DIY Portable Waterproof Power Bank

       When I was a kid, I built a few versions of a portable battery power source based on sealed lead acid batteries. I started with a 6V 4Ah cell... Later growing to a 12V 7Ah and stopped there when I always kept destroying batteries.

       I've been contemplating on building a new version of the portable power source for a long time but did not really have a purpose. I wanted to build one "just because".

       It will come in handy though as a portable test supply or jump starter? Or even an emergency power source during blackouts (yeah, like I still needed one, with the home RE setup working great).

       But I still wanted to make one :) This time around, it will be charged by either mains or via its own solar panel and will use the newer LiFePO4 cells.

I started by ordering twelve Headway 40152 cells which are 3.2V 15Ah LiFePO4 batteries used for electric bikes.

Wired them into a 4S3P configuration with solid copper bussbars.

Temporarily connected my DIY version of the balancer circuit. This is basically the same as those in the market that bleeds the excess voltage to keep the cells balanced. This one drains about 900mA of balance current.

Here are the schematic and PCB layout images. You'd need one per parallel bank of cells. Since this is a 4S3P battery, I used four identical balance modules.

Top bussbars. The battery is connected to my bench supply to charge up the cells for the initial balancing of the pack.

Yep, I've been keeping that circuit for so long. I made it back in late 2012.

Here it is. I started with an imitation Pelican case which is decent enough but much cheaper.

Created a panel from acrylic and added switches, circuit breakers and outlets.

The breakers are 15A each. One for the cig lighter outlet, the other for the binding posts. The red button is used to turn the meter on or off.

The wiring is neatly zip tied.

Here it is ready to be connected.

Another look at the balancing module.

This balancing module has a green LED that turns on when the cell reaches 3.65V and turns off at about 3.5V. The green LED indicated when the resistors are dissipating the excess voltage. There is also a red LED that lights up when the voltage is below 2.5V indicating that the cell is empty and is nearing damage due to over discharge.

All wired up!

Here it is ready for use.

I found out that the resistors dissipate a lot of power and gets really hot so I added heatsinks.

I also added a DIY solar charge controller based on this schematic but parts and layout are shrunk down to a 3A capacity. This was also used in my early RE setup at home. The PCB is dated late 2010!

Here it is again ready to be put back to the case.

To prevent damaging the battery, I used optocouplers to sum up the low voltage signal from the red LEDs.

The two wires travel to the other side of the pack....

And go into this shrink wrapped module which is basically three IRF1503 MOSFETs in parallel. This disconnects the load when any cell reaches 2.5V to prevent over discharging.

And it is, again, time to put back in the case.

But then! The amphour meter I ordered a while ago arrived and found that it is perfect to use here as the discharge profile of LiFePO4 is pretty constant voltage and then it suddenly drops at near fully dicharged which makes it hard to tell SOC from a voltmeter.

So I had to tear everything apart. Again.

I started with a fresh new acrylic sheet. Marked and drilled the holes.

Milled the square and rectangular openings for the switches and LCD display.

After some cleaning, sanding and filing, I now have a new panel.

The amphour meter came with a 200A 75mV shunt which is pretty compact but still too large to fit in my application.

So I had to go back to how I did it before. I used a stainless allthread and made my own shunt from it.

After calibrating the meter (Found the manual online searching for "TF01N battery meter") it worked pretty well.

Finally, it is back in the case.

The completed power bank weighs in at a little over 8kg (17.6lbs).

A little heavy but think about 45Ah worth of lead acid will be heavier even just for the battery alone. A quick google for a 45AH SLA weighs about 14.6kg (32.12lbs) and that is just the battery alone! Although it is more expensive, When I factored in cycle life at 100%DOD and price, it ends up costing the same as used SLA batteries!

Here is a pic of the LCD.

For a complete portable power source, you'd need a way to charge it when there is no power available. I made my own folding solar panel from two 20W PV modules, some cabinet handles and hinges.

The two PV modules are wired in parallel.

Here it is on the roof for testing.

The handles also double as cord spool and when the cable is looped around it, also serves as a lock to keep the modules closed.

The solar charge controller is working! It is set to regulate at 14.45V.

       Currently, it is being used for running a 12V fan or two for the bedroom just so that it is being used. It is then charged during the day via the folding PV temporarily placed on the roof.


       18 May 2015:

As with everything else.... I could not leave this alone. I made a new shunt using two 10mOhm resistors and a PCB.

Here it is installed. The idea is that the higher resistance makes better use of the ADC bits for a more accurate current reading.

The sense lines are connected right at the copper PCB foil.


       16 Sep 2016:

Tragedy struck!

I have this in daily use for more than a year now powering various gadgets just so that it gets used. This afternoon, Looked at the display and it was reading 16.6V and the balancer modules are very hot!

For some reason, the charge controller did not cut off to 14.45V but instead went right up. It happened one time but it was lucky since it froze in the OFF position, this time, it was stuck in ON. I'm not really sure if that is what happened or the equalize switch was accidentally turned on when I cleaned it one time.

The cells taken out and each one was tested for internal resistance.

Out of the twelve, 9 were still good at 4mOhm, one was 5mOhm and the other two was high impedance. Great.

So, I had my older pack that I used for my HID bike light but only occassionally used for other things (converted cordless drill, etc) and tore it apart for the cells. Turned out it had three 5mOhm cells and one 7mOhm cell so I divided the ones that measured 5mOhm so each parallel bus had two 4mOhm and one 5mOhm cell. Back in business! But I will have to buy another set for the old pack eventually.

While I was at it and it was disassembled, I modified the charge controller oscillator to reduce the likelihood of it stopping again.

I also painted the acrylic black.

I think the black looks really nice.


       04 Jun 2016:

Took this powerbank with me on a roadtrip along with the folding PV to power gadgets and stuff rather than rely on the car for power to preven draining the car battery and getting stranded. Would also work well as emergency jumpstart power.


       21 Sep 2016:

Before I damage it further. I modified the over discharge protection and also added over charge protection.

This is a back to back quad of IRF1503 MOSFETs to do the over charge and over discharge control.

The balance module LEDs are removed and wired to quad optocouplers.

It took a while to connect all those fine wires.

Modules all connected up.

Three wires go around to the other side to the MOSFET bank.

MOSFET bank is heatshrunk for protection and tucked under.


       03 Feb 2017:

       We are planning a roadtrip up north soon so I needed a better way of having an auxilary power rather than the folding PV like last year. This time, I plan to mount a 50W panel on the car roof using magnets. They will be used for powering gadgets, phones and laptops rather than using the car battery. This would also allow me to jumpstart the car incase it gets drained for whatever reason.

I got a bunch of these N38 grade Neodymium magnets for cheap.

There's more underneath.

Made a bunch of these magnetic mounts to hold a PV on the car roof.

Arranged the magnets NSNS then SNSN to get a really strong pull then covered it in self adhesive felt to protect the paint finish.

The piece of braided hose acts as a flexible coupling.

Trying it out. First, I thought of putting it right behind the LED bar but it looks kind of off.

So I placed it further back. I think that looks better.

Takes a lot of force to pull off unless you twist the magnets to "peel" it off the roof. I think it should handle highway speeds just fine.

Page created and copyright R.Quan © 15 May 2015.