swap out the entire pack, even though only 1 of the cells in my vehicle pack is lagging significantly, and one other is a bit behind.

So now about the charging. I am hesitating to hook up my Noco Genius 3500 even though it says Li-Ion, as it also says 12V Li Ion. I didn't know there was such a thing. Maybe a 3 cell battery? I can't measure what it puts out because when you turn it on and put a voltmeter across it, the voltage varies as it appears to be probing the load to figure out what it is. I don't see how hooking it up could do any harm, if it doesn't put out enough voltage to charge anything?

When I look up the specs of the charger, it seems to put out 14.4V when in Li Ion mode. That's not enough

There is a "repair" mode which puts out 16.5V 1.5A and it then acts as a "DC Power Supply". Someone said put a "small resistor" in series .. how small, and what does this accomplish? (I guess it reduces the voltage when the current is high, and then the voltage rises as the current decreases??) Is a vice grip a good enough resistor?
vh2q said:
I am hesitating to hook up my Noco Genius 3500 even though it says Li-Ion, as it also says 12V Li Ion. I didn't know there was such a thing. Maybe a 3 cell battery? ... it seems to put out 14.4V when in Li Ion mode. That's not enough
I'd say it's designed for 4-cell LFP (Lithium Iron Phosphate, which is a type of lithium ion cell); those cells are 3.2 V nominal and 3.6 V max, so 4S is 12.8 V nominal and 14.4 V max. Not a bad fit for an automotive auxiliary battery.

But as you say, that's not enough for all the 4.2 V max lithium chemistries. (And way too high for 4S of lithium titanate, also a form of lithium ion, which is 2.4 V nominal). Cathodes ain't cathodes.
Hi vh2q, sorry for the delay in responding - I've been diverted as life is 'interesting' around here...

Good that your four cells measure an identical 3.56v. That means that the four-cell module is sitting at 14.24vdc.

Since the four cells are already 'balanced', let's see if we can charge or discharge this new module without disturbing the individual cells.

Step Zero: measure the voltage of a module (any good module), presently inside your i-MiEV, with the same DVM. Be careful, as the voltages inside the i-MiEV pack are LETHAL.

Your objective is to have your new module's voltage match the existing modules' voltage, EXACTLY, before installing the new module into the i-MiEV.

By far the best way of doing this is as kiev suggested: use a power supply that you simply dial in the voltage you need to achieve and the current you wish to charge at. Simply turn it on and let it take its time to get there and you're all done.

Alternatively, if your target voltage is around a14.4v then you can simply use a conventional 12v battery charger to bring up your module's voltage.

Whereabouts are you located? Perhaps one of us can come by and help you out?

Now, a dangerous alternative is to use a couple of 12v batteries in series as the voltage source and feed your new module THROUGH A RESISTOR. A vice-grip is NOT a resistor, as its resistance is probably lower than 0.000001 ohms and terrible things could happen...

The purpose of the resistor in series with your voltage source is to reduce the current flowing from the source into your module. Without a resistor, things burn and smoke happens (ouch, those are scary words right now in California).


Let's say you measure and find that each module inside your existing i-MiEV pack is sitting at, say 15.6vdc. For reference, that means that each cell is sitting at 3.900vdc.

Let's now take two plain old flooded-lead-acid 12v batteries (12.65vdc fully charged) and connect them in series. That give you 25.3vdc as a voltage source which you want to use to charge your pack.

Let's be conservative and say that we want to limit the current flow from your source into your four-cell pack to 2A, for example.

Recall that R = E/I, where R is resistance in ohms, E is voltage across the resistor in volts, and I is current in amps.

The size of the resistor you want to put in series is:

(25.3 - 15.6)/2 = 4.85ohms

Thus a 5ohm resistor will do the job.

Now, this resistor will be dissipating a significant amount of power as heat.

P= I^2 * R, where P is Power in watts

P = (2 * 2) * 5 = 20 watts

It's going to get hot, so I would use, for example, maybe a 5 ohm 100-watt resistor (funny, as that's what sitting on my bench as we speak..)

2A will only put in 2Ah in one hour into your 45Ah pack, so this could take a long time in this example. Depending on your target voltage you may want to speed things up and increase the current flow. Just follow the math above to get your new resistor values for maybe 5A or 10A (I would not go higher than 10A). Maybe somebody can suggest what to use for a resistor substitute?

I've sent you a PM with my contact information and I can email you a diagram of how to set this up. Feel free to call me directly.

What is VERY IMPORTANT is to NOT leave this setup unattended after you have attached the wires and are charging your new module. You must monitor the module voltage and disconnect the setup as soon as your target voltage is reached.

Sorry if I'm being too basic about this with you, but this is a completely manual setup I've described which can destroy your new module if not controlled carefully. Once again, it would be best to use a regulated power supply for charging this module.

I am assuming we are not talking about a tiny electronic resistor here, the heat would burn it up? that's why I was looking for a common object that might serve as a resistor and take the heat ... and the way I measure them my vise grips have a resistance of about 1. 4 ohms. With the added benifit that they will clamp onto my battery terminal.

