FAQ

TL;DR – Absolutely.

The cells are internally wrapped with non-conductive fiberglass boards, then carefully packaged inside the battery and foam lined to prevent any movement and protect against shock. The BMS is then stacked on top of the cells, with an air gap between the battery and BMS which provides a thermal advantage compared to just placing the BMS directly on the battery. Direct placement and then heavy usage can cause the heated BMS to heat the battery cell and potentially cause damage. We protect from that with the air gap.

The Battery Management System is like the crossing guard at a school. It controls the charge and discharge, only allowing electricity to flow if it’s safe to do so. It runs 24/7 monitoring 2 temperature probes, voltage, amp draw, and looking for any possible short circuit. If any negative condition occurs, the BMS stops the flow of electricity temporarily. Once the negative condition is gone, the battery will automatically begin allowing the electricity to flow.

Which is better Lithium Ion or Lithium Iron?

This is a great question. In short, Lithium Iron Phosphate has many distinct advantages over Lithium-Ion, safety being the biggest advantage.

Newcastle Systems has a great write up on the pro’s and con’s of each technology and why the LiFePO4’s are a better choice.

In an nutshell, the phosphate has much better stability which makes it far less likely to catch fire. Combine that with good density and long life expectancy, and the LiFePO4 is a much better battery.

Our batteries are not designed to handle the large amp draw that engines require when starting up.

There are specialty LiFePO4 batteries that can handle the high current demands of starters but they are not very mature in the industry and longevity is still a question.

In general, no you can NOT use LiFePO4 batteries to start an engine on a vehicle or boat.

However, small engines with smaller Amp requirements may work if you connect multiple LiFePO4 batteries together. For instance, the small RV generators could possibly be started if you have 3 or more of our 12V 100 Ah batteries connected together. This is an advanced use of our battery, and you should always contact an electrical expert for advice before attempting to do something like this.

For our 12 Volt batteries we recommend any commercially available charger that has a setting for 12V lithium.

Yes, a proper BMS should limit the max current being drawn from the battery cells.

However, they do not slow down or regulate the Amps.

For example, our BMS’s have two parameters that can be seen in the Bluetooth app called “Charge Overcurrent” and “Discharge Overcurrent”. If that value is exceeded, the BMS will shut off the charge or discharge paths respectively. The paths can be selectively turned on/off by the BMS to protect itself and the cells.

If you want to regulate the Amps being pulled off your alternator, you'll need a DC-DC converter. There are several tutorails on that subject easily found with a quick search.

If you are keeping a close eye on the BMS app during the final stages of charging, you may notice that one or more cells tend to go up faster than the rest. Seeing this can appear alarming, however, it’s perfectly normal.

Having the advantage of peeking inside the battery with Bluetooth raises a lot of questions and this is a common concern.

First, realize that 3.4V per cell is considered to be the “full” voltage of a battery cell or 13.6V for a 4 cell pack. There is very little capacity above 3.4V. In fact, there is less than 1% of additional capacity to be gained between 3.5V and 4.2V per cell.

There is a misconception that 14.2V must be reached to be considered full. 14.2 to 14.6V is what most charger manufacturers will set their chargers to. This number comes from the max that 4 cells could potentially reach if they all went to the accepted absolute max voltage of a LiFePO4 cell of 3.65V. (3.65 x 4 = 14.6). The technical max is 4.2V, beyond that excessive heating and damage will occur.

So then, why does one cell “race” in the App?
Because, at some point over 3.4V, at least one of the cells is going to be at 99%. And that last 1 percent of capacity exists from around 3.4 to 4.2V, as that last bit is pushed in, it appears to be “racing”. This can also be seen on a graphical representation of the LiFePO4 charge profile, which is generally referred to as a “flat” charge profile. Imagine watching that last portion, beyond the “knee” as it was charging, it would appear to be “racing” on a meter or in the app.


So, are the LiFePO4 cells matched?
Yes, the cells are matched for internal resistance. You’ll notice as soon as the batteries begin to discharge the cells will typically come back to within .005V of each other and stay there until you get close to 3.0V (empty). Towards the bottom of the charge profile, one cell will again “race” through the voltage range as it loses its last 1-2% of capacity.

