DIY battery balancing circuit. Ronsimpson

NiCad Battery bank Balance circuit (1.2v 32Ah x 20 cells)

I have 20 x NiCad (flooded cells) of 1,2v 32Ah each, wired in series into a 24v bank. Float voltages per cell is 1.40. 1.42 V/cell.

(I actually have 80 cells connected in 4 x 20 cells each iaw for a total of 128 Ah. charged with a 24v 3KWh Invertor )

Have looked at off the shelf BMS’s but is above my pay grade. (most are designed for other batteries types and I really only need balancing)

I had a look at the attached circuit found it on YT. Can this circuit be modified to balance 1.4v cells even if I need to build 1 each for every cell.

I am aware that the cell voltage of 1.4v is low for most components to function on. which means that 24v from the bank will be needed to power the circuit

will need to use the bank voltages of 24 vdc via n ± LM7805 or something similar circuit to drive the BMS’s for each of the cells.

Then a censing circuit to read the cell voltage 1.40 v adjustable (preferably multiturn pot) and use this to activate the BMS in 1. to balance each cell via a load resistor.

I think both circuits need to be isolated from each other by an OPTO or something.

Any ideas please share. A simple but effective circuit that I can build to balance each cell would be appreciated.

My electronics knowledge is quite limited component wise. I can build and test n circuit thanks to YT and other platforms that I have learned from ect. But the intricacy’s of component values etc. not well versed.

Thank you for your interest and help in this project


Well-Known Member

Battery Balancer I can’t find it now but I saw your circuit from china for very little money. Adjustable over a voltage range. Search or amazon for battery balancer.

Your circuit has a TL431 inside. It can not work below 2.5V or maybe 3V. There are other versions of the part that should work to 1.25V. If you need the circuit modified one of us could take a look at it more. Ron Simpson ps. The battery Balancers I saw on line will not work below 3V so watch out.


New Member

Battery Balancer I can’t find it now but I saw your circuit from china for very little money. Adjustable over a voltage range. Search or amazon for battery balancer.

Your circuit has a TL431 inside. It can not work below 2.5V or maybe 3V. There are other versions of the part that should work to 1.25V. If you need the circuit modified one of us could take a look at it more. Ron Simpson ps. The battery Balancers I saw on line will not work below 3V so watch out.

I noticed that most BMS dont work that low a voltage, I would really appreciate if it can be looked at. If one can power the main circuit from the 24v supply side and the sensor side from the 1.4v from the cel tie self can this be n workable !

What version of the TL431 can go that low? or is there maybe a nother work arround ?

Ron thanks for looking in on a Saturday night evening much appreciated.

that balancer looks nice but min voltage is 1.8v

The Friendly Neighborhood Charger

There’s a whole bunch of ways to charge the cells you’ve just added to your device – a wide variety of charger ICs and other solutions are at your disposal. I’d like to FOCUS on one specific module that I believe it’s important you know more about.

You likely have seen the blue TP4056 boards around – they’re cheap and you’re one Aliexpress order away from owning a bunch, with a dozen boards going for only a few bucks. The TP4056 is a LiIon charger IC able to top up your cells at rate of up to 1 A. Many TP4056 boards have a protection circuit built in, which means that such a board can protect your LiIon cell from the external world, too. This board itself can be treated as a module; for over half a decade now, the PCB footprint has stayed the same, to the point where you can add a TP4056 board footprint onto your own PCBs if you need LiIon charging and protection. I do that a lot – it’s way easier, and even cheaper, than soldering the TP4056 and all its support components. Here’s a KiCad footprint if you’d like to do that too.

This is a linear charger IC – if you want 1 A out, you need 1 A in, and the input-output voltage difference multiplied by current is converted into heat. Thankfully, the TP4056 modules are built to handle high temperatures reasonably well, and you can add a heatsink if you want. Maximum charging current is set by a resistor between ground and one of the pins, default resistor being 1.2 kΩ resulting in 1 A current; for low-capacity cells, you can replace it with a 10 kΩ resistor to set a 130 mA limit, and you can find tables online for intermediate values.

