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Coulometer error creep

May 04, 2020 — BarryK

I posted about installation and calibration of the coulometer, that displays battery state-of-charge (SoC):

I used the Amptron 15A LifePO4 mains-input battery charger to charge the battery to 100%, then set the coulometer to 100% and capacity 50AH.

There is however, a problem: a battery charge/discharge cycle is not 100% efficient, there is some loss as heat. The battery may be capable of supplying 50AH, however, it will require more than 50AH to charge it fully from empty.

Lead-acid technology battery are very inefficient, only about 70-80% efficient in the discharge/recharge cycle. LiFePO4 batteries are much more efficient, 90-92% round trip, see this document:

The problem is that the heat loss is going to mean the coulometer will read incorrectly, becoming more incorrect with each discharge/recharge cycle. This is what will happen if we start with a coulometer reading of 100%, discharge to a reading of 20% SoC, then charge back up to a reading of 100%, then down to a reading of 20%, and so on:


...this is showing the actual battery capacity (the values given are for illustration only). After some charge and discharge cycles, when the coulometer drops to reading 20%, the battery will actually have reached 0%.

This is bad news, however, the coulometer will self-correct every time the battery is charged to full. For example, I plugged in the Amptron mains-input LiFePO4 12V 15A charger, and plotted voltage, current and state-of-charge (latter as read by the coulometer):


The coulometer has reached a reading of 100% before the mains charger has finished charging. As you can see above, the coulometer is reading 100% at 29 minutes, and the charger keeps going and turned itself off at about 31.5 minutes.

Note, after about a minute, the charger turned back on momentarily, then did so again after about another minute. I didn't catch the voltage/current readings, it was a very short duration.

Reaching 100% reading on the coulometer before the charging completes is a correction mechanism, however, a problem is that this self-correction is only going to happen if the battery is charged to full, or nearly full.

We really need another mechanism for the coulometer to self-correct. I don't see how we can use the charging voltage, however, the coulometer could be set to read 0% if the battery voltage drops below a certain value. This web page is a good read, and has some information on discharge voltage:

...that link has a graph of battery voltage while discharging, though it must be recognized that this is dependent on the discharge current. The graph is reproduced here:


The Battery Management System (BMS) built-in to the battery will automatically disconnect the output if the voltage falls below a certain value -- from a bit of reading, this seems to be in the range of 2.5 to 2.8 volts per cell, which will be 10.0 to 11.2 volts for a 12 volt battery.

My Amptron 50AH battery has a BMS that turns the battery off at 10.0 volts.

The coulometer setup allows entry of a minimum voltage, below which the % charge will immediately drop to 0%. So, how about 11.2V?

OK, I have done that, have programmed 11.2V as the 0% limit in the coulometer. This will give an automatic correction when discharging the battery. The battery will be down at around 5% actual charge to trigger this.

Discharging a LifePO4 battery to 5% SoC will reduce the lifetime, but this should be a correction factor that is only triggered occasionally. Hopefully. Even if it is triggered often, I will still expect to get about 2,000 cycles, and even if I cycle the battery every day, it is going to last several years. 

One more point to make about coulometer setup: does a battery rated at "50AH" actually deliver 50AH? If not, then the capacity entry in the coulometer will have to be adjusted accordingly.

It should be noted that the "error creep" that I have reported in this post is a known problem, and it occurs on many devices, such as mobile phones and electric scooters. 

Tags: nomad

Powerbox front-page in nomad section

May 03, 2020 — BarryK

I have created a front-page for the lithium powerbox project, in the "nomad" section of my website:

The nomad section is a gradual accumulation of documents for and about "nomads", especially "grey nomads" -- these are people who have chosen an alternative lifestyle on-the-road, periodically or permanent. Here is the front-page to the "nomad" section:

Ha ha, I am in lockdown, as are all the grey nomads in Australia, but restrictions are easing. 

Tags: nomad

Powerbox circuit diagram updated

May 03, 2020 — BarryK

As the "protected +bus" has been added, and the binding-posts are not shown in the original circuit diagram, I have updated the diagram:


The earlier blog post has been edited and the latest Dia and SVG files posted: 

Tags: nomad

Lithium powerbox photo-shoot

May 02, 2020 — BarryK

The powerbox is complete, for now anyway. There have been a series of blog posts as the project progressed, and the intention is to create a front-page for the project, that will link to all of the blog posts, in the correct sequence for anyone who wants to build one.

Here is the previous post:

There were some refinements to improve battery-terminal accessibility and wiring-board layout, and I edited the second-from-last post:

Now for a photo-shoot, with the powerbox posing at various angles for the camera:





Need to do some more testing of course, and go camping! 

