Charge 18650 3s8p DIY pack
First off, New user. This answer may be posted somewhere else, but at this point I have watched tons of videos, and searched that my head hurts. I can’t seem to post a jpg or png photo. New user restriction? Nevertheless.
How do I charge a 3s8p 18650 cell pack with BMS wired into it? I assembled my pack, wired up the BMS listed below, but am confused as to the best way to charge my pack. I see people charging using laptop chargers, etc. Just supplying voltage to the BMS circuit.I have RC chargers. My most powerful being the Reaktor 300w 20A
Can I use this charger to charge my pack with it’s built in BMS? The charger balances the cells as it charges. Does this Double balancing have any ill effects?
Also, would I need to wire balance leads to get the charger to initiate? Most chargers don’t allow charging multi cell packs without said balance port in use.
I do have many laptop chargers at my disposal, but those are usually low amperage chargers. With the RC charger I could charge at any Amp up to 20a.
I intend on powering an AC plug to plug devices into. I am using a cheap Banggood 40a 3s BMS.
To whomever reads this, thank you! I hope I can get some clarity on the best way to charge 18650 packs, as this is my first. If I have left out any pertinent info, please let me know.
Mods: If I have violated any rules with links, or photo embedding, this is unintended.
OffGridInTheCity
You hook balance leads (as shown below) and run them to a connector. If you have an RC balancer, you can plug the balance (leads) connector into the balance port. You hook the main power to the power leads (12v) of the battery. The main charging power goes in thru the main leads. IF you hook up the balance leads. and the charger has balance capability. it will use the smaller/balance leads to adjust the voltage of each individual (4v) set of parallel cells during the charge process.
If your charger only has 1 balance port. with perhaps 8 pin connector. (instead of several 8pin, 7pin, 6pin. 4pin) then you can typically still use it with just 4 wires. You just have to determine which end is the ‘most negative’ and wire your 4 balance leads to the 8pin connector so the most negative matches what the charger is expecting.
robertjpjr
New member
Thanks for the reply. You don’t address the fact that the battery pack has a BMS soldered into the circuit. What is the best means of charging it, knowing that fact? I understand wiring the pack in order to use balance leads. If indeed they are necessary. If I wire the pack with balance leads, then charge using a RC charger, what happens with the BMS wired into the circuit?
Also, is this the correct location for this thread? I am new to this particular site.
How to make Lithium Ion Battery using 3S BMS and 4S BMS
How to make Lithium Ion Battery using 3S BMS and 4S BMS- In this article, I am going to explain how to use different BMS modules for making different size battery packs. This article is going to be a little longer but after reading this article, you will become an expert and then you will be able to make any size battery using Lithium-Ion Batteries. You can also read my article on how to make a Lead Acid Battery at Home.
Using Lithium-Ion Battery cells you can make any size battery pack of your choice, as you have full control over the voltage and current selection. You can simply connect the lithium-Ion batteries in series to increase the voltage or connect them in parallel to increase the current Or you can connect them in series and parallel at the same time to increase the voltage and current. So you can make a small 3.7v battery pack for powering up your Arduino, ESP32, STM32, ESP8266, etc or you can make a Heavy-duty 14 volts battery pack to power up your entire house, or a heavy-duty 72 volts battery pack to power up your electric Car.
Since each Lithium-Ion Battery cell is only 3.7 volts but comes with different mAh capacities from a few hundred mAh to let’s say 10000 mAh. The Lithium-Ion Batteries I am going to use for making 3S and 4S battery packs are 3.7V and 5000mAh. In most applications, this voltage and current are not enough to power up your devices and then you start thinking about connecting these Lithium-Ion battery cells in series and parallel. At this point, you can use the soldering or spot welding technology for making these series and parallel connections.
Personally, I don’t recommend the soldering technique for this job. What I recommend is Spot welding technology. In my previous article, I made a semi-automatic Spot welding machine and I explained each and every detail including, wiring explanation with the help of a simulation designed in Proteus, Spot welding electrodes selection, and how to weld lithium Ion Battery cells using this DIY Homemade Spot welding machine. Now, with this Automatic spot welding machine, I can start making battery packs for my upcoming electric Bikes, Scooters, Go-Karts, and so on.