There’s some cool things about the TP4056 IC that most people don’t know about if they’re using the modules as-is. The IC’s CE pin is hardwired to 5 V VIN, but if you lift that pin, you can use it to disable and enable charging with a logic level input from your MCU. You can monitor the charging current by connecting your MCU’s ADC to the PROG pin – the same pin used for the current setting resistor. There’s also a thermistor pin, typically wired to ground, but adaptable for a wide range of thermistors using a resistor divider, whether it’s the thermistor attached to your pouch cell or one you added externally to your 18650 holder.

There’s problems with the TP4056 too – it’s a fairly simple IC. Efficiency isn’t an imperative where wall power is available, but the TP4056 does waste a decent bit of power as heat. A switching charger-based module avoids that, and often also lets you charge at higher currents if ever required. Connecting a cell in reverse kills the chip, and the protection circuit too – this mistake is easy to make, I’ve done that aplenty, and this is why you need spares. If you reverse the cell contacts, throw the board out – don’t charge your cells with a faulty IC.

Also, given the TP4056’s popularity, copies of this IC are manufactured by multiple different chip vendors in China, and I’ve observed that some of these copy ICs break more easily than others, for instance, no longer charging your cells – again, keep spares. The TP4056 also doesn’t provide charging timers like other, more modern ICs do – a subject we touched upon in the comment section of the first article.

All in all, these modules are powerful and fairly universal. It’s even safe to use them to charge 4.3 V cells, as due to the CC/CV operation, the cell simply won’t charge to its full capacity – prolonging your cell’s life as a side effect. When you need to go beyond such modules, there’s a myriad of ICs you can make use of – smaller linear chargers, switching chargers, chargers with built-in powerpath and/or DC-DC regulator features, and a trove of ICs that do LiIon charging as a side effect. The world of LiIon charger ICs is huge and there’s way more to it than the TP4056, but the TP4056 is a wonderful starting point.

The Protection Circuit You Will See Everywhere

Just like with charging ICs, there’s many designs out there, and there’s one you should know about – the DW01 and 8205A combination. It’s so ubiquitous that at least one of your store-bought devices likely contains it, and the TP4056 modules come with this combo too. The DW01 is an IC that monitors the voltage of your cell and the current going to and from it, and the 8205A is two N-FETs in a single package, helping with the actual “connect-disconnect the battery” part. There’s no additional current sensing resistor – instead, the DW01 monitors voltage across the 8205A junction. In other words, the same FETs used to cut the cell from the outside world in case of failure, are used as current sensing resistors. This design is cheap, prevalent, and works wonders.

The DW01 protects from overcurrent, overdischarge and overcharge – the first two happen relatively often in hobby projects, and that last one’s handy if your charger ever goes rogue. If something wrong happens, it interrupts the connection between the cell’s negative terminal and GND of your circuit, in other words, it does low-side switching – for a simple reason, FETs that interrupt GND are cheaper and have lower resistance. We’ve also seen some hacks done with this chip – for instance, we’ve covered research from a hacker who figured out that the DW01 can be used as a soft power switch for your circuit – in a way that doesn’t compromise on safety. You only need to connect a GPIO pin of your MCU to the DW01, preferably through a diode – this comment describes an approach that seems pretty failure-resistant to me.

When you first connect a LiIon cell to the DW018205A combination, sometimes it will enable its output, but sometimes it won’t. For instance, if you have a holder for 18650s and a protection circuit connected to it, it’s a 50/50 chance that your circuit will power up once you insert the battery. The solution is simple – either connect a charger externally, or short-circuit the OUT- and B- with something metal (I often add an external button), but it’s annoying to deal with. Just like TP4056, the DW018205A combo dies if you connect the battery in reverse. Also, the DW01 is internally wired for 2.5 V overdischarge cutoff, which technically isn’t changeable. If you don’t have a separate software-controlled cutoff, the FS312 is a pin-compatible DW01 replacement with 3.0 V overdischarge point, helping you prolong your cell’s life.