Tags: nomad

iTECHBCDC25 DC-DC charger problem resolved

May 01, 2020 — BarryK

In the last post in the lithium powerbox project, I reported a couple of issues with the DC-DC charger:

I sent an email to iTECHWORLD Service department, reporting two problems:

  1. The battery-type selector button did not work, it was stuck on "calcium"
  2. The MPPT regulator was not tracking at the peak power point of the panel

I received a reply from Jason, that has resolved the first issue, number-2 is looked at a bit more today.

Before receiving the reply from Jason, early this morning I thought it necessary to perform a more thorough test of the DC-DC charger, solar charging...

Nice sunny day today, last chance as rain forecast over the next several days. In fact, almost exactly one year ago, on an almost identical day, mid-winter, sunny, hardly any breeze, ambient 16 degC, I tested three solar panels, including the Atem Power "250W" panel:

...and recorded a peak power output for the Atem Power panel of 157.2W.

MPPT charging

Early this morning, Friday 1st May, 2020, 9.10am - 9.30am, have tested charging the iTECHBCDC25 with this same panel. Ambient is 17 degC, irradiance is 820W/m2 -- a year ago it was almost the same, 800W/m2.

As the DC-DC charger is stuck on "calcium" battery type, I used my Waeco Coolpower 44 battery box, which has a deep cycle 44AH AGM battery, a closer choice to the calcium charging profile than the lithium battery. Here is a photo:


As the battery is fully charged, I loaded it down by a 40W globe running via an inverter. The panel was outputting 20.3V at 4.99A (101W), and the battery voltage was 14.7V.

I needed to load the battery down a bit more, bring down the battery voltage, so that the DC-DC charger will think the battery state-of-charge is low, and will then pull everything it can from the panel. So I attached my air compressor. Now getting 15.4V at 9.6A (148W) from the panel, battery voltage 12.1V.

Good, but a year ago, I determined the peak power point of the panel to be 16.21V, power output of 157W. So that issue is still there, the iTECHBCDC25 is pulling at a bit off the peak power point.

But, to be 100% certain about this, I really should test for the peak power point at the same time as testing with iTECHBCDC25.


Moving on, the issue of the battery-type button being stuck on "calcium". Here is a photo:


Fast forward to this evening, I read my emails, and here is the reply from Jason:

In regards to the DCDC charger being stuck on calcium, to set it to the correct profile, you will need to disconnect the solar input and auxiliary battery connections, and only have the main(starting battery) connected, in order to change the settings.

...riiiight. Tonight, I used the Waeco Coolpower as the "car battery", connected to the red and black cable of the iTECHBCDC25, and hey, was able to change to "Lithium" type. Good, but the User Manual does not state that the "car battery" only has to be connected. Quoting from the User Manual:

Once correctly installed, the iTECHBCDC25 has MPPT solar
charging for increased solar efficiency.
Once your solar panel has been connected to the solar
input section of the iTECHBCDC25 the charger will
transfer to solar mode when the vehicle is not running.
The initial default setting is for AGM/GEL batteries.
If you are charging a battery of a different chemistry
simply change the battery type by pressing the battery
type selector button on the front panel of the
Once you have selected the new battery type the
iTECHBCDC25 will remain on this battery type until it is
Solar charging requires a solar input of 16 to 25v.
Your solar panel will need to be unregulated when
connecting to the iTECHBCDC25.

Well, that is a relief, the charger is not broken.

My Hyundai Getz has a radio/CD-player that, if the car battery is disconnected, after reconnection, the security-code has to be be re-entered for the radio/CD-player to work again. Of course, that is for security, but it is to be hoped that the DC-DC charger will remember the battery-type setting if the "car battery" is removed! ...yay, it does! 

Tags: nomad

DC-DC charger issues and powerbox improvements

April 30, 2020 — BarryK

Ha ha, I am an inveterate tinkerer -- in the sense of always wanting to experiment, modify and improve whatever I have designed and constructed. In fact, the journey is often as much or more fun than the destination. Camping, for example -- preparing for camping is as much fun as actually going camping.

Nice sunny day, so this morning took the powerbox outside and connected the Atem Power "250W" solar panel. Yes, it does charge, but there are two serious issues. I have sent an email to iTECHWORLD technical support, and will await their response before deciding if these "issues" can be described as faults. Stay tuned for developments.

The previous blog post for this powerbox project:

In earlier posts, I have identified some limitations in the physical design of the powerbox and the wiring layout. Decided to fix them,,,

Getting at those battery terminals is just too awkward and dangerous. The top-plate of the box is screwed on from the underneath. In retrospect, this was dumb, but there was a reason originally -- wanted to place the DC-DC charger flat on the top of the box, without protruding screws getting in the way.