So, I have these Lithium Ion Batteries which I salvaged from my dead Hoverboard; Nickel-plated Strips, and this Automatic Spot welding machine. So, what else I need is the BMS module, BMS stands for Battery Management System.
Here, I have these different BMS modules; you can see the sizes are different. Don’t get confused if you see smaller or bigger BMS modules, their use is 100% the same. These smaller and bigger BMS modules are the 3S BMS modules, the only difference is that the smaller one is for low current applications and this bigger BMS module can deal with high currents. While this other BMS module is 4S.
3S means, 3 batteries in series, and 4S mean 4 batteries in series, and if you are making a battery pack for your electric bike or electric scooter then you will need 13S BMS, which means you will need to connect 13 lithium-ion batteries in series to get 48 volts. So, before purchasing a BMS module first make sure how much voltage and current you need. So, I am sure, you have fully understood why 3S is written on these two BMS modules and why 4S is written on this other BMS module. These are printed on the backside of the module.
Before, I am going to share with you some other useful information and the test results, first a few words about the sponsor of this article for helping me purchase the required components and tools.
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Here is my 3S Lithium Ion Battery pack made of three Lithium-Ion Battery cells connected in series, each cell has 5000mAH capacity. For this battery pack, I used a 3S BMS module. Since I am using only 3 lithium-ion cells in series and there are no batteries connected in parallel so the mAh will remain the same which is 5000mAh. As you know in series the current remains the same. If you want to increase the current capacity up to 10000 mAh then you will have to connect 3 more batteries but this time in parallel. So the battery pack will become 3S and 2P. You can make 3S and 3P, 3S and 4P, and so on. This will increase the current capacity.
This one is 4S battery pack as you can see I have connected 4 lithium-ion battery cells in series. As I am using 4 cells, that’s why I used a 4S BMS module. This battery pack gives me more voltage than the 3S battery pack, but the current capacity is still 5000mAH as I am not using any cells in parallel.
After, sharing with you, some basic information; now it’s time to explain the BMS modules Pinouts and wiring. Without any further delay let’s get started.
S BMS Module Wiring:
Let’s first start with these 3S BMS modules. Both the modules are designed for the same job that is to protect the Lithium-Ion Batteries from Overcharge and Over-discharge. Both the modules got the same contacts. The same P and P- contacts, the same B and B- contacts, and the same B1 and B2 contacts. The only difference between the two BMS modules is that the smaller BMS module is designed for 10A applications while the bigger BMS module is designed for 25A applications. So, let’s start with this smaller 10A BMS module.
The P and P- contacts are used for the input and output. Through these contacts, you will charge your batteries and to these contacts, you will also connect your output loads, but one at a time. You can either connect the load or you can charge the batteries. When a charger is connected then you cannot connect the loads. So, first, charge the batteries using these P and P- contacts, then remove the charger and connect the load which you want to power up.
The B and B- contacts will be connected with the Battery pack main positive and main ground terminals. The B1 contact will be connected with the 3.7Volts, and B2 contact will be connected with 7.4 volts. Don’t get confused, I know what exactly you are thinking. Let me explain this in a more easy way.
Since we are making a 3S battery pack, which means we need to connect 3 lithium-ion cells in series. Connect the positive side of the first battery with the negative side of the 2 nd battery, now connect the positive side of the 2 nd battery with the negative side of the 3 rd battery. The positive side of the third battery is the main positive of the battery pack and it should be connected with the B contact on the BMS and the negative side of the first battery is the main negative of the battery pack and it should be connected with the B- contact on the BMS. The B1 should be connected with the first cell and B2 should be connected with the 2 nd cell.
No matter if you have got the new or old Lithium-Ion Battery cells, first check the voltages. Select batteries which gives you approximately the same voltages.
All these batteries are in useable condition.
I just love building a battery pack using these Lithium-Ion Battery cells, I can connect them in different styles. For the sake of this article, I am going to go with this configuration, so that you can easily understand the connections.