You can buy a batch of ready-to-go protection circuit modules, or just use the protection circuit laid out on the TP4056 module PCB. You can also accumulate a decent stock of protection circuits by taking them out of single-cell batteries whenever the cell puffs up or dies – take caution not to puncture the cell while you do it, please.

All The Ways To Get 3.3 V

For a 4.2 V LiIon cell, the useful voltage range is 4.1 V to 3.0 V – a cell at 4.2 V quickly drops to 4.1 V when you draw power from it, and at 3.0 V or lower, the cell’s internal resistance typically rises quickly enough that you will no longer get much useful current out of your cell. If you want to get to 1.8 V or 2.5 V, that is not a problem, and if you want to get to 5 V, you’ll use a boost regulator of some sort. However, most of our chips still run at 3.3 V – let’s see what our options are here.

When it comes to LiIon range to 3.3 V regulation, linear regulators closely trail switching regulators in terms of efficiency, often have lower quiescent (no-load) current if you seek low-power operation, and lower noise if you want to do analog stuff. That said, your regular 1117 won’t do – it’s an old and inefficient design, and the 1117-33 starts grinding its gears at about 4.1 V. Instead, use pin-compatible, low dropout voltage replacements like AP2111, AP2114 and BL9110, or AP2112, MIC5219, MCP1700 and ME6211 if you’re okay with SOT23 stuff. All of these are linear regulators comfortable providing 3.3 V with input down to 3.5 V and sometimes even 3.4 V, if you’d like to power something like an ESP32. It’s hard to deny the simplicity of using a linear regulator – one chip and a few caps is all it takes.

If you want 500 mA to 1000mA or even more current on an ongoing basis, a switching regulator will be your best friend. My personal favourite is PAM2306 – this regulator is used on the Raspberry Pi Zero, it’s very cheap and accessible, and even has two separate output rails. Given its capability to do 100% duty cycle operation, it can extract a lot of juice out of your cells, often desirable for higher-power projects where runtime matters. And hey, if you got Pi Zero with a dead CPU, you won’t go wrong snipping a part of the PCB off and soldering some wires to it. When designing your own board, use datasheet recommendations for inductor parameters if the whole “picking the right inductor” business has you confused.

So, the PAM2306 is the regulator on the Pi Zero, and it’s also LiIon-friendly? Yep, you can power a Pi Zero directly from a LiIon battery, as all the onboard circuitry works down to 3.3 V on the “5 V” pins. I’ve tested it extensively in my own devices, and it even works with the Pi Zero 2 W. Combined with this powerpath and a charger, you have a complete “battery-powered Linux” package, with all the oomph that a Raspberry Pi provides – at cost of only a handful of components. One problem to watch out for is that MicroUSB port VBUS will have battery voltage – in other words, you’re best off filling the MicroUSB ports with hot glue just in case someone plugs a MicroUSB PSU there, and tapping the USB data testpoints for USB connectivity.

SO, What process can achieve battery balance?

Both Battery Management System(BMS) and charging controller can achieve battery balance.

In this article, we will talk about BMS more. The following are types of technologies of BMS.

Types of Battery Balance Technology

Battery balancing is one of the core functions of a BMS. Here are two mainly types of battery balancing: active balancing and passive balancing. The main difference between them is if they will waste battery energy or not.

APPLY FOR high-series, large-capacity power-type lithium battery pack

In other words, burns off excess energy from the higher energy cell through a resistive element until the charge matches the lower energy cell.

APPLY FOR low-series, small-capacity lithium battery pack

Preliminary judgment, active balance is better because it does not waste energy.

Disadvantages of active balance and passive balance

components are required than passive equalization, and the cost is higher

2.The reliability is slightly worse and takes up more space.