Will the top-balance get better over time?
Yes. the top balance will get slightly better over time but probably never be perfect.

So, what is acceptable?
You’re paying for watts when purchasing a battery, so as long as you are getting the rated capacity, you should consider that acceptable. Conducting a simple LiFePO4 Capacity Test with a $20-40 device would show you if your battery is meeting its rated capacity.

TLDR; Short answer is no, you do not need a special charger with LiFePO4 batteries.
But, it’s recommended to always use a charger designed for your chemistry.

This is a very common question, and a very good question to be asking.

Our batteries come with a built-in Battery Management System (BMS) that protects the battery from under and overcharging. You can think of the BMS as the brains of the battery, and these brains have the sole purpose of keeping the battery safe, always on watch 24/7, and never taking a break. You can watch a short LiFePO4 BMS video we made discussing all the settings available on our 50Ah and up batteries.

So, let’s say you hook up a 24V charger on accident, to one of our 12V batteries, the BMS would detect high voltage, go into “overvoltage protect” and shut off the charge path to prevent damage to the battery cells. Pretty cool right?

So, you can see why you don’t need a “special” charger for our LiFePO4 batteries with built-in BMS.

What about charging LiFePO4 batteries with a lead-acid charger?
The problem with lead-acid chargers is that they run a “charge profile” and vary amperage based on the voltage of the battery. If the voltage is low, they go into “bulk” charge mode, which means the charger sends max amps. Then as the voltage increases the charger will begin to send fewer amps to the battery, this is called the “absorption” phase. Finally, once the charger gets near a predetermined voltage of about 13.1 volts, the charger will transition into a “float” stage where it just barely keeps any amperage on the battery to keep it “topped off”.

Due to these steps down in charging amps, the first problem arises, that is that LiFePO4 batteries are being charged much more slowly than they could be. A lead-acid charger will typically charge a LiFePO4 battery 4-5 times slower. That is obviously a huge disadvantage of using a lead-acid charger on LiFePO4 batteries.

You’re next question might be “Isn’t it better to charge slower?” No, not with LiFePO4, there are no negative effects of charging at high charge rates on LifePO4 batteries.

The next issue is that even at 13.1 volts, a LiFePO4 is not fully charged. A fully charged LiFePO4 sits around 13.6 volts. Between 13.1V and 13.6V is a pretty good bit of power and worth noting that you won’t get 100% of your available power if charging with lead-acid chargers. The total amount of capacity varies based on your charger and what the float voltage is set to.

Summary
No, you do NOT need a “special” charger to charge a LiFePO4 battery, the BMS will protect itself at all times. However, you will have significantly longer charge times and decreased total capacity using lead-acid chargers.

Not all batteries are created equal and if you want find out how much juice your LiFePO4 battery, or any battery for that matter, can really produce then read on.

There are a couple of ways you can check capacity, both involve fully charging the battery and then depleting it down to it’s cut-off voltage. Typically, tests are done at a .2C discharge rate, which is to say 20% of the capacity of the battery. So, if you have a 100Ah battery, you typically want to discharge it at 20 Amps per hour (20Ah). LiFePO4 batteries are a bit more robust than lead-acid and you can discharge at a higher C rate without much change in the useable capacity. It has to with Puekert Effect and a little beyond the scope of this article.

The Real World LiFePO4 Capacity Test
The advantages of the real-world test are that it doesn’t cost anything, you can do it with just about any known load source, and they’re fairly reliable. They’re reliable, as long as you have an accurate amp rating from the load, so be sure you know how many amps the load your using is consuming. Disadvantages are that you have to keep a close eye on the battery so you know exactly when it stopped outputting power for the calculations. This is also not a very technical test with specific numbers to report, it’s more of a feel-good test.

With that, the steps are simple.

Charge the Battery to 100%
Note the time
Put the load on the battery
Wait for the battery to be expended
Note the time again
The formula is Amps x Time (hours) = Amp Hours

If you know with certainty, you were pulling 20 amps for 5 hours, then you got 100 Amp hours (Ah).
2 Amps for 3.5 hours, then you get 7Ah.