First I connected all three batteries in series using my homemade spot welding machine. The connection diagram I have already shared above. Anyways, It took me a few minutes to complete the welding. After that, I fixed the 3S BMS module on the battery pack, and completed the wiring as per the connection diagram. I first soldered the Battery pack main voltage and Ground wires which you can confirm using the digital multimeter. Then I looked for the first cell and confirmed the voltage which was around 3.7 volts and I connected it to the B1 contact on the BMS module. Then I looked for the 2 nd battery and again I confirmed the voltage which was around 7.4 volts and I connected it to the B2 contact on the BMS. Finally, I connected the XT-60 connector with the P and P- contacts of the BMS module. After, I was done with the wiring then I checked the short circuit and continuity, and I also checked all the voltages. You can watch video tutorial for the practical demonstration which is given at the end of this article. So, after doing all the testing, then I charged the battery pack for a few minutes and then I used it to power up 775 Motor and 12V dc Light bulb.
S BMS Module Wiring:
Now, look at this 10A, 4S BMS Module, it has got the same contacts as the 3S BMS module. This module also has these voltages printed which makes it easier to use. The only difference between the 3S and 4S BMS modules is that, the 4S BMS module has this one more contact which is labeled as B3. The connection diagram is pretty much the same.
I have already made this 4S battery pack. The P and P- contacts are connected to the XT-60 connector. The B and B- contacts are connected with the battery pack main positive voltage and Gnd. B1 is connected with the positive of Battery1, B2 is connected with the positive of Battery2, and finally the B3 contact is connected with the positive of Battery3. Before charging this battery pack, first I confirmed all the voltages using my digital multimeter. I checked it with different loads and it worked flawlessly.
V LiFePO4 Battery Voltage Chart
Here’s a printable version of the above chart:
And here it is graphed out:
48V batteries are more popular for larger solar systems. They rarely make sense for small-scale projects. Designing a higher voltage solar system allows you to keep amperage low, thereby saving you money on wiring and equipment costs.
48V LiFePO4 batteries are fully charged at 58.4 volts and fully discharged at 40 volts. They are made by connecting 16 3.2V LiFePO4 cells in series.
48V LiFePO4 Battery Charging Parameters
- Charging voltage: 56.8-58.4V
- Float voltage: 54.4V (or disabled)
- Maximum voltage: 58.4V
- Minimum voltage: 40V
- Nominal voltage: 48V or 51.2V
V LiFePO4 Cell Voltage Chart
Here’s a printable version of the above chart:
And here it is graphed out:
Individual LiFePO4 cells have a nominal voltage of 3.2 volts. They are fully charged at 3.65 volts and fully discharged at 2.5 volts.
You can buy individual LiFePO4 battery cells online. They’re best used for making your own lithium batteries. You can wire cells in series and parallel to make LFP batteries with your desired voltage and capacity combinations.
3.2V LiFePO4 Cell Charging Parameters
- Charging voltage: 3.55-3.65V
- Float voltage: 3.4V (or disabled)
- Maximum voltage: 3.65V
- Minimum voltage: 2.5V
- Nominal voltage: 3.2V
Ways to Check LiFePO4 Battery Capacity
Measure Battery Open Circuit Voltage with a Multimeter
Pros: Moderately accurate
Cons: Must disconnect all loads and chargers and let battery rest
A battery’s voltage changes depending on its charge and discharge rate. Plus, LiFePO4 batteries have a relatively flat discharge curve from around 99% to 20% capacity. Because of these factors, it can be hard to estimate their state of charge from voltage alone.
To get an even somewhat accurate estimate of LiFePO4 battery capacity based on voltage, you first need to disconnect any loads and chargers from the battery. (Don’t forget to disconnect your solar panels from your charge controller first!)
Let the battery rest for a little while — I usually wait 15-30 minutes — and then measure its open circuit voltage with a multimeter.
Compare your measurement to the right voltage curve above, or the state of charge chart in your battery manual. Use it to get a rough estimate of your battery’s remaining capacity.
For example, I own the Ampere Time 12V 100Ah LiFePO4 Deep Cycle Battery (Ampere Time has since rebranded to “LiTime”). I wanted to check its capacity after having stored it for a few weeks. I brought it out of storage and measured its voltage with a multimeter. I got 13.23 volts.
I then compared this number to the 12V LiFePO4 state of charge chart above, as well as the one in the battery manual.
Based on the charts, I’d estimate my battery’s state of charge was somewhere around 80%.
