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New OFFICIAL Bluetooth apps (open Beta)

We are happy to announce that our new official Overkill Solar Mobile apps are now live on the Google Play Store and the Apple App Store!

The apps are completely FREE, do not require an account or login, and don’t require an internet connection.

Install the new app today:

iOS App: Overkill Solar Mobile App

Android App: Overkill Solar Mobile App

Release Notes

30 Nov 2022: The latest update adds “background location” permissions. This was necessary for some (not all) Android devices to discover the Bluetooth modules.

17 Nov 2022: The first release has all the basic functions that allow you to view your BMS status, change parameters, and calibrate.

The Android app requires a bunch of permissions to work correctly. Please allow precise location, media/files, and Bluetooth discovery. All this is necessary for Bluetooth to work on all Android versions. We DO NOT collect any personal information with the app, and it does not track your location in any way.

It also automatically unlocks password protected BMS firmware, and will resolve any issues with lost or incorrect BMS passwords.

Note: the password/PIN function has been completely bypassed by this release. If you are concerned about unauthorized access to your BMS via Bluetooth, you should unplug the Bluetooth module when it’s not in use.

Bug Reports

UPDATE: We think we have fixed the bug that stops some Android devices from finding the Bluetooth modules. Anyone with this issue should update and try again. Your app should say “V1.1.1 -V10” at the top of the HOME tab. If not, uninstall and reload from the play store.

The new app is still in the beta test stage. We have found that some Android devices are unable to find the Bluetooth modules using our new app. The issue is with specific permissions that need to be requested by the app.

If you have a bug to report, please email and describe the bug plus which device and Android/iOS version you are using. Thanks!

Need the old app? Check the Support/Downloads Page.

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Cells Labeled Backwards: This kills the BMS

Please check your cell polarity with a volt meter before connecting the BMS!

This type of cell has been shipped by other suppliers with the labels on backwards.

We have seen an excessive number of BMSs ruined recently because the customer’s cells had the labels on backwards

Most cheap prismatic cells on the market have black plastic on the POSITIVE terminals and beige plastic on the NEGATIVE terminals.

Note: This is not our product. Overkill Solar has never supplied this type of cell.

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About Cell Balancing

Yes, the BMS has a built-in balancing function.

HOWEVER no, it is not capable of doing an initial balance on new cells.

The balancer works by connecting a tiny bleed resistor to the cells with the highest voltage, and the excess energy in those cells turns into waste heat. This is a slow process. The intention is that the BMS can maintain the balance on the cells as they slowly drift over their lifetime.

Bleed Resistors

A batch of new cells needs to be top-balanced before they can be expected to charge properly as a battery pack.

Why top balance?

Why? Because of the nature of the LiFePO4 voltage curve. At the top end of a charge cycle, the cell voltage spikes quickly, and charging must be stopped to prevent damage to the cells. If one cell is at a higher state of charge, (in terms of amp-hours or coulombs), even by a small amount, it will spike while the other cells are still in the “bulk” phase of their charge cycle. On the graph below, the red line is the highest cell, which triggers a “cell overvoltage” alarm before the pink/green cells get to a full charge. The BMS must then disconnect to protect the high cell, and the battery pack will be at a lower voltage than expected. You want all the cells to spike up at the same time, and the only way this can happen is for them to be well balanced.

Imbalance at end of charge

There are several ways to manually balance cells, depending on what equipment you have access to.

Method 1 – cc power supply

The best way in my opinion, is to use a regulated power supply to charge the cells to 3.65 volts each. The cells would be connected in parallel as a single cell and charged together (without the BMS), then re-assembled into the series-connected pack with the BMS.

Will Prowse demonstrates in this video:

Method 2 – manual bleed resistor

Cheapest way: Connect a load to the high cell in your pack to quickly bleed off the excess energy. I tried this method using a random car light bulb with some alligator clips on the leads. You need to watch the cell voltages closely because its easy to go too far.