3.The energy loss caused during the hot standby period may be more energy consumed thanthe equivalent equilibrium.

1.Waste energy and consume costs.

2.High equilibrium current level. Energy is converted into heat and causes loss, which affects the operation of the battery pack.

3.Unable to increase the cell capacity with a small residual amount

Four Forms of Active Balancing

According to energy flow direction, here are four Forms of Active Balancing— you will be more clear about active balance.

Energy is transmitted between single cells.

Suitable for small-capacity batteries.

Energy is transmitted from the single battery with the highest degree of charging to the entire battery pack.

The simplest and most efficient.

Energy is transmitted from the entire battery pack to the single battery with the lowest degree of charging.

The performance is best when using multiple single batteries and multiple output chargers.

single battery to battery pack or battery pack to single battery can be provided according to demand.

The performance is best when redistributing.

Above, we have know the main technologies about battery balance.

Next, common battery balance methods of both passive and active balance will be introduced.

Resistance Consumption Balance Method-passive balancing method

Through the resistance connected to the battery cell, the energy higher than that of other cells is released to achieve the equilibrium of each cell.

Through connecting resistance and the battery cell, the releasing energy of that battery will be higher. Then battery balance achieves.

4.Multiple monomers can be discharged at the same time.

2.The monomer can be discharged but can not be charged

3.Other battery cells must be based on the lowest cell as the standard to achieve balance

Active cell balance circuits are typically based on capacitors, inductors or transformers, and power electronics interface. These entail:

Advantage–simple: uses a single capacitor

Disadvantage–requires a large number of switches and intelligent control of the switches

Connect multiple capacitors to each battery in order to transfer unequal energy

Advantage–do not require a voltage sensor or closed-loop control.

Advantage– small volume, low cost

Advantage–fast balancing speed, decent cell balancing efficiency

Advantage– fast balancing speed with low magnetic losses.

Advantage –- fast equalizing speed.

Disadvantage–requires an expensive and complex circuit

Flyback/ forward converter – the energy of a high voltage cell is stored in the transformer.

battery, balancing, circuit

Advantage– high reliability.

Advantage– fast balancing speed, high efficiency

Above are simple descriptions of those battery balance methods which are references for your battery design.

battery, balancing, circuit

Before you DIY your battery pack,

You should pay attention to

battery, balancing, circuit

RC LiPo Battery Balancing Plugs/Taps

Okay, so now we know why an RC LiPo battery has to be balanced, the question now is how to do it?

As pictured above, every 2S and higher multi celled RC LiPo battery will have what is called a balance plug; also called a balance tap. This plug allows individual charging or discharging of each cell in the battery pack.

The balance plug will have one extra pin/wire than there are number of cells in the pack. The battery in that photo is a 3S (3 cells), and as you can see it uses a 4 pin/wire balance plug. A 4S pack would have a 5 pin plug, a 5S would have a 6 pin plug, and so on.

Shown below is how these balance wires are connected to the individual cells of a 3S LiPo pack, allowing both charging or discharging of each cell independent of the other/s.

RC LiPo Battery Wiring Schematic

Charging LiPo Batteries With Balancer The Three Primary Ways

Lipo’s can be balanced while charging the pack through the balance plug with a balancing charger.

This is the usual charging method as well on devices that have internal LiPo charging circuitry such as RC radios that support lithium battery packs.

This method uses the charger (or internal charge circuitry) to individually charge each cell and ensure the voltages are the same in each cell as they charge.

Pictured right a dedicated 3 cell charger is charging a 3 cell RC LiPo battery through the balancing plug/tap. The limitation using the balance tap for charging is the maximum charge rate.

Since the gauge of balance plug wiring and the plug itself are small, this method only works on smaller LiPo’s or charge rates not much higher than 2.5 amps maximum.

A good clue if you are pushing too many amps through the balance leads would be a warm/hot balance plug/wiring.

LiPo’s can be balanced with a stand alone balancer such as a Blinky Balancer while the pack is being charged through the main power plug.