Be aware, there are a lot of environmental factors that can affect a test like this, temperature, wire size, wire length, etc., all play a factor in actual results. The real-world test is a really good litmus test to know if you’re getting close to what you paid for and typically provides the warm fuzzy that things are as they should be. Anything grossly off should be investigated further.

The Metered Bench Test
Using something like a Hall Effect meter or a dummy load capacity tester can be a more accurate way to measure how many amp-hours you’re getting from your LiFePO4 battery. They typically give you Amp hours and Watts.


$17 Amp/Volt Meter

$31 Hall Sensor Meter

$56 Dummy Load Tester

Each of the above devices will show with pretty good accuracy, what the capacity of your battery is. Each product has advantages and disadvantages. The last, the dummy load tester is probably the most accurate and flexible option. You can simply connect the battery, set an Amp draw, reset the counter and walk away. When the battery shuts off, the tester stops the clock. It will even retain the info if the power goes out. The other two are not bad, nor overly complicated, but results and opinions vary on their accuracy, so buyer beware and read the reviews on Amazon.

The general steps for this LiFePO4 capacity test are similar to the first:

Fully charge the battery
Reset your test device
Put a load on the battery
Wait for the battery to hit the low voltage cut-off
Check your results
If you’re doing a capacity test, be sure to charge the battery until the battery reaches 100%. Then discharge the device until the battery is fully depleted. If you’re using a LiFePO4 reBel Battery with Bluetooth, you will be able to see through the App when you are 100% as well as when you are at 0%.

When the test is complete, you should have a reading at or above what the battery is rated for. You may see slight variations based on temperature, wire sizing, wire length, and other factors. If you’re more than 5% off the rated capacity, try a couple more cycles as sometimes “kinks” are worked out by simply cycling the battery. Less than 5% variations can generally be attributed to environmental factors.

You may notice that the percentage showing on the JBD Xiaoxiang app will drop over about 2-24 hours after reaching full charge.

This is due to the internal cells settling out and very normal. Google search: why do lifepo4 cells drop after charging?

So, why does the percentage drop in the app?

In the parameter section within the app, there is a parameter called “cell full voltage” which is set to 3.4V. So the percentage meter uses that value to determine 100%. After the cells have settled, usually between 3.35V and 3.4V, the meter falsely sees that as lost watts and it will show somewhere between 75-100%. Another setting is for the 80% mark and the app is interpolating the difference to give a percentage.

There is no loss of capacity and the battery will still meet it’s rated amount of watts, it is still considered fully charged.

You can change the “cell full voltage” if you want to ensure you always read 100%, 3.35V would probably be sufficient to edit it to and would not void your warranty, this is simply an informational setting.

If you need the admin version of the JBD app, please email support@rebelbatteries.com to get the download link.

A Battery Management System (BMS) is arguably the most important piece of a smart battery. It protects the battery from outside forces that can damage the internal cells.

If the battery short circuits for example, the BMS can detect this and shutoff for a predetermined amount of time.

Lithium Iron Phophate batteries are sensitive to extreme temperatures, so there are temperature probes connected to the BMS, and predefined settings determine when the battery should shutoff to protect itself.

There are also over/under voltage protections that will prevent discharging the LiFePO4 batteries to deeply or overcharging to dangerous levels.

Think of the BMS as a micro-computer that’s running 24/7 protecting itself and your investment.

You can use 123456 to login.

If it repeatedly asks for the pin, try rebooting your phone.

The JBD BMS pin was an uncompleted feature by the manufacturer. The app never actually uses it, you can put in any 6 digits and it will work. We only tell people 123456 in case they someday do implement it, at least then we are all on the same page. In fact, the iOS App never asks for a pin and logins pretty flawlessly, even on our iPhone 5 for testing.

This is not an uncommon problem with the XiaoXiang play store App. There are a couple things to try.

Enable Location in your system settings
Reboot the device
If it asks for a pin, use 123456

If you still can not log in to your BMS, please submit a trouble ticket and we can provide you with an “admin” version of the app that has a much higher success rate. It comes with a LOT of options that can alter settings within the BMS.

Below is an App walkthrough we did comparing the “consumer” version to the “admin” version.