My charger has a DC supply mode .. I am assuming it has some way of limiting the current.
vh2q said:
I am assuming we are not talking about a tiny electronic resistor here, the heat would burn it up?
Yep, a tiny resistor would vaporize! Here's an example of what I was thinking: Hey, with two of these you could parallel them and get 2.5Ω and double your current.
vh2q said:
that's why I was looking for a common object that might serve as a resistor and take the heat ... and the way I measure them my vise grips have a resistance of about 1. 4 ohms. With the added benifit that they will clamp onto my battery terminal.
Nice idea, but that resistance measurement is very suspect as it's just a chunk of metal and I bet it's not repeatable and dependent on where you touch those vice-grips. Did you zero-out your DVM before taking the reading? What does it read when you short out the probe tips?
vh2q said:
My charger has a DC supply mode .. I am assuming it has some way of limiting the current.
Sorry, can't help you there. What does the nameplate or manual say?

Admire you for tackling the module replacement, but please do be careful. There's a reason manufacturers don't let their service techs open up the battery pack.

You might consider updating your profile so we can see whereabouts in the world you're locatedL
I am in Texas ... car is in Bahamas. No option but to fix it myself.

NOCO Genius 3500 ... "repair mode" supposed to emulate a DC supply at fixed voltage 16V but I a pretty sure it's current limited. Is there any (safe) way to test this mode? In open circuit, you can't put it into this mode. Has to be hooked up to a load. All I have is a multimeter.

Oh and I also have some nichrome wire that i used to make a foam cutter. I don't know the resistance per foot but I do know the thing gets nice and hot on a lionel train dc supply !!
You have the solution! According to this NOCO Genius 3500 User Guide, the "12v Repair Mode" has a fixed 16.5v 1.5A output. That is 4.125 volts per cell. So, you're in business and can ignore the workaround discussion above.

You'll still have to watch it and unplug to ensure you don't overshoot your target voltage.

Please do make sure that your DVM is properly calibrated. The cheap (even free) Harbor Freight DVMs are useful, but some I've tested have been off by as much as 0.25v when reading 12v.

BTW, how are you getting the battery to the car? You might check the regs.

Edit: The name that NOCO Genius 3500 assigned to that mode is interesting as Repair Mode seems have been designed not only for attempting to jolt lead-acid batteries back to life, but also for 4S Lithium Polymer or Lihium Ion ( I guess they wanted to make sure that someone would not confuse names and inadvertently use it for LiFePO4 batteries, which would be disastrous, so there is no use of the word "Lithium" in that Repair Mode's description.
Great! Thanks.

I can put it on a boat or there is an air charter service that says it's AOK to bring on board with me.

Will charge that sucker up to 4V per cell, and when I get to Bahamas, will charge the rest of it up (or discharge) to match (have hobdrive).

Anyone who wants a free vacation and is willing to pay their way to Abaco, is welcome to join me wrenching on this car. Food, beer and accommo is on me. Great bonefishing and offshore as well (charter, i don't have a boat). I think it will be late Oct before they open customs to visitors though.
When charging the 4S module, be careful: when charging this module, be sure to monitor each of the four cells to ensure that they are tracking; i.e., that the voltages of each of the four cells are identical as the overall module voltage goes up.

How do I know this might be a problem? Earlier today I was slowly charging an old 11P16S LiFePO4 module with built-in BMS. After half-a-day of slow charging at one amp with amazing identicity of all sixteen voltages, I increased the current up to four amps to speed things up (still well under the 110A max spec) and it suddenly started to run away on me as the voltages on a few of the cell groups suddenly and dramatically increased. Luckily I was checking this every few minutes and shut it down before any damage was done (I hope). I need to go back and review the specifications as I'm shocked that the BMS didn't compensate. :shock:
Moral of the story: monitor the individual cell voltages carefully as you're charging your module.
Hooked up NOCO in "repair mode" and it's bring up the 4 pack slowly. Took all morning to get from 4.06V to 4.15V. It's delivering 1.5A. So I think that's the ticket. I'll bring it up to 16V, check the balance, and then ship it over to the island. I'll bring the existing pack to 4V per cell and then swap out the bad module. It's a pretty piss poor design when ONE cell can bring the entire pack to its knees.
vh2q said:
Took all morning to get from 4.06V to 4.15V. It's delivering 1.5A. So I think that's the ticket. I'll bring it up to 16V...
Are you switching between volts per cell and total volts? Because if the 4-cell module total was at 4.06 V, then you have at least 3 ruined cells. My guess is three cells at 0V, and they aren't likely to come up in voltage. Even if they did, they're likely ruined, with very little capacity. As already mentioned, watch the individual cell voltages closely, or you could have a fire, even charging at 1.5 A.

It's a pretty piss poor design when ONE cell can bring the entire pack to its knees.
All EV batteries will be like this. It's an unfortunate situation, but failure like this is very rare.
Pack seem to be OK, never went below 3.3V per cell, all cells coming up and equalized even though I am gang charging.

Going to swap this pack out soon. Meanwhile the car has stopped taking a charge at all .. lights on charger cord don't come on at all neither do fans. I am assuming voltage of the bad pack fell below min. Car sat for a year during covid mess.