Pictrued left is a non-balancing computerized charger charging a 3 cell LiPo pack through the main power plug with the Blinky balancer hooked up to the balance plug.

The Blinky will monitor the voltage of each cell in the pack and apply a small load to discharge any cell that is indicating a charge voltage higher than the other cells in the pack, trying to keep all cells within about 0.02 volts (20 mV) of each other.

The biggest issue with these little external balancers, is they can’t produce enough resistive load to balance out larger packs at higher charge rates.

Basically, don’t use them on anything much larger than a 2000 mAh pack while charging it @ 1C or lower rates. It can also take a longgg. time to balance out the pack because the balancing load is so small.

Finally the best most popular way to balance and charge an RC LiPo battery is by using a good computerized RC charger with built-in balance circuitry.

The Best LiPo Battery Balancing Method

The balance plug is plugged into the computerized charger’s balance port (a balance or para-board could also be used).

The charger then puts a load (through the balance plug) on any cell/s the are drifting past the voltage of the others keeping them all equalized.

The 4 main reasons this LiPo battery balancing method is best:

  • It’s very adaptable for many different capacity and cell count/voltage packs.
  • You can balance charge large packs at high currents as all the charge current is being sent through the large main power plug/connector.
  • Balancing discharge currents are generally higher with good computerized charges which allows faster and more accurate balancing resulting in shorter charging times and heathier LiPo’s.
  • It’s much safer! If the balance drain current for example can’t keep up to the charger’s larger charge current in order to keep all the cells balanced, the computerized charger will automatically throttle back the main charge current.

Most good computerized RC chargers with built in balance circuitry will also automatically select the correct cell count of battery (since they detect the number of cells through the balance plug); or warn you if you have the wrong cell count selected and thus wrong charge voltage selected.

This auto cell detect feature offers one more very useful level of goof-proofness (not sure if that’s a real word, but for me it should be). This feature has saved my butt (and my LiPo packs) too often to count!

LiPo Battery Balancing Current. What is it?

Thought I better touch on balancing current because I do get a fair number of questions about it.

As I mentioned above with the balance current limitation of that little Blinky/similar external balancer; balance current is an important consideration when choosing your battery charger.

RC Charger LiPo Balancing Drain Current Specification

The more balance drain current (passive resistive load) a charger can place on each cell of the battery, the faster it will be able to balance out your LiPo, LiFe, Lion LiHV multi celled battery packs as they charge.

If the balance current is very low, it can actually take longer to balance the pack than it does to charge it!

As I mentioned on my best RC battery charger page, I like to see at least a 1.0 Amp balance drain current when looking at any RC charger I might be considering on the specifications as pictured above with this iCharger charger example (red arrow).

RC LiPo Battery Main Power Connectors

Let’s finish off this LiPo battery balancing topic with the main LiPo power plug/connector because they are also an important part of most LiPo balance charging methods.

I cover RC Battery Connector types on my LiPo Battery Connector Page in detail.

That page includes RC connector types, power ratings, pros/cons of various connectors, and how-to videos if you are unsure of how to properly solder, splice, or crimp RC connectors.

Many RC LiPo batteries and ESC’s actually don’t come with any connector/s (just the two wire ends insulated with heat shrink).

If you purchase a battery/ESC like that, make sure you purchase the correct connector/type and ensure your soldering skills are up to the task. Otherwise, better search for a battery/ESC that comes with the correct connector/plug type already in place.

Speaking of soldering ; with all these LiPo battery main and balance plugs that will need replacing from time to time as they wear out; you’ll soon find out how necessary good soldering skills are once into electric powered RC.

If you’re already a good solderer, great! If not, you better learn. Soldering truly is one of the most important skills to acquire in this hobby.

Again, I cover soldering with helpful videos on the RC connector page. I also cover soldering equipment in detail on my RC soldering for beginners page.

That’s it! Happy Safe LiPo Battery Balancing 🙂

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