Cheap and dirty balancing

Method 3 – passive equalization

What does NOT work is the old recommendation of connecting your new cells in parallel and letting them passively equalize for hours or days. This does not work because of the flat charge curve. They are at almost the same voltage even if they are far apart in state-of-charge. Basically the cells don’t know that they aren’t balanced unless you can push them into the very top end of the charge cycle.

Matched cells

What about cell matching? Cells have a certain internal resistance. Grade-A cells are tested at the factory to confirm that their internal resistance is acceptable, usually <1 milliohm. If your battery pack is made of grade-B cells or cells of different ages or if they have been damaged before, then they are not matched. Mismatched cells will quickly become unbalanced when the pack is cycled. This is one reason why you should pay for good grade-A cells.

I bought 4 of the very cheapest low grade garbage cells from aliexpress, just for experimenting. I balanced them several times, but after even 1 cycle of charging and discharging, they are way out of balance. This is because they are not matched at all. Some cells have a high internal resistance, so they get hotter than the better cells, and this puts them at a lower state of charge. If you are trying to use crappy cells like this, you will only be able to charge them up to ~80% to avoid constant cell over-voltages. This might be good for a big cheap solar storage bank, but it can cause big problems for a pack that you cycle daily, or use with large loads.

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Unknown PIN or password

If you are using the Android app version 3.1.1026 or the one from google play, there is a conflict with the newer BMS firmware version 2.1.

The BMS does not have a password function, but the app still expects it to be there. It will send the blank PIN (000000) and get a negative response from the BMS, and the app gets stuck in this loop.

This has been resolved in the iOS app.

We are also coming out with our own new android app soon that will solve this problem.

For now, you can try the older version of the android app 3.1.1015, or try the iOS app, or the desktop app via USB. Links to these applications can be found here:

If you need to use the USB desktop app to fix this, you can email and request a free USB module.

Note that the existing Android app version 3.1.1026 will never work with BMS firmware 2.1 even if you reset the password.

We apologize for the inconvenience.

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Password Reset , communication failure.

Starting with firmware version 0x17 sometime in 2021, JBD added a password function to the BMS firmware, without telling anybody.

The BMS doesn’t have a physical reset button, so there really isn’t a practical way to implement password protection. (without risking a permanent lockout)

This has caused some of our customers to be locked out of the settings on their BMS if an unknown password is set.

The basic monitoring functions are unaffected, however the JBD android app will not display the basic info without the correct password. The iOS app and the desktop app(s) will still display basic info without the correct password, but will not display parameters.

This is sometimes reported by the app(s) as a communication error because the BMS is not returning the requested data, only the “bad password” message.

The solution may be a password reset

The newest version of JBD’s desktop app includes a tool to clear the password, which actually makes the whole password function useless at best.

There is a screenshot included in the zip file with instructions, since the app does not display english labels on that page.

You can now use our release of Eric Poulson’s BMS-Tools desktop app to send a password reset command.

Either way, You will need a USB communication module.

Find the application download links here:

What’s next

After the next version update, the xiaoxiang-bms iOS app will have a password reset tool.

We do not know when a new version of the JBD android app will include the reset function.

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How to charge LiFePo4

LiFePO4 does not need a multistage charging profile.

The manufacturer of our 100 ah cells, and the MFG of the common 280ah cells specifies charging at a rate of 0.5c up to a maximum voltage limit of 3.650 per cell.

Based on the LiFePo4 charge curve the cells will reach nearly 100% charge at 3.500v per cell, so this is our recommended target voltage (14.0v per 4-cell battery, 28v per 8 cell battery).

Note that there is no mention of “float” charging. This is only applicable to lead acid yet most charger designs include a float setting for LiFePo4, usually 13.2v for a 4 cell battery.

Why is this a problem? Because at 13.2v the battery will be significantly discharged, and so you will observe the battery going through a deep cycle after every full charge, even though it remains plugged into shore power.

Therefore the ideal charger for LiFePo4 batteries (in our opinion) is a current limited power supply set to 14.0 to 14.2 volts.