I like this method best for estimating the state of charge of an LFP battery I’ve just received or just pulled out of storage. The battery is already at rest and not connected to anything. I find it too inconvenient to disconnect everything once the battery is in use.
DIY lithium battery builders will also measure the voltage of used (and new) battery cells — such as LFP cells and 18650 lithium batteries — to see which are good and which are duds.
Use a Battery Monitor
Pros: Most accurate, convenient
Cons: Good battery monitors are expensive
The best way to track battery capacity is to connect a good battery monitor — such as the Victron SmartShunt or Victron BMV-712. For my recommendations, check out my review of the best battery monitors.
Battery monitors track the amount of amp hours consumed to accurately estimate the state of charge. They also display useful system specs such as battery voltage and current. Some connect via Bluetooth to your phone so you can check your LiFePO4 battery’s capacity in a mobile app.
Use a Solar Charge Controller
Pros: Convenient
Cons: Inaccurate
“My solar charge controller already measures battery voltage. I can just use it to check battery capacity.”
This voltage reading is largely inaccurate. It suffers from all of the problems mentioned above, plus it’s done while the battery is connected to loads and chargers.
(Not to mention that some charge controllers have incorrect voltage readings.)
For example, recall that when I checked my battery’s voltage with a multimeter at the battery terminals, I got a voltage reading of 13.23 volts. That correlates to a roughly 80% state of charge.
But when I connected my battery to an MPPT charge controller, the controller measured 13.0 volts. That correlates to a roughly 30% state of charge — a difference of 50%! Granted, some charge controllers have much more accurate battery voltage readings than others.
After all, voltage drops under load. And a charge controller is a load. If I were to connect a solar panel and start solar charging the battery, its voltage would quickly jump.
Checking battery capacity this way is convenient. But beware that it can be quite inaccurate. I generally use this voltage reading just to make sure my battery isn’t close to being fully discharged.
LiFePO4 Voltage FAQ
What is the voltage of a fully charged 12V LiFePO4 battery?
A fully charged 12V LiFePO4 battery will have a charging voltage of around 14.6 volts and a resting voltage of around 13.6 volts.
What is the charging voltage of a 12V LiFePO4 battery?
The charging voltage for 12V LiFePO4 batteries is 14.2 to 14.6 volts. This works out to a charging voltage of 3.55 to 3.65 volts per cell.
Most often, you’ll see LiFePO4 battery chargers and solar charge controllers use a charging voltage of 14.4 volts for 12V lithium batteries.
What is the minimum voltage of a 12V LiFePO4 battery?
The minimum voltage of many 12V LiFePO4 batteries is around 10 volts. The battery’s BMS should detect when the battery voltage falls to around 10 volts and trigger low-voltage cutoff. (Low-voltage cutoff is also called low-voltage disconnect, which you’ll sometimes see abbreviated LVD.)
Note: Some batteries have higher cutoff voltages, such as 10.6V. So the limit in your battery manual may not be exactly 10V.
LiFePO4 batteries in low-voltage cutoff enter a sleep mode to protect the battery cells from over discharge. LFP batteries in sleep mode can have very low voltage readings, usually less than 5 volts. You may think that the battery is dead, but really it’s just sleeping.
Once a battery enters sleep mode, it needs to be woken up. Refer to your battery manual for instructions on how to do this. If your manual doesn’t have instructions, check out our tutorial on how to wake up a sleeping LiFePO4 battery.
What is the float voltage of a 12V LiFePO4 battery?
LiFePO4 batteries don’t need to be float charged because they don’t leak charge the way lead acid batteries do.
If you can, disable float charging on your charge controller or battery charger. If you can’t, prevent the battery from entering float charge by setting the float voltage to that recommended in the battery manual — usually 13.6 volts ± 0.2 volts.
How much can you discharge a LiFePO4 battery?
Many LiFePO4 batteries can discharge 100% of their rated capacity every time with no ill effects.
However, many manufacturers recommend discharging only 80% to maximize battery life. In fact, some brands state the cycle life of their batteries based on 80% depth of discharge (DoD).
For comparison, lead acid batteries can only discharge 50% of their rated capacity. So a 12V 100Ah LFP battery has as much usable capacity as a 12V 200Ah lead acid battery.