Screwing from the top allows the screws to be removed and top panel lifted off. This allows access to the battery terminals. So, I pulled off all of the side panels, removed the screws holding on the top panel:


...yay, easy access to the battery terminals. You can't see the positive terminal in the photo, as it is covered with black electrical tape.

Late night right now, tomorrow will apply screws from the top. It will be easy to remove them and lift up the top panel any time it is desired to access the battery terminals, or anything else.

Another problem is that extra terminal block just floating there, looking messy, see this earlier snapshot:


Another task for tomorrow will be replacement of that floating terminal block with a small busbar mounted on the spare space on the wiring-board. And a bit of a tidy-up of the wiring. 

EDIT 2020-05-02:
The top plate has been modified, with screws on the top, so that it can be opened up. I have edited the original blog post:

To get rid of that floating terminal block, I have installed "protected +bus", using one of the 8-way busbars from Jaycar, though (currently) only need three ways:


"+bus" is connected directly to the battery positive terminal, whereas "protected +bus" is on the other side of a 30A circuit breaker. Or rather, is, once it is wired up:


And, after screwing the front and back panels into place:


That is better. It is still crowded of course, but that was part of the exercise, to build a very compact powerbox.   

Tags: nomad

Installation of the coulometer LCD display

April 29, 2020 — BarryK

The previous post in the powerbox project:

The finishing touch is to install and calibrate the Amptron coulometer. As already discussed, the shunt is already mounted on the wiring-board. This is what I have:

If you search on eBay and elsewhere, for "battery coulometer", you will find this available from many vendors, and there are also setup videos on YouTube.

Installing the coulometer

There is a shielded cable to connect the shunt to the LCD display, and this is a problem, as it is very long. This is for the situation where you want the display somewhere else, maybe on the dashboard of the vehicle, or whatever.

I want to cut it right down, as the LCD display is close to the shunt. So, I cut the plugs off each end, and joined the cable. I am familiar with delicate electrical work, as I was an electronic engineer in my working days. I have a small soldering iron and fine solder for the purpose. The wires inside this cable are so tiny, this photo blows it up, but in actuality they are so tiny. The photo shows after I have stripped each wire and twisted them together:


...I then very carefully soldered each join, then put heatshrink tubing over each join, then bound it all together with electrical tape.

I know that such delicate work will be a challenge for many -- well, you could just go and buy a ready-made powerbox! But it is not insurmountable. The wires are so tiny, I stripped them with my thumbnail, pressing onto the table -- my cable stripper does not handle such tiny wire.

Calibrating the coulometer

I watched one of the YouTube videos on how to calibrate the coulometer, but it is misleading. They set "full" and "empty" corresponding to certain battery voltages, but this could be very inaccurate.

Here are the instructions:



What I did was first charge the battery to 100%, using the Amptron mains-input lithium 15A battery charger:

...this has eyelet lugs, that I replaced with a black Anderson plug -- very convenient, just plug it into the powerbox, and it will charge until 100%.

Following the above instructions, I then pressed the "up" key for three seconds to set the display at 100%.

Following the "User Settings", I then chose battery capacity as 50AH -- note, the buttons have to be very firmly pressed, and getting out of "engineering mode" proved to be difficult.

That's it, I did not set "FULL" and "ZERO" voltages, as that is not a reliable indicator of 100% and 0%.

Using the coulometer

I plugged my Powertech 150W inverter (purchased many years ago from Jaycar, and used on many camping trips) into a cigarette lighter socket, and plugged in a desktop lamp:


Fantastic, working! Here is the LCD display:


Very nice! That snapshot is after running the lamp for a short time, the capacity has dropped a bit below 50AH.

EDIT 2020-05-04:
After further reflection, I decided it is a good idea to minimize "error creep" of the State-Of-Charge, by setting a lowest voltage for 0%. See this later post:   

Tags: nomad

Wiring-board installed in powerbox

April 29, 2020 — BarryK

In the previous post, I wrote about some compromises when wiring-up the wiring-board:

Now for some photos showing the wiring-board installed. Firstly, here is the wiring-board all ready to be installed:


And here it is installed:


...quite crowded, but more so because of that extra terminal block. The previous post discussed how that could have been improved, with a distribution bus mounted on the wiring-board -- there is some space bottom-right of the board.

I also discussed in the previous post, the limited space to access the battery terminals. The unconnected eyelet lug in the above photo is waiting to be bolted to the battery positive terminal. I managed, just, and solutions to obtain better access are mentioned in the previous post.

EDIT 2020-05-02:
Both of these problems, battery terminal access and messy floating terminal block, have been fixed. See this later blog post: 

Tags: nomad